Acids, bases, salts and electrolysis

Question 1

How can soluble salts be made? (4)

Answer 1

Reacting acids (1) with:
Some metals (some are too reactive and some are too unreactive) 
Insoluble bases (the base is added to the acid until no more will react and the excess solid is filtered off)
Alkalis - an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution

Question 2

How can solid salts be produced from salt solutions?

Answer 2

Crystallisation

Question 3

How can insoluble salts be made? How might this be useful?

Answer 3

Mixing appropriate solutions of ions so that a precipitate is formed. Precipitation can be used to remove unwanted ions e.g. treating water

Question 4

What’s the difference between a base and an alkali?

Answer 4

Metal oxides and hydroxides are bases and they will neutralise acids. Soluble hydroxides are called alkalis

Question 5

On what does the salt produced in a reaction between an acid and a base or alkali depend?

Answer 5

The acid used (HCl produces chlorides, nitric acid produces nitrates, sulphuric acid produces sulphates)
The metal in the base or alkali

Question 6

How are ammonium salts formed and how are they useful?

Answer 6

Ammonia dissolves in water to produce an alkaline solution, then the salts are produced from it. Used as fertilisers.

Question 7

Representative ionic equation for neutralisation

Answer 7

H^+(aq) + OH^-(aq) –> H20(l)

Question 8

Define electrolyte

Answer 8

The substance broken down by electrolysis (with electricity)

Question 9

What does the mnemonic PANIC stand for? What does this mean?

Answer 9

Positive = Anode, Negative Is Cathode

Question 10

What does the mnemonic OIL RIG stand for? What does it mean?

Answer 10

Oxidation Is Loss, Reduction Is Gain
At the cathode, positively charged ions gain electrons (reduction) and at the anode, negatively charged ions lose electrons (oxidation)

Question 11

Why must ionic compounds be electrolysed in solution or molten?

Answer 11

Ions are free to move around to the electrodes

Question 12

In a solution containing K+ and H+ ions, which will go to the cathode? Why?

Answer 12

H+ because hydrogen is less reactive than potassium

Question 13

What is the order of discharge at the positive electrode?

Answer 13

halide ions (group VII ions) > hydroxide > all other negatively charged ions

Question 14

Half equations for electrolysis of lead bromide? (PbBr2)

Answer 14

Negative electrode: Pb^2+ + 2e- ––> Pb

Positive electrode: 2Br- – 2e- ––> Br2

Question 15

Why is oxygen gas often given off at the positive electrode?

Answer 15

Hydroxide ions are (often) discharged

Question 16

5 uses of aluminium metal

Answer 16

Pans, aeroplanes, cooking foil, drink cans, bicycle frames

Question 17

How does aluminium occur naturally and why is not found pure?

Answer 17

Found in bauxite, which is then separated to give alumina (aluminium oxide). It is quite reactive

Question 18

How and why is the melting point reduced in electrolysis of alumina (aluminium oxide)?

Answer 18

Mixing it with molten cryolite (an ionic compound) in order to save energy

Question 19

Why do the positive carbon electrodes need to be replaced regularly in an electrolytic cell used for aluminium extraction?

Answer 19

Oxygen gathers there and at high temperatures it reacts with the carbon in the electrodes to form carbon dioxide. They gradually burn away.

Question 20

Word equation for electrolysis of brine?

Answer 20

sodium chloride solution ––> hydrogen + chlorine + sodium hydroxide solution

Question 21

What happens, during the electrolysis of brine, when chlorine and hydrogen gas have escaped?

Answer 21

The remaining Na+ and OH- ions are left, forming sodium hydroxide solution

Question 22

Half equations for brine

Answer 22

Positive electrode: 2Cl- (aq) ––> Cl2 (g) + 2e-

Negative electrode: 2H+ (aq) + 2e- ––>H2 (g)

Question 23

Uses of the products of brine electrolysis

Answer 23

Chlorine: bleach (+sodium hydroxide), disinfectants, PVC
Hydrogen: margarine (added to veg oils)
Sodium hydroxide: soap, paper, bleach (+chlorine)

Question 24

Why electroplate?

Answer 24

Protect metal from corrosion
Make object more attractive
Harden surface and scratch-resistance
Save money with thin layer of precious metal

Question 25

Copper or (nickel/zinc) electroplating half equations

Answer 25

Positive anode: Cu (s) – 2e- ––> Cu^2+ (aq)

Negative cathode: Cu^2+ (aq) +2e- ––> Cu (s)