AS & A Level Chemistry - 9701 Paper 5 2009 Winter Zone 2

(a) Hydrochloric acid, nitric acid and sulfuric acid are all strong acids. Predict how $\Delta H_{\text{neutralisation}}$ values for these acids would compare with each other.

Predict and explain how the expected $\Delta H_{\text{neutralisation}}$ for a weak acid, such as ethanoic acid or ethanedioic acid, compares with that obtained for hydrochloric acid.
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(b) In a series of experiments to determine the enthalpy change of neutralisation for each acid with $2.0 \, \text{mol} \, \text{dm}^{-3}$ sodium hydroxide, NaOH, the following aqueous acids are available.

hydrochloric acid, $\text{HCl}$
sulfuric acid, $\text{H}_2\text{SO}_4$
nitric acid, $\text{HNO}_3$
ethanoic acid $\text{CH}_3\text{CO}_2\text{H}$

Ethandioic acid, $(\text{CO}_2\text{H})_2.2\text{H}_2\text{O}$, is also available as a hydrated crystalline solid.

When determining the enthalpy change of neutralisation using the acids listed above in reaction with sodium hydroxide,

the independent variable is, .................................................................................................................................
the dependent variable is. .................................................................................................................................

(c) Draw and label a diagram of the apparatus you would use to determine the temperature change, $\Delta T$, when each of the acids reacts with $30.0\, \text{cm}^3$ of $2.0 \text{mol} \text{dm}^{-3}$ sodium hydroxide. Any experimental method that is normally carried out in a school or college laboratory may be described.

Identify three aspects of the experimental method or its use that must be kept the same to ensure comparable results.

At least one of these should minimise heat loss from the apparatus.

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(d) In each experiment $30.0 \text{cm}^3$ of $2.0 \text{mol} \text{dm}^{-3}$ NaOH is to be used.
There is to be an excess of sodium hydroxide, NaOH.

Suggest appropriate volumes and concentrations for each of the acids to be used in the individual experiments.
In order that the experiments are equivalent, the total final volumes should be the same in each experiment.

[Table_1]

acid	volume/ \text{cm}^3	concentration /\text{mol dm}^{-3}
hydrochloric $\text{HCl}$
sulfuric $\text{H}_2\text{SO}_4$
nitric $\text{HNO}_3$
ethanoic $\text{CH}_3\text{CO}_2\text{H}$
ethanedioic $(\text{CO}_2\text{H})_2$

(e) Ethanedioic acid is a hydrated crystalline solid, $(\text{CO}_2\text{H})_2.2\text{H}_2\text{O}$.

Outline, step by step, how you would prepare, in a graduated flask, $250\,\text{cm}^3$ of a solution of ethanedioic acid with exactly the concentration you have given in the table above. [\text{A}_r: C, 12.0; H, 1.0; O, 16.0]
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(f) Show the mathematical expression for the enthalpy change of neutralisation of sodium hydroxide with hydrochloric acid, using the volume and concentration from (d) and $\Delta T$ to represent the temperature change.

[4.3 J of heat energy raise the temperature of $1 \, \text{cm}^3$ of any solution by $1 ^\circ C$.]

(g) Hydrochloric acid is labelled harmful as it is an irritant.
Aqueous sodium hydroxide is labelled corrosive.

Suggest a suitable precaution that should be taken when carrying out the experiment using these two solutions.
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A student investigates the effusion time for a number of dry gases using the following apparatus.

The instructions given to the student for the experiment are as follows.
• Attach two markers to the scale on the gas syringe, 70cm³ apart.
• Completely fill the syringe with the dry gas under investigation.
• Use the piston on the syringe to push all of the gas out of the syringe.
• Completely fill the syringe with the dry gas a second time.
• Clamp the syringe in an upright position, keeping hold of the piston of the syringe.
• Release the piston – as it descends gas effuses from the small hole at the end of the effusion tube.
• Record the time taken for the piston to travel between the two marks.
• Repeat the experiment a number of times for each gas under investigation.
Mean times, to the nearest second, obtained by the student are given in the table below.

| | M_r | effusion time/s | (M_r)² | √M_r |
|---|---|---|---|---|
| hydrogen | H₂ | 2 | 19 | | |
| oxygen | O₂ | 32 | 76 | | |
| carbon dioxide | CO₂ | 44 | 89 | | |
| butane | C₄H₁₀ | 58 | 102 | | |
| chlorine | Cl₂ | 71 | 113 | | |

(a) Complete the table above.
Calculate $(M_{\text{r}})^2$ to 3 significant figures.
Calculate $\sqrt{M_{\text{r}}}$ to 4 significant figures. [1]

(b) Plot the following graphs using data from the table.
(i) Effusion time against relative molecular mass, $M_{\text{r}}$.
[Graph_1]
(ii) Effusion time against the square of the relative molecular mass, $(M_{\text{r}})^2$.
[Graph_2]
(iii) Effusion time against the square root of the relative molecular mass, $\sqrt{M_{\text{r}}}$.
[Graph_3] [3]

(c) From the graphs drawn, deduce and explain the relationship between effusion time and the relative molecular mass of the gas.
Credit will be given for stating the relationship in mathematical terms. [2]
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(d) Suggest a reason for using dry gas in each of the experiments. [1]
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(e) State and explain the effect on the effusion time of filling the syringe with hydrogen from a flask in which zinc is reacting with dilute hydrochloric acid. [1]
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A group of students reduce samples of copper oxide by passing hydrogen gas over a weighed sample of the oxide contained in a porcelain boat.



The results of the experiment are given below.

[Table_1]

(a) Complete the table above to find, for each sample of the oxide,

(i) the mass of copper,

(ii) the mass of oxygen,

(iii) the mass of copper combined with $1.00\, \text{g}$ of oxygen.

[2]

(b) Give the number of any student whose results you consider to be anomalous. By making reference to the experimental method for reducing the copper oxide, explain a possible cause for this anomaly.

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[1]

(c) From the results in (a)(iii), calculate an appropriate mean for the mass of copper combined with $1.00\, \text{g}$ of oxygen. Show all your working, indicating clearly the results used in determining this mean value.

The mean mass of copper, combined with $1.00\, \text{g}$ of oxygen is ............................................ g

[2]

(d) Use your answer to (c) to determine the formula of the copper oxide. Show all your working.
[$A_r$: O, 16.0; Cu, 63.5]

The formula of the copper oxide is ............................................

[2]