Chemistry 9701/33 — May/June 2010

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations · Qualitative Analysis

Q1Presentation of Data and ObservationsManipulation, Measurement and ObservationAnalysis, Conclusions and EvaluationFree sample

You are required to determine the molar enthalpy change of solution for ammonium chloride, FA 1.

When an exothermic reaction takes place in a container such as a beaker, some of the evolved heat energy is absorbed by the beaker.

When an endothermic reaction takes place some of the required heat energy is supplied by the beaker.

The amount of heat energy evolved or supplied for a 1 °C change in temperature is known as the heat capacity of the beaker.

In preparation for your experiment to determine the molar enthalpy change of solution for FA 1 you will first need to determine the approximate heat capacity of a 250 cm³ beaker.

Before starting any practical work read through the instructions in (a) and draw up a table to record your results.

(a)

Determining the approximate heat capacity of the 250 cm³ beaker

When samples of hot and cold water are mixed in the 250 cm³ beaker, some heat is lost to the beaker in raising its temperature. To determine the approximate heat capacity of your 250 cm³ beaker, you will determine the maximum temperature rise when a sample of hot water is added to cold water in the beaker.

Use a 50 cm³ measuring cylinder to transfer 50 cm³ of cold water into the 250 cm³ beaker.
Use the 50 cm³ measuring cylinder to transfer 50 cm³ of cold water into a 100 cm³ beaker. Note the temperature of the water in this 100 cm³ beaker and heat it carefully and gently until the temperature of the water in it has increased by 45–50 °C then stop heating, e.g. if the water is at 20.0 °C you should warm it to 65–70 °C.
Stir the cold water in the 250 cm³ beaker with the thermometer.
Record the temperature of the cold water (this is the temperature at $t = 0$ min).
Record the temperature each minute for 3 minutes.
After you have taken the reading at $t = 3$ min, use the thermometer to stir the hot water in the 100 cm³ beaker.
At $t = 4$ min, measure the temperature of the hot water and record this value in the box below.
Immediately add the hot water from the 100 cm³ beaker to the cold water in the 250 cm³ beaker. Stir with the thermometer but do not record the temperature.
Continue to stir the water throughout the experiment.
Record the temperature at $t = 5$ min, and then every ½ minute until $t = 8$ min.
Empty and rinse the 250 cm³ beaker. Dry it using a paper towel.
Record all measurements of time and temperature obtained.

The temperature, $T_1$ , of the hot water at $t = 4$ min is ............ °C.

Table of results

(b)

Graph plotting

Plot a graph of the temperature of the water in the 250 cm³ beaker ( $y$ -axis) against time ( $x$ -axis) on the grid below.
Do not plot the temperature, $T_1$ , of the hot water at $t = 4$ min.
Draw two straight lines of best fit; one through the points up to $t = 3$ min; the second through the points from $t = 5$ min to $t = 8$ min. Extrapolate both lines to $t = 4$ min.
From the extrapolated lines read the minimum and the maximum temperatures at $t = 4$ min. Record these values in the spaces provided below.
Determine the values for the two temperature changes at $t = 4$ min.

Fig. 1.1

Minimum temperature, $T_2$ , at $t = 4$ min is ............... °C.

Maximum temperature, $T_3$ , at $t = 4$ min is ............... °C.

Temperature rise for 50 cm³ of cold water in the 250 cm³ beaker, $(T_3 - T_2)$ is ............... °C.

Temperature fall for 50 cm³ of hot water from the 100 cm³ beaker, $(T_1 - T_3)$ is ............... °C.

(c)

Calculations

Working should be shown in all calculations.

[4.2 J are absorbed or released when the temperature of 1.0 cm³ of water changes by 1.0 °C.]

(i)

Calculate the heat energy gained by the 50 cm³ of cold water in the 250 cm³ beaker.

The heat energy gained by the cold water = ............ J.

(ii)

Calculate the heat energy lost by the 50 cm³ of hot water from the 100 cm³ beaker.

The heat energy lost by the hot water = ............ J.

(iii)

The difference between the values calculated in (i) and (ii) is an approximate value for the total heat energy absorbed by the 250 cm³ beaker during the experiment. The heat capacity of the beaker is the amount of heat energy absorbed for a 1 °C change in temperature.

\text{approximate heat capacity of the } 250\text{ cm}^3\text{ beaker} = \frac{(\text{heat energy lost}) - (\text{heat energy gained})}{(T_3 - T_2)} \text{ J }^\circ\text{C}^{-1}

Use your answers to (i) and (ii) and the temperature rise from (b) to calculate the approximate heat capacity of the 250 cm³ beaker.

The approximate heat capacity of the 250 cm³ beaker = ............ J °C⁻¹.

(d)

Determining the enthalpy change of solution for ammonium chloride

Follow the instructions below to find the temperature change when a known mass of solid ammonium chloride dissolves in water.

You are provided with two samples of ammonium chloride. You should use the sample labelled $\text{NH}_4\text{Cl}$ in experiment 1 and the sample labelled FA 1 in experiment 2.

Experiment 1

Enter all results in the table below.
Weigh the stoppered tube containing ammonium chloride, which is labelled $\text{NH}_4\text{Cl}$ .
Use the 50 cm³ measuring cylinder to transfer 100 cm³ of cold water into the rinsed and dried 250 cm³ beaker used in (a).
Stir the water in the beaker with the thermometer and record the temperature.
Add the solid from the weighed tube to the water.
Stir the mixture constantly with the thermometer.
Record the minimum temperature obtained in the solution.
Reweigh the tube labelled $\text{NH}_4\text{Cl}$ , its stopper and any residual ammonium chloride.
Empty and rinse the beaker and dry it using a paper towel.

Experiment 2

Enter all results in the table below.
Weigh a clean, dry, boiling-tube.
Weigh between 9.8 g and 10.2 g of FA 1, ammonium chloride, into the boiling-tube.
Repeat the procedure in experiment 1 and record the minimum temperature obtained when this mass of FA 1 dissolves in 100 cm³ of water.
Reweigh the boiling-tube and any residual ammonium chloride.

Results

	experiment 1	experiment 2
mass of tube + ammonium chloride / g
mass of empty tube / g
mass of tube + residual ammonium chloride / g
mass of ammonium chloride / g
initial temperature of water / °C
minimum temperature obtained / °C
temperature fall, $\Delta T$ / °C

(e)

Calculations

Working should be shown in all calculations.

(i)

Use the temperature fall from (d), experiment 1, to calculate the change in heat energy of the solution.
[4.3 J are absorbed or released when the temperature of 1.0 cm³ of solution changes by 1.0 °C.]

The change in heat energy of the solution = ................. J.

(ii)

To calculate the total change in heat energy as ammonium chloride dissolves in water, the change in heat energy of the 250 cm³ beaker has to be added to the change in heat energy of the solution.
Explain why these two changes in heat energy have to be added together.

(iii)

Use your answer in (i) above and the approximate heat capacity of the 250 cm³ beaker calculated in (c)(iii) to calculate the combined change in heat energy of the beaker and solution.

The combined change in heat energy of the beaker and solution = ................. J.

(iv)

Calculate how many moles of FA 1, $\text{NH}_4\text{Cl}$ , were used in (d), experiment 1.
[ $A_r$ : $\text{Cl}$ , 35.5; $\text{H}$ , 1.0; $\text{N}$ , 14.0]

................ mol of FA 1 were used in experiment 1.

(v)

Calculate the enthalpy change when 1 mol of FA 1 dissolves in an excess of water. This is the molar enthalpy change of solution, $\Delta H_{\text{solution}} (\text{NH}_4\text{Cl})$ .
Make certain that your answer is given in $\text{kJ mol}^{-1}$ and has the appropriate sign.

\Delta H_{\text{solution}} (\text{NH}_4\text{Cl}) = \text{........ } \text{............................ kJ mol}^{-1}

\text{sign } \text{ calculated value}

(vi)

Explain the significance of the sign you have given in (v) and how it is related to your experimental results.

(f)

Evaluation

A data book value for the molar enthalpy change of solution, $\Delta H_{\text{solution}} (\text{NH}_4\text{Cl})$ , is $+15.2 \text{ kJ mol}^{-1}$ .

The value you have obtained may be significantly different from this value.

Calculate the difference between your value of $\Delta H_{\text{solution}} (\text{NH}_4\text{Cl})$ and the data book value. Record this difference below. Express this difference as a percentage of the data book value.

difference = ................. $\text{kJ mol}^{-1}$

percentage difference = ................. %

(g)

Sources of error

Describe one major source of error in this experiment. Suggest an improvement which would significantly increase the accuracy of the experiment. Explain why your suggestion would produce a more accurate value.

description of major source of error

suggested improvement

explanation of why suggestion would increase experimental accuracy

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