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Copy of Chemical Systems and Equilibrium
Transcript of Copy of Chemical Systems and Equilibrium
In most acid-base reactions, a proton (H+ ion is transferred from one reactant to another.)
A Bronsted-Lowry acid is a proton donor
A Bronsted-Lowry base is a proton acceptor
Amphoteric (amphiprotic substance)- a substance capable of acting as an acid or base depending on the chemical reaction; a substance that can donate or accept an electron.
Conjugate acid-base pair- two substances that differ only by one proton. The acid has one more proton than its conjugate base. The stronger an acid, the weaker its conjugate base. Conversely, the weaker the acid, the stronger its conjugate base. nvn The environment behaves as a defined chemical system. In this unit we develop mathematical equations that quantify chemical movement as they seek to achieve an equilibrium distribution. Weak Base is a weak electrolyte that does not ionize completely in water to form H+(aq) ions a compound that doesn't react completely with water to form an equilibrium that includes OH-(aq) ions Percent Ionization Since weak acids don't ionize completely in water. We can figure out the percentage of the acid that ionized using the following equation
p = ([H+(aq)]/[HA(aq)])*100
Where p = percent ionization (%)
[H+(aq)] = concentration of H+ ions (M)
[HA(aq)] = concentration of the acid (M) Example Problem
Calculate the percent ionization of propionic acid, HC3H5O2(aq), if a 0.05 M solution has a pH of 2.78
HC2H5CO2(aq) <--> H+(aq) + C2H5CO2-(aq)
[HC2H5CO2(aq)] = 0.05M
pH = 2.78
[H+(aq)] = 10^-pH
[H+(aq)] = 10^-2.78
[H+(aq)] = 1.7*10^-3M p= ([H+(aq)]/[HC2H5CO2(aq)])*100
p= 3.3% Therefore propionic acid ionizes 3.3% in a 0.05M solution Acid and Base Ionization Constant (Ka) (Kb) Is the equilibrium constant for the ionization of an acid (Ka) or a base (Kb) Ka = ([H+(aq)][prod]/[react])
Kb = ([OH-(aq)][prod]/react]) Relationship between Ka and Kb For acids and bases whose chemical formulas differ only by a hydrogen (conjugate acid-base pairs)
KaKb = Kw
Kw = 1*10^-14 For example
Barbituric acid, HC4H3N2O3(aq), an organic acid used to manufacture hypnotic drugs and some plastics, is a weak acid with a Ka of 9.8*10^-5. AN industrial process requires a 0.25M solution of barbituric acid. Calculate the [H+(aq)] and the pH of this solution
HC4H3N2O3(aq) <--> H+(aq) + C4H3N2O3-(aq) Ka = 9.8*10^-5
H2O(l) <--> H+(aq) + OH-(aq) Kb = 1.0*10^-14 ([H+(aq)][C4H3N2O3-(aq)])/[HC4H3N2O3(aq)] = Ka Ice Table
HC4H3N2O3(aq) <--> H+(aq) + C4H3N2O3-(aq)
I 0.25 0.00 0.00
C -x +x +x
E 0.25-x x x (x(x))/0.25-x = 9.8*10^-5
since 0.25>>9.8*10^-5 then 0.25-x = 0.25
x^2/0.25 = 9.8*10^-5
x^2 = 2.4*10^-5
x = 4.9*10^-3 Since x = 4.9*10^-3 then [H+(aq)] = 4.9*10^-3 pH = -log[H+(aq)]
pH = -log(4.9*10^-3)
pH = 2.31
The [H+(aq)] of a 0.25M solution of barbituric acid is 4.9*10^-3M, and the pH is 2.31 Polyprotic Acids Is an acid that posses more than one ionizable proton (H+(aq)).
Each of these ionizable protons possess their own acid ionization constant. Ka1 for the first ionization reaction and Ka2 for the second... etc.
Ka1 usually has the highest ionization constant followed by Ka2... etc.
Ka1> Ka2> Ka3... etc. For example
H2CO3(aq) has two ionizable protons, Ka1 and Ka2
Ka1 occurs when the first H+(aq) ion ionizes
H2CO3(aq) <--> H+(aq) + HCO3-(aq) Ka1 = 4.4*10^-7
Ka2 occurs when the second H+(aq) ion ionizes
HCO3-(aq) <--> H+(aq) + CO3 2-(aq) Ka2 = 4.7*10^-11
Notice how Ka1 is much larger than Ka2, because of this Ka1 is usually used to figure out the pH of the acid Salts That Form Neutral Solutions Dynamic Equilibrium: a balance between forward and reverse processes occurring at the same rate Solubility Equilibrium: a dynamic equilibrium between a solute and a solvent in a saturated solution in a closed system. How Buffers are Made Buffers are solutions that have a highly stable pH that will resist changes in pH when small amounts of acid or base are added to the solution. Adding water to a buffer will generally not change the pH of the buffer. Buffers are created by mixing a large amount of a weak acid or base together with its conjugate. The weak acid and its conjugate base or weak base and conjugate acid can remain in a solution without neutralizing the other. How Buffers Work When hydrogen ions are added into the buffer solution, they will be neutralized by the basic part of the buffer. If hydroxide ions are added to the buffer, they will be neutralized by the acidic part of the buffer. These neutralization reactions will affect the overall pH of the buffer solution very minimally until all the buffer has been used up. Acidic & Basic/Alkaline Buffer Solutions An acidic buffer solution has a pH of less than 7. Acidic buffers are composed of a weak acid and a conjugate base (often a salt). The conjugate base is a strong base.
Alkaline, or basic buffer solutions have a pH of greater than 7. They are most commonly made from a weak base and one of its conjugate acid. The conjugate acid is a strong acid. Uses and Applications of Buffers Blood is a natural buffers that exist in many living beings including humans. Blood contains carbonic acid (H2CO3) and hydrogen carbonate (HCO3-) which work together to keep the pH of blood at a semi neutral 7.4. Keeping the pH of the bloodstream neutral is important because many proteins and enzymes will only work at a certain pH; therefore, changing the pH of blood can cause serious problems and potentially death. During extreme amounts of exercise, it is possible for the pH to be lowered to a dangerous level as hydrogen atoms begin to accumulate in the bloodstream. This can cause the blood to reach a point at which a buffer is no longer effective. All buffers have a limit, and once reached, the pH will begin to change substantially. Buffers are important in many commercial products as well. Shampoos, for example, contain buffers of citric acid and sodium hydroxide which help to balance out the natural alkalinity (baseness) of the soap as it would otherwise burn your scalp. Baby lotion is good for relieving rashes because it is buffered to a slightly acidic pH of 6. This prevents the growth of bacteria and other pathogens. Buffers can also be used in the production of alcohol. If the process of fermentation, which causes sugars to convert to alcohol in the absence of oxygen, went unregulated, a spoilage of material being fermented would likely occur. To prevent this, many manufacturers add a buffer to regulate the pH level making this spoilage essentially impossible. Buffering Capacity Buffer solutions are able to maintain a near constant pH when small amounts of acid or base is added to them. Quantitative measures of this resistance to pH changes is known as buffer capacity. The equation for buffer capacity is given by;
n represents the number of equivalents of added strong base/acid. Buffered vs. Unbuffered As shown by the diagram, buffers increase the amount of acid or base that must be added in order to cause a large jump in the pH. The higher the buffer concentration, the longer it takes until the jump/equivalence point is reached. Henderson-Hasselbach Equation The equilibrium constant (K) is the value that represents the concentrations of the products, over the concentrations of the reactants. It was first created by two Norwegian chemists named Cato Maximilian Guldberg and Peter Wage. This equation is shown as:
K = [C]^c[D]^d/ [A]^a[B]^b
In the above equation, A,B,C, and D are chemical substances in gaseous or aqueous phases, a,b,c, and d are their coefficients.
K is classified as the equilibrium constant because if you experiment with the same elements at the same temperature, using this law, the value of K will remain the same for that particular type of experiment. Key Characteristics of Equilibrium A few characteristics of the equilibrium law are as follow:
-The molar concentrations of the products are always multiplied together, and are always in the numerator, while those of the reactants are always multiplied together and are always in the denominator.
-The coefficients in the balanced equation are equal to the exponents of the equilibrium law expression
-Concentrations represent that of the substances at equilibrium
Remember that the concentrations, as usual for aqueous substances, are in mol/L Since a dynamic equilibrium is a balance between opposing processes occurring at the same rates, the reaction can be both forward and reversible. That is why the mathematical expression is written as A + B C + D. This means that the reaction can just go forward, reverse, and then go forward again perpetually unless something happens to disturb it. Generally, when you find the value of K for the forward reaction, it will be the reciprocal of the reverse reaction.
There are, however, limitations to the accuracy of the equilibrium constant. For example, if it does not hold true when the temperature changes. Taking a look at ammonia (NH), its value of K is 4.26 X 10 ^8 at 25 degrees Celsius whereas it is 8 X 10^ -7 at 400 degrees Celsius. Now don’t get confused, the value of K still remains constant no matter the concentrations of the products and reactants (although the percent reaction does also depend on concentrations), as long as the temperature remains constant. This is to say that the value of K and the temperature that the reaction is taking place at are somewhat proportional. Different types of Equilibrium and Magnitude There are two types of equilibria, heterogeneous and homogeneous. A heterogeneous equilibria is when the reactants and products are in more than one phase, and a homogeneous equilibria is when everything is in the same state.
Something to take note of is that equilibrium law expressions are always written from the net ionic form of reaction equations, and the constant concentrations of solids (and any spectator ions), don’t appear in the expression.
If you are not sure how much of the reactants are reacting with the products and vice versa, you need only look at the magnitude of the equilibrium constant. If it is greater than one, the concentrations of the products are much greater than those of the reactants. If it is close to or exactly one, the concentrations of reactants and products should be roughly equal. If it is less than one, the concentrations of the reactants will be much greater than the products. Solubility Constant The solubility product constant is represented as Ksp, and it represents the value obtained from the equilibrium law applied to a saturated solution. One topic that this idea can be applied to, and a very vast one at that, is the solubility of different sorts of salts in nature (and in animals). Many different animal species have different techniques for dealing with chemicals that would normally be very harmful to their body. For instance, most aquatic animals convert the toxic amino group into ammonia, which is highly soluble in water. This means that it never builds up in their tissues, and can be easily excreted.
Ksp is calculated when excess solute is in equilibrium with its aqueous solution. The equation to find the value of the solubility constant is the same as that of finding K for the equilibrium constant. The difference is that this constant represents the concentrations of the ions in the solution, to the power equal to the coefficient of each when in the balanced equation. Determining Precipitates The solubility product constant is represented as Ksp, and it represents the value obtained from the equilibrium law applied to a saturated solution. One topic that this idea can be applied to, and a very vast one at that, is the solubility of different sorts of salts in nature (and in animals). Many different animal species have different techniques for dealing with chemicals that would normally be very harmful to their body. For instance, most aquatic animals convert the toxic amino group into ammonia, which is highly soluble in water. This means that it never builds up in their tissues, and can be easily excreted.
Ksp is calculated when excess solute is in equilibrium with its aqueous solution. The equation to find the value of the solubility constant is the same as that of finding K for the equilibrium constant. The difference is that this constant represents the concentrations of the ions in the solution, to the power equal to the coefficient of each when in the balanced equation. These salts are made up of the cations of strong bases and the anions of strong acids. This causes them to have no effect on the pH of an aqueous solution because they cancel each other out.
NaCl (s)Na+ (aq) is a cation in NaOH (aq) which is a strong base
Cl- (aq) is an anion in HCl (aq) which is a strong acidTherefore NaCl (s) forms a neutral solution Salts That Form Acidic Solutions These salts contain cations that are conjugate acids of weak molecular bases. These conjugate acids are what cause the pH of the solution to lower.
NH4Cl(s) contains a NH4+ (aq) ion which is the cation of a the weak base NH3 (aq) Salts That Form Basic Solutions These salts contain the opposite of the salts that form acidic solutions. They contain anions that are conjugate bases for weak acids. These conjugate bases cause the pH of the solution to go up.
NaC2H3O2 (aq) contains a C2H3O2- (aq) ion which is the conjugate base HC2H3O2 (aq) (acetic acid) Salts That Act as Acids and Bases These salts contain both the cation of a weak base and the anion of a weak acid, both of which can hydrolyze. To determine the pH of the solution, you might figure out which ion is a stronger conjugate base or acid. Comparing the Ka and Kb values and seeing which one is bigger will tell you which one is stronger.
NH4CN(s) has both NH4+ (aq) and CN- (aq) ions. The NH4+ ion is the conjugate acid of the weak base NH3 (aq) and the CN- ion is the conjugate base for HCN (aq). Hydrolysis of Amphoteric Ions An amphoteric ion is an ion that can either donate or accept a hydrogen ion (proton). This allows the ion to hydrolyze into an acid or a base. To figure out whether or not it hydrolyzes into an acid or base you must write out the hydrolysis equation of the ion being an acid and a base. This will allow you to find the Ka and Kb. Depending on which one is the larger number will tell you if the solution will be basic or acidic.
HCO3-(aq) is an amphoteric ion
HCO3-(aq) + H2O(l) CO32-(aq) + H3O+(aq) Ka = 4.7*10^-11
That is the hydrolysis reaction as an acid
HCO3-(aq) + H2O(l) H2CO3(aq) + OH-(aq) Kb = 2.7*10^-8
That is the hydrolysis reaction as a base
Since the Kb > Ka the solution will be basic Hydrolysis of Metal and Nonmetal Oxides Metal oxides react with water to produce basic solutions
Nonmetal oxide react with water to produce acidic solutions Solubility: How Fish Breathe There are examples of equilibrium everywhere in our society, many of which we don’t think about or notice. There are the obvious examples of things such as salt or a pill dissolving in water, but there are also more complex examples. These can be very interesting in their complexity, and I will attempt to explain one of them now.
Humans breathe by taking oxygen from the air. That is a fact that no one can argue, as it is scientifically proven, but our bodies don’t really have to go through a lot to separate the oxygen from the nitrogen in the air. It may come as a surprise to some people that fish also breathe oxygen, and they do so by taking it from the water around them. Some people may have thought that fish somehow breathe in water, and while that isn’t true, I wouldn’t contradict you too harshly. I mean, you would presume that an entity would live/breathe using what it is in majority surrounded by. When I say fish “take the oxygen from the water around them”, I don’t mean that they use the O from the HO molecules, not at all. Water is a stable compound, which means that it is hard to change. Fish actually take dissolved oxygen that forms in the water. The amount of this breathable oxygen is roughly 10 out of every one million molecules of water (so 10/1,000,000 or 1/100,000 molecules of oxygen dissolve in water).
A few factors that affect how soluble the oxygen in the water can be are temperature, pressure, and salinity. The solubility of the oxygen gas ( O g) increases with decreasing temperature. In chemistry, this is known as an inversely proportional relationship. If one goes down, the other goes up and vice versa. An easy way to remember this is to just think that cold water is able to hold more oxygen, maybe think that the ice on top of the lake is trapping the oxygen in. That isn’t exactly the case, but whatever works to remember. As for the salinity, it is also an inversely proportional relationship. As the salinity (salt content) of the water decreases, the solubility of oxygen increases. Finally, the relationship between the solubility of oxygen and pressure is a directly proportional one, which means that if one goes up, so does the other. If you are wondering how the pressure of water can change, altitude is a large contributing factor, along with how fast it is moving.
I will try my best to not get into the biology of how a fish breathes, which can get very boring as you list off a million unpronounceable body parts, and stick to the chemistry, but I do want to touch on one thing. As water passes through a fish’s gills, it goes past the gill filaments which are thin, disc-like membranes filled with a capillary network known as lamellae. The gill filaments that are projected out to the water allow water to flow across these lamellae, which (getting to the point of this deviation) results in a gas exchange between oxygen and carbon dioxide. As the water flows past the gills, the dissolved oxygen goes into the fish’s bloodstream. At the same time, carbon dioxide passes into the water, deoxygenating it and carries it away and out of the body through the operculum. If it helps to have a visual, here are a couple of diagrams: (below text)
Not all fish are created equal. By this I mean that some species of fish are better suited to different levels of oxygen intake. In a way, their bodies are only so “oxygen soluble” as well. Some species thrive in cold water as they need water that is able to dissolve higher levels of oxygen, while others like hotter habitats.
One last explanation, which is probably one that doesn’t really need explaining, is why salt water contains less oxygen than fresh water. This is due to the fact that the salt in salt water is already saturating the water, which leaves less room for additional substances. Fresh water is much less saturated, and so it is easier for it to dissolve oxygen. If you take a cup of water and pour a small amount of salt into it, it will quickly dissolve it. If you then pour a few tablespoons in to that same cup, you will see that the rate at which the salt is dissolved slows down and then comes to a stop. In saltwater, it isn’t at the point where it stops dissolving things, but it is much closer to that than is fresh water. That is the simple way to explain why the salinity of water affects oxygen dissolvability.
In conclusion, solubility plays a much bigger part in the workings of everyday life than most people may think. It can be related to, or rather is the root of, many processes. This is just one extremely small, but hopefully interesting, piece of information in the world of solubility and its applications. Without understanding the chemistry behind why things are soluble, we would have a lot of trouble explaining things that we today see as very basic. This, obviously (or hopefully obviously!) includes how animals, specifically fish in this case, are able to breathe. (Detailed Process) There are examples of equilibrium everywhere in life. Above are just bits and pieces of the overall puzzle that is life, which in itself is a constantly evolving and changing chemical system. From the basic equilibriums of chemicals we study in class, to those of plants and animals, to that of the Earth and the cosmos as a whole, it is safe to say that there is an almost majestic balance to it all. A yin and yang if you will, and it is truly an honour to witness and be part of it all, even for the blink of an eye that is our life in this ever mysterious universe References:
Edmondson, R. (2006, March 04). How do fish breathe?. Retrieved from http://www.myuniversalfacts.com/2006/03/how-do-fish-breathe.html
Burr. (2008, January). Fish, water temperature and oxygen. Retrieved from http://www.newton.dep.anl.gov/askasci/gen06/gen06478.htm Formation of Limestone Caves By: Austin Balcaen Caves have always been a wonder to the human race. They have housed our ancestors and many different species of organisms ever since a living creature first step foot on this planet. We’ve created horror movies about them, some people make a living off of them, and many people have made it their hobby to explore them. The most abundant type of cave is called the solutional cave. This cave type is a cave that forms in rock that is soluble. Some of these rocks are limestone, chalk, dolomite, and marble. Limestone being the most commonly found solutional cave. These rocks don’t just dissolve into normal water though. They start to dissolve into solutions that are slightly acidic to acidic. The acidic properties of nonmetal oxides can be thanked for their ability to start the formation of these caves.
When carbon dioxide gets dissolved into rain, the effect is acid rain (carbonic acid). The acid rain is then able to dissolve limestone, CaCO3(s), because it is slightly acidic. The equations for these two reactions are shown below
CO2(g) + H2O(l) H+(aq) + HCO3-(aq)acidification of the rain/groundwater
H+(aq) + CaCO3(s) Ca2+(aq) + HCO3-(aq)CaCO3(s) dissolves in acidic solution
Once the limestone is dissolved it starts to create a hole in the earth. This hole gets bigger and bigger until eventually stalactites and stalagmites are able to form in it. Stalactites are a formation of solid limestone that starts to form from the ceiling of a cave and then continue to form down because of gravity. They almost always have pointed tips. Stalagmites, on the other hand, are a formation of solid limestone that starts to form from the base of a cave and then continues to form in the upward direction. They almost always have a rounded tip. The chemistry behind this formation is that the aqueous solution evaporates allowing the carbon dioxide to escape. This then shifts the equilibrium of the equations above to the left, causing the limestone to precipitate.
We see pictures of these caves with massive stalagmites and stalactites and don’t even realize how long these fascinating structures take to form. It is extremely slow, a lot slower than most people would guess; a couple of hundreds of thousands of years to be exact. The growth rate on average is about one millimeter per century.
For whatever application or purpose these caves may serve. They are and have always been an awe inspiring sight and fascination. They gave many different organisms and us shelter over the centuries as we sought refuge from the extreme cold or extreme heat and from predators. The process of equilibrium salt solution chemistry may have saved our species and many other species from extinction over the many centuries that we have endured. Adventure Caves USA. (n.d.). Formation of a limestone cave. Retrieved from http://www.adventure-caves-usa.com/limestone-cave.html
Austin, S. (n.d.). Origin of limestone caves. Retrieved from http://www.icr.org/article/origin-limestone-caves/ The Autoionization of Water
The reaction between two water molecules producing a hydronium ion and a hydroxide ion. Chemists often view the H3O+ ion as a water molecule with an H+ ion (a proton) attached by a coordinate covalent bond to the oxygen atom. Chemists often omit the water molecule that carries the H+ ion for convenience, therefore we write this equilibrium equation as: H2O(l) H+(aq) + OH-(aq).
Like all chemical equilibria, water obeys the equilibrium law. Therefore, a new constant is derived and is called the ion product constant for water: Kw=1.0 X 10-14 at SATP. We can use this constant to calculate either hydrogen ion concentration or hydroxide ion concentration in an aqueous solution of a strong or weak acid or base at SATP, if another concentration is known.
Since Kw= [H+(aq)][OH-(aq)] simply rearrange the equation to solve for the unknown concentration. Strong Acids
A strong acid is an acid that ionizes completely in water to form hydrogen ions. The %ionization of strong acids is greater than 99%, but in calculations we assume that it is 100%. The most familiar strong acids are: hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, and phosphoric acid. A Monoprotic acid is an acid that has only one ionisable (acidic) proton such as HCl(aq).
A strong base is an ionic substance that dissociates completely in water to release hydroxide ions. All of the group 1 elements are strong bases, and when they dissolve in water, one mole of hydroxide ion is produced for every mole of metal hydroxide that dissolves. These metal hydroxides are highly soluble in water. Group 2 elements form strong hydroxides, and when these bases dissolve in water, two moles of hydroxide ions are formed for every one mole of metal hydroxide that dissolves in solution. Hydrogen Ion Concentration and pH
In 1909, the pH scale was developed for communicating concentrations. pH can be measured in several different ways, and some are more precise than others. A substance that changes colour when they react with acids or bases are called acid-base indicators. The most common indicator used in schools is litmus paper. A solution that is acidic will turn blue litmus paper pink when dipped in solution, and a solution that is basic will turn red litmus paper blue when dipped in a solution. Neutral solution pH=7.00
acidic solution pH<7.00
basic solution pH>7.00 pH values are calculated from the hydrogen ion concentration, and is calculated with this equation:
pOH values are calculated from the hydroxide ion concentration, and is calculated with this equation:
a quicker way for finding pOH is with the equation:
pOH= 14-pH Quantitative Changes in Equilibrium Systems Combining the knowledge of equilibria with the understanding of stoichiometric techniques, we can predict how much of the reactants will be consumed and how much product will be formed in any chemical system that reaches equilibrium. For the general reaction:
aA + bB cC + dD
This tells us that when all things are equal, the larger the K value, the more the reaction favours products. In this example:
H2(g) + I2(g) 2HI(g) K=50 at 450°C
CO2(g) + H2(g) CO(g) + H2O(g) K=1.1 at 900°C
For the first reaction, the larger the K value indicates that the first reaction proceeds farther to completion by the time equilibrium is established.
A very large K value means a reaction favouring products, and a very small K value means a reaction favouring reactants.
Temperature is something you must consider when calculating K. Temperature is the only factor that changes the value of K for a given reaction. Other changes such as change in concentration, or the presence/absence of a catalyst. A temperature change that increases the value of K for a reaction shifts equilibrium to the right (more complete.) a temperature change that decreases the value of K for a reaction shifts equilibrium to the left (less complete.) The Reaction Quotient, Q
When a chemical system begins with reactants only, the reaction will obviously shift to the right towards products. When reactants and products are both present, the direction in which the reaction proceeds is unsure. In this case, we can substitute the concentrations into the equilibrium law expression too produce a trial value called a reaction quotient, Q. Q is thought as to be similar to K, yet K is calculated using concentrations at equilibrium, but Q may or may not be at equilibrium. The same mathematical equation is used for calculating both K and Q, and the result of such a trial calculation will be one of these three possible solutions:
1. Q = K, system is at equilibrium
2. Q>K, system must shift left towards reactants to reach equilibrium, since the product to reactant ratio is too high
3. Q<K, system must shift right towards products to reach equilibrium, since the product to reaction ratio is too low. Acid-Base Titration
Equivalence and Endpoint
In a titration, the equivalence point is the ideal stopping point when completing a titration. For accurate results, the equivalence point must be found precisely and with accuracy. The known concentration, the titrant, is added to the solution of unknown concentration, very slowly in order to be able to get the most accurate and precise results possible. To be at the equivalence point, the correct amount of titrant solution must have been added in order to fully react with the unknown concentration. Since the equivalence point isn't fully noticeable and nearly impossible to get exactly in a lab setting, one will instead want to stop at the endpoint. The endpoint is when you can qualitatively see a slight change in colour of the solution from a clear to a minor colour like really light pink for example. This indicated the equivalence point has been reached and will be fairly close to the actual equivalence point. There are other indicators besides colour, but colour change is generally the main indicator. The type of indicator used will vary depending on what type of titration is being done, and how accurate it must be. Endpoint and equivalence point aren't equal but represent the same general idea. Endpoint is indicated by the visible or qualitative change at the end of a titration whereas an equivalence point is shown when the moles of the titrant are equal to the unknown concentration.
Acid-Base Titration Process
In order to do an acid-base titration you will first need a clean buret. This buret will be filled with a measured amount of the known titrant solution. It is important to check for air bubbles and leaks prior to the titration in order to prevent inaccurate results from occurring. Record the intitial volume in the buret of your known solution. It is now a good idea for one to calculate the theoretical endpoint of the reaction using the balanced equation and the formula Concentration = moles / volume (C = n/V) to find the theoretical volume needed, as this will give an approximate stopping point. The unknown solution should be placed in the Erlenmeyer flask with the indicator. Start the titration with a good stream and stop watching for how long the temporary change lasts. As you get closer to the equivalence point and endpoint, add solution at a much slower rate (eventually a drop at a time). Stop titrating the moment you get a faint, noticeably different, and permanent change. Once the change is permanent, record the mL of solution that remain in the buret and subtract this number from the original amount of solution to get your actual volume of titrant. You can use your known volume and concentration for the known solution to find its moles by modifying the C=n/V equation so it becomes n = CxV. This will give you the moles of the known which can be used to find the moles of the unknown solution using the balanced equation. If there was a 2:1 ratio with the known being 2 and the unknown being 1, then the moles of the unknown will be equal to ½ the moles of the known solution. This process can also be done using a pH meter to record changes in pH from adding the titrant on a graph. Using the points placed on the graph, it should be easier to detect the endpoint more accurately as well as providing a useful graph.
An acid-base indicator is a weak acid or weak base that change colour over a range of hydrogen ion concentration. When titrating a weak acid or weak base, an acid-base indicator will change under slight alkaline or slight acid conditions respectively. Some acid-base indicators include thymol blue, phenol red, and phenolphthalein. Indicators can be used in more than one pH range, and the quantity used with vary in aqueous or alcohol solutions as shown in the following table.
Applications of Acid-Base Titration
Acid-base titration is very useful in an area of fields including; pharmacy & medicine, food industries, science and education, bio-diesel production, and water testing to name a few. Titration is used in pharmacy to achieve a desired mix of compound drugs. Doctors will use titration to get correct proportions of medicine, and blood glucose levels of patients with diabetes can be monitored with titration. Titration can be used in food to test chain lengths of fatty acids, test for amounts of salts and sugars, amounts of vitamins, and to test foods like cheese to see if they are ready for consumption. Titration is used in biology to determine the proper concentrations for things like anaesthetizing animals. It is also used in high school chemistry as it is an important tool for any chemist. For bio-diesel production, titration is used in determining the acidity of vegetable oil. Aquariums have their pH monitored often with titration to make sure it is at a safe level. A buffer is a solution that can keep a constant pH when small amounts of acids or bases added. Buffers contains both a weak acid and its conjugate base. The pH changes slightly when a small amount of acid or base are added and absorbed by a buffer. When there is a high or low pH, only solutions of strong acids or bases are used. pH = pKa + log ([A-]/[HA])
pH = pKa + log ([C2H3O2-] / [HC2H3O2])
pH = -log (1.8 x 10-5) + log (0.50 M / 0.20 M)
pH = -log (1.8 x 10-5) + log (2.5)
pH = 4.7 + 0.40
pH = 5.1 Example of Applying the Henderson-Hasselbach Equation
Calculate the pH of a buffer solution made from 0.20 M HC2H3O2 and 0.50 M C2H3O2- that has an acid dissociation constant for HC2H3O2 of 1.8 x 10-5 Acid-base titrations occur in laboratory experiments where a known acid or base concentration is used to identify the concentration of an unknown. This can be useful in finding concentrations and various other things as well the equivalence point/neutralization point. Moles are used to calculate the amount of acid and base as well as litres or millilitres. A buret/pipette is used to perform a titration and a pH indicator is used to determine when a noticeable neutralization has occurred. Blood-Glucose Monitoring (Detailed Process) Glucose (sometimes called dextrose) is a carbohydrate and is the most important sugar in the human metabolism. It serves as the main energy source for the brain, and it also fuels cells throughout the body.
Insulin, is a hormone that carries this sugar into the liver, fat, and muscle cells to be stored for energy. However, people with Diabetes cannot make enough insulin, or their cells do not respond with insulin normally, so then glucose can’t be transported to be stored throughout all the different organs in the human body. This results in high blood sugar levels which cause several symptoms such as blurry vision, hunger, fatigue, weight loss, frequent urination, and excessive thirst. After many years of these symptoms, diabetes can lead to very serious problems:
• Eye problems, such as trouble seeing and light sensitivity, which leads to blindness.
• Skin and feet can get infected with painful sores, and sometimes, your foot or leg must be amputated.
• Nerves in the body become damaged causing pain, tingling, or even entirely lose your sense of feeling. This nerve damage can give you problems with digesting food, going to the bathroom, or for men, making it hard to get an erection.
So clearly, people with diabetes find the equilibrium involving glucose in the blood stream extremely important. The concentration for equilibrium must be 4-6 mmol/L to be considered healthy. When this concentration rises, people with diabetes must take medication to lower the blood sugar levels in the blood stream and bring it back to normal. Depending on what type of diabetes you have, depends what kind of medication needed. For type 1 diabetes, insulin must be taken because the body does not make it. For type 2 diabetes, the body doesn’t use insulin the way it should. A drug called metformin is a common drug used for people with type 2 diabetes.
In modern society, devices such as “One Touch Profile” are commonly used by diabetics to test their blood sugar concentration. After analyzing a single drop of blood, the meter shows the glucose concentration in mmol/L. On this device, multiple readings, with date and time, are stored at can be accessed at any time. This data can also be downloaded to their computer, then emailed to their doctor so they can monitor their blood levels.
Overall, advances in technology have had a huge positive impact on people with diabetes lives. Devices such as the “One Touch Profile” allow diabetics to monitor their blood levels at any time, so that they know when to take medication to establish equilibrium in the blood sugar levels in the body. Even further, this has also reduced the amount of the prolonged effects that diabetes can have making the day to day life of a diabetic much safer, and easier. The kinetic molecular theory states that particles are always moving and collisions are always occurring in a system even if the solution is a closed system. When a solvent is placed in a solution collisions are occurring between water molecules and ions that make up the crystals placed in the solution. Every time dissolved ions collide with the crystals, more ions will break from the crystal dissolving it further and further until the dissolution has ended. Once the equilibrium has been reached, water molecules are still colliding with the crystal surface, but at this point the rate of dissolution is equal to the rate of crystallization. When the system is in a state of dynamic equilibrium, the dissolving and crystallizing processes are taking place at the same rate. There would be no changes observable to the concentration of the ions in the solution or in the quantity of the solid present. Phase Equilibrium A common type of phase change is the evaporation/condensation equilibrium in a closed system:
H2O(l) = H2O(g)
Evaporation only occurs at first, certain molecules in the liquid gain enough energy by colliding with each other to leave the surface of the liquid and move into the space (air) above the liquid, which changes it to the gas phase. As the molecules in the gas phase continue to increase, they collide with the liquid surface and lose energy until they reach the condensed phase. In an open system this could never happen because the molecules would escape from the system after leaving the surface of the liquid. Chemical Reaction Equilibrium Quantative reactions are reactions in which virtually the entire limiting reagent is consumed. These kinds of reactions will occur in an open system so that both reactants and products are used up, whereas in a closed system both reactants and products are still present at the end of a reaction. At equilibrium, the products are colliding to form reactants while competing with the reactants colliding to form products. This system has to be closed so that the reactants and products don’t escape from the container.
Closed System: CaCO3(s) CaO(s) + CO2(g)
Open System: CaCO3(s) CaO(s) + CO2(g)
Equilibrium: CaCO3(s) = CaO(s) + CO2(g)
*For a given overall system composition, the same equilibrium concentrations are reached whether equilibrium is approached in the forward or the reverse direction.* Percent Reaction at Chemical Equilibrium Hydrogen and iodine gas have been extensively studied by chemists because the molecules are relatively simple and the reaction takes place entirely in the gas phase. It is a rapid reaction where the purple colour of the iodine vapour fades, then becomes constant.
H2(g) + I2(g) = 2HI(g)
Percent reaction: the yield of product measured at equilibrium compared with the maximum possible yield of product.
Percent reaction equation = actual product yield/theoretical product yield x 100
There are 3 categories of reactions:
1. Reactions that favour reactants very strongly
2. Reactions that favour products very strongly
3. Reactions that achieve noticeable equilibrium conditions
Calculating equilibrium concentrations are solved by using ICE tables (Initial Concentration, Change in concentration and Equilibrium concentration). You use the table to solve for ‘x’ and use the value of ‘x’ to calculate the equilibrium concentrations of the two other entities. Qualitative Changes in Equilibrium Systems Le Chaȃteliers Principle. “When a chemical system at equilibrium is disturbed by a change in a property, the system adjusts in a way that opposes the change”
When applying this principle, it involves an initial equilibrium state, a shifting ‘non-equilibrium state, and a new equilibrium state. It provides a method of predicting the response of a chemical system to a change of conditions. This had advanced the technical world by making technological processes more efficient and more economical. Concentration Changes Le Chateliers Principle predicts that if more of a reactant is added to a system at equilibrium, then that system will undergo an equilibrium shift. An equilibrium shift is the movement of a system at equilibrium resulting in a change in the concentrations of reactants and products. The concentration is usually only partially counteracted which means that the final concentrations of the reactants and products in the equilibrium state are different from the original equilibrium state. Equilibrium shifts happen everywhere, including inside your body. When your heart pumps oxygenated blood from the lungs to body tissue, the deoxygenated blood returns to the heart and is pumped back to the blood. There are always equilibrium shifts happening.
Rate Theory and Concentration Changes: When an equilibrium shift occurs when a reactant concentration is increased, collisions become more frequent because of there are more reactant particles per unit volume after the reactant is added. This increases the forward rate significantly. Therefore the reverse reaction rate is not immediately changed, so rates aren’t equal anymore; products increase. When this happens the reaction rate increases as well. Temperature Changes Endothermic reaction: reactants + energy = products
Exothermic reaction: reactants = products + energy
By heating or cooling a container, energy can be added or removed from a system. When cooled, the systems attempts to warm itself up by shifting in the direction that produces heat. If heat is added, the equilibrium will shift in the direction that absorbs heat.
Rate Theory and Energy Changes: reactions rates will slow down when the temperature is low (cool) and reactions speed up at a higher temperature (hot). When cooled, reaction rates do not only slow down, the forward and reverse rates become unequal. Concentration changes occur that will increase the reverse rate and decrease the forward rate until they are equal again. Gas Volume Changes According to Boyle’s law, the concentration of a gas in a container is directly related to the pressure of the gas. When the volume of every gas in the container is decreased by 50%, it doubles the concentration of every gas. This may cause a shift in equilibrium. In order to predict if a pressure change will affect the equilibrium state, the total number of moles of gas reactants and the total number of moles of gas products must be considered. A system that has equal number of gas molecules on each side of the equation will not shift after a change in volume.
Rate Theory and Gas Volume Changes: when a decrease in volume occurs, both the forward and reverse reaction rates will increase since the concentrations of the reactants and products will increase. The forward rate will tend to increase more than the reverse rate because there are more particles involved in the forward reaction. In other words, there are more collisions happening in the forward reaction which makes the rates unequal. Therefore, there is production of more product. Changes with No Effects • Adding catalysts – will decrease the time required to reach the equilibrium position, but will not actually change the final position of the equilibrium. Forward and reverse rates are increased equally.
• Adding inert gases – adding gas while keeping the volume constant can be change the pressure of a gaseous system at equilibrium. When an inert gas is added, it changes the probability of successful collisions for both the reactants and products equally. Acid-base titration may not be the hardest to execute properly, however, it can vary in procedure and reagents needed based on what it is you are looking for. Titration has many applications in society and is vital to many scientific applications and procedure. Without titration many situations in chemistry would be difficult to solve and many useful and important industrial and medical processes would potentially not exist. Due to this it is quite apparent that titration, though not too difficult to perform, is very useful in society. Helmenstine, A. (2011, August 4). Aboutchemistry.com. Retrieved from
Callahan, R. (2011). Real life uses of titration. In Retrieved from http://www.ehow.com/list_5968981_real-life-uses-titration.html
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Titrations. (2012). Retrieved from http://www.sparknotes.com/chemistry/acidsbases/titrations/section1.rhtml Zaykoski, L. (2010, May 28). The importance of glucose. Retrieved from http://www.livestrong.com/article/133891-the-importance-glucose/
Diabetes. (2012, June 27). Retrieved from http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002194/
Haines, S. (2012, February 16). What i need to know about diabetes medicines. Retrieved from http://diabetes.niddk.nih.gov/dm/pubs/medicines_ez/