Page 76 - Science Coursebook

5.3 Endothermic processes

Endothermic reactions
Some chemical reactions take heat from their surroundings and store it as chemical
energy. These are called endothermic reactions. When an endothermic reaction
takes place, the temperature decreases.

Activity 5.3
Carrying out an endothermic reaction

1 Place some citric acid or lemon juice in a test tube.
2 Measure and record the temperature.
3 Add three spatulas of sodium hydrogencarbonate and stir.
4 Measure and record the temperature.

Questions

A1 What was the difference between the temperatures
at the start and the end of the experiment?
A2 Is heat energy given out or taken in during this
reaction?

This is the word equation for the reaction between sodium hydrogencarbonate and
citric acid.
sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide

During this reaction, energy is taken from the environment. This
results in the contents of the tube having a lower temperature.
When we eat refreshing sweets we use this reaction. Inside
sherbet sweets there is a mixture of dry citric acid and sodium
hydrogencarbonate. When you eat them, these substances
dissolve in the water in your saliva, and react together. This gives
a cool ‘fizzy’ feeling in your mouth, which is refreshing.

Questions
1 What are the reactants in the reaction shown in the
word equation above?
2 Which are the products in the reaction shown in the
word equation above?
3 What is an endothermic reaction?
4 Explain why eating sherbet sweets makes your mouth
feel cooler.

A+I 5 You may also get a ‘fizzy’ feeling in your mouth when
you eat sherbet. Why is this?
Sherbet sweets.

74 5 Energy changes

A+E SE