2020 November Chemistry Exam Question
Below is an image of a chemistry exam question from November 2020. This image contains questions regarding the effect of chain length on the equilibrium constant Kc.
(a) Identify the independent and dependent variables in this experiment.
(b) 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.
(c) Suggest why the titration must be conducted quickly even though a low temperature is maintained.
(d) An additional experiment was conducted in which only the sulfuric acid catalyst was titrated with NaOH(aq). Outline why this experiment was necessary.
(e) Calculate the percentage uncertainty and percentage error in the experimentally determined value of Kc for methanol.
(f) Comment on the magnitudes of random and systematic errors in this experiment using the answers in (e).
(g) Suggest a risk of using sulfuric acid as the catalyst.
Answers
(a) Independent variable: chain length OR number of carbon atoms in alcohol.
Dependent variable: volume of NaOH OR Kc/equilibrium constant OR equilibrium concentration/moles of CH3COOH.
(b) Dilution/lower concentrations, less frequent collisions per unit volume.
(c) Equilibrium shifts to the left OR more ethanoic acid is produced as ethanoic acid is neutralized OR prevents/slows down ester hydrolysis.
(d) To determine volume/moles of NaOH used up by the catalyst/sulfuric acid in the titration OR to eliminate/reduce systematic error caused by acid catalyst.
(e) Percentage uncertainty: (0.4 ÷ 6.5) × 100 = 6%.
Percentage error: ((6.5 − 5.3) ÷ 5.3) × 100 = 23%.
(f) Any two:
- Large percentage error means large systematic error in procedure.
- Small percentage uncertainty means small random errors.
- Random errors smaller than systematic error.
(g) Corrosive/burns/irritant/strong oxidizing agent/carcinogenic OR disposal is an environmental issue OR causes other side reactions/dehydration/decomposition.
2019 November Chemistry Exam: Effect of Acid on Limestone
A student investigated how the type of acid in acid deposition affects limestone, a building material mainly composed of calcium carbonate.
Experiment Details
The student monitored the mass of six similarly sized pieces of limestone. Three were placed in beakers containing 200.0 cm3 of 0.100 mol dm-3 nitric acid, HNO3(aq), and the other three in 200.0 cm3 of 0.100 mol dm-3 sulfuric acid, H2SO4(aq).
Questions
(a) Draw a best-fit line on the graph.
(b) (i) 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.
(ii) Explain why the rate of reaction of limestone with nitric acid decreases and reaches zero over the period of five days.
(iii) Suggest a source of error in the procedure, assuming no human errors occurred and the balance was accurate.
(c) (i) The student hypothesized that sulfuric acid would cause a larger mass loss than nitric acid. Justify this hypothesis.
(ii) The student obtained the following total mass losses:
| Acid | Limestone sample | Total mass loss / g |
|---|---|---|
| Nitric acid | 1 | 0.17 |
| Nitric acid | 2 | 0.14 |
| Nitric acid | 3 | 0.15 |
| Sulfuric acid | 4 | 0.10 |
| Sulfuric acid | 5 | 0.07 |
| Sulfuric acid | 6 | 0.08 |
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.
2019 November Chemistry Exam: Answers
1. a best-fit smooth curve
1. b (i) tangent drawn at time zero, g day-1, 0.16
1. b (ii) acid used up OR acid is the limiting reactant
concentration of acid decreases OR less frequent collisions
1. b (iii) surface area not uniform OR limestone pieces do not have same composition/source OR limestone absorbed water (which increased mass) OR acid removed from solution when limestone removed 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
1. c (i) 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+
1. c (ii) calcium sulfate remained/deposited on limestone (in sulfuric acid) OR reaction prevented/stopped by slightly soluble/deposited/layer of calcium sulfate
2019 May Chemistry Exam: Lead Alloy Mug and Bromine Reaction
Question 1: Lead Alloy Mug
This question is about a mug made of a lead alloy. The rate of lead dissolving in common beverages with various pH values was analyzed.
Lead dissolving in beverages at various times and temperatures:
| Experiment | Beverage | pH | Time / min | Temp. / °C | Lead concentration / mg dm-3 |
|---|---|---|---|---|---|
| 1 | Cola | 2.5 | 5 | 16 | 6 |
| 2 | Cola | 2.5 | 30 | 16 | 14 |
| 3 | Cola | 2.5 | 60 | 16 | 23 |
| 4 | Cola | 2.5 | 5 | 18 | 11 |
| 5 | Lemonade | 2.9 | 5 | 18 | 14 |
| 6 | Orange juice | 3.7 | 5 | 18 | 18 |
| 7 | Beer | 4.2 | 5 | 18 | 2.3 |
| 8 | Tap water | 5.9 | 5 | 18 | 15 |
(a)
Identify the experiment with the highest rate of lead dissolving.
(b)
(i) Suggest why the relationship between time and lead concentration for Cola at 16°C is not linear.
(ii) Examine, giving a reason, whether the rate of lead dissolving increases with acidity at 18°C.
(c)
(i) Lead(II) chloride, PbCl2, has very low solubility in water. Explain why the presence of chloride ions in beverages affects lead concentrations.
(ii) 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.
Question 2: Bromine and Methanoic Acid Reaction
Bromine and methanoic acid react in aqueous solution:
Br2(aq) + HCOOH(aq) → 2Br-(aq) + 2H+(aq) + CO2(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 s, in cm3 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.
(ii) Identify one error associated with the use of an accurate stopwatch.
2019 May Chemistry Exam: Answers
1. a 6
1. b (i) equilibrium is being established between lead in solution and in mug
solution becoming saturated OR concentration of lead ions/[Pb2+] in the solution has increased over time
acid concentration has decreased as reacted with lead
surface lead has decreased/formed a compound/forms insoluble layer on surface
acid reacts with other metals because it is an alloy
1. b (ii) no AND experiment 7/beer has lowest rate and intermediate acidity/pH
no AND experiment 6/orange juice has fastest rate but lower acidity/higher pH than lemonade
no AND experiment 6/orange juice has highest rate and intermediate acidity/pH
1. c (i) equilibrium shifts to the left/towards reactants
lead compounds/ions precipitate OR concentration of lead ions/[Pb2+] decreases
1. c (ii) daily limit = 5.0 × 10-6 g kg-1 × 80.0 kg = 4.0 × 10-4 g of lead
volume = 4.0 × 10-4 g / 1.5 × 10-2 g dm-3 = 2.7 × 10-2 dm3
2. a tangent drawn to curve at t = 20 s
slope/gradient calculation
0.35 cm3 s-1
2. b ALTERNATIVE 1: colour, Br2/reactant is coloured, Br-(aq) is not
ALTERNATIVE 2: conductivity, greater/increased concentration of ions in products
ALTERNATIVE 3: mass/pressure, gas is evolved/produced
ALTERNATIVE 4: pH, methanoic acid is weak AND HBr is strong OR increase in [H+]
2. 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 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
2. c (ii) human reaction time/delay starting/stopping the stopwatch
2018 May Chemistry Exam: Hydrochloric Acid and Marble Chips
Question 2: Reaction of Hydrochloric Acid with Marble Chips
Students were asked to investigate how a change in concentration of hydrochloric acid, HCl, affects the initial rate of its reaction with marble chips, CaCO3.
They decided to measure how long the reaction took to complete when similar chips were added to 50.0 cm3 of 1.00 mol dm-3 acid and 50.0 cm3 of 2.00 mol dm-3 acid.
Two methods were proposed:
- Using small chips, keeping the acid in excess, and recording the time taken for the solid to disappear.
- Using large chips, keeping the marble in excess, and recording the time taken for bubbles to stop forming.
(a)
Annotate the balanced equation below with state symbols.
CaCO3 (__) + 2HCl (__) → CaCl2 (__) + CO2 (__) + H2O (__)
(b)
Neither method actually gives the initial rate. Outline a method that would allow the initial rate to be determined.
(c)
(i) 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.
(ii) State a factor that has a significant effect on reaction rate, which could vary between marble chips of exactly the same mass.
(d)
A group recorded the following results with 1.00 mol dm-3 hydrochloric acid:
| Trial | Time / s ±0.01 s |
|---|---|
| 1 | 120.56 |
| 2 | 136.83 |
| 3 | 108.49 |
| Mean | 121.96 |
(i) Justify why it is inappropriate to record the uncertainty of the mean as ±0.01 s.
(ii) 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.
(iii) 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.
2018 May Chemistry Exam: Answers
2. a CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
2. b Measure the volume of gas at different times and plot a graph and extrapolate OR measure the mass of the reaction mixture at different times and plot a graph and extrapolate
2. c (i) Method 2 AND the marble is in excess so a little extra has little effect OR large chips AND the 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
2. c (ii) Surface area OR purity of the marble
2. d (i) Variation of individual values is much greater than this uncertainty OR uncertainty does not take into account student reaction time
2. d (ii) 121.96s / 2 = 60.98s ≈ 61s
2. d (iii) 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