Identify the compound responsible for the acidity of gastric juice, and state whether it is a strong or weak acid.

An antacid contains calcium carbonate and magnesium carbonate.

Write the equation for the reaction of magnesium carbonate with excess stomach acid.

Outline how ranitidine reduces stomach acidity.

Calculate the pH of a buffer solution which contains 0.20 mol dm⁻³ ethanoic acid and 0.50 mol dm⁻³ sodium ethanoate. Use section 1 of the data booklet.

pK_a (ethanoic acid) = 4.76

hydrochloric acid/HCl «(aq)» AND strong «acid» ✔

MgCO₃ (s) + 2HCl (aq) → MgCl₂ (aq) + CO₂ (g) + H₂O (l) ✔

NOTE: Accept ionic equation.

blocks/binds to H2-histamine receptors «in cells of stomach lining»
OR
prevents histamine molecules binding to H2-histamine receptors «and triggering acid secretion»
OR
prevents parietal cells from releasing/producing acid ✔

NOTE: Do not accept “antihistamine” by itself.
Accept “H2-receptor antagonist/H2RA” OR “blocks/inhibits action of histamine”.
Accept “blocks receptors in parietal cells «from releasing/producing acid»”.
Do not accept “proton pump/ATPase inhibitor”.

«pK_a = 4.76»
«pH = pK_a + log $\left(\frac{\left[{\text{CH}}_3{\text{COO}}^{-}\right]}{\left[{\text{CH}}_3\text{COOH}\right]}\right)$»
«pH = 4.76 + 0.40 =» 5.16 ✔

Identify the independent and dependent variables in this experiment.

The ice bath is used at equilibrium to slow down the forward and reverse reactions. Explain why adding a large amount of water to the reaction mixture would also slow down both reactions.

Suggest why the titration must be conducted quickly even though a low temperature is maintained.

An additional experiment was conducted in which only the sulfuric acid catalyst was titrated with $\mathrm{NaOH}\hspace{0.17em}\left(\mathrm{aq}\right)$. Outline why this experiment was necessary.

Calculate the percentage uncertainty and percentage error in the experimentally determined value of $K_{\mathrm{c}}$ for methanol.

Comment on the magnitudes of random and systematic errors in this experiment using the answers in (e).

Suggest a risk of using sulfuric acid as the catalyst.

Independent variable:
chain length OR number of carbon «atoms in alcohol»
AND
Dependent variable:
volume of $\mathrm{NaOH}$ OR $K_c$/equilibrium constant OR equilibrium concentration/moles of ${\mathrm{CH}}_3\mathrm{COOH}$ ✔

dilution/lower concentrations ✔

less frequent collisions «per unit volume» ✔

Accept “lowers concentration of acid catalyst” for M1. M2 must refer to increase in activation energy or different pathway.

Do not accept responses referring to equilibrium.

equilibrium shifts to left
OR
more ethanoic acid is produced «as ethanoic acid is neutralized»
OR
prevents/slows down ester hydrolysis ✔

Accept “prevents equilibrium shift” if described correctly without direction.

to determine volume/moles of $\mathrm{NaOH}$ used up by the catalyst/sulfuric acid «in the titration»
OR
to eliminate/reduce «systematic» error caused by acid catalyst ✔

Do not accept “control” OR “standard” alone.

Percentage uncertainty:
$«\frac{0.4\times 100}{6.5}=»6«\%»$ ✔

Percentage error:
$«\frac{6.5-5.3}{5.3}=»23«\%»$ ✔

Award [1 max] if calculations are reversed OR if incorrect alcohol is used.

Any two:

large percentage error means large systematic error «in procedure» ✔

small percentage uncertainty means small random errors ✔

random errors smaller than systematic error ✔

Award [2] for “both random and systematic errors are significant.”

corrosive/burns/irritant/strong oxidizing agent/carcinogenic
OR
disposal is an environmental issue
OR
causes other side reactions/dehydration/decomposition ✔

Do not accept just “risk of accidents” OR “health risks” OR “hazardous”.

Well answered. Students mostly identified (alcohol) chain length as the independent variable and K_c at the dependent. For the latter [ethanoic acid] at equilibrium was another popular choice with some students neglecting to clarify "equilibrium" which was needed for the mark. This evidences an issue already identified in the Internal assessment that very often students only identify the processed variables. The proportion of students referring to volume of NaOH was too low for expectations.

A significant number of students scored at least one mark, usually the first and many both. Weaker students lost the second mark by referring to less collisions instead of less frequent collision or other words to this effect. Very few students referred to more diluted catalyst and of those even less were able to provide an adequate explanation in terms of the increased Ea. Many students tried to answer this question in terms of equilibrium instead of kinetics. There were also several responses that replied as if the dilution would only occur for part of the reaction or individual reactants instead of the entire solution.

Not well answered and of the few students that replied correctly most referred to preventing equilibrium shift and few candidates identified the direction of the shift. It was rather common to see answers where Le Chatelier's principle was stated without any attempt in adapting it to the context. Very few students described the specific equilibrium shift that could occur during the titration, changing the results.

Some students achieved on mark. Many answers referred simple to "control" or "standard" underlining the lack of some skills as also identified in the Internal assessment. Once again very few students had the specific details necessary to explain why this separate titration was needed in their response to receive a mark.

Many students scored both points and others at least one. Weaker students inverted the calculations.

Of the many students that obtained the mark most did through the first alternative and a lesser percentage through the third. Many students were unable to relate their calculations from 2e (percentage error and percentage uncertainty) to systematic error and random error. They either compared the calculations to incorrect errors or in some cases did not discuss the errors at all. Once again this points to a general lack on experimental understanding.

Most students received a mark for this question base on specific hazards. Very few students related disposal to environmental issues which isn't surprising as this is often missed in the Internal Assessment. Weaker students provided vague answers related to health issues which did not receive a mark. Some students misunderstood the question.

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 which region, A or B, corresponds to each type of radiation by completing the table.

Oceans can act as a carbon sink, removing some CO₂(g) from the atmosphere.

CO₂(g) $\rightleftharpoons$ CO₂(aq)

Aqueous carbon dioxide, CO₂(aq), quickly reacts with ocean water in a new equilibrium reaction. Construct the equilibrium equation for this reaction including state symbols.

Describe how large amounts of CO₂ could reduce the pH of the ocean using an equation to support your answer.

Suggest an equation for the production of syngas from coal.

The Fischer-Tropsch process, an indirect coal liquefaction method, converts CO(g) and H₂(g) to larger molecular weight hydrocarbons and steam.

Deduce the equation for the production of octane by this process.

Suggest a reason why syngas may be considered a viable alternative to crude oil.

Accept “B” alone for incoming radiation from sun.

All three correct answers necessary for mark.

[1 mark]

CO₂(aq) + H₂O(l) $\rightleftharpoons$ H₂CO₃(aq)

State symbols AND equilibrium arrow required for mark.

Accept

CO₂(aq) + H₂O(l) $\rightleftharpoons$ H⁺(aq) + HCO₃^–(aq).

CO₂(aq) + H₂O(l) $\rightleftharpoons$ 2H⁺(aq) + CO₃^2–(aq).

[1 mark]

CO₂(aq) + H₂O(l) $\rightleftharpoons$ 2H⁺(aq) + CO₃^2–(aq)
OR
CO₂(aq) + H₂O(l) $\rightleftharpoons$ H⁺(aq) + HCO₃^–(aq)
OR
H₂CO₃(aq) + H₂O(l) $\rightleftharpoons$ H₃O⁺(aq) + HCO₃^–(aq)
OR
H₂CO₃(aq) $\rightleftharpoons$ H⁺(aq) + HCO₃^–(aq)
OR
H₂CO₃(aq) + 2H₂O(l) $\rightleftharpoons$ 2H₃O⁺(aq) + CO₃^2–(aq)
OR
H₂CO₃(aq) $\rightleftharpoons$ 2H⁺(aq) + CO₃^2–(aq)

equilibrium shifts to the right causing increase in [H₃O⁺]/[H⁺ ] «thereby decreasing pH»

Equilibrium sign needed in (b) (ii) but penalize missing equilibrium sign once only in b (i) and (ii).

Do not accept “CO₂(aq) + H₂O(l) $\rightleftharpoons$ H₂CO₃(aq)” unless equation was not given in b (i).

[2 marks]

C(s) + H₂O(g) → CO(g) + H₂(g)
OR
3C(s) + H₂O(g) + O₂(g) → 3CO(g) + H₂(g)
OR
4C(s) + 2H₂O(g) + O₂(g) → 4CO(g) + 2H₂(g)
OR
5C(s) + H₂O(g) + 2O₂(g) → 5CO(g) + H₂(g)

Accept other correctly balanced equations which produce both CO AND H₂.

[1 mark]

8CO(g) + 17H₂(g) → C₈H₁₈(l) + 8H₂O(g)

[1 mark]

coal more plentiful than crude oil
OR
syngas can be produced from biomass/renewable source
OR
syngas can undergo liquefaction to form octanes/no need to transport crude
OR
syngas can be produced by gasification underground, using carbon
OR
capture/storage «to not release CO₂ to the atmosphere»
OR
coal gasification produces other usable products/slag

[1 mark]

Deduce why more heat was produced in mixture B than in mixture A.

Deduce why the temperature is higher in mixture C than in mixture D.

more «moles/amount of» acid have been added/reacted
OR
more of the limiting reagent is present
OR
more «of the exothermic» reaction has occurred

[1 mark]

no more reaction/same energy released AND cold/colder/cooler liquid added
OR
no more reaction/same energy released AND greater total volume of liquid

Accept “no more reaction/same energy released AND greater heat loss «to the surroundings in mixture D»”.

[1 mark]

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]

Outline why the initial reaction should be carried out under a fume hood.

Deduce the equation for the relationship between absorbance and concentration.

Outline how a solution of 0.0100 mol dm⁻³ is obtained from a standard 1.000 mol dm⁻³ copper(II) sulfate solution, including two essential pieces of glassware you would need.

The original piece of brass weighed 0.200 g. The absorbance was 0.32.

Calculate, showing your working, the percentage of copper by mass in the brass.

Deduce the appropriate number of significant figures for your answer in (d)(i).

Comment on the suitability of using brass of this composition for door handles in hospitals.

If you did not obtain an answer to (d)(i), use 70 % but this is not the correct answer.

Suggest another property of brass that makes it suitable for door handles.

Copper(II) ions are reduced to copper(I) iodide by the addition of potassium iodide solution, releasing iodine that can be titrated with sodium thiosulfate solution, Na₂S₂O₃ (aq). Copper(I) iodide is a white solid.

4I⁻ (aq) + 2Cu²⁺ (aq) → 2CuI (s) + I₂ (aq)

I₂ (aq) + 2S₂O₃²⁻ (aq) → 2I⁻ (aq) + S₄O₆²⁻ (aq)

Deduce the overall equation for the two reactions by combining the two equations.

Suggest why the end point of the titration is difficult to determine, even with the addition of starch to turn the remaining free iodine black.

NO₂/NO/NO_x/HNO₃/gas is poisonous/toxic/irritant ✔

Accept formula or name.

Accept “HNO₃ is corrosive” OR “poisonous/toxic gases produced”.

Accept “reaction is harmful/hazardous”.

Slope (gradient):

40 ✔

Equation:

absorbance = 40 × concentration

OR

y = 40x ✔

Accept any correct relationship for slope such as $\frac{1.00}{0.025}$.

Award [2] if equation in M2 is correct.

dilute 1.00 cm³ «of the standard solution with water» to 100 cm³

OR

dilute sample of standard solution «with water» 100 times ✔

«graduated/volumetric» pipette/pipet ✔

volumetric flask ✔

Accept any 1 : 100 ratio for M1.

Accept “mix 1 cm³ of the standard solution with 99 cm³ of water” for M1.

Do not accept “add 100 cm³ of water to 1.00 cm³ of standard solution” for M1.

Accept “burette/buret” for M2.

Accept “graduated/measuring flask” for M3 but not “graduated/measuring cylinder” or “conical/Erlenmeyer flask”.

concentration of copper = 0.0080 «mol dm^–3» ✔

mass of copper in 250.0 cm³ = «0.0080 mol dm^–3 × 0.2500 dm³ × 63.55 g mol^–1 =» 0.127 «g»

OR

mass of brass in 1 dm³ = «4 × 0.200 g =» 0.800 g AND [Cu2+] = «0.0080 mol dm^–3 × 63.55 g mol^–1 =» 0.5084 g dm^–3 ✔

«% copper in this sample of brass $=\frac{0.127}{0.200}\times 100=$» 64 «%»

OR

«% copper in this sample of brass $=\frac{0.5084}{0.800}\times 100=$» 64 «%» ✔

Accept any value in range 0.0075–0.0085 «mol dm^–3» for M1.

Accept annotation on graph for M1.

Award [3] for correct final answer.

Accept “65 «%»”.

two ✔

Do not apply ECF from 1(d)(i).

«since it is greater than 60%» it will reduce the presence of bacteria «on door handles» ✔

resistant to corrosion/oxidation/rusting

OR

low friction surface «so ideal for connected moving components» ✔

Accept “hard/durable”, “«high tensile» strength”, “unreactive”, “malleable” or any reference to the appearance/colour of brass (eg “gold-like”, “looks nice” etc.).

Do not accept irrelevant properties, such as “high melting/boiling point”, “non-magnetic”, “good heat/electrical conductor”, “low volatility”, etc.

Do not accept “ductile”.

2I⁻ (aq) + 2Cu²⁺ (aq) + 2S₂O₃²⁻ (aq) → 2CuI (s) + S₄O₆²⁻ (aq)

correct reactants and products ✔

balanced equation ✔

M2 can only be awarded if M1 is correct.

precipitate/copper(I) iodide/CuI makes colour change difficult to see

OR

release of I₂/iodine from starch-I₂ complex is slow so titration must be done slowly ✔

The graph shows the relationship between the temperature and the rate of an enzyme-catalysed reaction.

State one reason for the decrease in rate above the optimum temperature.

Explain why a change in pH affects the tertiary structure of an enzyme in solution.

State one use of enzymes in reducing environmental problems.

enzyme denatures
OR
change of conformation/shape of active site
OR
substrate cannot bind to active site/binds less efficiently ✔

NOTE: Accept “change in structure” or “substrate doesn't fit/fits poorly into active site”

Any two of:
acidic/basic/ionizable/COOH/carboxyl/NH₂/amino groups in the R groups/side chains «react» ✔
exchange/lose/gain protons/H⁺ ✔
change in H-bonds/ionic interactions/intermolecular forces/London dispersion forces ✔

NOTE: Do not accept “enzyme denatures” OR “change of conformation/tertiary structure” OR “substrate cannot bind to active site/binds less efficiently” as this was the answer to 8(a).

breakdown of oil spills/industrial/sewage waste/plastics
OR
production of alternate sources of energy «such as bio diesel»
OR
involve less toxic chemical pathway «in industry» ✔

NOTE: Accept “«enzymes in» biological detergents can improve energy efficiency”.