Salts and electrolysis

Question 1

What are acids?

Answer 1

Substances that produce hydrogen ions, H+ when they are added to water.

Question 2

What are alkalis?

Answer 2

Bases that produce OH- ions when they are added to water, and make alkaline solutions that pH values greater than 7.

Question 3

What is formed when a substance is dissolved in water?
What does the state symbol aq show?
What do hydrogen ions do to solutions?

Answer 3

An aqueous solution.
The state symbol aq shows that the ions are in an aqueous solution.
They make solutions acidic and they have pH values less than 7.

Question 4

What are indicators?

What is different about universal indicator and full range indicators?

Answer 4

Indicators have different colours in acidic and alkaline solutions.
Universal and full range indicators have different colours in different pH values.

Question 5

Acids react with metals that are above which element in the reactivity series?
What reactions between acids and metals are too reactive to be done safely?

Answer 5

Hydrogen

The reactions between acids and very reactive metals such as sodium and potassium.

Question 6

What are the products of reactions between metals and acids?

Answer 6

A salt and hydrogen.

Question 7

What are bases?

What are the products of reactions between acids and bases?

Answer 7

Metal oxides and hydroxides that neutralise acids when they react.
A salt and water.

Question 8

How can the reactions between a metal or a base and an acid be used to make salts? (3)

From which acids can the following salts be made: Chlorides, nitrates and sulfates?

Answer 8

1) A metal, or base that is insoluble in water, is added a little at a time to the acid until all the acid has reacted.
2) The mixture is then filtered to remove the excess solid reactant, leaving a solution of salt.
3) The solid salt is made when the water is evaporated from the solution so that it crystallises.

Chlorides - hydrochloric acid
Nitrates - nitric acid
Sulfates - sulfuric acid

Question 9

How can soluble salts be made and what is the other product of this type of reaction?
How can the reaction between any acid and alkali be represented?

Answer 9

By reacting an acid with an alkali, the other product is water.
H+ (aq) + OH- (aq) => H20(I)

Question 10

There is no visible change when an acid reacts with an alkali so what can be used to show when the reaction is complete?
How can the solid salt be obtained from the solution in a neutraliasation reaction?

Answer 10

An indicator or a pH meter is used to show when the reaction is complete.

The solid salt can be obtained from the solution in a neutralisation reaction by crystallisation.

Question 11

What is ammonia solution?

What does it produce when it is reacted with acids, what can these be used for?

Answer 11

Ammonia solution is an alkali that does not contain a metal.

It reacts with acids to produce ammonium salts which are used as fertilisers.

Question 12

How can we make insoluble salts?

Give an example.

Answer 12

Insoluble salt can be made by mixing solutions of soluble salts that contain the ions needed.
For example, we can make lead iodide by mixing solutions of lead nitrate and potassium iodide, the lead iodide forms a precipitate that can be filtered from the solution, washed with distilled water and dried.

Question 13

How can some pollutants, such as metal ions be removed from water?
What is done to the water?

Answer 13

Some pollutants, such as metal ions can be removed from water by precipitation.
The water is treated by adding substances that react with the pollutant metal ions dissolved in the water to form insoluble salts.

Question 14

What is electrolysis?

When does electrolysis take place?

Answer 14

Electrolysis is the process that uses electricity to break down ionic compounds into elements.
Electrolysis takes place when electricity is passed through a molten ionic compound or a solution containing ions.

Question 15

What is an electrolyte?

Answer 15

The substance that is broken down in the process of electrolysis.

Question 16

How is the electrical circuit in electrolysis set up?

What are the electrodes made from?

Answer 16

The electrical circuit has two electrodes that make contact with the electrolyte.
The electrodes are often made of an inert substance that does not react with the products.

Question 17

What happens in the process of electrolysis? (3)

Answer 17

1) The ions in the electrolyte move to the electrodes where they are discharged to produce elements.
2) Positively charged ions are attracted to the negative electrode where they form metals or hydrogen, depending on the ions in the electrolyte.
3) Negatively charge ions are attracted to the positive electrode where they lose their charge to form non-metallic elements.

Question 18

What happens when positively charged ions reach the negative electrode?
What is this process called?
How many electrons does an ion with a single positive charge gain?

Answer 18

When positively charge ions reach the negative electrode they gain electrons to become neutral atoms.
Gaining electrons is called reduction, so the positive ions have been reduced.
1

Question 19

What happens at the positive electrode?
What is this process called?
What do some non-metals form? Example.

Answer 19

At the positive electrode, negative ions lose electrons to become neutral atoms.
This process is called oxidation, some non-metal atoms combine to form molecules.
E.g. Bromine forms Br2.

Question 20

How can the changes at the electrodes be represented?

Half equations for lead bromide? Lead has 2+ charge and bromine has 1- charge.

Answer 20

The changes at the electrodes can be represented using half equations.

Negative electrode: Pb2+ + 2e- => Pb
Positive electrode: Br- => Br2 + 2e-

Question 21

What ions does water contain?

When solutions of ions in water are electrolysed what may be produced at the negative electrode, when does this happen?

Answer 21

Hydrogen ions and hydroxide ions.
When solutions of ions in water are electrolysed, hydrogen may be produced at the negative electrode if the other positive ions in the solution are those of a metal more reactive than hydrogen.

Question 22

What is usually formed at the positive electrode from the electrolysis of aqueous solutions?
What will be produced at the positive electrode if the solution contains a reasonably high concentration of a halide ion?

Answer 22

At the positive electrode oxygen is usually produced from aqueous solutions.
If the solution contains a reasonably high concentration of halide ions then a halogen will be produced at the positive electrode.

Question 23

Why must aluminium be extracted from its ore by electrolysis?
What does aluminium ore contain and what must be done to it?

Answer 23

Aluminium must be extracted from its ore by electrolysis because it is more reactive than carbon.
Aluminium ore contains aluminium oxide which must be purified and then melted so that it can be electrolysed.

Question 24

What is the melting point of aluminium oxide what is the problem with this?
What is aluminium oxide mixed with and why is this done? (2)

Answer 24

The melting point of aluminium oxide is over 2000ºC, which requires a lot of energy to produce.

1) Aluminium oxide is mixed with another ionic compound called cryolite, so that the mixture melts at about 850ºC.
2) The mixture can be electrolysed at the lower temperature and produces aluminium and oxygen.

Question 25

What happens at the negative electrode during the process of electrolysis of aluminium?

Answer 25

At the negative electrode, aluminium ions are reduced to aluminium atoms by gaining electrons, the molten aluminium is collected from the bottom of the cell.

Question 26

What happens at the positive electrode during the process of electrolysis of aluminium?

Answer 26

At the positive electrode, oxide ions are are oxidised to oxygen atoms by losing electrons and the oxygen atoms form oxygen molecules.

Question 27

Half equations for the electrolysis of aluminium.

Answer 27

At the negative electrode: Al3+(I) + 3e- => Al(I)

At the positive electrode: 2O2-(I) => 2O2(g) + 4e-

Question 28

What are the positive electrodes used in the cell for the extraction of aluminium made of?
At the high temperatures of the cell what do the carbon electrodes react with and what is the product formed?
What are the consequences of this?

Answer 28

The positive electrodes used in the cell are made of carbon.
At the high temperatures of the cell the oxygen reacts with the carbon electrodes to produce carbon dioxide.
This means that the carbon electrodes gradually burn away and have to be replaced regularly.

Question 29

What is brine?
What ions does brine contain?
What is produced at the electrodes during the electrolysis of brine?
What is the solution left after the electrolysis of brine?

Answer 29

Brine is a solution of sodium chloride in water.
The solution contains:
sodium ions, Na+(aq)
chloride ions, Cl- (aq)
hydrogen ions, H+(aq)
hydroxide ions, OH-(aq)
Hydrogen is produced at the negative electrode and chlorine is produced at the positive electrode.
This leaves a solution of sodium ions and hydroxide ions, NaOH(aq)

Question 30

Half equations for the electrolysis of brine.

Answer 30

At the positive electrode: 2Cl- => Cl2 + 2e-

At the negative electrode: 2H+ + 2e- => H2

Question 31

What is sodium hydroxide and what are its uses (4)?

Answer 31

Sodium hydroxide is a strong alkali and it has many uses including:

1) Making soap
2) Making paper
3) Making bleach
4) Neutralising acids and controlling pH.

Question 32

What are the uses of chlorine? (4)

Answer 32

Chlorine is used

1) To kill bacteria in drinking water and in swimming pools. 2) To make disinfectants, bleach and plastics.

Question 33

What are the uses of hydrogen? (2)

Answer 33

Hydrogen is used to make margarine and hydrochloric acid.

Question 34

What is electroplating?

What three metals are often used?

Answer 34

Electroplating uses electrolysis to put a thin coating of metal onto an object, gold silver and chromium plating are often used.

Question 35

Give four reasons why electroplating is done.

Answer 35

1) To make the object look more attractive.
2) To protect a metal object from corroding.
3) To increase the hardness of a surface.
4) To reduce costs by using a thin layer of metal instead of the pure metal.

Question 36

How is electroplating carried out? (5)

Answer 36

1) The object to be plated is used as the negative electrode.
2) The positive electrode is made from the plating metal.
3) The electrolyte is a solution containing ions from the plating metal.
4) At the positive electrode , atoms of the plating metal lose electrons to form metal ions which go into the solution.
5) At the negative electrode, metal ions from the solution gain electrons to form metal atoms which are deposited on the object to be plated.

Question 37

Half equations for nickel electroplating. Nickel has charge of 2+.

Answer 37

At the positive nickel electrode: Ni(s) => Ni2+(aq) + 2e-

At the negative electrode to be plated: Ni2+(aq) + 2e- => Ni (s)