Chemistry C2 Revision

C2.1.1 Structure and bonding

a) Compounds are substances in which atoms of two or more elements are chemically combined.

b) Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels (shells) of atoms in order to achieve the electronic structure of a noble gas.

c) When atoms form chemical bonds by transferring electrons, they form ions. Atoms that lose electrons become positively charged ions. Atoms that gain electrons become negatively charged ions. Ions have the electronic structure of a noble gas (Group 0).

d) The elements in Group 1 of the periodic table, the alkali metals, all react with non-metal elements to form ionic compounds in which the metal ion has a single positive charge.

e) The elements in Group 7 of the periodic table, the halogens, all react with the alkali metals to form ionic compounds in which the halide ions have a single negative charge.

f) An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions.
These forces act in all directions in the lattice and this is called ionic bonding.

g) When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong. Some covalently bonded substances consist of simple molecules such as H2, Cl2, O2, HCl, H2O, NH3 and CH4. Others have giant covalent structures (macromolecules), such as diamond and silicon dioxide.

h) Metals consist of giant structures of atoms arranged in a regular pattern.

The electrons in the highest occupied energy levels (outer shell) of metal atoms are delocalised and so free to move through the whole structure. This corresponds to a structure of positive ions with electrons between the ions holding them together by strong electrostatic attractions.

C2.2.1 Molecules

a) Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.

b) Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.

c) Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge.

C2.2.2 Ionic compounds

a) Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions. These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.

b) When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current.

C2.2.3 Covalent structures

a) Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points.

b) In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.

c) In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other because there are no covalent bonds between the layers and so graphite is soft and slippery.

d) In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.

e) Carbon can also form fullerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, e.g. in tennis rackets.

C2.2.4 Metals

a) Metals conduct heat and electricity because of the delocalised electrons in their structures.

b) The layers of atoms in metals are able to slide over each other and so metals can be bent and shaped.

c) Alloys are usually made from two or more different metals. The different sized atoms of the metals distort the layers in the structure, making it more difficult for them to slide over each other and so make alloys harder than pure metals.

d) Shape memory alloys can return to their original shape after being deformed, e.g. Nitinol used in dental braces.

C2.2.5 Polymers

a) The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced using different catalysts and reaction conditions.

b) Thermosoftening polymers consist of individual, tangled polymer chains. Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when they are heated.

C2.2.6 Nanoscience

a) Nanoscience refers to structures that are 1–100 nm in size, of the order of a few hundred atoms. Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors, stronger and lighter construction materials, and new cosmetics such as sun tan creams and deodorants.

C2.3.1 Atomic structure

a) Atoms can be represented as shown in this example:

b) The relative masses of protons, neutrons and electrons are:

c) The total number of protons and neutrons in an atom is called its mass number.

d) Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.

e) The relative atomic mass of an element (Ar) compares the mass of atoms of the element with the 12C isotope. It is an average value for the isotopes of the element.

f) The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

g) The relative formula mass of a substance, in grams, is known as one mole of that substance.

C2.3.2 Analysing substances

a) Elements and compounds can be detected and identified using instrumental methods. Instrumental methods are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small.

b) Chemical analysis can be used to identify additives in foods. Artificial colours can be detected and identified by paper chromatography.

c) Gas chromatography linked to mass spectroscopy

(GC-MS) is an example of an instrumental method:

* Gas chromatography allows the separation of a mixture of compounds

* The time taken for a substance to travel through the column can be used to help identify the substance

* The output from the gas chromatography column can be linked to a mass spectrometer, which can be used to identify the substances leaving the end of the column

C2.3.3 Quantitative chemistry

a) The percentage of an element in a compound can be calculated from the relative mass of the element in the formula and the relative formula mass of the compound.

b) The empirical formula of a compound can be calculated from the masses or percentages of the elements in a compound.

c) The masses of reactants and products can be calculated from balanced symbol equations.

d) Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:

* The reaction may not go to completion because it is reversible

* Some of the product may be lost when it is separated from the reaction mixture

* Some of the reactants may react in ways different from the expected reaction.

e) The amount of a product obtained is known as the yield. When compared with the maximum theoretical amount as a percentage, it is called the percentage yield.

f) In some chemical reactions, the products of the reaction can react to produce the original reactants.
Such reactions are called reversible reactions and are represented:
A + B C + D
For example:
Ammonium chloride ammonia + hydrogen chloride

C2.4.1 Rates of reaction

a) The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed over time:

b) Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy. The minimum amount of energy particles must have to react is called the activation energy.

c) Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of reaction.

d) Increasing the pressure of reacting gases increases the frequency of collisions and so increases the rate of reaction.

e) Increasing the concentration of reactants in solutions increases the frequency of collisions and so increases the rate of reaction.

f) Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

g) Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts.

h) Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce costs.

C2.5.1 Energy transfer in chemical reactions

a) When chemical reactions occur, energy is transferred to or from the surroundings.

b) An exothermic reaction is one that transfers energy to the surroundings. Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation. Everyday uses of exothermic reactions include self-heating cans (e.g. for coffee) and hand warmers.

c) An endothermic reaction is one that takes in energy from the surroundings. Endothermic reactions include thermal decompositions. Some sports injury packs are based upon endothermic reactions.

d) If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case.

For example:

C2.6.1 Making salts

a) The state symbols in equations are (s), ( l ) , (g) and (aq).

b) Soluble salts can be made from acids by reacting them with:

* Metals – not all metals are suitable; some are too reactive and others are not reactive enough

* Insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off

* Alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution.

c) Salt solutions can be crystallised to produce solid salts.

d) Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed. Precipitation can be used to remove unwanted ions from solutions, for example in treating water for drinking or in treating effluent.

C2.6.2 Acids and bases

a) Metal oxides and hydroxides are bases. Soluble hydroxides are called alkalis.

b) The particular salt produced in any reaction between an acid and a base or alkali depends on:

* The acid used (hydrochloric acid produces chlorides, nitric acid produces nitrates, sulphuric acid produces sulphates)

* The metal in the base or alkali.

c) Ammonia dissolves in water to produce an alkaline solution. It is used to produce ammonium salts. Ammonium salts are important as fertilisers.

d) Hydrogen ions, H+(aq), make solutions acidic and hydroxide ions, OH–(aq), make solutions alkaline. The pH scale is a measure of the acidity or alkalinity of a solution.

e) In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. This reaction can be represented by the equation:

C2.7.1 Electrolysis

a) When an ionic substance is melted or dissolved in water, the ions are free to move about within the liquid or solution.

b) Passing an electric current through ionic substances that are molten, for example lead bromide, or in solution breaks them down into elements. This process is called electrolysis and the substance that is broken down is called the electrolyte.

c) During electrolysis, positively charged ions move to the negative electrode, and negatively charged ions move to the positive electrode.

d) Electrolysis is used to electroplate objects. This may be for a variety of reasons and includes copper plating and silver plating.

e) At the negative electrode, positively charged ions gain electrons (reduction) and at the positive electrode, negatively charged ions lose electrons (oxidation).

f) If there is a mixture of ions, the products formed depend on the reactivity of the elements involved.

g) Reactions at electrodes can be represented by half equations, for example:

h) Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite.
Aluminium forms at the negative electrode and oxygen at the positive electrode. The positive electrode is made of carbon, which reacts with the oxygen to produce carbon dioxide.

i) The electrolysis of sodium chloride solution produces hydrogen and chlorine. Sodium hydroxide solution is also produced. These are important reagents for the chemical industry, e.g. sodium hydroxide for the production of soap and chlorine for the production of bleach and plastics.