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Exothermic and endothermic reactions

In this chapter you will learn:

  1. What are Exothermic and endothermic reactions?

  2. Effect of Pressure Change

  3. Reaction profile

  4. Energy changes in reactions

Exothermic and endothermic reactions

When a chemical reaction takes place, energy is transferred to or from the surroundings.

Energy is conserved in chemical reactions. This means that the amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. Therefore, if a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the same amount as was transferred.

In an exothermic reaction, the temperature of the surroundings increases, as energy is transferred to the surroundings.

Examples of exothermic reactions include: combustion of fuels, many oxidation reactions and acid-alkali neutralisation reactions.

In an endothermic reaction, the temperature of the surroundings decreases, as energy is taken in from the surroundings.

Examples of endothermic reactions include: thermal decomposition of compounds, the reaction between citric acid and sodium hydrogencarbonate and sports injury packs for cooling effect.

Therefore, we can say that exotherimic and endotherimic reactions are mutually opposite. In endothermic reactions, energy enters, whereas in exothermic reactions, energy exits.

Effect of Pressure Change

An increase in pressure causes the equilibrium to shift towards the side that produce the smaller number of molecules of gas to decrease the pressure again.

A decrease in pressure causes the equilibrium to shift in the direction that produce the larger number of molecules of gas to increase the pressure again.

Reaction profile

Chemical reactions can happens only when reacting particles collide each other with sufficient energy.

Reaction profiles are used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

A reaction profile includes the activation energy, which is the minimum amount of energy needed for a reaction to occur.

The difference between the energy levels of the reactants and products gives the overall energy change for the reaction.

An energy level diagram shows whether a reaction is exothermic or endothermic.

In an exothermic reaction, the energy level decreases, because energy is given out to the surroundings.

in an endothermic reaction, the energy level increases because energy is taken in from the surroundings.

Energy changes in reactions

Energy is transferred when bonds are broken or formed.

During a chemical reaction, when bonds in the reactants are broken, energy must be supplied to break bonds.

Energy is released when new bonds in the products are formed.

The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.

If the energy released from making new bonds is greater than the energy needed to break existing bonds, the reaction is exothermic.

If the energy needed to break existing bonds is greater than the energy released from forming new bonds, the reaction is endothermic.

A bond energy is the amount of energy needed to break one mole of a particular covalent bond.

The energy change in a reaction can be calculated using bond energies.

To calculate an energy change for a reaction:

You can add together the bond energies for all the bonds in the reactants - this is the 'energy in'

You can add together the bond energies for all the bonds in the products - this is the 'energy out'

energy change = energy in - energy out

energy change = energy released on bond formation - energy absorbed in bond breaking

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