5 Salts and electrolytes

Question 1

What are alkalis?

Answer 1

soluble hydroxides which are alkaline solutions
produce OH- ions when we add them to water
e.g.sodium hydroxide solution

Question 2

What are bases?

Answer 2

include alkalis, substances which can neutralise acids
metal oxides and metal hydroxides are bases
e.g.iron oxide and copper hydroxide

Question 3

What pH is pure/distilled water?

Answer 3

neutral

pH 7

Question 4

What are acids?

Answer 4

substances which produce H+ ions when we add them to water

e.g. citric (in citrus fruit and fizzy drinks) acid, sulfuric acid, ethanoic (in vinegar) acid

Question 5

How do we know from the pH scale what is an alkali and what is an acid?

Answer 5

less than 7 = acid
7 = neutral
more than 7 = alkali

Question 6

What is the Bronsted-Lowry theory of acids and alkalis?

Answer 6

Acids are normally called ‘something’ acid

Alkalis are normally called ‘something’ oxide

Question 7

What are indicators?

Answer 7

substances which change colour when we add them to acids and alkalis
e.g. litmus paper

Question 8

What are examples of products on the pH scale?

Answer 8

0
1 hydrochloric acid very acidic
2 citric acid (lemon juice)
3 orange juice and vinegar
4
5 black coffee slightly acidic
6 rainwater
7 pure water neutral
8 seawater and baking soda
9 milk of magnesia and soap slightly alkaline
10
11
12 washing soda
13 very alkaline
14 oven cleaner and sodium hydroxide

Question 9

Explain how neutralisation takes place between acids and alkalis in terms of ions….

Answer 9

Acids dissolve in water to form hydrogen ions (H+)
Soluble bases (alkalis) dissolve in water to form hydroxide ions (OH-)
These ions then combine to form water
If there are the same number of these ions each then the solution will become neutral

Question 10

What does an acid and a metal react together to make?

Answer 10

a salt + hydrogen

if the metal is very reactive e.g. sodium or potassium, the reaction with acid is too violent to carry out safely

Question 11

What does an acid and an insoluble base react together to make?

Answer 11

a salt + water

Question 12

What does the salt formed from a reaction depend on?

Answer 12

the metal or base that we use as well as the acid

so bases containing sodium ions will a;ways make sodium salts etc.

Question 13

In terms of the acid used, what type of salt will be formed from a reaction with hydrochloric, sulfuric, nitric and ethanoic acid?

Answer 13

hydrochloric = chlorides
sulfuric = sulfates
nitric = nitrates
ethanoic = ethanoates

Question 14

What happens when ammonia reacts with water?

Answer 14

forms a weakly alkaline solution

NH3 (aq) + H2O (l) –> NH4+ (aq) + OH- (aq)

Question 15

What happens when ammonia solution reacts with an acid (e.g.nitric acid)?

Answer 15

acid + ammonia solution = an ammonium salt + water

HNO3 (aq) + NH4+ (aq) + OH- (aq) –> NH4NO3 (aq) + H2O (l)

Question 16

What is the reaction called when we react an acid with an alkali to make a salt and water?

Answer 16

a neutralisation reaction

Question 17

Why do we need to use an indicator?

Answer 17

to tell us when the reaction is complete and that the acid and alkali have completely reacted

Question 18

How do we make insoluble salts?

Answer 18

by reacting two soluble salts together

Question 19

What is the name given to the reaction making an insoluble salt called and why is this?

Answer 19

a precipitation reaction

because the salt is formed as a solid

Question 20

Why is the a precipitation reaction important?

Answer 20

when we want to remove metal ions from polluted water (the salt formed is a solid making it easier to extract)

Question 21

In a precipitation reaction, what does each reactant contain?

Answer 21

one of the ions of the insoluble salt
e.g. LEAD nitrate solution + potassium IODIDE solution –> LEAD IODIDE precipitate + potassium nitrate solution
(Pb2+ + 2I- –> PbI2) key ions separated from spectator ions

Question 22

What is electrolysis?

Answer 22

the splitting up of a compound/breaking down of a substance using electricity

Question 23

What must happen for electrolysis to work?

Answer 23

• there must be ions to have a charge to attract to an electrode(not covalent molecules)
• the ions must be able to move (to the electrode)
• the electrodes must be inert (unreactive)

Question 24

Why is it hard to predict what will form at the electrodes when we electrolyse a solution?

Answer 24

water can also form ions

• the less reactive element, between hydrogen (H+) ions and the metal will usually be produced at the cathode
• at the anode, we often get oxygen gas given off from discharged hydroxide ions

Question 25

What happens during electrolysis?

Answer 25

positively charged ions move to the negative electrode (cathode) and at the same time the negative ions move to the positive electrode (anode)

Question 26

What is oxidisation?

Answer 26

a loss of electrons at the anode the ions give away electrons impure copper dissolves e.g. Cu(s) --> Cu2+(aq) + 2e-

Question 27

What is reduction?

Answer 27

a gain of electrons at the cathode the ions pick up electrons pure copper is plated e.g. Cu2+(aq) +2e- --> Cu(s)

Question 28

What are the uses of electrolysis?

Answer 28

- to make pure metal | - to split up elements e.g. aluminium oxide, lead bromide

Question 29

What is important about the electrolysis of sodium chloride solution, brine?

Answer 29

``` Produces 3 very useful products: Sodium chloride(aq) -->(electrolysis) hydrogen(g) + chlorine(g) + sodium hydroxide(aq) ```

Question 30

What are the half equations in the electrolysis of brine?

Answer 30

at the cathode: 2H+ + 2e- --> H2 (hydrogen ions are less reactive than sodium ions) at the anode: 2Cl- --> Cl2 + 2e-

Question 31

What are the uses of hydrogen?

Answer 31

- pure so is useful in the food industry to manufacture margarine by reacting hydrogen with vegetable oils - can also be used to make hydrochloric acid...

Question 32

What are the uses of chlorine?

Answer 32

- used in water and sewage treatment - used in the manufacture of: PVC, textiles, solvents, titanium dioxide, pesticides - used in drugs to treat leukaemia, heart disease and arthritis - can also be used to make hydrochloric acid and bleach...

Question 33

What are the uses of sodium hydroxide?

Answer 33

- used for purifying petroleum and natural gas - used for degreasing metals - used in water and sewage treatment - used in the manufacture of: paper, cellulose, wrapping, artificial fibres, soaps, detergents, aluminium and titanium dioxide - can also be used to make bleach...

Question 34

What are the uses of hydrochloric acid?

Answer 34

- used in cleaning materials - used for water and efficient treatment - used in the manufacture of: medicine, food products (e.g. soy sauce, gelatin), cosmetics, synthetic rubber, precious metals (e.g. gold)

Question 35

What are the uses of bleach?

Answer 35

- used for treating water in swimming pools | - used for household bleach

Question 36

What are the 3 useful products made from the electrolysis of brine?

Answer 36

- chlorine) used to make bleach which kills bacteria and to make plastics - hydrogen) used to make margarine - sodium hydroxide) used to make bleach, paper and soap

Question 37

What are the uses of aluminium?

Answer 37

- pans - overhead power cables - aeroplanes - cooking foil - drink cans - window and patio door frames - bicycle frames and car bodies

Question 38

What makes aluminium useful?

Answer 38

- low density - very strong - oxidised layer prevents it from rusting so does not corrode - shiny so it reflects heat

Question 39

How is aluminium extracted?

Answer 39

electrolysis

Question 40

What is bauxite ore?

Answer 40

ore mined by open cast mining containing mainly aluminium oxide with a few other rocky impurities such as iron oxide

Question 41

How is aluminium produced?

Answer 41

- aluminium oxide is dissolved in molten cryolite in order to use less energy to melt it (from 2000 degrees C to 850 degrees C) - during electrolysis, the aluminium metal is collected at the negative electrode in the cells whilst oxygen is formed at the positive electrode

Question 42

During the electrolysis of aluminium oxide, why do the carbon anodes need regular replacement?

Answer 42

because they burn away as they react with oxygen to form carbon dioxide

Question 43

What is electroplating?

Answer 43

when an object is coated with a thin layer of metal by electrolysis (an electroplated object)

Question 44

Why do we electroplate objects?

Answer 44

- to protect the metal beneath from corroding - to make the object look more attractive - to increase the hardness of the surface and its resistance to scratching - to save money by using a thin layer of precious metal instead of the pure expensive metal (also helps people who are allergic to metals such as nickel used in cheap jewellery)

Question 45

How does electroplating work?

Answer 45

- the positive electrode (anode) is made from the plating metal (they are oxidised and lose electron(s) to form ions which go into the solution) - the metal to be plated is used as the negative electrode (cathode) (the plating ions from the solution are reduced, gaining electron(s) to form atoms deposited on the cathode) - the electrolysis takes place in a solution (electrolyte) containing ions of the plating metal