5.1 Exothermic & endothermic reactions
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What is the conservation of energy principle?
Energy is conserved in chemical reactions. The amount of energy at the end of a chemical reaction is the same as before the reaction takes place.
What is an exothermic reaction? Give examples
A reaction where energy is transferred to the surroundings so that the surrounding temperature increases Examples include combustion, oxidation reactions and neutralisation (acid + alkali) reactions.
What is an endothermic reaction? Give examples
A reaction where energy is taken in from the surroundings so the surrounding temperature decreases Examples include thermal decomposition and the reaction of citric acid and sodium hydrogen carbonate.
What is activation energy?
Minimum amount of energy that particles need to react
What is a reaction profile?
A reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.
What occurs in a chemical reaction in terms of bond energies? (Higher)
Energy is supplied to break bonds and energy is released when bonds are made.
Describe the breaking/forming of bonds in exothermic reactions. (Higher)
Energy released from forming bonds is greater than that needed to break the bonds.
Describe the breaking/forming of bonds in endothermic reactions. (Higher)
Energy needed to break bonds is greater than energy released making them.
What is the equation to find enthalpy change in terms of bond energies? (Higher)
Energy of reaction = sum of bonds broken – sum of bonds made
What is enthalpy change? (Higher)
The difference between the energy of the reactants and the energy of the products is called the enthalpy change (∆H) of the reaction. For an exothermic reaction, the enthalpy change is always negative. In an endothermic reaction, the products are at a higher energy than the reactants, so it’s positive.