Exothermic and endothermic reactions - AQAExplaining energy changes in reactions - Higher
Exothermic reactions transfer energy to the surroundings and the temperature of the surroundings increases. Endothermic reactions take in energy and the temperature of the surroundings decreases.
energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. is transferred when bondThe chemical link that holds molecules together. are broken or are formed.
Breaking bonds
Forming bonds
Type of process
Endothermic
Exothermic
Heat energy transferred
Taken in
Given out
Type of process
Breaking bonds
Endothermic
Forming bonds
Exothermic
Heat energy transferred
Breaking bonds
Taken in
Forming bonds
Given out
During a chemical reaction:
bonds in the reactantA substance that reacts together with another substance to form products during a chemical reaction. are broken
new bonds are made in the productA substance formed in a chemical reaction.
The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.
A reaction is:
exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
endothermicReaction in which energy is taken in. if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
