Section 8 - Reaction Types

Question 1

Reaction types - Addition

Answer 1

This is a reaction in which atoms are added to an unsaturated bond so that the bond becomes saturated.

Question 2

Reaction types - Combustion

Answer 2

• This is the chemical reaction between a fuel and oxygen.
• Normally the fuel is an organic compound and the products are carbon dioxide and water - this is complete combustion.
• Without enough oxygen, incomplete combustion occurs, producing poisonous carbon monoxide.

Question 3

Reaction types - Condensation

Answer 3

This is similar to an addition reaction in which a simple molecule like water is also formed.

Question 4

Reaction types - Cracking

Answer 4

This is the (thermal) decomposition of long-chain hydrocarbon molecules from crude oil into shorter-chain alkanes and alkenes. This requires high temperatures and pressures and a catalyst (usually aluminium oxide), and makes hydrocarbons that are more useful.

Question 5

Reaction types - Dehydration

Answer 5

This is the removal of water from a compound by heating. In organic molecules it usually results in the formation of a C=C bond.

Question 6

Reaction types - Displacement

Answer 6

This is a reaction where one element displaces another, less reactive, element from a compound. This usually takes place between metals, but also with halogens.

Question 7

Reaction types - Disproportionation

Answer 7

This is a rare type of chemical reaction where an element in a reactant is oxidised and reduced at the same time. Chlorine can undergo disproportionation reactions.

Question 8

Reaction types - Electrolysis

Answer 8

This is a process that uses electricity to break down a compound. The reactant or reactants must be in the liquid state - either molten or in solution. The particles have to be able to move. An example is the electrolysis of bauxite to obtain pure aluminium.

Question 9

Reaction types - Elimination

Answer 9

This is just the removal of a small molecule from a larger molecule. Usually water or hydrogen is removed (and not replaced by anything else).

Question 10

Reaction types - Endothermic

Answer 10

Any chemical reaction that takes in energy. This means that the reactants will have less energy than the products. The enthalpy change of an endothermic reaction is always positive.

Question 11

Reaction types - Exothermic

Answer 11

Any chemical reaction that gives out heat energy. This happens because the products have less energy than the reactants. The enthalpy change of an exothermic reaction is always negative.

Question 12

Reaction types - Hydrogenation

Answer 12

This is the addition of a molecule of hydrogen across a C=C bond. One atom attaches to each carbon.

Question 13

Reaction types - Neutralisation

Answer 13

This is the reaction between a basic compound and an acid. The products always include the salt of the acid, water and other products dependent on the acid and base.

Question 14

Reaction types - Oxidation

Answer 14

There are two possible definitions for this - the best is loss of electrons. Another useful one is the gain of oxygen. It is the opposite of reduction.

Question 15

Reaction types - Precipitation

Answer 15

A precipitate is a solid that is formed in a solution by a chemical reaction or by a change in temperature affecting solubility. Precipitates are insoluble in the solvent. A precipitation reaction is simply any reaction that produces a precipitate.

Question 16

Reaction types - Radical (Chain) Reactions

Answer 16

Reactions involving radicals - an atom or compound with an unpaired electron. Often, one of the products of the reaction is also a radical which can perform further reactions. This makes the process a chain reaction.

Question 17

Reaction types - Redox

Answer 17

This is the name for a reaction that involves both reduction and oxidation processes. It is usually used to describe reactions that just involve electron transfer.

Question 18

Reaction types - Reduction

Answer 18

There are two possible definitions for this - the best is the gain of electrons. The other useful one is the loss of oxygen. Important point: oxidation and reduction always happen together - it is impossible to have one without the other.

Question 19

Reaction types - Reversible

Answer 19

This is the name given to any chemical reaction that can go forwards and backwards at the same time. That means that the reactants will form the products, but that the products will also react (or decompose) to give the reactants.

Question 20

Reaction types - Substitution

Answer 20

This is simply a reaction in which an atom (or group of atoms) in a molecule is swapped for a different atom (or group of atoms).

Question 21

Reaction types - Thermal decomposition

Answer 21

This is where one compound breaks down, under heating, into two or more simpler compounds. A classic example is the breakdown of any carbonate compound.