C4: Chemical changes

Question 1

What colour are acids on the pH scale?

Answer 1

Red.

Question 2

What colour are alkalis on the pH scale?

Answer 2

Blue/purple.

Question 3

What does pH mean?

Answer 3

Power of hydrogen- a measure of the concentration of H+ ions in the solution.

Question 4

What are 4 measuring instruments for pH?

Answer 4

Litmus paper, phenolphthalein, universal indicator and a pH probe (goes in the solution) attached to a pH meter.

Question 5

What colour does blue litmus paper turn in acids?

Answer 5

Red.

Question 6

What colour does blue litmus paper turn in alkalis?

Answer 6

Stays blue.

Question 7

What colour does red litmus paper turn in acids?

Answer 7

Stays red.

Question 8

What colour does red litmus paper turn in alkalis?

Answer 8

Turns blue.

Question 9

What is useful about universal indicator?

Answer 9

It matches a shade on the pH scale.

Question 10

What is the benefit of measuring pH digitally?

Answer 10

It is more accurate and gives numerical values rather than a colour, which is harder to interpret.

Question 11

What is the equation for neutralisation?

Answer 11

Acid + base = salt + water.

Question 12

How can neutralisation be shown in terms of hydrogen and hydroxide ions?

Answer 12

H⁺_(aq) + OH^-_{(aq) ⇒} H₂0_(l)

Question 13

What is the formula of sulfuric acid?

Answer 13

H₂SO₄

Question 14

What is the formula of nitric acid?

Answer 14

HNO₃

Question 15

What is the formula of hydrochloric acid?

Answer 15

HCl

Question 16

What is the formula of copper hydroxide?

Answer 16

Cu(OH)₂

Question 17

What is the formula of sodium hydroxide?

Answer 17

NaOH

Question 18

What is the formula of magnesium oxide?

Answer 18

MgO

Question 19

What makes a substance acidic?

Answer 19

H⁺ ions.

Question 20

What salt does sulfuric acid produce in neutralisation reactions?

Answer 20

Sulphate

Question 21

What salt does hydrochloric acid produce in neutralisation reactions?

Answer 21

Chloride

Question 22

What salt does nitric acid produce in neutralisation reactions?

Answer 22

Nitrate

Question 23

What do acids donate in neutralisation reactions?

Answer 23

H⁺ ions.

Question 24

Why are strong acids more reactive than weak acids of the same concentration? What does this mean?

Answer 24

They dissociate more H⁺ ions so the rate of reaction is quicker.

Question 25

What makes an acid strong?

Answer 25

Strong acids ionise- dissociate hydrogen ions- completely in water. The percentage of dissociated H+ ions is higher per litre.

Question 26

What does a value of 1 on the pH scale mean in terms of hydrogen ions?

What does an increase in 1 mean?

Answer 26

There is one mole of dissociated hydrogen ions per litre of solution.

The concentration of dissociated H+ ions has decreased by a factor of 10.

Question 27

Is the pH of a stronger acid lower or higher than of a weaker acid (for the same concentration in a solution)?

Answer 27

Lower.

Question 28

Name 3 strong acids.

Answer 28

Sulfuric, nitric and hydrochloric acid.

Question 29

Name 4 weak acids.

Answer 29

Ethanoic, citric, tartaric and carbonic acid.

Question 30

What is the difference between bases and alkalis?

Answer 30

Alkalis are soluble metal hydroxides. Bases are metal oxides or hydroxides and can neutralise acids. All alkalis are bases but not all bases are alkalis.

Question 31

Potassium hydroxide + hydrochloric acid ->

Answer 31

Potassium chloride + water.

Question 32

What reacts to form copper chloride and water?

Answer 32

Hydrochloric acid and copper oxide.

Question 33

What is the balanced symbol equation for the reaction between potassium hydroxide and sulfuric acid?

Answer 33

2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

Question 34

What is the reactivity series?

Answer 34

It lists metals in order of their reactivity towards other substances:

Question 35

What determines metals’ reactivity?

Answer 35

How easily they lose electrons (oxidise).

Question 36

Acid + metal ->

Answer 36

Salt + hydrogen

Question 37

___ -> salt + hydrogen

Answer 37

Acid + metal

Question 38

Acid + metal → salt + hydrogen. What indicates the speed of this reaction?

Answer 38

The rate at which bubbles of hydrogen are given off, or the temperature change.

Question 39

Metal + water ->

Answer 39

Metal hydroxide + hydrogen

Question 40

___ -> metal hydroxide + hydrogen

Answer 40

Metal + water

Question 41

Which metals will react with water?

Answer 41

The more reactive ones: potassium, sodium, lithium and calcium.

Question 42

What is tarnishing?

Answer 42

Rusting- what happens to the surface of metals when they react with water.

Question 43

What is oxidation?

Answer 43

1. A loss of electrons. 2. When a metal reacts to bond with oxygen, forming an oxide.

Question 44

What is reduction?

Answer 44

1. A gain in electrons. 2. When a metal breaks a bond with oxygen.

Question 45

How can metals be tested to see how reactive they are in comparison to others?

Answer 45

They can be tested in dilute acids.

Question 46

What does the reactivity of a metal depend upon?

Answer 46

Its tendency to give away outer electrons and form positive ions- oxidise.

Question 47

Why do metals have to be extracted?

Answer 47

Metals react with oxygen to form oxides (oxidation), forming ores from which they have to be extracted.

Question 48

What is the name given to a reaction that separates a metal from its oxide?

Answer 48

A reduction reaction.

Question 49

Acid + metal oxide ->

Answer 49

salt + water

Question 50

___ -> salt + water

Answer 50

Acid + metal oxide/hydroxide

Question 51

Acid + metal hydroxide ->

Answer 51

salt + water

Question 52

Acid + metal carbonate ->

Answer 52

salt + water + carbon dioxide

Question 53

___ -> salt + water + carbon dioxide

Answer 53

Acid + metal carbonate

Question 54

Hydrochloric acid + sodium carbonate ->

Answer 54

sodium chloride + water + carbon dioxide

Question 55

Give the equation for the oxidation of magnesium.

Answer 55

2Mg + O₂ -> 2MgO

Question 56

Which metals can be extracted from their ores chemically by reduction with carbon?

Answer 56

Metals below carbon in the reactivity series- zinc, iron, copper.

Question 57

How do reduction reactions work?

Answer 57

Carbon reacts with a metal oxide to produce carbon dioxide and the pure metal. This only works if carbon is more reactive than the metal.

Question 58

Describe the practical: making soluble salts using an insoluble base.

Answer 58

_{^{1) Fill a beaker with boiling water (heat gives energy which will speed up the reaction).}}

_{^{2) Measure 10ml of sulfuric acid into a test tube. Put the test tube into the beaker of water.}}

_{^{3) Allow the acid to heat up before adding copper oxide, which is an insoluble base.}}

_{^{4) The sulfuric acid and copper oxide react, producing a blue salt solution. Excess copper oxide sinks to the bottom.}}

_{^{5) Filter the salt solution from the excess copper oxide- this makes a filtrate.}}

_{^{6) Bring the solution almost to the boil over a bunsen burner, which makes the solution more concentrated (some water boils off) and gives it a higher temperature gradient so that more crystals form.}}

_{^{7) Place the solution into a petri dish; after time, it will become crystalline.}}

Question 59

How can the reactivity of metals be tested?

Answer 59

1) Reacting them with acids: the violence indicates the metal’s reactivity. Increasing heat and concentration of acid can make the reaction faster. The rate of reaction is indicated by the rate at which hydrogen bubbles are released.
2) Reacting them with acid/water and measuring the temperature change. Temperature change will be greater with more reactive metals. Controls should be the time period, the mass and the surface area of the metal.

Question 60

Give the balanced symbol equation for the extraction of iron by reduction with carbon.

Answer 60

2Fe₂O₃ + 3C → 4Fe + 3CO₂

Question 61

Which metals can be extracted by reduction with carbon?

Answer 61

Those less reactive than carbon, because carbon needs to bond with the oxygen. These are zinc, iron and copper.

Question 62

Why has gold been mined for longer throughout history than other metals?

Answer 62

It is found in its elemental form, due to its low reactivity; it does not oxidise to form ores.

Question 63

A gain of electrons =

Answer 63

reduction.

Question 64

A loss of electrons =

Answer 64

oxidation.

Question 65

What are redox reactions and why do they have this name?

Answer 65

A more reactive metal is put into the solution of a less reactive metal, which is displaced from its compound. re=reduction and ox= oxidation; these happen at the same time, hence redox.

Question 66

Redox reactions are the same as…

Answer 66

Displacement reactions.

Question 67

In displacement reactions, it’s always the metal ___ that is reduced (gains electrons) and the metal ___ that is oxidised (loses electrons).

Answer 67

1) Ion.
2) Atom.

Question 68

What are spectator ions?

Answer 68

Ions which don’t change in a reaction.

Question 69

Write the ionic equation for this displacement reaction:

Iron sulfate + zinc →zinc sulfate + iron

Answer 69

1) Write symbol equation, including ions:

FeSO_4(s) + Zn_(aq) → ZnSO_4(aq) + Fe_(s)

Fe²⁺SO₄^2- + Zn → Zn²⁺SO₄^2- + Fe

2) Take out and ignore the spectator ion: SO₄^2-

3) Write the ionic equation, including only the atoms/ions which are reduced or oxidised:

Fe²⁺ + Zn → Zn²⁺ + Fe

4) The iron ions are reduced by the zinc atoms.
5) The zinc atoms are oxidised by the iron ions.

Question 70

What does electrolysis mean?

Answer 70

Splitting up with electricity.

Question 71

What is the negative electrode?

Answer 71

The cathode.

Question 72

What is the positive electrode?

Answer 72

The anode.

Question 73

What is an electrolyte?

Answer 73

A molten or dissolved ionic compound.

Question 74

What type of power supply is used for electrolysis?

Answer 74

D.C.

Question 75

Why is electrolysis necessary for extracting some metals from their ores?

Answer 75

Some are too reactive for displacement reactions (more reactive than carbon).

Question 76

What are ores?

Answer 76

Compounds in which most metals exist, combined with other elements, typically oxygen or sulphur.

Question 77

What happens in electrolysis?

Answer 77

An electric current is passed through an electrolyte. Each ion present is attracted to the electrode (charged metal plate) with an opposite charge. Ions are reduced/oxidised to become pure, uncharged elements.

Question 78

Positive ions will move towards the ___ and are ___.

Answer 78

cathode, reduced.

Question 79

Negative ions will move towards the ___ and are ___.

Answer 79

anode, oxidised.

Question 80

Why is cryolite added to aluminium oxide during electrolysis?

Answer 80

It lowers the melting point of aluminium, reducing the energy cost.

Question 81

Give the half equation for chloride ions at the anode.

Answer 81

2Cl^- → Cl₂ + 2e^-

Question 82

Is aluminium oxidised or reduced during electrolysis?

Answer 82

All metals are reduced.

Question 83

Is chlorine oxidised or reduced during electrolysis?

Answer 83

All non-metals are oxidised.

Question 84

What is the downfall of electrolysis?

Answer 84

It is expensive because it requires lots of energy to both produce the current and heat the ionic compound.

Question 85

What is different about electrolysis in aqueous solutions?

Answer 85

There are ions in the water too, which may react with the electrodes, depending on the reactivity of ions in the electrolyte.

Question 86

What is the rule for the reaction at the cathode in electrolysis of aqueous solutions?

Answer 86

Metal and H⁺ ions are present.

If hydrogen is less reactive than the elemental form of the metal, hydrogen gas is produced.

If hydrogen is more reactive, a solid layer of the pure metal forms on the cathode.

Whatever is less reactive reacts with the cathode.

Question 87

What is the rule for the reaction at the anode in electrolysis of aqueous solutions?

Answer 87

OH^- and non-metal ions are present at the anode.

If halide (Cl^-, Br^-, I^-) ions are present, halogens will be produced.

If not, oxygen gas and water will form.

Question 88

What are titrations?

Answer 88

Experiments which allow you to find the exact volumes of two solutions needed for them to react completely.

Can be used to find the unknown concentration of an acid/alkali, by finding the volume of this needed to neutralise a volume of alkali/acid of known concentration.

Question 89

Describe how you would carry out a titration to find the concentration of an alkali needed to neutralise a volume of an acid of known concentration.

Answer 89

1) Add a set volume of the alkali to a conical flask using a pipette and pipette filler, then add 3 drops of single indicator (phenolphthalein/litmus/methyl orange).
3) Wearing safety goggles, use a funnel to fill a burette with the acid of known concentration. Record the volume in the burette.
4) Use the burette to add the acid to the alkali a bit at a time, regularly swirling the flask. When all the alkali has been neutralised, the indicator changes colour. When this point is nearing, add only a drop at a time.
5) Once the colour change has occurred, record the final volume of acid in the burette and subtract it from the initial volume to find the volume of acid needed to neutralise the alkali.
6) Repeat steps 1-5 three times to increase the accuracy; your results should be within 0.1cm³ of each other. Omit any anomalous results and calculate the mean volume.
7) Calculate the concentration of the alkali (moles of acid = conc. x volume. Use ratios from balanced symbol equation to find moles of alkali. Conc. of alkali = moles/volume).

Question 90

Why are single (rather than universal) indicators used for titrations?

Answer 90

You want to see a sudden and definitive colour change at the point when the acid/alkali has been neutralised.

Question 91

What colour does phenophthalein turn in:

• Acids?
• Alkalis?

Answer 91

• Acids: colourless
• Alkalis: pink

Question 92

What colour does litmus turn in:

• Acids?
• Alkalis?

Answer 92

• Acids: red
• Alkalis: blue

Question 93

What colour does methyl orange turn in:

• Acids?
• Alkalis?

Answer 93

• Acids: red
• Alkalis: yellow