Salts and Electrolysis

Question 1

What is an acid?

Answer 1

Substances with a pH less than 7

They are proton donors, so produce H⁺ ions when added to water

They are the chemical opposite of an alkali

Question 2

What is a base?

Answer 2

Proton acceptors with a pH value above 7

They will react with acids to neutralise them and produce salt as one of the products

Question 3

What is an alkali?

Answer 3

Bases that dissolve in water

They have a pH value greater than 7

They produce OH^- ions when in solution

Question 4

Which metals in the reactivity series will acids react with?

Answer 4

Those above hydrogen

Question 5

Why wouldn’t you react sodium or potassium with an acid in a lab?

Answer 5

Because the reaction is too violent to be done safely

Question 6

What is the reaction that occurs when metals react with acids?

Answer 6

Metal + Acid → Salt + Hydrogen

MASH

Question 7

What is produced in a neutralisation reaction?

Answer 7

Acid + Alkali/Base → Salt + Water

Question 8

How can salt be made from a metal or base that is insoluble in water?

Answer 8

• It is added a little bit at a time to the acid until the acid has reacted
• The mixture is then filtered to remove the excess solid reaction
• This leaves a solution of the salt
• The solid salt is made when water is evaporated from the solution so that is crystallises

Question 9

How can salts be crystallised from solutions?

Answer 9

By evaporating off water

Question 10

How can we make soltuble salt?

Answer 10

By reacting an acid with an alkali

Question 11

What compound is produced in every neutralisation reaction?

Answer 11

Water

Question 12

Why do we need to use an indicator to show what neutralisation is complete?

Answer 12

Because there is no visible change

Question 13

What is ammonia solution?

Answer 13

An alkali that does not contain a metal

Question 14

What is the chemical formula for ammonium nitrate?

Answer 14

NH₄NO₃

Question 15

What are ammonium salts used for?

Answer 15

As fertilisers

Question 16

How can insoluble salts be made?

Answer 16

• By mixing solutions of soluble salts that contain the ions needed
• This forms a precipitate
• The precipitate can then be filtered from the solution, washed with distilled water, and dried

Question 17

Give a useful application of precipitation

Answer 17

It can be used to remove some pollutants, such as metal ions, from water.

The water is treated by adding substances that react with the pollutant metal ions dissolved in the warer to form insoluble salts

Question 18

What is electrolysis?

Answer 18

A process that uses electricity to break down ionic compounds into elements

Question 19

What must happen to ionic compounds before they can be electrolysed and why?

Answer 19

They must be molten or in a solution so that their ions are free to move to the electrodes

Question 20

What is an electrolyte?

Answer 20

A substance containing free-moving ions that is broken down by electrolysis

Question 21

Describe what happens during electrolysis/how it works

Answer 21

• The electrical circuit has two electrodes that make contact with the electrolyte.
• The ions in the electrolyte move to the electrodes where they are discharged to produce elements
• Positively charged ions are attracted to the negative electrode, where they are reduced to form either metals or hydrogen
• Negatively charged ions are attracted to the positive electrode where they are oxidised to form non-metallic elements

Question 22

What is reduction and oxidation?

Answer 22

Reduction is losing electrons (positive ions)

Oxidation is gaining electrons (negative ions)

Oxidation Is Loss

Reduction Is Gain

(OIL RIG)

Question 23

Write out the haf equation for the oxiation of lead at the negative electrode

Answer 23

Pb²⁺ + 2e^- → Pb

Question 24

Complete the half equation for the reduction of bromine at the positive electrode

Answer 24

2Br → Br₂ + 2e^-

Question 25

When aqueous solutions are electrolysed, what is produced at the positive electrode?

Answer 25

Oxygen gas is always produced unless the solution contains halide ions, in which case the halogen will be produced

Question 26

When aqueous solutions are electrolysed, what is produced at the negative electrode?

Answer 26

Hydrogen gas is always produced unless the solution contains ions of a metal that is less reactive than hydrogen

Question 27

What is aluminium oxide mixed with before it is electrolysed and why?

Answer 27

Cryolite This lowers its melting point from 2000°C to 850°C This means less energy is used

Question 28

What is produced by the electrolysis of aluminium?

Answer 28

Aluminium and oxygen

Question 29

What are the positive electrodes used in the electrolysis of aluminium made of and why does this cause problems?

Answer 29

Carbon At the high temperatures at which the reaction takes place, the oxygen reacts with the carbon to produce CO₂ This means that the electrodes gradually burn away and have to be replaced

Question 30

What is brine?

Answer 30

A solultion of sodium chloride (salt) in water

Question 31

What is produced during the electrolysis of brine?

Answer 31

Hydrogen at the negaive electrode Chlorine at the positive electrode Sodium hydroxide solution is left behind

Question 32

Why is hydrogen produced when sodium chloride is electrolysed?

Answer 32

Because the solution contains hydrogen ions which are discharged in preference to the sodium ions as sodium is more reactive than hydrogen

Question 33

What is the sodium hydroxide produced from the electrolysis of brine used for?

Answer 33

* Making soap * Making paper * Making bleach * Neutralising acid * Controlling pH

Question 34

What is the chlorine produced from the electrolysis of brine used for?

Answer 34

* To kill bacteria in drinking water and swimming pools * To make bleach * To make disinfectant * To make plastics

Question 35

What is the hydrogen produced from the electrolysis of brine used for?

Answer 35

* To make margerine * To make hydrochloric acid

Question 36

Why might electroplating be done?

Answer 36

* To make an object look more attractive * To protect a metal object from corroding * To increase the hardness of a surface * To reduce costs by using a thin layer of metal instead of pure metal

Question 37

What is electroplating?

Answer 37

Using electrolysis to put a thin coating of metal onto an object

Question 38

How is electroplating done?

Answer 38

* The object to be plated is used as the negative electrode * The positive electrode is made from the plating metal * The electrolyte is a solution containing ions of the plating metal * At the positive electrode, ions of the plating metal lose electrons to form metal ions which go into the solution * At the negative electrode, metal ions from the solution gain electrons to form metal atoms which are depositied on the object to be plated