How fast? the rate of chemical change: notes and practice questions

An experiment was conducted to investigate the effect of temperature on the rate of reaction between excess calcium carbonate chips and 100 cm³ of 0.5 mol dm⁻³ hydrochloric acid. The volume of carbon dioxide gas produced was collected in a gas syringe. The results for the reaction at 25 °C (Experiment 1), 35 °C (Experiment 2), and 45 °C (Experiment 3) are plotted on the graph below.

(a) By annotating the graph, determine the initial rate of reaction for Experiment 2, in cm³ s⁻¹.

(b) Estimate the time required for Experiment 1 to produce half of the total volume of CO₂.

(c) Suggest two reasons, in terms of collision theory, why the rate of reaction in Experiment 3 (45 °C) is greater than in Experiment 1 (25 °C).

(d) Calculate the percentage increase in the volume of CO₂ produced after 20 seconds when the temperature is increased from 25 °C (Experiment 1) to 45 °C (Experiment 3).

(e) The experiment was performed in a sealed flask connected to the gas syringe. Sketch a graph of the total mass of the apparatus against time for Experiment 1 and provide a reason for its shape.

A student investigates the decomposition of hydrogen peroxide, H₂O₂, in a commercial antiseptic solution when exposed to light over a period of 7 days. The reaction is:

$2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$

The concentration of H₂O₂ is determined by titration with a standard solution of acidified potassium manganate(VII), KMnO₄. Several identical flasks are prepared. One flask is kept in darkness, while others are exposed to a light source for different durations each day.

(a) Identify two variables, other than the volume of the solution, that should be controlled in this experiment.

(b) (i) The antiseptic solution is diluted with deionized water before titration. Suggest why this is necessary.

(b) (ii) Identify a possible systematic error associated with the control sample kept in darkness.

(b) (iii) Suggest how the experimental setup could be improved to check if the systematic error identified in (b)(ii) is significant.

(c) The following data are collected for one of the titrations:

Final burette reading = $24.50 \pm 0.05~\text{cm}^3$

Initial burette reading = $2.30 \pm 0.05~\text{cm}^3$

Calculate the percentage uncertainty of the titre.

(d) The initial concentration of H₂O₂ in all flasks was $0.88~\text{mol}~\text{dm}^{-3}$. The concentration after 7 days was measured for each flask.

(i) Calculate the average rate of decrease in hydrogen peroxide concentration, in $\text{mol}~\text{dm}^{-3}~\text{day}^{-1}$, for the sample exposed to light for 4 hours daily over the 7-day period.

(d) (ii) The student's hypothesis is: "A greater decrease in hydrogen peroxide concentration will be observed in solutions exposed to light for longer durations, due to photochemical decomposition."

Discuss, with reference to the data, the extent to which the results support this hypothesis.

(d) (iii) State one implication of the results for the storage of hydrogen peroxide solutions.

(e) Suggest a relevant extension to this investigation that would provide further information on the stability of hydrogen peroxide solutions.

A student investigated the effectiveness of three different brands of antacid tablets (Brand A, Brand B, and Brand C) in neutralizing hydrochloric acid, which simulates stomach acid. The active ingredient in all three brands is calcium carbonate, $CaCO_3$.

The following procedure was used for each brand:

1. One tablet was crushed using a mortar and pestle.

2. The crushed tablet was transferred to a conical flask containing $50.00 \text{ cm}^3$ of $0.500 \text{ mol dm}^{-3}$ HCl(aq). This is an excess of acid.

3. The mixture was stirred for a fixed time until the reaction appeared complete.

4. The resulting solution was titrated with $0.250 \text{ mol dm}^{-3}$ NaOH(aq) using a suitable indicator.

5. The experiment was repeated three times for each brand.

(a) State the independent and dependent variables for this investigation.

(b) Suggest two experimental conditions, other than the volumes and concentrations of the acid and alkali, that must be controlled to ensure a fair comparison between the brands.

(c) Suggest a reason for crushing the tablet before adding it to the acid.

The average results for the titrations are shown in the table.

(d) Based on the data, deduce which brand of antacid is the most effective, giving a reason.

(e) Calculate the amount, in mol, of HCl neutralized by one tablet of Brand B.

(f) The manufacturer of Brand A claims their tablet contains $0.900 \text{ g}$ of calcium carbonate, $CaCO_3$. Determine if the student's results for Brand A support this claim. The equation for the reaction is: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$.

A student investigates the rate of reaction between magnesium and sulfuric acid. The reaction produces hydrogen gas.

(a) Write the balanced chemical equation, including state symbols, for the reaction between magnesium and sulfuric acid.

(b) Describe two different experimental methods to measure the rate of this reaction. For each method, state the variable that would be measured.

The student carries out the reaction using 0.50 g of magnesium and excess 1.0 mol dm⁻³ sulfuric acid. The experiment is performed twice: once with a single ribbon of magnesium and once with the same mass of magnesium powder. The results are plotted on the graph below.

(c) (i) Identify which curve, A or B, represents the reaction with magnesium powder. Explain your answer using collision theory.

(ii) Explain why both curves reach the same final volume of hydrogen gas.

Sulfur trioxide, $SO_3$, is a key intermediate in the industrial production of sulfuric acid. It is produced by the reversible reaction of sulfur dioxide, $SO_2$, and oxygen, $O_2$, in the Contact process.

$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$

(a) State the effect of increasing the pressure on the rate of the forward reaction, assuming the temperature is kept constant.

(b) Explain your answer to (a) using collision theory.

(c) The reaction is typically carried out in the presence of a vanadium(V) oxide, $V_2O_5$, catalyst. State the effect of the catalyst on the rate of the reaction and explain how it achieves this effect.