AS & A Level Chemistry - 9701 Paper 5 2010 Summer Zone 1

The neutralisation of an acid by a base is exothermic.

In this experiment the following solutions are available.

2\,\text{mol}\,\text{dm}^{-3} \text{ sulfuric acid, } \text{H}_{2}\text{SO}_{4}
3\,\text{mol}\,\text{dm}^{-3} \text{ sodium hydroxide, } \text{NaOH}

The equation for the reaction is:

$\displaystyle 2\text{NaOH(aq)}+\text{H}_{2}\text{SO}_{4}\text{(aq)}\rightarrow \text{Na}_{2}\text{SO}_{4}\text{(aq)}+2\text{H}_{2}\text{O(l)} $

(a) $2\,\text{mol}\,\text{dm}^{-3} \text{ H}_{2}\text{SO}_{4}$ is gradually added to a fixed volume of $3\,\text{mol}\,\text{dm}^{-3} \text{ NaOH}$ in a $150\,\text{cm}^{3}$ plastic cup, while stirring continuously. The temperature of the solution, measured with a thermometer, increases until the alkali is just neutralised. On further addition of the cold acid the temperature of the solution slowly falls.

Select an appropriate volume, $x\,\text{cm}^{3}$, of $3\,\text{mol}\,\text{dm}^{-3}$ NaOH to use in the experiment.

\text{....................................... cm}^3

Calculate the volume of $2\,\text{mol}\,\text{dm}^{-3}$ H$_{2}$SO$_{4}$ that will just neutralise $x\,\text{cm}^{3}$ of $3\,\text{mol}\,\text{dm}^{-3}$ NaOH.

Sketch the graph you would expect to obtain as the acid is added. Label the neutralisation point.



(b) This experiment can be used to determine the enthalpy change of neutralisation for the reaction. To ensure reliable results the experiment should be repeated a number of times.

When sulfuric acid is added to the fixed volume of aqueous sodium hydroxide in this experiment

(i) the independent variable is ..........................................................,
(ii) the dependent variable is ................................................................,
(iii) the other variables that need to be controlled are ........................................
................................................................................................................. .
[3]

(c) In carrying out the experiment, what apparatus would you use to accurately measure the independent variable?
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[1]

(d) Explain how you would use this apparatus to control the independent variable.
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[1]

(e) Identify and assess

(i) a risk associated with the plastic cup used in this experiment,
..................................................................................................................................................
(ii) a risk associated with the $3\,\text{mol}\,\text{dm}^{-3}$ NaOH.
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[1]

(f) Describe how the risks in (e) can be kept to a minimum for

(i) the plastic cup,
..................................................................................................................................................
(ii) the $3\,\text{mol}\,\text{dm}^{-3}$ NaOH.
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[1]

(g) In the space below draw a table to show column headings for all of the measurements you would make during the experiment. Include in the table one or more columns for any calculated values needed to determine the enthalpy change of neutralisation.
[2]

(h) Show how you would calculate the total heat energy produced in the plastic cup up to the point when the sulfuric acid has just neutralised the sodium hydroxide. You may use $\Delta T$ to represent the temperature change.

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

[1]

(i) The enthalpy change of neutralisation is the energy change associated with the reaction shown by the following equation.

$\text{NaOH(aq)} + \frac{1}{2} \text{H}_{2}\text{SO}_{4}\text{(aq)} \rightarrow \frac{1}{2} \text{Na}_{2}\text{SO}_{4}\text{(aq)} + \text{H}_{2}\text{O(l)}$

Show how you would convert the energy change expressed in (h) into a value, in kJ mol$^{-1}$, for the enthalpy change of neutralisation, $\Delta H_{\text{neutralisation}}$. Show clearly the sign and the expression for the enthalpy change.

$\Delta H_{\text{neutralisation}} \;=\; \text{.........}\;\; \text{...................................}\;\; \text{kJ\ mol}^{-1}$
[2]

A student reads in a text-book about ideal and non-ideal mixtures of liquids.

When two different liquids are mixed, bonds between molecules in each of the different liquids are broken and new bonds are formed between the different molecules in the mixture.

In an ideal mixture the energy used to break bonds between molecules in the different liquids is approximately the same as the energy released when new bonds are formed between molecules in the mixture.

In a non-ideal mixture the energy used to break bonds in the different liquids is not the same as that released in bond formation.

The breaking of bonds is an endothermic process.
The formation of bonds is an exothermic process.

In order to test this information, the student investigates a number of mixtures of
(i) propan-1-ol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$, and propan-2-ol, $\text{CH}_3\text{CHOHCH}_3$,
(ii) ethanol, $\text{CH}_3\text{CH}_2\text{OH}$, and cyclohexane, $\text{C}_6\text{H}_{12}$.

The boiling point of each pure liquid and each mixture is measured. The thermometer used has graduations at 0.2°C.

The results for the experiment with propan-1-ol and propan-2-ol, which was carried out four times, are shown on the next page.



(a) Indicate below any results that you consider to be anomalies and that should not be included when calculating the mean boiling temperature.
..........................................................................................................................................
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[1]

(b) Complete the table above to show the following.
(i) the percentage (%) by volume of propan-1-ol in each of the liquids/mixtures
(ii) the mean boiling temperature for each of the liquids/mixtures

[2]

The experiment is repeated using mixtures of ethanol and cyclohexane. The results of these experiments are given in the table below.



(c) For both graphs below, use as much of the y-axis as possible for experimental data.
Do not start your temperature scales at 0°C.
Draw a graph of mean boiling temperature (y-axis) against the % of propan-1-ol (x-axis) in the mixture of propan-1-ol and propan-2-ol.



Draw a graph of mean boiling temperature (y-axis) against the % of ethanol (x-axis) in the mixture of ethanol and cyclohexane.

[3]

(d) The graph you have drawn for propan-1-ol and propan-2-ol is typical of ideal mixtures of liquids.
The graph you have drawn for ethanol and cyclohexane is typical of some non-ideal mixtures of liquids.
From the shape of your graph explain whether the mixing of ethanol and cyclohexane is overall an endothermic or an exothermic process.
The mixing of ethanol and cyclohexane is ................................................................................... ,
because ......................................................................................................................................
..............................................................................................................................................
..............................................................................................................................................

What type of intermolecular forces exist between the molecules of
(i) pure ethanol, ..........................................................................................................................
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(ii) pure cyclohexane, ................................................................................................................
..........................................................................................................................

(iii) ethanol and cyclohexane in the mixture?
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[3]

A group of students perform an experiment to confirm that the formula of magnesium oxide is MgO. Each student is provided with a different length of magnesium ribbon which is coiled and heated in a crucible fitted with a lid. The magnesium reacts with oxygen to form magnesium oxide.
The instructions for the experiment are as follows.

• Weigh the empty crucible and lid.
• Coil the length of magnesium ribbon and place it in the bottom of the crucible.
• Reweigh the crucible and lid with the magnesium.
• Heat the crucible with a Bunsen burner.
• Periodically lift the crucible lid for a very short period of time. This allows air to enter the crucible.
• Each time the lid is lifted, take care to minimise the loss of any white smoke which is some of the powder formed.
• When the reaction appears to have stopped, remove the crucible lid and heat the crucible and its contents strongly for 2 minutes.
• Cool and reweigh the crucible, lid and the contents.