Chemical reactions involve the breaking and forming of chemical bonds. Breaking a bond requires energy, which is 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
If the energy required to break the bonds in the reactants is less than the energy released by the formation of new bonds in the products, then the reaction is exothermic. This means that 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 reactions is endothermic. This means that the energy change is positive.
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:
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.
Hydrogen and chlorine react to form hydrogen chloride gas, represented by the chemical equation:
Hydrogen + Chlorine → Hydrogen chloride
H2 + O2 → 2HCl
The table below shows the bond energies for the reaction.
|Bond||Bond energy (KJ/mol)|
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.