Chemical changes

Question 1

What does the pH scale measure?

Answer 1

How acidic or alkaline a solution is

Question 2

What is an indicator?

Answer 2

A dye that changes colour depending on whether it’s above or below a certain pH

Question 3

Other than indicator, how can you test the pH of a solution?

Answer 3

A pH probe attached to a pH meter which measures the pH electronically, giving it a numerical value

Question 4

What is an acid?

Answer 4

A substance that forms an aqueous solution with a pH less than seven and forms H+ ions in water

Question 5

What is a base?

Answer 5

A substance with a pH greater than seven

Question 6

What is an alkali?

Answer 6

A base that dissolves in water to form a solution with a pH greater than seven and forms OH- solutions in water

Question 7

What will a neutralisation reaction produce?

Answer 7

Acid + base –> salt + water

Question 8

What can titrations be used for?

Answer 8

To find out exactly how much acid is needed to neutralise a quantity of alkali

Question 9

How do you increase the accuracy of a titration experiment?

Answer 9

Use the first titration as a practice (do not include in results), repeat the experiment a few times (making sure the same answer is got each time) and calculating a mean of your results, ignoring any anomalies

Question 10

Why should a single indicator be used for a titration?

Answer 10

It allows you to see a sudden colour change at the end-point, rather than a gradual change

Question 11

Give three examples of a single indicator

Answer 11

Phenolphthalein, litmus or methyl orange

Question 12

What is the difference between a strong and a weak acid?

Answer 12

Strong acids ionise completely in water (all acid particles dissociate to release H+ ions) and weak acids do not fully ionise in solution (only a small proportion of particles dissociate to release H+ ions)

Question 13

Will a strong acid or a weak acid be more reactive and why?

Answer 13

A strong acid because reactions of acids involves the reaction of H+ acids with substances so as the concentration of H+ ions is higher, the rate of reaction will be faster

Question 14

What is the rule for the concentration of H+ ion son the pH scale?

Answer 14

Factor H+ ion concentration changes by = 10^-x

Question 15

Is a high concentration acid and a strong acid the same thing?

Answer 15

No, acid strength tells you what proportion of the acid molecules ionise in water and concentration measures how much acid there is in a certain volume of water

Question 16

What is produced from the neutralisation of an acid and an alkali?

Answer 16

All metal oxides and metal hydroxides (alkaline compounds) react with acids to form a salt and water

Question 17

Give three examples of reactions between metal oxides or hydroxides and acid

Answer 17

hydrochloric acid + copper oxide –> copper chloride + water, sulfuric acid + potassium hydroxide –> potassium sulfate + water, nitric acid + sodium hydroxide –> sodium nitrate + water

Question 18

What does a reaction between an acid and a metal carbonate (a type of base) produce?

Answer 18

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

Question 19

Give two examples of acid and metal carbonate reactions

Answer 19

hydrochloric acid + sodium carbonate –> sodium chloride + water + carbon dioxide, sulfuric acid + calcium carbonate –> calcium sulfate + water + carbon dioxide

Question 20

What is the reactivity series?

Answer 20

Potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen and copper

Question 21

How is reactivity determined for metals?

Answer 21

How easy they lose their electrons, forming positive ions

Question 22

What will how metals react with acids tell you about their reactivity?

Answer 22

The speed of the reaction is indicated by bubbles of hydrogen being given off or the temperature change; the more reactive a metal, the faster a reaction will go

Question 23

What will the reaction between a metal and water produce?

Answer 23

Metal + water –> metal hydroxide + hydrogen

Question 24

Which metals on the reactivity series will react with water?

Answer 24

The only ones reactive enough are potassium, sodium, lithium and calcium

Question 25

What is an oxidation reaction?

Answer 25

When a metal reacts with oxygen to form an oxide (gain of oxygen)

Question 26

What is a reduction reaction?

Answer 26

A reaction that separates a metal from its oxide (loss of oxygen)

Question 27

Give an example of an oxidation reaction and a reduction reaction (in symbols)?

Answer 27

2Mg +O2 –> 2MgO

2CuO + C –> 2Cu + CO2

Question 28

What is the most common of extracting metals?

Answer 28

By using reduction with caron: the ore is reduced as oxygen is removed from it and the carbon is oxidised as it gains oxygen

Question 29

Which metals in the reactivity series can be reduced with carbon and why?

Answer 29

Metals below carbon as carbon can only take oxygen away from metals that are less reactive than carbon itself

Question 30

What are redox reactions in terms of electrons?

Answer 30

A loss of electrons is oxidation and gaining electrons is reduction (OIL RIG)

Question 31

Why is it called a redox reaction (give an example)?

Answer 31

Reduction and oxidation are happening at the same time e.g. Fe + 2H^+ –> Fe^2+ + H2: the iron atoms lose electrons whilst the hydrogen ions gain electrons

Question 32

What is the rule of displacement?

Answer 32

A more reactive metal will displace a less reactive metal from it’s compound

Question 33

In a displacement reaction, what is reduced and what is oxidised?

Answer 33

The metal ion always gains electrons (reduced) and the metal atom always loses electrons (oxidised)

Question 34

What does electrolysis mean?

Answer 34

Splitting up with electricity

Question 35

How does electrolysis work?

Answer 35

A current is passed through an electrolyte (molten or dissolved ionic compound). The ions move towards the electrodes, where they react and the compound decomposes.

Question 36

What happens at the cathode?

Answer 36

As it is negative, the positive ions will move towards it where they gain electrons

Question 37

What happens at the anode?

Answer 37

As it is positive, the negative ions will move towards it and lose electrons

Question 38

What are the pros and cons of using electrolysis to extract metals from their ores?

Answer 38

It is a way of extracting metals if they are too reactive to be reduced with carbon, however, it can be very expensive as it requires lots of energy to melt the ore and produce the current

Question 39

Describe in detail the process of extracting aluminium

Answer 39

It is extracted as aluminium oxide from bauxite, then as it has a very high mp, it is mixed with cryolite to lower the mp. As a molten mixture, it contains free ions so can conduct electricity. The positive ions are attracted to the cathode where they pick up three electrons and turn into aluminium atoms.

Question 40

Give the two electron equations and the overall equation for the electrolysis of aluminium

Answer 40

Al^3+ + 3e^- –> Al
2O^2- –> O2 + 4e^-
2Al203 –> 4Al + 302

Question 41

At the cathode, what happens if the metal ions form an elemental metal that is more or less reactive than hydrogen?

Answer 41

If it is more, hydrogen gas will be produced and if it is less, a solid layer of the pure metal will be produced instead

Question 42

How can you test for any gases present in an electrolysis experiment?

Answer 42

Chlorine bleaches damp litmus paper, hydrogen makes a squeaky pop with a lit splint and oxygen will reight a glowing splint