5.1 Exothermic & Endothermic Reactions

A student investigated what happened when sodium thiosulphate dissolved in water.

Experiment 1

By using a measuring cylinder, 20 cm³ of distilled water was poured into a polystyrene cup.
Use the thermometer diagram to record the temperature of the water in the table.

1 g of powdered sodium thiosulphate was added to the cup and the mixture stirred with a thermometer. Use the thermometer diagram to record the temperature of the solution.

Experiment 2

Experiment 1 was repeated using 2 g of powdered sodium thiosulphate. Record the temperature in the table.

Experiments 3, 4 and 5

Experiment 1 was repeated using 3 g, 4 g and 5 g of powdered sodium thiosulphate respectively. Record the temperatures in the table.

Plot the results of the experiments on the grid below. Draw a straight line graph. 

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What type of chemical reaction occurs when sodium thiosulphate dissolves in water?

Explain how the temperature changes would differ in the experiments if 40 cm³ of water were used.

Explain why the sodium thiosulphate was powdered before being used.

Predict what the temperature of the reaction mixture in Experiment 5 would be after 1 hour. Explain your answer.

Suggest one change you could make to the apparatus used in the experiments to obtain more accurate results.

A small piece of limestone was heated strongly and left to cool. A few drops of cold water were added. The solid expanded and gave off steam. 

Complete the empty boxes to identify the pieces of apparatus labelled.

What type of chemical reaction takes place when water is added?

A student investigated the reaction of aqueous sodium hydroxide with two different acids, acid C and acid D.

Two experiments were carried out.

Experiment 1

By using a measuring cylinder, 20 cm³ of aqueous sodium hydroxide was poured into a conical flask and the initial temperature of the solution was measured.

A burette was filled with acid C up to the 0.0 cm³ mark.

5 cm³ of acid C was added to the sodium hydroxide in the flask. The temperature of the mixture was measured.

Further 5 cm³ portions of acid C were added to the mixture in the flask, stirring with the thermometer until a total volume of 30 cm³ of acid C had been added. The temperatures after each 5 cm³ portion had been added were measured.

Use the thermometer diagrams to record the temperatures in the table of results.

Table of results

Experiment 2

The burette was emptied and rinsed with water. Experiment 1 was repeated using acid D. 

Use the thermometer diagrams to record the temperatures in the table of results.

Table of results

Plot the results for Experiments 1 and 2 on the grid and draw two smooth line graphs.
Clearly label your graphs.

From your graph, deduce the temperature of the mixture when 3 cm³ of acid C reacted with sodium hydroxide in Experiment 1.

Show clearly on the graph how you worked out your answer.

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Why was the burette rinsed with water in Experiment 2?

Predict the temperature of the reaction mixture in Experiment 2 after 1 hour. Explain your answer.

An experiment was carried out to measure the temperature changes during the neutralisation of sodium hydroxide solution with dilute hydrochloric acid. Both solutions were allowed to stand in the laboratory for about 30 minutes.

25 cm³ of sodium hydroxide solution was added to a polystyrene beaker and the temperature was measured. 10 cm³ of hydrochloric acid was added to the beaker and the highest temperature reached measured.

The experiment was repeated using different volumes of acid. 

Why were the solutions left to stand for about 30 minutes before the experiments?

Why was a polystyrene beaker used instead of a glass beaker?

The results of the experiments are shown plotted on the grid below.  

What type of chemical reaction occurs when sodium hydroxide is neutralised by hydrochloric acid?

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