SL Paper 3
Body fluids have different pH values.
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.
pKa (ethanoic acid) = 4.76
An investigation was carried out to determine the effect of chain length of the alcohol on the equilibrium constant, , for the reversible reaction:
The reactants, products and the catalyst form a homogeneous mixture.
Fixed volumes of each alcohol, the ethanoic acid and the sulfuric acid catalyst were placed in sealed conical flasks.
At equilibrium, the flasks were placed in an ice bath, and samples of each flask titrated with to determine the ethanoic acid concentration present in the equilibrium mixture.
The following processed results were obtained.
© International Baccalaureate Organization 2020
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 . Outline why this experiment was necessary.
Calculate the percentage uncertainty and percentage error in the experimentally determined value of 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.
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.
The combustion of fossil fuels produces large amounts of CO2, a greenhouse gas.
The diagram below illustrates a range of wavelengths in the electromagnetic spectrum.
Synthesis gas, or syngas, mainly composed of CO(g) and H2(g), is an alternative form of fuel. It can be produced by coal or biomass gasification, passing steam over the source material in a low oxygen environment.
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) 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.
A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.
Volume of aqueous NaOH = 25.0 ± 0.5 cm3
Concentration of HCl = 1.00 ± 0.01 mol dm−3
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.
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.
Alloys containing at least 60 % copper reduce the presence of bacteria on their surface.The percentage of copper in brass, an alloy of copper and zinc, can be determined by UV-vis spectrometry.
A sample of brass is dissolved in concentrated nitric acid and then made up to 250.0 cm3 with water before analysis.
Cu (s) + 4HNO3 (aq) → Cu(NO3)2 (aq) + 2NO2 (g) + 2H2O (l)
3Zn (s) + 8HNO3 (aq) → 3Zn(NO3)2 (aq) + 2NO (g) + 4H2O (l)
The concentration of copper(II) ions in the resulting solution is then determined from a calibration curve, which is plotted by measuring the light absorbance of standard solutions.
Titration is another method for analysing the solution obtained from adding brass to nitric acid.
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.
Enzymes are biological catalysts.
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.