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Acids & Bases

for 10 MYP Science
by

Angela De Jong

on 24 October 2012

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Transcript of Acids & Bases

slight increases in acidity can kill crustaceans, insects and some plankton species, disrupting the food web of lakes
higher increases in acidity can kill fish species or prevent them from reproducing sulfur dioxide reacts with water vapour to form fine particles of sulfate → these particles contribute to smog, and can also become lodged in the respiratory tract and contribute to asthma and other respiratory problems
acid precipitation can damage buildings and statues leading to expensive repairs
acid precipitation ruins car finishes by ‘eating away’ at the paint Acids & Bases Acids and bases are types of compounds that have characteristic formulas and similar chemical behaviours. Both are very important reactants and catalysts in industrial processes, such as in the production of pharmaceutical drugs. What are Acids & Bases? What are some common bases that we may encounter in our daily lives?
baking soda
ammonia
toothpaste
Windex® What are some common acids that we may encounter in our daily lives?
vinegar
lemon juice
battery acid
Coca-cola® Common Acids & Bases Characteristics of Acids & Bases Acids Bases sour tasting
good conductors due to H+ ion
turn blue litmus paper red
do not change phenolphthalein indicator – remains clear
formulas often begin with a hydrogen bitter tasting & slippery feel
good conductors due to OH- ion
turn red litmus paper blue
change phenolphthalein indicator pink
formulas often contain a hydroxide ion (OH-) or a bicarbonate group (HCO3) According to the Arrhenius theory, all acids release hydrogen ions (H+) when dissolved in water. For example:

HCl + H2O --> H+(aq) + Cl-(aq)

According to the theory, all bases release hydroxide ions (OH-) when dissolved in water. For example:

NaOH + H2O --> Na+(aq) + OH-(aq) One of Many Theories of Acids and Bases: The Arrhenius Theory A strong acid dissociates completely in water. This means that none of the acid is left in solution; rather, 100% of the acid ionizes into a hydrogen ion (H+) and an anion.

There are six common strong acids:
HCl - hydrochloric acid
HNO3 - nitric acid
H2SO4 - sulfuric acid
HBr - hydrobromic acid
HI - hydroiodic acid
HClO4 - perchloric acid
A weak acid does not dissociate completely in aqueous solution. Acetic acid (CH3COOH) is an example of a weak acid. Strong vs. Weak Acids Acids and bases are often referred to as strong or weak. What does this actually mean? A strong base dissociates completely in water to form hydroxide ions (OH-) and a cation.
There are six common strong bases:
LiOH - lithium hydroxide
NaOH - sodium hydroxide
KOH - potassium hydroxide
Ca(OH)2 - calcium hydroxide
Sr(OH)2 - strontium hydroxide
Ba(OH)2 - barium hydroxide

A weak base does not dissociate completely in aqueous solution. Ammonium hydroxide (NH4OH) is an example of a weak base. Strong vs Weak Bases Water, H2O, seems to pose a strange dilemma.

When water molecules are ionized, they form equal quantities of hydrogen ions and hydroxide ions, though in reality, hydrogen ions usually attach themselves to another water molecule, forming the hydronium ion, H3O+. Water? http://bio1152.nicerweb.net/Locked/media/ch03/03_UN53WaterDissociation_L.jpg Because of the equal nature of both ions, water is considered to be neutral – it is not acidic or basic.

Because both ions are always present in aqueous solution, the only way to determine whether a solution is acidic or basic is to look at the ratio of hydrogen ions and hydroxide ions.
H+ > OH- acidic
H+ < OH- basic
H+ = OH- neutral The pH scale is a logarithmic scale with values ranging from 0-14. A change of one unit on the scale represents a tenfold effect on the pH. An acid with a pH of 3 is ten times stronger than an acid with a pH of 4
pH stands for “potenz Hydrogen” (potential for hydrogen), or the concentration of hydrogen ions in a solution. An increase of 1 on the scale represents dividing the concentration of H+ ions by 10, and a decrease of 2 represents multiplying the concentration by 100. A strong acid has a low pH, whereas a strong base has a high pH. pH Scale pH < 7 ... acidic
pH = 7 ... neutral
pH > 7 ... basic pH Scale http://commons.wikimedia.org/wiki/File:PH_Scale.svg There are various methods to measure pH, such as using pH paper or a pH meter.

If you know the concentration of H+ in solution, you can use the following equation to calculate pH:
pH = -log10 [H+]

where [H+] is the number of hydrogen ions present in 1 liter of solute, otherwise known as concentration. Measuring pH Example:
What is the pH of a solution in which the hydrogen ion concentration is 0.001? Calculating pH

What type of reaction is the following?

HCl(aq) + NaOH(aq) --> H2O(l) + NaCl(aq)

This is a special type of double displacement reaction that occurs between acids and bases, and is also called a neutralization reaction. These reactions form a salt and often water. Neutralization Reactions When a strong base and a strong acid react, they form a neutral solution. There are many practical uses for neutralization reactions. For example, baking often requires sodium bicarbonate (NaHCO3), or baking soda, and some sort of acid, such as vinegar, cream of tartar, or lemon juice. The baking soda and the acid react to form a gas, which is trapped in the batter and causes the baked good to rise.
Antacids also take advantage of neutralization reactions. Sometimes the acid in the stomach becomes too concentrated and causes pain. An antacid is a mild base that helps to neutralize the acid and calm the stomach pain. When a metal is burned in oxygen, it forms a metal oxide. A metal oxide reacts in water to form a base:
4K(s) + O2(g) --> 2K2O(s)
K2O(s) + H2O(l) --> 2KOH(aq)

When a non-metal is burned in oxygen, it forms a non-metal oxide. A non-metal oxide reacts in water to form an acid:
C(s) + O2(g) --> CO2(g)
CO2(g) + H2O(l) --> H2CO3(aq) How Can We Make an Acid or Base? +/- log . 0 0 1 = Calculator tip: pH = -log10 [H+]
pH = -log10 [0.001]
pH = 3 Magnesium burns in oxygen to produce magnesium oxide. Example 2Mg + O2 --> 2MgO
MgO + H2O --> Mg(OH)2

Magnesium hydroxide is a base. Acid precipitation is the term used to describe any precipitation (rain, snow, dew, fog), which is more acidic than normal.

All precipitation is normally slightly acidic, with normal rain having a pH of about 5.6. This acidity is due to CO2 in the atmosphere. The gas reacts with water in the following way:

CO2(g) + H2O(l) --> H2CO3(aq) What is Acid Precipitation? However, the increased acidity is due mostly to sulfur dioxide (SO2) and nitrogen oxides (NOx).

These gases are transformed into sulfuric acid (H2SO4), nitric acid (HNO3) and nitrous acid (HNO2) when they come into contact with water in the atmosphere. http://tragerwaterreport.files.wordpress.com/2010/04/epa-acid-rain-chart.jpg http://www.epa.gov/acidrain/education/site_students/images/phscale.gif Where do these pollutants come from? SO2 is a by-product of industrial processes and burning of fossil fuels.
Ore smelting, coal-fired power generators and natural gas processing are the main contributors.
In eastern Canada, over 50% of the acid deposition originates from the United States. Preventing Acid Precipitation There are many strategies for reducing SO2 and NOx emissions. The Canadian government has been working on this since the launch of The Eastern Canada Acid Rain program in 1985.
SO2 emissions have decreased significantly since this time, and acid precipitation has also decreased. The cooperation of the U.S. was also needed, and in 1991 the Canada-US Air Quality Agreement was signed. NOx emissions come from combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electrical-utility boilers and engines, and other equipment.
Again, U.S. emissions have a huge impact on Canadian atmosphere. Effects on Humans: Effects of Acid Precipitation Effects on Aquatic Environments: Effects on Forests acid precipitation damages the surfaces of leaves and needles, reducing a tree's ability to withstand cold, and inhibiting plant germination and reproduction
acid precipitation causes leaching of soil nutrients The Chemistry of Acid Rain When considering these effects of acid precipitation, it is clear that they are all interconnected. Acid precipitation is therefore a significant problem that cannot be ignored. http://commons.wikimedia.org/wiki/File:Hedon_Church%27s_Western_Door_Archway_-_geograph.org.uk_-_1270321.jpg http://upload.wikimedia.org/wikipedia/commons/d/d4/Silberwald.jpg http://upload.wikimedia.org/wikipedia/commons/8/84/Northland_waste_water.jpg Formation of Oxides in Atmosphere:

S(s) +O2(g) --> SO2(g)
2SO2(g) + O2(g) --> 2SO3(g)

N2(g) +O2(g) --> 2NO(g)
NO(g) +0.5O2(g) --> NO2(g) Formation of Acids in Atmosphere:

CO2(g) + H2O(l) --> H2CO3(aq)
SO2(g) + H2O(l) --> H2SO3(aq)
SO3(g) + H2O(l) --> H2SO4(aq)
2NO2(g) + H2O(l) --> HNO2(aq)
+ HNO3(aq)
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