Exploring and designing: notes and practice questions

A student investigates the effect of temperature on the rate of reaction between magnesium ribbon and hydrochloric acid.

$\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2(\text{aq}) + \text{H}_2(\text{g})$

In each trial, a $2.0\text{ cm}$ piece of magnesium ribbon is added to $30.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}\text{ HCl(aq)}$ in a conical flask maintained at a specified temperature. The volume of hydrogen gas collected in a gas syringe after $30\text{ s}$ is recorded. The procedure is repeated at five different temperatures between $20\text{ }^\circ\text{C}$ and $60\text{ }^\circ\text{C}$.

(a) State the independent variable in this investigation.

(b) State the dependent variable in this investigation.

A student investigates the effect of hydrochloric acid concentration on the rate of reaction with calcium carbonate:

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

In each trial, the student adds $2.00\text{ g}$ of marble chips to $50.0\text{ cm}^3$ of hydrochloric acid in a conical flask connected to a gas syringe. The volume of carbon dioxide gas produced in the first $60\text{ s}$ is measured.

(a) State one variable related to the solid reactant, other than its mass, that must be controlled across all trials.

(b) Explain why the temperature of the acid solution must be controlled to ensure a valid comparison between trials at different concentrations.

A student designs an experiment to investigate the effect of temperature on the rate of decomposition of hydrogen peroxide, $\text{H}_2\text{O}_2\text{(aq)}$, in the presence of a manganese(IV) oxide catalyst:

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

The student measures the volume of oxygen gas evolved over time using a gas syringe at five different temperatures: $20\ ^\circ\text{C}$, $30\ ^\circ\text{C}$, $40\ ^\circ\text{C}$, $50\ ^\circ\text{C}$, and $60\ ^\circ\text{C}$. At each temperature, the volume of gas is recorded every 10 seconds for 2 minutes.

(a) Suggest why the student chose to test five different temperatures across the range rather than only two.

(b) Suggest why the volume of gas was recorded at 10-second intervals rather than at 60-second intervals.

A student investigated the concentration of ethanoic acid in three different commercial brands of white vinegar (Brand 1, Brand 2, and Brand 3).

In each experiment, a $25.00\text{ cm}^3$ sample of the vinegar was transferred to a conical flask and titrated against a standardized $0.100\text{ mol dm}^{-3}\text{ NaOH(aq)}$ solution using phenolphthalein indicator. The volume of $\text{NaOH(aq)}$ required to reach the end point was recorded. Three concordant titres were obtained for each brand.

(a) State the independent variable in this investigation.

(b) State the dependent variable in this investigation.

A student investigates the effect of temperature on the rate of reaction between aqueous sodium thiosulfate and dilute hydrochloric acid:

$\text{Na}_2\text{S}_2\text{O}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{SO}_2\text{(g)} + \text{S(s)} + \text{H}_2\text{O(l)}$

In each trial, $25.0\text{ cm}^3$ of $0.150\text{ mol dm}^{-3}\text{ Na}_2\text{S}_2\text{O}_3\text{(aq)}$ and $5.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}\text{ HCl(aq)}$ are equilibrated to a chosen temperature using a thermostatted water bath. The two solutions are mixed in a conical flask placed over a piece of paper marked with a black cross. The student records the time taken for the precipitate of sulfur to obscure the cross.

(a) State the independent variable in this investigation.

(b) Explain why the initial concentration of the sodium thiosulfate solution must be controlled across all trials to ensure a valid comparison.

A student investigates the effect of temperature on the rate of reaction between zinc granules and dilute hydrochloric acid:

$\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$

In each trial, an excess of zinc is added to $40.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}\text{ HCl(aq)}$ in a conical flask connected to a $100\text{ cm}^3$ gas syringe.

The student selects five temperatures across the range $20.0\text{ }^\circ\text{C}$ to $60.0\text{ }^\circ\text{C}$ ($20.0\text{ }^\circ\text{C}$, $30.0\text{ }^\circ\text{C}$, $40.0\text{ }^\circ\text{C}$, $50.0\text{ }^\circ\text{C}$, and $60.0\text{ }^\circ\text{C}$) and carries out three trials at each temperature.

(a) Suggest why the student chose to investigate at least five different temperatures across this range rather than only three.

(b) Suggest two reasons why the student chose $60.0\text{ }^\circ\text{C}$ as the upper limit of the temperature range rather than extending it to higher temperatures, such as $85.0\text{ }^\circ\text{C}$.

A food scientist uses paper chromatography to analyze the food dyes present in four different commercial brands of candy (Brand P, Brand Q, Brand R, and Brand S). A concentrated drop of dye extracted from each brand is placed onto the baseline of a sheet of chromatography paper. The chromatogram is developed in a sealed tank using a solvent mixture of water and ethanol. After the solvent front travels 8.0 cm, the distance travelled by each dye component from the baseline is measured to determine its retention factor, $R_{\text{f}}$.

(a) State the independent variable in this investigation.

(b) State the dependent variable in this investigation.

A student determines the enthalpy of combustion of ethanol using a spirit burner and a metal calorimeter containing water.

The student places a draft shield around the spirit burner and calorimeter, and covers the calorimeter with an insulating lid.

(a) Outline why the draft shield was placed around the spirit burner.

(b) Outline why the calorimeter was covered with an insulating lid.

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 monitors the pH during the titration of $25.00\text{ cm}^3$ of $0.100\text{ mol dm}^{-3}$ ethanoic acid, $\text{CH}_3\text{COOH(aq)}$, with $0.100\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH(aq)}$, using a digital pH meter.

(a) Outline why the pH meter must be calibrated using standard buffer solutions before the titration is carried out.

(b) Outline why the pH probe is rinsed with distilled water between taking measurements in different 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 reaction between zinc and excess dilute sulfuric acid:

$\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$

In Experiment 1, the student adds $0.50\text{ g}$ of zinc granules to $50.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}\text{ H}_2\text{SO}_4\text{(aq)}$ at $20.0\text{ }^\circ\text{C}$ and measures the volume of hydrogen gas evolved over time until the reaction ceases.

In Experiment 2, the student repeats the procedure under identical conditions using $0.50\text{ g}$ of zinc powder instead of zinc granules.

(a) Predict the effect of using zinc powder instead of zinc granules on the initial rate of hydrogen gas production.

(b) Predict the effect of using zinc powder instead of zinc granules on the total volume of hydrogen gas collected when the reaction is complete. Justify your prediction.

A student investigates the separation of two organic compounds, compound X and compound Y, using thin-layer chromatography (TLC).

Compound X is strongly polar, whereas compound Y is non-polar.

The stationary phase is a polar silica plate, which interacts more strongly with polar substances. The mobile phase is hexane, a non-polar solvent, which interacts more strongly with non-polar substances.

(a) Predict, with a reason, whether compound X or compound Y will have a higher retardation factor, $R_\text{f}$, in this experiment.

(b) The student repeats the experiment using a new silica plate, but replaces the pure hexane mobile phase with a solvent mixture containing ethanol, which is significantly more polar than pure hexane.

Predict the effect of using this more polar solvent on the $R_\text{f}$ value of compound X.

A food chemist is analyzing the components of a new artificial sweetener mixture using paper chromatography. A small spot of the mixture is applied to a chromatography paper, and the paper is placed in a sealed chamber with a solvent system consisting of $70\%$ ethanol and $30\%$ water. After some time, the solvent front moves up the paper, separating the components.

(a) Define the stationary phase and the mobile phase in this paper chromatography experiment.

(b) Explain why it is important that the initial spot of the sweetener mixture is applied above the solvent level in the chamber, and why the solvent front should not be allowed to reach the very top of the chromatography paper.

(c) After the chromatography run, the solvent front travelled $9.0\text{ cm}$ from the start line. Two distinct components, Sweetener A (yellow) and Sweetener B (blue), were observed to have travelled $7.2\text{ cm}$ and $4.5\text{ cm}$ respectively from the start line.

Calculate the retardation factor, $R_F$, for Sweetener A and Sweetener B. Show your working.

(d) The stationary phase (paper) is polar due to cellulose, and the mobile phase is a moderately polar mixture of $70\%$ ethanol and $30\%$ water.

Deduce, with a reason, which sweetener (A or B) is more polar.

(e) If Sweetener A and another component had very similar $R_F$ values and were not well separated, suggest one modification to the experiment that could improve their separation.