C4 - Chemical Changes

Question 1

How can you measure the pH of a solution?

Answer 1

• Use an indicator.
• Use a pH probe attached to a pH meter.

Question 2

What is an acid?

Answer 2

A substance that forms aqueous solutions with a pH of less than 7. Acids form H+ ions in water.

Question 3

What is a base?

Answer 3

A substance with a pH greater than 7.

Question 4

What is an alkali?

Answer 4

A base that dissolves in water to form a solution with a pH greater than 7. Alkalis from OH- ions in water.

Question 5

What is the reaction between acids and bases and what is it called?

Answer 5

Neutralisation:

Acid + Base ———> Salt + Water

Question 6

How can a neutralisation reaction be seen in terms of ions?

Answer 6

H+ (aq) + OH- (aq) ———> H20 (l)

Question 7

What are titrations used for?

Answer 7

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

Question 8

Describe an investigation to find out the concentration of some alkali.

Answer 8

1) Using a pipette and pipette filler, add a set volume of the alkali to a conical flask. Add a few drops of an indicator such as phenolphthalein.
2) Use a funnel to fill a burger with some acid or a known concentration. This must be done below eye level to prevent acid from going into the eyes. Safety glasses must be worn too. Record the initial volume of the acid in the burette.
3) Using the burette, add the acid to the alkali a bit at a time whilst swirling the conical flask. Add the acid in dropwise when approaching the end-point.
4) When the alkali turns colourless, the alkali has been neutralised. Immediately stop adding any more acid.
5) Record the final volume of acid in the burette and calculate the volume of acid used to neutralise the acid by subtracting the final volume from the initial volume.

Question 9

How do you increase the accuracy of titrations?

Answer 9

Repeat the titrations:
- The first titration should be a rough titration to get an approximate idea of where the solution changes colour / reaches the end point.
- Repeat the whole titration a few times.
- Calculate a mean, ignoring anomalous results.

Question 10

Why can’t you use universal indicator during titrations?

Answer 10

It is made from a mixture of indicators, so it gradually changes colour. In titrations, you need to see a sudden change, so you should use single indicators such as phenolphthalein.

Question 11

What do acids do in water?

Answer 11

They ionise in water / aqueous solutions, producing H+ ions.

Question 12

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

Answer 12

• Strong acids ionise completely in water. All acid particles dissociate to release H+ ions.
• Weak acids do not fully ionise in solution. Only a small proportion of acid particles dissociate to release H+ ions.

Question 13

Why does a weak acid have partial ionisation?

Answer 13

The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid. Since only a few acid particles release H+ ions, the position of equilibrium lies well to the left.

Question 14

What happens to the reaction of the concentration of H+ ions in an acid is higher and why?

Answer 14

The rate of reaction will be faster because reactions of acids involve the H+ ions reacting with other substances.

Question 15

What is the ruling on the change of pH on the pH scale for acids?

Answer 15

For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of 10.

Question 16

What is the difference between strong acids and concentrated acids?

Answer 16

• Acid strength tells you what proportion of the acid molecules ionise in water.
• Concentration measures how much acid there is in the solution.

Question 17

What are metal oxides and hydroxides? What does this mean?

Answer 17

They are bases. Some dissolve in water. These soluble compounds are alkalis. Even bases that don’t dissolve in water will still take part in neutralisation with acids. So all metal oxides and metal hydroxides react with acids to form a salt and water.

Question 18

What does the salt produced depend on?

Answer 18

The acid and the metal ion in the oxide / hydroxide:

• Hydrochloric acid makes a chloride salt.
• Sulfuric acid makes a sulfate salt.
• Nitric acid makes a nitrate salt.

Question 19

What do acids and metal carbonates produce?

Answer 19

Salt, water AND carbon dioxide.

Question 20

Describe an investigation to make soluble salts using an insoluble base.

Answer 20

1) Gently warm the dilute acid using a Bunsen burner, then turn off the Bunsen burner.
2) Add the insoluble base to the acid a bit at a time in excess whilst stirring so all the acid is neutralised.
3) Filter out the excess solid to get the salt solution.
4) Gently heat the solution using a water bath to evaporate some of the water to make the solution less concentrated.
5) Stop heating the solution and leave the solution to cool and crystallise.
6) Filter out the solution and dry the crystals.

Question 21

What is the reactivity series?

Answer 21

It lists the metals in order of their reactivity towards other substances.

Question 22

Name the metals in order or reactivity on the reactivity series.

Answer 22

• Potassium
• Sodium
• Lithium
• Calcium
• Magnesium
• Carbon
• Zinc
• Iron
• Hydrogen
• Copper

Question 23

What do some metals that react with acids produce?

Answer 23

Salt and hydrogen.

Question 24

How does the reactivity of the metal affect the reaction?

Answer 24

The more reactive the metal, the faster the reaction will go. The speed of reaction is indicated by the rate at which the bubbles of hydrogen are given off.

Question 25

Other than the speed of the reaction, how can you tell how reactive the metal is in a reaction?

Answer 25

Measuring the temperature change of the reaction with an acid or water over a set period of time.

Question 26

Which metals react with water?

Answer 26

• Potassium
• Sodium
• Lithium
• Calcium

Question 27

Which metals don’t react with water?

Answer 27

• Zinc
• Iron
• Copper

Question 28

How are metals usually found?

Answer 28

Metal oxides. These oxides are often the ores that the metals need to be extracted from.

Question 29

What is an ore?

Answer 29

A type of rock that contains metal compounds.

Question 30

What is a reaction that separates a metal from its oxide called?

Answer 30

A reduction reaction.

Question 31

What does oxidation and reduction mean?

Answer 31

Oxidation: Gain of oxygen, loss of electrons.
Reduction: Loss of oxygen, gain or electrons.

Question 32

How is a metal extracted with carbon?

Answer 32

• The ore is reduced as oxygen is removed from it.
• Carbon gains oxygen and loses electrons, so it is oxidised.
• Therefore, the metal loses oxygen and gains electrons, so it is reduced.

Question 33

When can a metal be separated from its ore with carbon and why?

Answer 33

If the metal is lower than carbon in the reactivity series. This is because carbon can only displace metals that are less reactive as more reactive metals displace less reactive metals in compounds.

Question 34

How are very unreactive metals found?

Answer 34

In earth as the metal itself.

Question 35

What is a displacement reaction?

Answer 35

A redox reaction. It is when a more reactive metal displaces a less reactive metal from its compound.

Question 36

What happens to the metal
ion and metal atom in a displacement reaction?

Answer 36

• The metal ion gains electrons and is reduced.
• The metal atom loses electrons and is oxidised.

Question 37

What is shown in an ionic equation?

Answer 37

Only the particles that react and the products they form.

Question 38

What are ions that don’t change in a reaction called?

Answer 38

Spectator ions. They can be ignored in ionic equations.

Question 39

What does electrolysis mean?

Answer 39

Splitting up with electricity.

Question 40

What happens during electrolysis?

Answer 40

1) An electric current is passed through an electrolyte.
2) The ions in the electrolyte move towards the electrodes.
3) The positive ions move towards the negative cathode and become reduced as they gain electrons.
4) The negative ions move towards the positive anode and become oxidised as they lose electrons.
5) This creates a flow or charge through the electrolyte as ions travel to the electrodes.
6) As ions gain or lose electrons, they form the uncharged element and are DISCHARGED from the electrolyte.

Question 41

What is an electrolyte?

Answer 41

A molten or dissolved ionic compound.

Question 42

Why do the ionic solids need to be molten or dissolved?

Answer 42

Ionic solids can’t be electrolysed because the ions are in fixed positions and cannot move.

Question 43

If a metal is more reactive than carbon, how is it separated from its ore?

Answer 43

Electrolysis.

Question 44

What is the problem with electrolysis?

Answer 44

It is very expensive as lots of energy is required to melt the ore and produce the required current.

Question 45

Describe how aluminium is separated from the ore bauxite by electrolysis.

Answer 45

1) Bauxite contains aluminium oxide.
2) Aluminium has a very high melting point, so it is mixed with cryolite to lower the melting point. This makes this process cheaper.
3) The molten mixture contains free ions, so it can conduct electricity.
4) The positive Al3+ ions are attracted to the negative cathode where they gain three electrons and are reduced to form aluminium. The element then sinks to the bottom of the electrolysis tank.
5) The negative O2- ions are attracted to the positive anode where they lose two electrons to form oxygen atoms which combine to form O2 molecules. These molecules then leave the solution as they are gases.
6) If the electrode is made out of carbon, the oxygen will then react with it to form carbon dioxide. Therefore, the electrode needs to be replaced regularly.

Question 46

What is the difference in using aqueous solutions than electrolytes?

Answer 46

Aqueous solutions also have hydrogen ions and hydroxide ions whereas electrolytes only contain the two ions.

Question 47

What happens during electrolysis when using aqueous solutions?

Answer 47

1) At the cathode, hydrogen gas is produced if the metal ions form an element that is more reactive than hydrogen. If the metal ions from an element that is less reactive than hydrogen, a solid layer of the pure metal will be produced instead. (The less reactive element is formed but don’t write this in the exam unless it asks.)
2) At the anode, if OH- and halide ions are present, molecules of the halogen are formed. If no halide ions ate present, then the OH- ions are discharged and oxygen is formed. Oxygen can be seen as bubbles.

Question 48

How can you test for any gaseous products produced from electrolysis?

Answer 48

Chlorine: Damp litmus paper - bleaches it white.
Hydrogen: Place a lit splint in it - makes a squeaky pop sound.
Oxygen: Place a glowing split in it - relights.

Question 49

What do half equations show?

Answer 49

The reactions at the electrodes.

Question 50

What is the important thing to remember when combining half equations?

Answer 50

The number of electrons need to be the same for each half equation.