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Chemistry Unit 2

Structure, Bonding, Properties, Energy, Salts, Electrolysis
by

Becky Cripps

on 9 January 2013

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Transcript of Chemistry Unit 2

Structure Goal Structure and Bonding How Much? Salts Chemistry Unit 2 Chemical Bonding Atoms form compounds to form stable electronic structures. They do this by reacting with other elements - they may either share, lose, or gain electrons. Two non-metallic elements form a covalent bond.
A non-metallic and metallic element bond via ionic bonding. Metal atoms have few electrons in their outer shell and therefore lose electrons to the non-metal. Metals form positive ions (because they have lost electrons, which have a negative charge), and non-metals form negative ions.
These ions are now oppositely charged, therefore they attract each other magnetically. Both ions have the stable electronic structure of a noble gas. The "group" of an element on the periodic table indicates how many electrons are in its outer shell. Group 1 elements have 1 electron in their outer shell, etc. Ionic Bonding Ions all pack together in a regular pattern to form a giant ionic structure. Positive ions are surrounded by negative ions and visa versa, meaning that STRONG ELECTROSTATIC FORCES OF ATTRACTION act in all directions and the giant structure is very strong.

It is important to know the ratio of atoms in an ionic compound. An ion with a 2+ (positive two/lost two electrons) will attract two ions with 1- (negative one/gained an electron). They always balance out.

For example, calcium ions are Ca^2+ and chloride ions are Cl-, so to form a giant structure, there must be two Chloride ions for every one Calcium ion. Formulae Of Ionic Compounds We know that all ionic compounds are neutral over all, therefore we can work out the formula of an ionic compound using the charges. The charges on each ion in an ionic compound always "cancel each other out", to become neutral. To work out the initial charge on the ion, we need to work out (using its atomic number) how many electrons it will gain/lose during ionisation. We then compare this to the ion it is being electrostatically attracted to and work out how many of each ion is needed for the overall ionic compound to be neutral. Brackets In Compounds Sometimes, the ion itself contains more than one element. Carbonate ions contain carbon and oxygen (even though C and O are non-metallic). Hydroxide ions are OH-. There are many examples of this.
When ions like this form an ionic compound, they sometimes have to be "multiplied" to make the compound neutral, just as normal ions must be multiplied. For example, Calcium and hydroxide ions make Ca(OH)2. The "OH-" ion must be double to neutralise the Ca2+ ion, but we keep it together with brackets to show that OH- is a single ion.
If ions like this do not need to be multiplied to form a neutral electronic structure, then there will be no brackets. Eg, CO3(2-) is present in CaCO3, but there are no brackets because Ca2+ and CO3(2-) already balance out to make a neutral compound. Covalent Bonding All non-metallic elements must gain electrons to form stable electronic structures. A covalent bond is when two atoms achieve this by sharing electrons. Atoms bonded this way are called molecules.
Again, we can work out how many electrons a non-metal needs to gain to form a stable electronic structure using the periodic table. Group seven atoms need to gain one electron, etc. This also reflects on how many shared pairs of electrons there will be. Group 4 elements will form up to 4 shares, etc.
Each pair of electrons shared forms a strong covalent bond between the two atoms. Covalent bonds act only between the atoms they are bonding, therefore most molecules are simple. Some can join together to form "giant covalent structures" known as macromolecules. Metals Metals form regular, giant structures in which many layers of metals are arranged.
Metals atoms together like this cause the electrons in the highest energy level (outer shell) to delocalise and form a sea of moving electrons.
As the metals have lost electrons, they become positive ions. The positive ions and sea of negative electrons are strongly attracted to each other via electrostatic forces of attraction, and this attraction means the metal structure is very well held together, giving it a high melting/boiling point. Structure and Properties Giant Ionic Structures Ionic compounds have high melting points and boiling points. They are solid at room temperature. Ionic compounds have many electrostatic forces of attraction, therefore they are very strongly bonded and it takes a lot of energy to break them down.
In their liquid form, ionic compounds conduct electricity because their electrons are free to move and carry electrical charge. Water can break down the bonds between SOME ionic compounds and therefore some dissolve in water. In this state also, ionic compounds conduct electricity.
Ionic compounds cannot move around whole solid because the ions cannott move; they only vibrate in fixed positions. Simple Molecules Molecules are covalently bonded. Covalent bonds, however, act only within the molecule itself, therefore molecules have little attraction to other molecules.
Because there are low forces between molecules, covalent substances often have low melting and boiling points. Molecules have no overall charge and therefore cannot conduct electricity.
Molecules in a solid substance (such as ice) are only attracted to each other through weak intermolecular forces. When this substance melts or boils, these forces are overcome. It does not take a lot of energy to do this, therefore most molecular substances are gases at room temperature. The larger the molecule, the greater the intermolecular forces. Particularly large molecules such as Iodine are liquid at room temperature. Giant Covalent Structures Certain atoms, such as carbon, can form multiple covalent bonds because of their electronic structure. In a giant covalent structure, every atom is covalently bonded to several other atoms, making the structure of this macromolecule extremely strong. It takes a lot of energy to break down the giant covalent structure and therefore they have high melting/boiling points.
Fullerenes are cage-like structures formed of hexagonal rings of carbon atoms. They vary in size, some being so small that they are nano-sized. Because of this property, they may have uses such as drug administration, lubricants, catalysts and materian reinforcement. Diamond and Graphite Diamond Properties:
Each carbon atom covalently bonded to four other carbon atoms
Does not conduct electricity because all available bonds are taken up
Hard and transparent
High melting/boiling point
Regular, three-dimensional giant structure. Graphite Properties:
Each carbon atom chemically bonded to three other carbon atoms, forming a flat sheet of hexagons
Does conduct electricity because of delocalised electrons - one electron from each carbon atom is delocalised, creating a sea of electrons.
There are no covalent bonds between the hexagonal layers, meaning they can slide over each other. This makes graphite slippery and grey. Giant Metallic Structures Metal atoms of the same element are able to slide over each other when force is applied to them. This is because they are all the same size and are in a regular structure. The metal may change shape without breaking apart so they are useful im making wires, etc.
Sometimes we need to make a metal stronger to use it for a specific purpose. Alloys are mixtures (no bonding involved) of multiple different substances. They cause the metal layers in the structure to become irregular, meaning they cannot slide over each other and therefore become more rigid.
Some alloys (memory shape alloys) can be bent in to any shape and then returned to their original shape upon heating. Applications of this type of allow include dental braces.
Remember the properties of metals: they are hard, regularly arranged in structure, and conduct heat and electricity quickly because of their sea delocalised . The Properties Of Polymers Properties of a polymer vary depending on the monomers used to make them, and the conditions they are made in (pressure, catalysts, etc). For example, poly(propene) is made from the propene monomer whereas poly(ethene) is made from the ethene polymer. Polymers vary in density depending on which catalysts and conditions they were made under. Obviously, high densitiy polymers have a higher melting point (called a "softening point") than low-density polymers. Thermosetting and Thermosoftening Polymers Thermosoftening polymers:
Made from individual polymer chains which are tangled together - forces between the polymer chains are only weak intermolecular forces. These are easily overcome when the polymer is heated, but re-form once the polymer has cooled down, meaning the polymer will set.
It softens when it is heated and can be moulded in to any shape, which will set once the polymer cools.
It can be re-heated in order to change its shape.
Poly(ethene) used in plastic bags is an example of this Thermosetting polymers:
Strong covalent bonds form cross-links between the polymer chains, holding them in position.
because of these bonds, thermosetting polymers do not melt or soften when we heat them, but stay in a fixed position. Nanoscience A nanometre is a billionth of a metre, and a nanoparticle is a particle 1-100 nanometres in size.
They contain only a few hundred atoms. Particles this small behave differently to others, even if they are made up of the same substance, and their new properties make them very useful materials. They have an extremely large surface area.
Nanoscience is the study of these extremely small particles. Nanoparticles uses of nanoparticles include:
highly selective sensors
efficient catalysts
coatings
may be used in cosmetics such as sun cream and deodorant
give special properties to construction materials Because nanoparticles are so small, they may be dangerous. If they found their way in to our bodies, eg through breathing them in, they could cause disease. Not enough testing has been done on nanoparticles yet to prove that they are safe. The Mass Of Atoms The relative mass and both protons and neutrons is 1. The mass number is the number of protons and neutrons in an atom. Electrons have very little mass and are not represented in the mass number.
Atomic number is the number of protons which is equal to the number of electrons. The number of neutrons can be worked out by subtracting the atomic number from the mass number. Atoms of the same element all have the same atomic number, meaning they all react in the same way.
The number at the top of the element on the periodic table is the mass number and the one on the bottom is the atomic number. The mass number is always greater than the atomic number.
An isotope is a variation of an element. It has the same number of protons and electrons (atomic number) but a different number of neutrons. Masses Of Atoms And Moles Carbon 12 is the basis for all of the relative atomic masses of the periodic table. The "relative atomic mass" or AR value is the same as the mass number of the element. Relative atomic mass is is the average of all of the isotopes of the element.
We can convert this "relative atomic" mass number to grams to give us one MOLE of the substance. Eg, The relative atomic mass of sodium is 23, so one mole of sodium is 23 grams.
For compounds, we use relative formula mass (Mr). The same basics apply, but we add up all the "relative atomic" mass numbers in its formula to give us an overall "relative atomic" mass for the compound. For example, to work out the Mr of CaCl2, we know that the Ar of Calcium is 40 and the Ar of 2 Chloride atoms is 71, therefore the overall relative formula mass (Mr) is 111. Percentages and Formulae We can work out the percentage of any element in a compound using its empirical formula, Mr and Ar values. If we wanted to work out the percentage of CARBON in CO2, we would first need the "relative atomic"mass number of carbon (12) and the overall relative formula mass(Mr) of CO2 (12+16+16)=44. We now know that 12 out of 44 of the mass of CO2 is made up of carbon, so we can do 12/44 x 100 = 27.3%
The opposite of this is working out the empirical formula of a substance using its percentage composition (next slide) Working out the Empirical Formula of a Substance using its Percentage Composition Empirical formula is always used for ions, but molecules are sometimes represented by molecular formula instead. Empirical formula is the simplest possible ratio of atoms in a compound.
When asked to work out the empirical formula of a compound, we will either be given its percentage formula or how much of a certain element there is in a given mount of a substance, which we can convert to a percentage (eg, there are 40 atoms of carbon. It is easiest to draw a table. carbon hydrogen
mass in 100g of compound 80 20
mass divided by Ar value 80/12=6.67 20/1=20
simplest ratio of this 6.67=6.67=1 20/6.67=3
empirical formula CH3 (1 carbon for every 3 hydrogen) REMEMBER: if the simplest ratio is not instantly obvious, divide all figures by the smallest figure. Equations and Calculations. 2 Mg + O2 = 2 MgO The number in front of the compounds in the balanced equation show us how many moles of the substance have gone in to the reaction.
Here, we can see that 2 moles of Magnesium have reacted with 1 mole of Oxygen to give us 2 Moles of Magnesium Oxide. 2 moles of magnesium is 2 x 24, which is 48. We know that this reacts with one mole of diatomic oxygen (2 x 16 = 32) to give 80g of magnesium oxide. 2 moles of magnesium reacts with one diatomic mole of oxygen to give 2 moles of magnesium oxide. 1 mole of magnesium oxide must be, therefore, 40g. Using this, we can work out how much MgO we can make with various amounts of Mg or O2. We know that 1g of magnesium would make one 48th of the original amount of MgO. The original amount of MgO was 80g, so we divide 80 by 48 to give us the product of the reactants. Call this calculation A. We may, however, want to work out what 5g of Magnesium would give us rather than 1g. Obviously, to work this out, we just need to multiply calculation A by 5. 80/48 x 5 = 8.33.
So we know that with 5g of Mg, we can produce 8.33g of MgO. 2 Ca + Cl2 = CaCl2 2 moles of Ca give 1 Mole of CaCl2. 2 moles of Ca is 40x2=80.
If we had 10g of Ca, we would not have enough to make a full 2 moles of CaCl2. we only have 10/80, or 1/8th, of a mole, therefore we only produce 1/8th of 2 moles of CaCl2.
2 moles of CaCl2 = 40=71=111
1/8th of 111 = 12.5% = 13.875g CaCl2. 3 CO2 + C2H4 = C5O6H4 3 moles of CO2 make 1 mole of C5O6H4, or 44g of CO2 makes (160/3)=53.33g of C5O6H4.
Therefore 4g of CO2 will produce 1/11th of a mole (9%) of C5O6H4.
1 mole of C5O6H4 = 160, and 9% of 160 =14.4 g of C5O5H4. The Yield Of A Chemical Reaction The percentage yield is the amount that could theoretically be made compared to the amount actually made.
This can be calculated using the equation

percentage yield = amount of product collected
maximum amount of product possible

We work out the maximum amount of product possible by calculating to masses from the theoretical chemical reaction (see previous slide) and the amount collected by actually doing the experiment. For example, a student may collect 8.9g of Magnesium Oxide use 7g of Magnesium. This is the actual amount collected. Theoretically, 2 moles of Mg make 2 Moles of MgO, so 48g of Mg makes 64g of MgO. 7g/48 = 14.5%, and 14.5% of 64 is 9.28g. The student has collected less MgO than anticipated. to work out the percentage yield, we divide the student's amount (8.9g) by the maximum amount possible (9.28g) to give us 95.9% yield. reasons percentage yield is usually not 100%:
some reactant/product may have been left on the apparatus
some reactant/product may escape
reactions are not always completed because it is lacking some reactant
some reactants may undergo different chemical reactions than the one intended, meaning they cannot contribute to the product. Why do we want reactions to have high percentage yield?
conserves resources
reduces waste reactant
reduces cost of buying reactant
reduces how many reactions will need to occur for a specific amount of product, therefore reducing costs
May reduce energy needed (endothermic reactions) therefore less fuel burnt for electricity/reduces pollution etc
Less energy means more sustainable Reversible Reactions reactants of a chemical reaction produce products, but sometimes, products can produce reactants. If this is possible, the reaction is called a reversible reaction.

One type of reaction is the reaction of ammonia and hydrogen chloride salt to make ammonium chloride. At cool temperatures, ammonia and hydrogen chloride react to make ammonium chloride with the bi-product of hydrogen. When ammonium chloride is heated, it breaks down to produce ammonia and hydrogen chloride again.
Reactions which require heat to break down (like this one) are called thermal decomposition reactions. Analysing Substances Paper chromatography and mass spectrometry can be used to identify food additives to ensure that they are safe to consume. Food additives may be synthetic or natural, but are added to food to improve the quality of the taste.
Paper chromotography
spot of artificial colouring placed on paper
solvent is allowed to move through the paper
colourings are displaced differently depending on how soluble they are
the colourings can then be identitfied
Mass spectrometry seperates substances depending on their mass and requires a mass spectrometer. Instrumental Analysis modern instrumental analasys:
quick
precise
sensitive
accurate
can use very small samples
BUT:
expensive equipment
often training needed for people to use it. Many compound substances need to be analysed. One way of doing this is by using gas chromotography and a mass spectrometer. Gas chromotography:
substance is vaporised and carried by a gas through a collumn packed with small solids
Individual compounds in the substance travel through at different speeds and therefore are separated, coming out at different times.
The amount of substance leaving the collumn per second is recorded, along with the difference substance's retention times.
these retention times can be compared with other known compounds to identify them.
A very small amount of a substance can be used for this. The gas chromotography output is linked directly to a mass spectrometer which provides further information which a computer can use to quickly identify individual compounds. A mass spectrometer gives the relative molecular mass (Mr) of different compounds and records them on a graph.
The peak furthest to the right on a mass spectrometer shows the "molecular ion peak" - this is the compound with the largest mass.
The largest mass of an ion is an ion with only one electron removed. Rates and Energy How Fast? The rate of a reaction is defined as the amount of reactant used/product made in an amount of time. It can be calculated by doing:
rate of reaction = amount of reactant used/time
OR amount of product made/time
It can also be calculated using a graph of reaction over time. The steeper the gradient, the faster the reaction.
Ways of observing a reaction has happened:
change in colour, change in temperature, change in pH, change in concentration, reactants converted to product. Collision Theory Particles must collide with sufficient energy to react. The minimum amount of energy required for a reaction to happen is called the activation energy Anything that increases the overall energy or the rate of collisions will cause a reaction to happen faster. These things include:
increase in temperature
increase in concentration
increase of pressure (in gasses)
increase in surface area
use of catalyst (speeds up reaction) The effect of temperature increasing the temperature has two effects on the rate of reaction:
particles will collide more because increase in temperature will cause them to have more kinetic energy and will therefore move around faster, as well as colliding with more energy.
increasing the proportion of particles in the reactant that have the activation energy required to react.
a small increase will make reactions go faster and a small decrease will make them go slower.
this is why foods are kept refrigerated.

The Effect Of Concentration Or Pressure Increasing the concentration of a solution increases the amount of reactant particles within the same volume. If particles are closer together, they are more likely to collide therefore the collision rate will increase.
The same goes for the pressure of gases - more molecules in the same volume, so a faster reaction The Effect Of Catalysts Catalysts speed up chemical reactions by lowering the activation energy required for a reaction to take place. This means that a greater proportion of the reactant particles have the activation energy required.
Catalysts are never used up by the reaction as they are not a reactant themselves. Different catalysts are more efficient in different types of reaction, so different reactions require different catalysts.
Catalysts are always solid, and have as large of a surface area as possible to make them most effective. Catalysts in Action advantages:
used in industry to reduce activation energy. Reducing energy use lowers price and cost to environment - fossil fuels etc burnt to produce energy, CO2 released, etc.
economical because they can be used many times, don't need to be replaced often
make reactions happen faster - supply meeting demand disadvantages:
can be expensive to buy
involve using transition metals - many of which are toxic and will harm the environment if they escape New catalysts are constantly being developed in industry to improve efficiency. Nanoparticles could create highly efficient catalysts. Enzymes, biological catalysts, work at normal temperatures. Exothermic and Endothermic Reactions Energy is transferred in all chemical reactions - bonds of reactants are broken and bonds of products are made.
Exothermic reactions are reactions that release heat which heats up the surroundings. Examples of this type of reaction are combustion, oxidation, and neutralisation.
Endothermic reactions need to take energy from the surroundings so reactions form products. Some merely take energy from around them whereas others require an actual source of energy. Thermal decomposition reactions are endothermic - they need a constant supply of energy to work. Energy and Reversible Reactions Reversible reactions require opposite but precisely equal energy transfers. A reaction which is endothermic one way will be exothermic the other. A reaction using up 500J of energy one way will release 500J in the other.
anhydrous copper sulfate and water react to produce hydrated copper sulfate. This is an exothermic reaction which releases heat.
When copper sulphate crystals are heated, they produce anhydrous copper sulphate and water. Using Energy Transfers From Reactions Exothermic Reactions:
used in products such as hand warmers and heat-able drinks
some of these use non-reversible reactions, such as the reaction of calcium oxide with water.
Others use reversible reactions, such as the crystallisation of a salt. This can be heated in boiling water to re-dissolve. These are re-usable.
Endothermic Reactions:
cold packs may contain ammonium nitrate and water. This is a reversible reaction reaction, but not in the pack, so reactants must be kept separated and cold pack cannot be re-used. Acids and Alkalis When a substance is dissolved in water it becomes an aqueous solution. When an acid is added to water it produced H+ ions, making the solution acidic
Some bases dissolve in water. When they are, the produce OH- ions and make the solution alkaline. Alkalis and acids react to produce neutral water.
Indicators change colour to show whether a solution is alkaline or acidic. Some only have one colour for "alkaline" and another for "acidic", but some have a full spectrum of colours to indicate the exact pH value.
The three most common acids are hydrochloric acid (HCl), sulphuric acid (H2SO4) and nitric acid (HNO3). Making Salts From Metals Or Bases Acids react with all metals above hydrogen in the reactivity series
When metals react with acids, they produce a salt and a hydrogen gas
Bases are metal oxides and metal hydroxides. When a base reacts with an acid, a neutralisation reaction occurs and a salt and water are produced.
Both of these reactions are used to produce salts
Method: add the metal or base a little at a time until all of the acid has reacted, then filter off the excess solid reaction, leaving a solution of the salt. The salt can then be separated by crystallisation - evaporating the water until only the salt is left. Making Salts From Solutions acids and alkalis react to produce a salt and water. This is a neutralisation reaction. When acids and alkalis react, there is not visible change. Therefore, we need to use an indicator to show when the reaction is complete. The product will be a salt dissolved in a solution, but the salt and water can be separated via crystallisation. Precipitation Certain salt solutions can be mixed so that one of the products will be a white precipitate. It causes a displacement reaction and we can use it to obtain a desirable product.
For example, salt solutions are produced in neutralisation reactions. Lead iodide can be made by mixing lead nitrate and potassium iodide. The lead iodine produces a precipitate which can be filtered off the solution, washed with distilled water and blotted dry.
This method involves two aqueous solutions reacting to form a salt and an aqueous solution.
An example of this being used in industry is to remove pollutants from water.
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