Paper 5

Question 1

Suggest how the student could confirm the eggshell is completely dry in step 2.

Answer 1

Reheat the eggshell & reweigh until mass remains constant.

Question 2

How to calculate mean titre.

Answer 2

• Do NOT use rough titre.
• Take average of the two readings that have +- 0.1 cm^3 difference

Question 3

Suggest how the experiment could be made to be more reliable. (P5 S1 Q2f)

Answer 3

Repeat experiment to show that the points (on the graph) are reproducible.

Question 4

Another student suggests that to compare the rates of diffusion between K2Cr2O7 & KMnO4, it would be easier to place solid crystals (instead of solutions) of each of these compounds into the holes in 2 petri dishes of permeable gel.

Suggest 2 practical problems that this would cause. (P5 S1 Q2hii)

Answer 4

• Rate of diffusion: solids diffuse too slowly (through the gel)
• Insolubility: solid crystals may not dissolve into the gel
• Uniformity / size of crystals: crystals are not of uniform size

Question 5

In decomposition of hydrogen peroxide, suggest how the student could ensure they collect pure oxygen gas in the conical flask (gas over water collection method). (P5 S2 Q1bii)

Answer 5

Do not collect the first gas to be delivered.

Question 6

Suggest the improvements that could be made to make the experiment more reliable. (P5 S2 Q2eii)

Answer 6

• More measurements taken in smaller intervals
• Anomalies eliminated

Question 7

Suggest why reagents are heated to the same temperature before mixing. (P5 S2 Q3c)

Answer 7

Changing the temperature of the reagents will affect rate of reaction (so it must be kept constant).

Question 8

The solution of sulfuric acid used in each mixture was of concentration 0.150 mol dm-3. This acid was prepared from a solution of concentration 1 moldm-3.

Briefly describe how to make the more dilute solution, stating the capacity of any apparatus used.

Answer 8

• Use the equation m1V1=m2V2 to calculate the volume of water required to dilute the stock solution.
• Use a 50 cm^3 burette to transfer 37.5 cm^3 of 1M sulfuric acid into a 250 cm^3 volumetric flask.
• Make up to the (calibration) mark with distilled water.
• Shake the mixture.

Question 9

Predict how ΔHr may change as Ecell increases. Give a reason for your prediction.

Answer 9

• Ecell increases
• as reactivity difference increases
• enthalpy becomes more exothermic

Question 10

The aqueous copper (II) sulfate & metal mixture should be stirred continuously. Explain why.

Answer 10

So heat spreads evenly through the solution.

Question 11

Explain why the enthalpy change determination & cell potential determination should be carried out at the same temperature as each other.

Answer 11

• To ensure that the rate is the same in both cases
• At same temperature, you can make a fair comparison

Question 12

What must be used in temperature-change experiments to reduce heat loss?

Answer 12

polystyrene cup

Question 13

Describe a suitable method to add precisely 20.00g of cyclohexane to the boiling tube. Assume that the balance is accurate to 2 d.p. & that common laboratory apparatus is available.

Answer 13

• Add dropwise around 20.00g mark

Question 14

Explain whether a burette is suitable for measuring 25.64 cm3.

Answer 14

• No
• A burette cannot measure to 0.01 cm3

Question 15

Describe a method to prepare 250.0 cm3 of solution A starting with a glass beaker which contains the known mass of lawn sand determined in (a)(ii) as shown.

Lawn sand is a mixture of sand & iron(II) sulfate crystals, FeSO4.7H2O. To determine the exact percentage by mass of FeSO4.7H2O present in a sample of lawn sand, a student devises the following experiment.
1. Use a known mass of lawn sand to prepare 250.0 cm3 of solution A containing Fe2+ (aq) ions. Solution A must have dilute sulfuric acid, H2SO4 (aq), added to it before it is made up to 250 cm3.

P5 S5 Q2

Answer 15

1. Dissolve the iron(II) sulfate crystals in a beaker with distilled water.
2. Filter.
3. Rinse the residue.
4. Add H2SO4.
5. Add to 250 cm3 volumetric flask & make up to mark with distilled water.

Question 16

Beginning with a large piece of lithium being removed from the oil, state how you would prepare a small piece of lithium for use in this experiment.

Answer 16

Use filter paper to remove the oil.

Question 17

State how you would ensure that your titration result was reliable.

Answer 17

0.10 cm3 difference between the 2 most accurate titre values.

Question 18

State whether you consider the results to be reliable. Explain your answer.
(P5 S4 Q2e)

Answer 18

If yes, reliable because most points fall on best fit line.

Question 19

Student X commented that data collected at higher temperatures (less time taken) in the experiment may be less accurate than that collected at lower temperatures (more time taken). State whether student X is correct. Explain why.
(P5 S4 Q2f)

Answer 19

• Higher temperature
• Rate increases
• Time decreases
• 1/t x 100% = % error increases

Question 20

The experiment in (a) is repeated using dilute ethanoic acid instead of dilute hydrochloric acid. The concentration of both acids is equal. The same temperatures are used as in (a). State the effect this change in acid will have on the initial rate values. Give a reason for this.

Answer 20

• Ethanoic acid has less H+ ions because of partial dissociation of weak acid in solution.
• Initial rate decreases.

Question 21

The reaction is first order & obeys the following rate equation. Rate = k [C6H5N2Cl]. Explain how the data in Table 2.1 supports this statement.

Answer 21

• Explanation using numbers from the table
• Whatever change is made to the concentration causes the same change in the rate (within experimental error).

Question 22

Benzenediazonium chloride, C6H5N2Cl, decomposes in water as shown in the following equation: C6H5N2Cl(aq) + H2O(l) C6H5OH(aq) + N2(g) + HCl(aq)

A solution of 0.0750moldm–3 of C6H5N2Cl(aq) decomposes at a constant temperature of 50°C.
The volume of nitrogen gas, N2(g), collected is recorded every 5 minutes for 45 minutes.

Explain why making a standard solution of 100.0cm3 of 0.200moldm–3
C6H5N2Cl(aq) starting from a solution of 0.500moldm–3 C6H5N2Cl(aq) should be carried out at a temperature below 5°C.

Answer 22

To make sure the concentration does not change (from 0.200 moldm-3) (by making sure C6H5N2Cl does not decompose).

Question 23

Suggest why a titration using an indicator is more accurate than a thermometric titration.

Answer 23

The indicator changes colour rapidly / detects end-point of neutralisation instantly.

Question 24

In a thermometric titration, suggest why the value you obtained (-53.5 kJ mol-1) for standard enthalpy change of neutralisation differs from the theoretical value (-57.6 kJ mol-1).
(9701/52/o/n/23 Q1e)

Answer 24

Heat is loss to the surroundings.

Question 25

Suggest why the standard enthalpy change of neutralisation determined using ethanoic acid is less exothermic than the standard enthalpy change using HCl.

Answer 25

• Ethanoic is a weak acid.
• Energy is required for dissociation of ethanoic acid.

Question 26

In a reaction of Zn with HCl, if a student does not remove the oxide layer, the initial rate of reaction is lower than it should be. Explain why the initial rate of reaction is lower than it should be.

Answer 26

• (ZnO + HCl) does not produce hydrogen (at the start).

Question 27

Suggest why water is added to the ethoxyethane layer in step 9 before the titration can take place.
(9701/53/m/j/23)

Answer 27

Butanedioic acid dissolves into the water (before reaction with NaOH).

Question 28

Explain why the student is only able to repeat the titration in step 8 once. (Total volume = 30 cm3, 10cm3 used for each titration)
(9701/53/m/j/23)

Answer 28

It is not possible to pipette all (30cm3) of the aqueous layer from the beaker (as some will always be left behind).

Question 29

A student forgets to shake the separating funnel in step 5. Describe the effect this would have on the calculated Kpc value. Explain your answer.
(9701/53/m/j/23)

Answer 29

• Kpc decreases
• Because (equilibrium had not been attained &) more butanedioic acid remains in aqueous layer.

Question 30

Suggest why each sample is applied to the chromatography paper using a thin capillary tube rather than a dropping pipette.
(9701/53/m/j/23)

Answer 30

The application of the hydrolysed samples (to the paper) will be more controlled.

Question 31

State the reason for overlapping spots in chromatography.
(9701/53/m/j/23)

Answer 31

The spots have similar Rf values.

Question 32

Suggest an improvement to the chromatography method that would allow overlapping spots to be distinguished clearly.
(9701/53/m/j/23)

Answer 32

• Use a longer stationary phase / (chromatography) paper
• Use a different solvent / mobile phase

Question 33

A scientist is asked to find the rate of decomposition of an aromatic diazonium compound and determine the order of the reaction with respect to the aromatic diazonium compound.

The scientist is given 1.02g of an aromatic diazonium compound in a 50cm3 beaker. Describe the steps the scientist should take to make a 100.0cm3 standard solution containing 1.02g of this compound.
(9701/53/m/j/23)

Answer 33

1. Add (a small volume of) distilled water (to the 50cm3 beaker) AND dissolve the compound.
2. Transfer the solution & washings (using a funnel) AND into a 100cm3 volumetric flask.
3. Make up to the (calibration) mark / line with (distilled) water AND mix the solution (by inverting the flask).

Question 34

Suggest why the scientist does not monitor the decomposition of benzenediazonium chloride by measuring the loss of mass.
(9701/53/m/j/23)

Answer 34

Not possible to weigh apparatus (while temperature is maintained) in water bath.

Question 35

Explain why a reaction is a first-order reaction.

Answer 35

Half-lives are constant (within experimental error).