Topic 4: Chemical changes

Question 1

What is the pH scale?

Answer 1

A measure of how acidic or alkaline a solution is

Question 2

What can be used to measure of pH of a solution? [2]

Answer 2

1. Universal indicator
2. pH probe

Question 3

What do acids produce in aqueous solutions?

Answer 3

H⁺ ions

Question 4

What do alkalis produce in water?

Answer 4

OH⁻ ions

Question 5

What is an alkali?

Answer 5

A base that dissolves in water

Question 6

What happens in a neutralisation reaction between an acid and an alkali?

Answer 6

Hydrogen ions react with hyroxide ions to produce water

Question 7

What is a strong acid?

Answer 7

Completely ionised in aqueous solution (all acid particles dissosiate to release H⁺ ions)

Question 8

What is a weak acid?

Answer 8

Only partially ionised in aqueous solution (small proportion of acid particles dissosiate and release H⁺ ions)

Question 9

What is pH a measure of?

Answer 9

The concentration of H⁺ ions in a solution

Question 10

For every decrease of 1 on the pH scale, how much does the concentration of H⁺ ions increase by?

Answer 10

A factor of 10

Question 11

3 examples of strong acids

Answer 11

1. Sulfuric acid
2. Hydrochloric acid
3. Nitric acid

Question 12

3 examples of weak acids

Answer 12

1. Ethanoic acid
2. Citruc acid
3. Carbonic acid

Question 13

Why is a pH probe better to use than universal indictor?

Answer 13

Removes human judgement, making results more reliable

Question 14

What is the neutralisation word equation?

Answer 14

Acid + base —> salt + water

Question 15

What are the 3 common acids?

Answer 15

1. Sulphuric acid
2. Hydrochloric acid
3. Nitric acid

Question 16

What are the 2 common bases?

Answer 16

1. Sodium hydroxide
2. Calcium carbonate

Question 17

What type of reaction do weak acids form?

Answer 17

Reversible reactions

Question 18

Why does equilibrium lie to the left in a weak acid reaction?

Answer 18

Only a few particles release H⁺ ions

Question 19

What is the equation for an acid + a metal oxide?

Answer 19

Acid + metal oxide —> salt + water

Question 20

What is the equation for an acid + metal hydroxide?

Answer 20

Acid + metal hydroxide —> salt + water

Question 21

What is the equation for an acid + metal carbonate?

Answer 21

Acid + metal carbonate —> salt + water + carbon dioxide

Question 22

What is the colour change of litmus? acid —> alkali

Answer 22

Red —> blue

Question 23

What is the colour change of methyl orange? acid —> alkali

Answer 23

Red —> yellow

Question 24

What is the colour change of phenolphthalein? acid —> alkaline

Answer 24

Colourless —> pink

Question 25

What is the equation for an acid + metal?

Answer 25

Metal + acid ---> salt + hydrogen

Question 26

What is the equation for a metal + water?

Answer 26

Metal + water ---> metal hydroxide + hydrogen

Question 27

How can you investigate the reactivity of metals? [2]

Answer 27

1. Rate at which hydrogen bubbles are produced 2. Temperature change

Question 28

What is oxidation?

Answer 28

Gain of oxygen

Question 29

What is reduction?

Answer 29

Loss of oxygen

Question 30

What happens in a reduction reaction?

Answer 30

A metal is separated from its oxide

Question 31

Which metals can be reduced using carbon?

Answer 31

Metals below carbon on the reactivity series

Question 32

Why can some metals be reduced using carbon?

Answer 32

Carbon can take the oxygen from metals which are less reactive than carbon itself, leaving behind a pure metal

Question 33

Why are metals such as gold only ever found as the metal itself?

Answer 33

They are too unreactive to react with things like oxygen

Question 34

What do metals above carbon on the reactivity series have to be reduced by?

Answer 34

Electrolysis

Question 35

What is the problem with electrolysis?

Answer 35

Very expensive

Question 36

What gas is produced if a metal is reduced?

Answer 36

CO₂

Question 37

What is oxidation in terms of electrons?

Answer 37

Loss of electrons

Question 38

What is reduction in terms of electrons?

Answer 38

Gain of electrons

Question 39

What is electrolysis?

Answer 39

Splitting up with electricity

Question 40

What is an electrolyte?

Answer 40

A molten or dissolved ionic compound

Question 41

Is the cathode positive or negative?

Answer 41

Negative

Question 42

What kind of ions move to the cathode?

Answer 42

Positively charged ions

Question 43

Is the anode negative or positive?

Answer 43

Positive

Question 44

What kind of ions move to the anode?

Answer 44

Negatively charged ions

Question 45

# THINK: electrode? reduced/oxidised? released as? Describe what happens when lead bromide (a simple ionic compound) is electrolysed in a molten state [2]

Answer 45

1. Lead is attracted to the cathode, reduced and falls to the bottom as pure lead 2. Bromine is attracted to the anode, oxidised and is released as bromine gas

Question 46

In electrolysis, which way do electrons travel?

Answer 46

Anode ---> cathode

Question 47

Why are the electrodes in electrolysis usually made from inert carbon?

Answer 47

Inert carbon is unreactive, so will not interfere with the reaction

Question 48

In electrolysis, why does the compound you're trying to separate need to be molten or dissolved?

Answer 48

So the ions are free to move and can conduct electricity

Question 49

# Draw and check on back Draw an electrolysis set-up? [5]

Answer 49

1. Beaker 2. Electrolyte 3. Anode and cathode 4. Wire 5. Battery (small line towards cathode)

Question 50

When would a metal be extracted from its ore by electrolysis? [2]

Answer 50

1. If the metal is too reactive to be reduced with carbon 2. If the metal reacts with carbon itself

Question 51

What ore is alumunium extracted from?

Answer 51

Bauxite

Question 52

What steps have to be taken before aluminium oxide can be electrolysed? [3]

Answer 52

1. Purify aluminium oxide from bauxite 2. Mix aluminium oxide with cryolite - to lower the melting temperature 3. Melt it to make it molten

Question 53

Why is cryolite mixed with aluminium oxide before electrolysis?

Answer 53

To lower the melting temperature

Question 54

# THINK: electrode? reduced/oxidised? released as? Describe what happens when aluminium oxide (a metal ore) is electrolysed in a molten state [2]

Answer 54

1. Aluminium is attracted to the cathode, reduced and falls to the bottom as pure metal 2. Oxygen is attracted to the anode, oxidised and is released as oxygen gas

Question 55

# Write and check Write the half equations for the electrolysis of aluminium oxide [2]

Answer 55

1. Al³⁺ + 3e⁻ ---> Al 2. 2O²⁻ ---> O₂ + 4e⁻

Question 56

Why must the positive electrode (anode) be continually replaced?

Answer 56

It is made from carbon so it will start to react with oxygen to produce carbon dioxide

Question 57

Why is electrolysis expensive?

Answer 57

It requires a large amount of energy

Question 58

In aqueous solutions, what else is present as well as the ions from the compound? [2]

Answer 58

1. Hydrogen ions 2. Hydroxide ions

Question 59

What is the general rule at the cathode in aqueous solutions?

Answer 59

Hydrogen gas will be produced if the metal ion is more reactive than hydrogen

Question 60

What is the general rule at the anode in aqueous solutions?

Answer 60

If no halide ions are present, then the hydroxide ions will be discharged as oxygen and water

Question 61

Describe what happens when a solution of copper sulfate is electrolysed [2]

Answer 61

1. Copper and hyrogen ions are attracted to the cathode. Copper is less reactive than hydrogen so it is discharged and coats the electrode 2. Sulfate and hydroxide ions are attracted to the anode. No halide is present so hydroxide ions are discharged as oxygen and water

Question 62

Describe what happens when a solution of sodium chlorine is electrolysed [2]

Answer 62

1. Sodium and hydrogen ions are attracted to the cathode. Sodium is more reactive than hydrogen so hydrogen gas is produced 2. Chloride and hydroxide ions are attracted to the anode. A halide is present so chlorine gas is produced