C3 Chemical Reactions

Question 1

Why is the formula for Bromine Br2 and not Br?

Answer 1

Bromine exists as a diatomic molecule

Question 2

What is the meaning of the Roman Numerals in Iron(II) and Iron (III)?

Answer 2

The number of electrons in its outer shell

Question 3

Explain why a wooden log has more mass before burning than the ashes it leaves after burning despite the Law of Conservation of Mass?

Answer 3

Some of the particles will have evaporated as a gas. If the reaction took place in a closed system, the mass would be the same.

Question 4

Explain, in terms of particles, why iron bars increase in mass after rusting.

Answer 4

Rust is the reaction between Iron and Oxygen - since there are now more particles since the oxygen has bonded to the iron, the mass is higher.

Question 5

Explain why an unbalanced chemical equation is not a completed chemical equation.

Answer 5

It does not abide to the Law of Conservation of Mass - the number of particles is not the same on both sides of the equation.

Question 6

What is a half-equation?

Answer 6

An equation to model what happens to one reactant in a chemical equation and its electrons

Question 7

What is a complete ionic equation ?

Answer 7

An ionic equation with all of the ions in the experiment present.

Question 8

What are spectator ions?

Answer 8

Ions that do not take part in the reaction (i.e. that are the same on both sides of the equation)

Question 9

What is a net ionic equation?

Answer 9

An ionic equation only showing the ions that change in the reaction (i.e. ignoring spectator ions)

Question 10

What is the value of Avogrado’s Constant?

Answer 10

6.02x10^23

Question 11

What is the mole?

Answer 11

The amount of any substance that has the same number of entities as there are atoms in Carbon-12

Question 12

What is the mass of one mole of a substance?

Answer 12

Its relative formula mass/relative atomic mass/molar mass

Question 13

What is the equation linking mass, moles and molar mass?

Answer 13

Mass (g) = moles (mol) x molar mass (g/mol)
OR
Moles (mol) = Mass (g) / molar mass (g/mol)

Question 14

What is a limiting reactant?

Answer 14

The reactant that detemines how much product is produced.

Question 15

What is a reactant that is in excess?

Answer 15

There are more moles of the reactant in excess than the limiting reactant, so it has no effect on how much product is produced.

Question 16

How do you work out which is the limiting reactant?

Answer 16

Calculate the number of MOLES of each substance - NEVER use the MASS

Question 17

What is the stoichiometry of an equation?

Answer 17

The relative amounts of each substance used.

Question 18

What is an endothermic reaction?

Answer 18

A reaction that takes in energy from the surroundings.

Question 19

What is an exothermic reaction?

Answer 19

A reaction that releases energy to the surroundings.

Question 20

What are some examples of exothermic reactions?

Answer 20

Combustion
Neutralisation

Question 21

What are some examples of endothermic reactions?

Answer 21

Thermal decomposition
Photosynthesis

Question 22

Does breaking bonds release or require energy?

Answer 22

Breaking bonds requires energy.

Question 23

Does making bonds release or require energy?

Answer 23

Making bonds releases energy.

Question 24

What is the formula for energy change?

Answer 24

Energy change = energy required to break bonds of reactants - energy released when making bonds

Question 25

Is the energy change of an endothermic reaction positive or negative?

Answer 25

Positive because more energy is required to break bonds than to make them, so energy is taken from the surroundings.

Question 26

Is the energy change of an exothermic reaction positive or negative?

Answer 26

Negative because more energy is released making bonds than is required to break bonds, so energy is released to the surroundings.

Question 27

What are bond energies?

Answer 27

The energy required to break 1 mol of a particular bond. OR The energy released when 1 mol of this bond is made

Question 28

What is oxidation?

Answer 28

The gain of oxygen OR The loss of electrons OR Both

Question 29

What is reduction?

Answer 29

The loss of oxygen OR The gain of electrons OR Both

Question 30

What is a redox reaction?

Answer 30

A reaction where reduction and oxidation happen at the same time.

Question 31

What is an oxidising agent?

Answer 31

A substance that oxidises another substance without getting oxidised itself.

Question 32

What is a reduction agent?

Answer 32

A substance that reduces another substance without getting reduced itself.

Question 33

What is an acid?

Answer 33

A substance that releases hydrogen ions when it dissolves in water to form a solution

Question 34

What is a base?

Answer 34

A substance that can neutralise an acid?

Question 35

What is an alkali?

Answer 35

A base that can dissolve in water. AN alkali releases hydroxide ions when it dissolves in water to form a solution.

Question 36

What is the pH scale?

Answer 36

A scale displaying the acidity or the alkalinity of a substance. 0-6 is acidic, 8-14 is alkaline and 7 is neutral. The closer to 0 you go, the more acidic the substance becomes. The closer to 14 you go, the more alkaline it becomes.

Question 37

How can you find the pH of a solution?

Answer 37

Using indicators e.g. Universal Indicator or an electronic probe.

Question 38

What is the general equation for neutralisation?

Answer 38

Acid + base ---> salt + water

Question 39

What is neutralisation?

Answer 39

The process where an acid and a base react to form a salt and water. The salt made depends on the acid and the base used.

Question 40

How can you predict the salt made in a neutralisation reaction?

Answer 40

The first part comes from the metal in the base/alkali and the second part comes from the acid used e.g. Sodium Hydroxide + Nitric Acid makes Sodium Nitrate.

Question 41

Why are all alkalis bases but all bases are not alkalis?

Answer 41

A base is a substance that neutralises an acid, whereas an alkali is a base that can also dissolve in water.

Question 42

What is the general equation for the reaction between an acid and a carbonate?

Answer 42

Acid + carbonate ---> salt + water + carbon dioxide

Question 43

What is the general equation for the reaction between an acid and a metal?

Answer 43

Acid + metal ---> salt + hydrogen

Question 44

What is the difference between a dilute and concentrated acid>

Answer 44

A dilute acid has a low acid-to-volume ratio - a concentrated acid has a high acid-to-volume ratio e.g. dilute acid: 1: 10, concentrated acid: 12:10

Question 45

What is the difference between weak and strong acids?

Answer 45

Weak acids are only partially ionised - only some of their molecules release hydrogen ions. Strong acids are completely ionised - all of their molecules release hydrogen ions.

Question 46

Explain why a dilute strong acid may have the same pH as a concentrated weak acid.

Answer 46

pH depends on both concentration and the strength of the acid. A strong acid will have a low pH but that will increase due to it being diluted. A weak acid will have a higher pH but that will decrease because it is concentrated.

Question 47

What is electrolysis?

Answer 47

A process where an electric current is passed through a compound, causing a chemical change.

Question 48

What components are required for electrolysis?

Answer 48

Electrolyte: A compound in a liquid state or in a solution, which contains mobile (able to move) ions and can conduct electricity. Electrodes: Two are required. Made from a metal or graphite that conducts electricity to the electrolyte. An electrical supply.