Determine from the graph the rate of reaction at 20 s, in cm³ s⁻¹, showing your working.

Outline, with a reason, another property that could be monitored to measure the rate of this reaction.

Describe one systematic error associated with the use of the gas syringe, and how the error affects the calculated rate.

Identify one error associated with the use of an accurate stopwatch.

tangent drawn to curve at t = 20 s  [✔]

slope/gradient calculation  [✔]

0.35 «cm³ s^–1»  [✔]

Note: Accept values in the range 0.32–0.42 «cm³ s^–1» 

ALTERNATIVE 1
colour  [✔]
Br2 /reactant is coloured «Br^– (aq) is not» [✔]

ALTERNATIVE 2
conductivity [✔]
greater/increased concentration of ions in products [✔]

Note: Do not accept “changes in temperature” or “number of bubbles”.

ALTERNATIVE 3
mass/pressure [✔]
gas is evolved/produced [✔]

Note: Do not accept “mass of products is less than mass of reactants”.

ALTERNATIVE 4
pH [✔]
methanoic acid is weak AND HBr is strong
OR
increase in [H⁺] [✔]

ALTERNATIVE 1
gas may leak/be lost/escape
OR
plunger may stick/friction «so pressure is greater than atmospheric pressure»
OR
syringe may be tilted «up» so plunger moves less «with gravity acting on plunger»
OR
CO₂ dissolved in water  [✔]
calculated rate lower [✔]

ALTERNATIVE 2
syringe may be tilted «down» so plunger moves more «with gravity acting on plunger»
OR
syringe is held in hand so gets warmer and gas expands [✔]
calculated rate higher [✔]

Note: Calculated rate is lower or higher must be stated for M2.

Do not accept “scale on syringe is inaccurate”, “errors in reading syringe”, or “bubbles in syringe”.

human reaction time/delay «starting/stopping the stopwatch»  [✔]

This part proved to be challenging for some candidates whereas other candidates were able to draw a tangent at 20 sec and then calculate the rate. A significant number of candidates calculated the average rate and achieved one mark.

Majority of the candidates stated another property, which could be monitored correctly. The most common error was changes in temperature, which was stated by some candidates.

This part was about the systematic error and was answered correctly, but many candidates failed to state how the error affected the calculated rate. Many candidates confused this with the concept of a random error and identified the uncertainty of reading the syringe, which is incorrect.

This part was well answered by most candidates although some candidates did not read the question clearly and commented on the stopwatch not working properly or not being accurate.

Identify the experiment with the highest rate of lead dissolving.

Suggest why the relationship between time and lead concentration for Cola at 16 °C is not linear.

Examine, giving a reason, whether the rate of lead dissolving increases with acidity at 18 °C.

Lead(II) chloride, PbCl₂, has very low solubility in water.

PbCl₂ (s) $\rightleftharpoons$ Pb²⁺ (aq) + 2Cl⁻ (aq)

Explain why the presence of chloride ions in beverages affects lead concentrations.

A mean daily lead intake of greater than 5.0 × 10⁻⁶ g per kg of body weight results in increased lead levels in the body.

Calculate the volume, in dm³, of tap water from experiment 8 which would exceed this daily lead intake for an 80.0 kg man.

6   [✔]

Note: Accept “orange juice”.

equilibrium is being established «between lead in solution and in mug»
OR
solution becoming saturated
OR
concentration of lead ions/[Pb²⁺] in the solution has increased «over time»
OR
acid concentration has decreased «as reacted with lead»
OR
surface lead has decreased/formed a compound/forms insoluble layer on surface
OR
acid reacts with other metals «because it is an alloy»   [✔]

Note: Do not accept “concentration of cola, orange juice, etc… has decreased”

Do not accept a response that only discusses mathematical or proportional relationships.

no AND experiment 7/beer has lowest rate and intermediate acidity/pH
OR
no AND experiment 6/orange juice has fastest rate but lower acidity/higher pH than lemonade
OR
no AND experiment 6/orange juice has highest rate and intermediate acidity/pH  [✔]

Note: Accept no AND any comparison, with experimental support, that concludes no pattern/increase with acidity

eg: “rate of Pb/lead dissolving generally decreases with acidity as tap water has highest rate (after orange juice) while lemonade (lower pH) has lower rate”.

equilibrium shifts to the left/towards reactants  [✔]

lead «compounds/ions» precipitate
OR
concentration of lead «ions»/[Pb²⁺] decreases  [✔]

Note: Award [2] for “equilibrium shifts to the left/towards reactants due to common ion effect”.

Accept “lead ions/[Pb²⁺] removed from solution” for M2.

«daily limit = 5.0 × 10^–6 g kg^–1 × 80.0 kg =» 4.0 × 10^–4 «g of lead»  [✔]

«volume $=\frac{4.0\times {10}^{-4}\text{ g}}{1.5\times {10}^{-2}\text{g d}{\text{m}}^{-3}}=$» 2.7 × 10^–2/0.027 «dm³»  [✔]

Note: Award [2] for correct final answer

This part was correctly answered by the majority of the candidates.

A surprising number of candidates gave evidence for the non-linearity but then did not go on to explain why, giving no reasons or causes rooted in chemical theory. The command term "suggest" involves proposing a solution or hypothesis. Here the instruction "suggest why" indicates that the reason has to be explained.

The candidates who examined the data and quoted it in their answer generally scored the mark but several candidates did not refer to the data table.

Several candidates missed that this question was based on the equilibrium and it will shift to the left in presence of chloride ions.

Majority of the candidates scored two marks but some struggled with the conversion of grams to milligrams.