Page 74 - Science Coursebook

5.2 More exothermic reactions

Reactions of metals with water
In Stage 8 you observed the reaction of potassium in water. In Unit 4 you compared
the reactions of lithium, sodium and potassium in water. In all of these reactions
heat energy is released to the environment and the reaction temperature increases.
They are exothermic reactions.

Here is the word equation for the reaction of potassium with water.
potassium + water → potassium hydroxide + hydrogen

Water is made of molecules containing hydrogen and oxygen. In this
reaction, the bonds between the atoms of oxygen and hydrogen in the
water break. The atoms rearrange to form potassium hydroxide and
hydrogen. Heat energy is released.

Potassium reacting with water.

potassium + water → potassium hydroxide + hydrogen

Reactions of metals with acid
You may have noticed that when you put magnesium in dilute hydrochloric acid the test tube
got hot. The reaction between hydrochloric acid and magnesium is an exothermic reaction.
magnesium + hydrochloric acid → magnesium chloride + hydrogen

Measuring rise in temperature during a reaction
Noor and Hanif each poured 10 cm of hydrochloric acid into
3
a test tube and measured the temperature. Then they added
identical pieces of magnesium ribbon. When the reaction thermometer
stopped they measured the temperature again.

dilute
hydrochloric
acid

magnesium
ribbon

Measuring the rise in temperature when
magnesium reacts with hydrochloric acid.

Noor’s results Hanif’s results
Start temperature / °C End temperature / °C Start temperature / °C End temperature / °C
18 42 21 45

72 5 Energy changes

A+E