Energy changes in chemistry - (CCEA)

• Exothermic reactions give heat out to the surroundings. Endothermic reactions take heat in from the surroundings.

• The energy changes that take place during a reaction can be shown on a reaction profile diagram.

• Energy change values can be calculated by comparing the energy required to break the bonds in the reactants, and the energy released by making the bonds in the products.

During a chemical reaction, heat energy can be given out or taken in by the chemicals.

This can cause the temperature of the surroundings to change.

An exothermic reaction gives heat out to its surroundings.

The temperature of its surroundings increases.

Exothermic reactions include:

• combustion reactions.

• many oxidation reactions.

• most neutralisation reactions.

An endothermic reaction takes heat in from its surroundings.

The temperature of its surroundings decreases.

Endothermic reactions include:

• thermal decomposition reactions.

• the reaction of ethanoic acid and sodium carbonate.

What are energy level diagrams?

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

It shows the energy in the reactants and products, and the difference in energy between them.

In an exothermic reaction heat energy is given out to the surroundings.

This means the chemicals in the reaction will have a lower energy at the end of the reaction.

Since the energy level of the chemicals decreases, exothermic reactions have negative energy change values.

In an endothermic reaction heat energy is taken in from the surroundings.

This means the chemicals in the reaction will have a higher energy at the end of the reaction.

Since the energy level of the chemicals increases, endothermic reactions have positive energy change values.

A reaction profile is similar to an energy level diagram, but it includes the activation energy – the minimum energy that particles needed for a reaction to occur.

The activation energy is shown by a line called the ‘reaction pathway.’

Activation energy appears as a hump in the line.

The activation energy is equal to the difference in energy between the top of the hump and the energy of the reactants.

The overall change in energy is the difference between the energy of the reactants and products.

Energy is transferred when chemical bonds are broken or are formed.

• Breaking bonds requires energy (endothermic).

• Making bonds gives out energy (exothermic).

During a chemical reaction:

• bonds in the reactants are broken (endothermic) and

• new bonds are made in the products (exothermic).

For example, in the reaction between hydrogen and oxygen (2H₂ + O₂ → 2H₂O)

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 exothermic if more heat energy is released in making the bonds in the products, than is absorbed when breaking bonds in the reactants.

A reaction is endothermic if less heat energy is released in making bonds in the products than is absorbed when breaking bonds in the reactants.

Bond energy

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

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

A bond energy value can also tell you the amount of energy that is given out when one mole of a particular covalent bond is formed.

For example, the bond energy of a H-H bond is 436 kJ. This means that:

• 436 kJ of energy is taken in to break one mole of H-H bonds.

• 436 kJ of energy is given out when one mole of H-H bonds is formed.

To calculate an energy change for a reaction:

• add together the bond energies for all the bonds broken in the reactants.

• add together the bond energies for all the bonds formed in the products.

• energy change = energy for bonds broken – energy for bonds made.

Exothermic reactions have negative energy change values.

Endothermic reactions have positive energy change values.

Hydrogen and chlorine react to form hydrogen chloride gas:

H−H + Cl−Cl → 2 × (H−Cl)

Use the bond energies in the table to calculate the energy change for this reaction.

Bonds broken (1 × H-H, 1 × Cl-Cl) = 436 + 243 = 679 kJ

Bonds made (2 × H-Cl) = (2 × 432) = 864 kJ

Energy change = bonds broken – bonds made

= 679 - 864

= -185 kJ

The energy change is negative.

This shows that the reaction is exothermic.