Energy Changes

7.1 Exothermic and Endothermic reactions

• Energy is always transferred in chemical reactions, as chemical bonds in the reactants are broken and new ones are made in the products

What is the difference between endothermic and exothermic?

• Exothermic products have a lower energy content than endothermic products, which have a higher energy content. Endothermic reactions transfer energy from the environment and reduce the temperature because energy is taken in; exothermic reactions transfer energy to the environment and increase the temperature because energy is released

Name an endothermic reaction.

• Thermal decomposition is an example of an endothermic reaction. The products have a higher energy content than the reactants, so energy is transferred from the surroundings

Name an exothermic reaction.

• Neutralisation and combustion are examples of exothermic reactions. The products of exothermic reactions have a lower energy content than the reactants, so energy's transferred to the surroundings

Required practical: finding out if a reaction is endothermic or exothermic

Mix different combinations and stir every 30 seconds for 5 minutes (eg). Record the initial and final temperature. Find out if the temperature decreased (endothermic) or increased (exothermic).

7.2 Using energy transfers from reactions

Types of handwarmers

• Handwarmers only used once are based on the energy transferred during the oxidation of iron. Sodium chloride handwarmers are disposable, used once, but last hours. Handwarmers lasting many hours are based on the formation of crystals in a solution; to reuse it, put it in boiling water to redissolve the crystals.

What else are exothermic reactions used for?

• They are also used in self-heating cans. The reaction for this is: calcium oxide + water = calcium hydroxide

The uses of…