Chemistry 9701/35 — October/November 2012

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

You are to investigate the temperature change when a piece of magnesium ribbon is added to hydrochloric acid. You will measure the temperature at regular intervals during the reaction.

FA 1 is approximately $2\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
You are also provided with magnesium ribbon.

Read through the instructions carefully before starting any practical work.

(a)

Method

Curl or fold the magnesium ribbon so that it will just fit in the bottom of the plastic cup. Remove the magnesium from the cup.
Support the plastic cup in a $250\text{ cm}^3$ beaker.
Use a measuring cylinder to transfer $50\text{ cm}^3$ of FA 1 into the empty plastic cup.
Measure and record in the table below, the initial temperature of the acid in the cup.
Start the stop watch. Measure and record the temperature of FA 1 in the cup after 1 minute, 2 minutes and 3 minutes.
At time $3\frac{1}{2}$ minutes, add the curled or folded magnesium to the FA 1 in the cup and stir the mixture. Make sure the magnesium is submerged in the acid.
From time 4 minutes, continue to measure the temperature of the contents of the cup to complete the table.

Results

time / min	0	1	2	3	4	5	6	7	8	9	10
temperature / $^{\circ}\text{C}$

(b)

(i)

On the axes opposite, plot the temperature ( $y$ -axis) against time ( $x$ -axis). The temperature axis should allow you to include a point at least $5\text{ }^{\circ}\text{C}$ greater than the maximum temperature recorded.

graph_grid

(ii)

Complete the graph to show how the temperature of the contents of the cup varies with time.

Draw one straight line through the points between time 0 minutes and 3 minutes.
Draw one straight line through the points between time 5 minutes and 10 minutes.
Extrapolate these two lines and draw a vertical line at time $3\frac{1}{2}$ minutes.

(c)

Calculation

(i)

Use your graph to determine the change in temperature at $3\frac{1}{2}$ minutes.

change in temperature = ............ $^{\circ}\text{C}$

(ii)

In the experiment you have just carried out, explain how you know that the hydrochloric acid was in excess.

(iii)

One source of error in this experiment is due to the accuracy to which the thermometer can be read.

What is the maximum error in a single temperature reading on a thermometer with graduations at $1\text{ }^{\circ}\text{C}$ ?

maximum error = ........... $^{\circ}\text{C}$

Calculate the maximum percentage error when measuring a temperature rise of $7.5\text{ }^{\circ}\text{C}$ .

maximum percentage error = .............................. %

(iv)

Apart from errors associated with the thermometer, suggest one significant source of error in the procedure used in this experiment. Suggest an improvement that could be made to reduce this error.

(v)

The experiment was repeated on another day when the temperature of the room was much higher than when the original experiment was carried out.

Discuss the effect of this higher room temperature on the results of the experiment.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations13M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation15M

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