Question
This question is about a mug made of a lead alloy.
The rate of lead dissolving in common beverages with various $\mathrm{pH}$ values was analysed.
a. Identify the experiment with the highest rate of lead dissolving.
$\mathrm{b}(\mathrm{i})$ Suggest why the relationship between time and lead concentration for Cola at $16^{\circ} \mathrm{C}$ is not linear.
$\mathrm{b}$ (iiExamine, giving a reason, whether the rate of lead dissolving increases with acidity at $18^{\circ} \mathrm{C}$.
c(i)Lead(II) chloride, $\mathrm{PbCl}_2$, has very low solubility in water.
$$
\mathrm{PbCl}_2(\mathrm{~s}) \rightleftharpoons \mathrm{Pb}^{2+}(\mathrm{aq})+2 \mathrm{Cl}^{-}(\mathrm{aq})
$$
Explain why the presence of chloride ions in beverages affects lead concentrations.
c(ii)A mean daily lead intake of greater than $5.0 \times 10^{-6} \mathrm{~g}$ per $\mathrm{kg}$ of body weight results in increased lead levels in the body.
Calculate the volume, in $\mathrm{dm}^3$, of tap water from experiment 8 which would exceed this daily lead intake for an $80.0 \mathrm{~kg}$ man.
▶️Answer/Explanation
Markscheme
a. $6[\backsim]$
Note: Accept “orange juice”.
$b(i) e q u i l i b r i u m$ is being established «between lead in solution and in mug»
OR
solution becoming saturated
OR
concentration of lead ions/[ $\left.\mathrm{Pb}^{2+}\right]$ 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» [ $\checkmark]$
Note: Do not accept “concentration of cola, orange juice, etc… has decreased”
Do not accept a response that only discusses mathematical or proportional relationships.
b(ii) AND experiment $7 / \mathrm{beer}$ has lowest rate and intermediate acidity/pH
OR
no AND experiment 6/orange juice has fastest rate but lower acidity/higher $\mathrm{pH}$ than lemonade
OR
no $A N D$ experiment 6/orange juice has highest rate and intermediate acidity/pH [ $\boldsymbol{V}]$
Note: Accept no AND any comparison, with experimental support, that concludes no pattern/increase with acidity
eg: “rate of $\mathrm{Pb} / \mathrm{lead}$ dissolving generally decreases with acidity as tap water has highest rate (after orange juice) while lemonade (lower $\mathrm{pH}$ ) has lower rate”.
c(i)equilibrium shifts to the left/towards reactants [ $\boldsymbol{U}$ ]
lead «compounds/ions» precipitate
OR
concentration of lead «ions»/[ $\left.\mathrm{Pb}^{2+}\right]$ decreases $[\boldsymbol{V}]$
Note: Award [2] for “equilibrium shifts to the left/towards reactants due to common ion effect”.
Accept “lead ions $/\left[\mathrm{Pb}^{2+}\right]$ removed from solution” for $M 2$.
c(ii)sdaily limit $=5.0 \times 10^{-6} \mathrm{~g} \mathrm{~kg}^{-1} \times 80.0 \mathrm{~kg}=» 4.0 \times 10^{-4}$ «g of lead» [ $[$ ]
«volume $=\frac{4.0 \times 10^{-4} \mathrm{~g}}{1.5 \times 10^{-2} \mathrm{~g} \mathrm{dm}^{-3}}=» 2.7 \times 10^{-2} / 0.027 « \mathrm{dm}^3 »[\boldsymbol{V}]$
Question
Bromine and methanoic acid react in aqueous solution.
$$
\mathrm{Br}_2(\mathrm{aq})+\mathrm{HCOOH}(\mathrm{aq}) \rightarrow 2 \mathrm{Br}^{-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{CO}_2(\mathrm{~g})
$$
The reaction was monitored by measuring the volume of carbon dioxide produced as time progressed.
a. Determine from the graph the rate of reaction at $20 \mathrm{~s}$, in $\mathrm{cm}^3 \mathrm{~s}^{-1}$, showing your working.
b. Outline, with a reason, another property that could be monitored to measure the rate of this reaction.
c(i)Describe one systematic error associated with the use of the gas syringe, and how the error affects the calculated rate.
c(ii)dentify one error associated with the use of an accurate stopwatch.
▶️Answer/Explanation
Markscheme
a. tangent drawn to curve at $\mathrm{t}=20 \mathrm{~s}[\mathrm{~V}]$
slope/gradient calculation [ $[$ ]
$0.35 \ll \mathrm{cm}^3 \mathrm{~s}^{-1} »[\boldsymbol{V}]$
Note: Accept values in the range $0.32-0.42 \ll \mathrm{cm}^3 \mathrm{~s}^{-1} »$
b. ALTERNATIVE 1
colour [/]
$\mathrm{Br} 2$ /reactant is coloured «Br $(\mathrm{aq})$ is not» $[\boldsymbol{\sim}]$
ALTERNATIVE 2
conductivity $[\boldsymbol{\sim}]$
greater/increased concentration of ions in products [ $\boldsymbol{\sim}$ ]
Note: Do not accept “changes in temperature” or “number of bubbles”.
ALTERNATIVE 3
mass/pressure [ $\boldsymbol{\sim}]$
gas is evolved/produced $[\boldsymbol{\swarrow}]$
Note: Do not accept “mass of products is less than mass of reactants”.
ALTERNATIVE 4
$\mathrm{pH}[\boldsymbol{\vee}]$
methanoic acid is weak $A N D \mathrm{HBr}$ is strong
OR
increase in $\left[\mathrm{H}^{+}\right][\boldsymbol{\sim}]$
c(i)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
$\mathrm{CO}_2$ dissolved in water $[\boldsymbol{V}]$
calculated rate lower $[\boldsymbol{\sim}]$
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 $[\boldsymbol{}]$
calculated rate higher $[\boldsymbol{V}]$
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”.
c(ii)uman reaction time/delay «starting/stopping the stopwatch» $[\boldsymbol{V}]$