SL Paper 3
Antacids react with hydrochloric acid in the stomach to relieve indigestion. A student investigated different brands of antacid to see which caused the largest increase in pH in a given time. She added the antacids to hydrochloric acid, and recorded the change in pH over five minutes.
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.
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:
(1) using small chips, keeping the acid in excess, and recording the time taken for the solid to disappear
(2) using large chips, keeping the marble in excess, and recording the time taken for bubbles to stop forming.
A group recorded the following results with 1.00 mol dm−3 hydrochloric acid:
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.
Penicillins and aspirin are important medicines.
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.
Palmitic acid has a molar mass of 256.5 g mol−1.
The apparatus in the diagram measures the surface pressure created by palmitic acid molecules on the surface of water. This pressure is caused by palmitic acid molecules colliding with the fixed barrier. The pressure increases as the area, A, available to the palmitic acid is reduced by the movable barrier.
When a drop of a solution of palmitic acid in a volatile solvent is placed between the barriers, the solvent evaporates leaving a surface layer. The graph of pressure against area was obtained as the area A was reduced.
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.
This question is about a mug made of a lead alloy.
The rate of lead dissolving in common beverages with various pH values was analysed.
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.
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.
A student investigated how the type of acid in acid deposition affects limestone, a building material mainly composed of calcium carbonate.
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).
The limestone was removed from the acid, washed, dried with a paper towel and weighed every day at the same time and then replaced in the beakers.
The student plotted the mass of one of the pieces of limestone placed in nitric acid against time.
[Source: © International Baccalaureate Organization 2019]
The student hypothesized that sulfuric acid would cause a larger mass loss than nitric acid.
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.
This question is about a mug made of a lead alloy.
The rate of lead dissolving in common beverages with various pH values was analysed.
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) 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.