Topic 7.9-7.16 - Heat energy changes in chemical reactions
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What changes are accompanied by changes in heat energy? And state whether each change is exothermic or endothermic
-Salt dissolving in water (exothermic or endothermic) -Neutralisation reactions (exothermic) -Displacement reactions (exothermic or endothermic) -Precipitation reactions (exothermic)
Describe an exothermic change
A change or reaction in which heat energy is given out (transfers energy to the surroundings so the temperature of the surroundings increase)
How can the change in heat energy of a reaction be measured when the reaction takes place in a solution?
Temperature changes can be measured to reflect the heat changes. This can be carried out in a polystyrene cup with a lid, and measured using a thermometer
Describe an endothermic change
A change or reaction in which heat energy is taken in. (takes in energy from the surroundings so the temperature of the surroundings decreases)
Describe how bonds relate to endothermic/exothermic reactions
The breaking of bonds is endothermic. The making of bonds is exothermic.
When is the overall heat energy change for a reaction exothermic?
It is exothermic if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants
When is the overall heat energy change for a reaction endothermic?
It is endothermic if more heat energy is taken in breaking bonds than is required to release in making bonds in the reactants
How do you calculate the energy change in a reaction given the energies of bonds?
Sum of energy taken in to break bonds - sum of energy released to form bonds = overall energy change. It is measured in kJ mol⎺¹ If the energy out > energy in, the energy change will be negative - showing an exothermic reaction and vice versa.
Explain the term activation energy
Activation energy is the minimum amount of energy that particles must have to react
What does a reaction profile for an exothermic reaction look like?
The x-axis is progress of reaction and the y-axis is energy. There is a high horizontal line marked ‘reactants’, then a curve up and down. The distance between the peak of this curve and the reactants line is marked ‘activation energy’. The curve dips down to a lower horizontal line marked ‘products’. The distance between reactants and products is ‘overall change in energy’.
What does a reaction profile for an endothermic reaction look like?
The x-axis is progress of reaction and the y-axis is energy. There is a lower horizontal line marked ‘reactants’, then a curve up and down. The distance between the peak of this curve and the reactants line is marked ‘activation energy’. The curve dips down to a higher horizontal line marked ‘products’.