State an equation for the reaction of magnesium hydroxide with hydrochloric acid.

Suggest two variables, besides the time of reaction, which the student should have controlled in the experiment to ensure a fair comparison of the antacids.

Calculate the uncertainty in the change in pH.

The student concluded that antacid B was the most effective, followed by A then C and finally D. Discuss two arguments that reduce the validity of the conclusion.

Mg(OH)₂ (s) + 2HCl (aq) → MgCl₂ (aq) + 2H₂O (l)

Accept full or net ionic equation.

Any two from:

volume «of HCl»

concentration «of HCl»/[HCl]

temperature «of HCl»

mass of antacid/tablets

size of antacid particles/tablets

OR

surface area of antacid «particles»/tablets

Accept “number of tablets/different doses”.

Do not accept “same pH meter” OR “initial pH” OR “concentration of antacid/[antacid]”.

A variable must be given so do not accept answers such as “stirring”, “whether tablets are whole or crushed” etc.

[Max 2 Marks]

(±) 0.04

OR

(±) 0.03

Any two of:

uncertainty «(±)0.04/(±)0.03» means A and C cannot be distinguished

each measurement was conducted once

stomach pH should not be raised a lot «so antacid B is not necessarily effective»

mass/number of tablets/dose «of antacid» used was not controlled

actual environment in stomach is different

Accept “amount of tablets” for “dose”.

Do not accept “nature/composition of tablets differs”.

Accept an answer such as “time frame is too short since some antacids could be long-acting drugs if they contain a gelatinisation/delaying agent” but not just “time frame is too short since some antacids could be long-acting drugs”.

[Max 2 Marks]

Annotate the balanced equation below with state symbols.

CaCO₃(__) + 2HCl(__) → CaCl₂(__) + CO₂(__) + H₂O(__)

Neither method actually gives the initial rate. Outline a method that would allow the initial rate to be determined.

Deduce, giving a reason, which of the two methods would be least affected by the chips not having exactly the same mass when used with the different concentrations of acid.

State a factor, that has a significant effect on reaction rate, which could vary between marble chips of exactly the same mass.

Justify why it is inappropriate to record the uncertainty of the mean as ±0.01 s.

If doubling the concentration doubles the reaction rate, suggest the mean time you would expect for the reaction with 2.00 mol dm⁻³ hydrochloric acid.

Another student, working alone, always dropped the marble chips into the acid and then picked up the stopwatch to start it. State, giving a reason, whether this introduced a random or systematic error.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Accept “CO₂(aq)”.

[1 mark]

measure the volume of gas at different times «plot a graph and extrapolate»

OR

measure the mass of the reaction mixture at different times «plot a graph and extrapolate»

Accept other techniques that yield data which can be plotted and extrapolated.

[1 mark]

method 2 AND marble is in excess «so a little extra has little effect»

OR

large chips AND marble is in excess «so a little extra has little effect»

OR

method 2 AND HCl is limiting reagent «so a little extra marble has little effect»

OR

large chips AND HCl is limiting reagent «so a little extra marble has little effect»

Accept, as a reason, that “as the mass is greater the percentage variation will be lower”.

[1 mark]

surface area

OR

purity «of the marble»

Accept “shape of the chip”.

[1 mark]

variation of individual values is much greater «than this uncertainty»

OR

«uncertainty» does not take into account «student» reaction time

[1 mark]

«$\frac{121.96\text{ s}}{2}$ = 60.98 s» = 61 «s»

[1 mark]

systematic AND always makes the time shorter «than the actual value»

OR

systematic AND it is an error in the method used «not an individual measurement»

OR

systematic AND more repetitions would not reduce the error

Accept, as reason, “it always affects the value in the same direction” OR “the error is consistent”.

[1 mark]

Describe how penicillin combats bacterial infections.

State how penicillins may be modified to increase their effectiveness.

State the type of reaction used to synthesize aspirin from salicylic acid.

Explain why aspirin is not stored in a hot, humid location.

«irreversibly» binds/bonds to enzyme/transpeptidase

OR

inhibits enzyme/transpeptidase «in bacteria» that produces cell walls

OR

prevents cross-linking of bacterial cell walls

cells absorb water AND burst

OR

cells cannot reproduce

[2 marks]

modify side chain

[1 mark]

condensation

OR

esterification

OR

nucleophilic substitution/nucleophilic displacement/S_N2

Do not accept just “substitution/displacement”.

[1 mark]

water causes hydrolysis

OR

aspirin reacts with water

heat increases the rate of hydrolysis

OR

heat increases the rate of the reaction with water

Accept “aspirin will convert into salicylic/ethanoic acid”.

Do not accept “aspirin dissolves in water” OR “aspirin absorbs water/is hygroscopic”.

[2 marks]

Part of this molecule is hydrophilic (bonds readily to water) and part hydrophobic (does not bond readily to water). Draw a circle around all of the hydrophilic part of the molecule.

When a small amount of palmitic acid is placed in water it disperses to form a layer on the surface that is only one molecule thick. Explain, in terms of intermolecular forces, why this occurs.

Suggest why there is a small increase in the surface pressure as the area is reduced to about 240 cm², but a much faster increase when it is further reduced.

The solution of palmitic acid had a concentration of 0.0034 mol dm⁻³. Calculate the number of molecules of palmitic acid present in the 0.050 cm³ drop, using section 2 of the data booklet.

Assuming the sudden change in gradient occurs at 240 cm², calculate the area, in cm², that a single molecule of palmitic acid occupies on surface of the water.

If you did not obtain an answer for (b)(ii) use a value of 8.2 × 10¹⁶, but this is not the correct answer.

Must cut CH₂–CO bond AND enclose all of the –COOH group.

[1 mark]

Any two of:

–COOH/CO/OH/carboxylate/carboxyl/hydroxyl/hydroxy group forms hydrogen bonds/H-bonds to water

London/dispersion/instantaneous induced dipole-induced dipole forces occur between hydrocarbon chains

hydrocarbon chain cannot form hydrogen bonds/H-bonds to water

strong hydrogen bonds/H-bonds between water molecules exclude hydrocarbon chains «from the body of the water»

Accept “hydrophilic part/group forms hydrogen bonds/H-bonds to water”.

Accept “hydrophobic section” instead of “hydrocarbon chain”.

Award [1 max] for answers based on “the –COOH group being polar AND the hydrocarbon chain being non-polar”.

[2 marks]

Above about 240 cm²:

greater collision frequency/collisions per second between «palmitic acid» molecules and the barrier «as area reduced»

At less than about 240 cm²:

molecules completely cover the surface

OR

there is no space between molecules

OR

force from movable barrier transmitted directly through the molecules to the fixed barrier

OR

«palmitic acid» molecules are pushed up/down/out of layer

For both M1 and M2 accept “particles” for “molecules”.

For M1 accept “space/area between molecules reduced” OR “molecules moving closer together”.

[2 marks]

amount of acid = «5.0 × 10^–5 dm³ × 0.0034 mol dm^–3» = 1.7 × 10^–7 «mol»

number of molecules = «1.7 × 10^–7 mol × 6.02 × 10²³ mol^–1 =» 1.0 × 10¹⁷

Award [2] for correct final answer.

Award [1] for “1.0 × 10²⁰”.

[2 marks]

«area = $\frac{240\text{ c}{\text{m}}^2}{1.0\times {10}^{17}}$ » 2.4 × 10^–15 «cm²»

[1 mark]

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.

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

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)/product is not»  [✔]

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

ALTERNATIVE 2
conductivity  [✔]

greater/increased concentration of ions in products  [✔]

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⁺]  [✔]

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 experiment 5/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”.

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»  [✔]

Note: Do not accept “inaccurate stopwatch”.

This question was challenging for many students. Quite a few candidates did draw a tangent line at 20s for 1 mark, show a slope/gradient calculation of the line for 1 mark, and had a reasonable final value for the final mark. Some candidates only found the average rate by finding the ratio of the value at that data point and received one mark (16/20=0.80 cm³ s^-1). Candidates also received one mark if they had a correct answer with no work since the question clearly asked students to show their work.

The reaction rate was originally monitored by measuring the volume of CO₂ produced. Students needed to propose another method for this reaction, with a reason, that could be used to measure the rate. There were several possible correct answers and most students received at least one mark with many receiving both marks. The most common incorrect answer involved changes in temperature.

This question required students to recognize the rate of lead dissolving did not increase with acidity and to refer to data in the table for the reason. Some students did not refer to data in the table and did not receive the mark because they did not have a reason, other students compared the rate of lead dissolving with temperature increasing which did not answer the question.

This question was asking about a systematic error. There were several possible correct answers for the error, but students also needed to clearly identify a specific error and if the rate increased or decreased for the second mark. Many students confused this with the concept of a random error and identified the uncertainty of reading the syringe which is incorrect. Teachers need to reinforce the concept of systematic versus random errors.

This question was well answered by most candidates although some students did not read the question clearly and commented on the stopwatch having problems or not being accurate.

Draw a best-fit line on the graph.

Determine the initial rate of reaction of limestone with nitric acid from the graph.

Show your working on the graph and include the units of the initial rate.

Explain why the rate of reaction of limestone with nitric acid decreases and reaches zero over the period of five days.

Suggest a source of error in the procedure, assuming no human errors occurred and the balance was accurate.

Justify this hypothesis.

The student obtained the following total mass losses.

She concluded that nitric acid caused more mass loss than sulfuric acid, which did not support her hypothesis.

Suggest an explanation for the data, assuming that no errors were made by the student.

best-fit smooth curve ✔

NOTE: Do not accept a series of connected lines that pass through all points OR any straight line representation. 

tangent drawn at time zero ✔
g day⁻¹ ✔
0.16 ✔

NOTE: Accept other reasonable units for initial rate eg, mol dm⁻³ s⁻¹, mol dm⁻³ min⁻¹, g s⁻¹ OR g min⁻¹.

M3 can only be awarded if the value corresponds to the correct unit given in M2.
Accept values for the initial rate for M3 in the range: 0.13 − 0.20 g day⁻¹ OR 1.5 × 10⁻⁶ g s⁻¹ − 2.3 × 10⁻⁶ g s⁻¹ OR 7.5 × 10⁻⁸ − 1.2 × 10⁻⁷ mol dm⁻³ s⁻¹ OR 4.5 × 10⁻⁶ − 6.9 × 10⁻⁶ mol dm⁻³ min⁻¹ OR 9.0 × 10⁻⁵ − 1.4 × 10⁻⁴ g min⁻¹ OR a range based on any other reasonable unit for rate.

Ignore any negative rate value.
Award [2 max] for answers such as 0.12/0.11 g day⁻¹, incorrectly obtained by using the first two points on the graph (the average rate between t = 0 and 1 day).
Award [1 max] for correctly calculating any other average rate.

acid used up
OR
acid is the limiting reactant ✔

concentration of acid decreases
OR
less frequent collisions ✔

NOTE: Award [1 max] for "surface area decreases" if the idea that CaCO₃ is used up/acts as the limiting reactant” is conveyed for M1.

Do not accept “reaction reaches equilibrium” for M2.

surface area not uniform
NOTE: Accept “acids impure.

OR
limestone pieces do not have same composition/source
NOTE: Accept “«limestone» contains impurities”.

OR
limestone absorbed water «which increased mass»

OR
acid removed from solution when limestone removed
NOTE: Accept “loss of limestone when dried" OR "loss of limestone due to crumbling when removed from beaker”.

OR
«some» calcium sulfate deposited on limestone lost

OR
pieces of paper towel may have stuck to limestone

OR
beakers not covered/evaporation

OR
temperature was not controlled ✔

sulfuric acid is diprotic/contains two H⁺ «while nitric acid contains one H⁺»/releases more H⁺ «so reacts with more limestone»
OR
higher concentration of protons/H⁺ ✔

NOTE: Ignore any reference to the relative strengths of sulfuric acid and nitric acid.
Accept “sulfuric acid has two hydrogens «whereas nitric has one»”.
Accept "dibasic" for "diprotic".

calcium sulfate remained/deposited on limestone «in sulfuric acid»
OR
reaction prevented/stopped by slightly soluble/deposited/layer of calcium sulfate ✔

NOTE: Answer must refer to calcium sulfate.

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.

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⁺] has increased «over time»
OR
acid concentration has decreased «as reacted with lead»
OR
surface lead has decrease/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 responses that only discusses mathematical or proportional relationships.

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 × 80.0kg =» 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.

Most candidates did well on this question, identifying the correct experiment by number or beverage.

Many candidates struggled with this question, answering it from a mathematical perspective rather than explaining why the rate would decrease over time from a chemical perspective. There were several possible correct answers (reaching equilibrium, acid concentration decreasing, solution becoming saturated with lead ions, etc.…)

This question was an equilibrium question. Many students received 1 mark for either concentration of lead decreased, or lead chloride was produced and quite a few recognized that the explanation was the reaction shifted to the reactant or left side for the second mark.

Most students receive one mark for this question, and many receive both marks. The most common mistakes involved incorrect conversions from gram to milligrams or milligrams to grams.