Chemistry 9701/35 — October/November 2018

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

questions

marks

120

minutes

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

Q1Presentation of Data and ObservationsAnalysis, Conclusions and EvaluationManipulation, Measurement and ObservationFree sample

In this experiment you will determine the percentage purity of a sample of impure anhydrous sodium carbonate. You will use two different methods to measure the enthalpy change of reaction when a sample of impure anhydrous sodium carbonate reacts with excess dilute hydrochloric acid.

FA 1 is a sample of the impure anhydrous sodium carbonate.
FA 2 is $2.00\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FA 3 is a second sample of the impure anhydrous sodium carbonate used in FA 1.

(a)

Method 1

Weigh the container with FA 1. Record this mass.

$\text{mass of container with FA 1} = \text{.............................. g}$

Support one of the plastic cups in the $250\text{ cm}^3$ beaker.
Use the measuring cylinder to place $25\text{ cm}^3$ of FA 2 into the cup.
Measure the temperature of the FA 2 in the cup. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. Record this temperature at time $t = 0$ .
Start the stopclock and leave it running for the whole experiment.
Measure and record the temperature of FA 2 in the cup every half minute for 2 minutes.
At $t = 2\frac{1}{2}$ minutes tip all the FA 1 into the cup. Stir the contents of the cup.
Measure and record the temperature of the contents of the cup at $t = 3$ minutes and then every half minute up to $t = 9$ minutes.
Weigh the container with any residual FA 1. Record this mass.

$\text{mass of container with residual FA 1} = \text{.............................. g}$

(b)

(i)

On the grid on page 3, plot a graph of temperature ( $y$ -axis) against time ( $x$ -axis). You should choose a scale that allows you to plot $2\text{ }^\circ\text{C}$ above the maximum temperature reached.

On your graph, draw two straight lines of best fit. One line is for the temperature before adding FA 1 and the other line for the cooling of the solution once reaction is complete.

Extrapolate these two lines to $t = 2\frac{1}{2}$ minutes.

grid_page_3

(ii)

From your graph, find the theoretical temperature rise at $t = 2\frac{1}{2}$ minutes.

$\text{theoretical temperature rise} = \text{............................... }^\circ\text{C}$

(c)

(i)

Calculate the energy released in the reaction.

(Assume $4.2\text{ J}$ of heat energy changes the temperature of $1.0\text{ cm}^3$ of solution by $1.0\text{ }^\circ\text{C}$ .)

$\text{energy released} = \text{.............................. J}$

(ii)

The equation for the reaction between anhydrous sodium carbonate and hydrochloric acid is shown.

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

The literature value for the enthalpy change of this reaction is $-27.0\text{ kJ mol}^{-1}$ .

Use this figure, and the value that you found in (i), to find the mass of anhydrous sodium carbonate you used in (a). You should assume that no energy was lost to the surroundings in your experiment.

$\text{mass Na}_2\text{CO}_3 = \text{.............................. g}$

(iii)

Calculate the percentage of anhydrous sodium carbonate present in FA 1.

$\text{percentage Na}_2\text{CO}_3\text{ in FA 1} = \text{.............................. \%}$

(d)

In your calculation in (c), what assumption have you made about the impurity present in FA 1?

(e)

Method 2

Weigh a clean, dry plastic cup and record the mass.
Add between $1.70\text{ g}$ and $1.90\text{ g}$ of FA 3 to the plastic cup and record the mass.
Support the plastic cup in the $250\text{ cm}^3$ beaker.
Pour $25\text{ cm}^3$ of FA 2 into the measuring cylinder.
Measure and record the initial temperature of FA 2 in the measuring cylinder.
Pour the $25\text{ cm}^3$ of FA 2 into the plastic cup.
Stir the contents of the cup and record the maximum temperature. Tilt the cup if necessary so that the bulb of the thermometer is fully covered.
Calculate and record the mass of FA 3 used and the change in temperature.

(f)

Use the temperature rise in (e), and the fact that the enthalpy change for the reaction between anhydrous sodium carbonate and hydrochloric acid is $-27.0\text{ kJ mol}^{-1}$ , to calculate the percentage of anhydrous sodium carbonate in FA 3.

$\text{percentage Na}_2\text{CO}_3\text{ in FA 3} = \text{.............................. \%}$

(g)

FA 1 and FA 3 are both samples of the same impure anhydrous sodium carbonate and so the percentage of anhydrous sodium carbonate found using Method 1 and Method 2 should be the same. In practice the percentages are sometimes different from each other.

In both methods, percentage errors occur due to measuring the mass of solid and the temperature rise.

Ignoring these errors, which method is more accurate?
Tick the correct box and explain your answer.

Method 1 more accurate
Method 2 more accurate
Method 1 and Method 2 equally accurate

(h)

A student decided to confirm by experiment the literature value for the enthalpy change of the reaction between anhydrous sodium carbonate and hydrochloric acid. By mistake the student weighed a sample of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}$ , instead of anhydrous sodium carbonate, $\text{Na}_2\text{CO}_3$ .

State what effect this would have on the calculated value of the enthalpy change for the reaction. Explain your answer.

(i)

A student used $3.00\text{ g}$ of anhydrous sodium carbonate that was $80.0\%$ pure by mass.

Calculate the minimum volume of $2.00\text{ mol dm}^{-3}$ hydrochloric acid that would be needed to react completely with this sample of impure anhydrous sodium carbonate.

$\text{volume of HCl} = \text{.............................. cm}^3$

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