### GCSE Chemistry

Quantitative Chemistry
Chemical Changes
Energy Changes
Organic Chemistry
Chemical Analysis
Earth and Atmospheric Science
Using Resources

# Energy Changes in Chemical Reactions

Chemical reactions involve the breaking and forming of chemical bonds. Breaking a bond requires energy, making it endothermic, whereas forming bonds releases energy, which is exothermic. Each chemical bond has an associated energy value, which tells us how much energy is required to break the bond.

We can calculate the total energy change to determine if a chemical reaction is exothermic or endothermic. To do this, we need to know the different bonds in the reaction and the bond energy for each bond, measured in kilojoules per mole (KJ/mol).

To calculate the energy change for the reaction:

1. Add up the bond energies for all the bonds in the reactants. This is the “energy in.”

2. Add up the bond energies for all the bonds in the products. This is the “energy out.”

3. Use the equation: Energy change = Energy in – Energy out

f the energy released by the formation of new bonds in the products is greater than the energy required to break the bonds in the reactants, then the reaction is exothermic. This means the energy change is negative.

In contrast, if the energy required to break the bonds in the reactants is greater than the energy released by the formation of new bonds in the products, then the reaction is endothermic. This means that the energy change is positive.

## Example

Consider the reaction of burning methane gas in air. The balanced equation is:

CH4 + 2O2 → CO2 + 2H2O

The bond energies for the individual bonds involved in this reaction are:

• C-H bond energy in methane (CH4): 435 kJ/mol
• O=O bond energy in oxygen (O2): 498 kJ/mol
• C=O bond energy in carbon dioxide (CO2): 799 kJ/mol
• O-H bond energy in water (H2O): 464 kJ/mol

Using these bond energies, we can calculate the energy change for this reaction:

Energy in = 1(4 x 435 kJ/mol) + 2(2 x 498 kJ/mol) = 2274 kJ/mol

Energy out = 1(2 x 799 kJ/mol) + 2(2 x 464 kJ/mol) = 2592 kJ/mol

Energy change = 2274 kJ/mol – 2592 kJ/mol = -318 kJ/mol

Since the energy change is negative, this reaction is exothermic.

## Example

Hydrogen and chlorine react to form hydrogen chloride gas, represented by the chemical equation:

Hydrogen + Chlorine → Hydrogen chloride

H2 + Cl2 → 2HCl

The table below shows the bond energies for the reaction.

Use the bond energies in the table to calculate the energy change for the reaction. Then find out whether the reaction is exothermic or endothermic.

To calculate the energy change for the reaction, we follow these steps:

1. Calculate the total energy required to break the bonds in the reactants (H2 and Cl2). This is the “energy in” and is equal to the sum of the bond energies for H−H and Cl−Cl, which are 436 KJ/mol and 243 KJ/mol respectively.

Energy in = 436 KJ/mol + 243 KJ/mol = 679 KJ/mol

2. Calculate the total energy released when the bonds in the products (HCl) are formed. This is the “energy out” and is equal to the bond energy for H−Cl, which is 432 KJ/mol, multiplied by the number of HCl molecules produced in the reaction, which is 2.

Energy out = 2 × 432 KJ/mol = 864 KJ/mol

3. Calculate the energy change for the reaction by subtracting the energy out from the energy in:

Energy change = Energy in – Energy out

= 679 – 864

= -185 KJ/mol

As the energy change is negative, the reaction is exothermic. So, the reaction releases energy to the surroundings.

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