Every time you snap a glow stick to produce light, you are witnessing the energy changes that occur when molecules break apart and form new structures. During any chemical reaction, existing chemical bonds must be broken, and entirely new bonds must be formed.
The overall energy change of a chemical reaction is simply the net difference between the energy taken in to break bonds and the energy given out when making them.
We can calculate the exact energy change of a reaction using values called bond energies. A bond energy is the mean amount of energy required to break one mole of a specific covalent bond in its gaseous state.
To find the enthalpy change (), which is the total energy change of the reaction, we use the stoichiometry (balancing numbers) from the chemical equation to multiply the specific bond energies. The standard unit for these calculations is kJ/mol.
The overall energy change is calculated using the following formula:
Calculate the overall energy change for the reaction between hydrogen and chlorine and state whether the reaction is exothermic or endothermic.
Equation:
Bond energies: , ,
Step 1: List and count all bonds broken in the reactants and calculate the total energy taken in.
Step 2: List and count all bonds made in the products and calculate the total energy released.
Step 3: Calculate the overall energy change using the formula.
Step 4: Identify the reaction type based on the sign of the final answer.
Students often try to explain energy changes by counting the total number of bonds made or broken, but examiners will only award marks for comparing the quantity of energy taken in versus released.
Always draw out the displayed formulas of every molecule in the equation; this allows you to physically cross out or count every single bond so you do not miss any multipliers.
A frequent calculation error is reversing the formula by doing 'Products minus Reactants' — remember the mnemonic 'Break minus Make' to ensure you always subtract the bonds formed from the bonds broken.
If you make an arithmetic error when calculating the reactant or product totals, you can still gain 'Error Carried Forward' (ECF) marks if you substitute your incorrect totals correctly into the final subtraction formula.
Bond breaking
An endothermic process where energy must be supplied from the surroundings to overcome the attractive forces between atoms.
Bond making
An exothermic process where energy is released to the surroundings as new chemical bonds form.
Activation energy (Ea)
The minimum energy required to break the initial bonds in reactant molecules so a chemical reaction can begin.
Exothermic reaction
A reaction where more energy is released when making bonds than is required to break existing bonds, resulting in a negative overall energy change.
Endothermic reaction
A reaction where more energy is required to break bonds than is released when making new bonds, resulting in a positive overall energy change.
Bond energy
The mean amount of energy required to break one mole of a particular covalent bond.
Enthalpy change (ΔH)
The overall change in energy of a reaction, representing the difference between the energy taken in and the energy given out.
Put your knowledge into practice — try past paper questions for Chemistry A
Bond breaking
An endothermic process where energy must be supplied from the surroundings to overcome the attractive forces between atoms.
Bond making
An exothermic process where energy is released to the surroundings as new chemical bonds form.
Activation energy (Ea)
The minimum energy required to break the initial bonds in reactant molecules so a chemical reaction can begin.
Exothermic reaction
A reaction where more energy is released when making bonds than is required to break existing bonds, resulting in a negative overall energy change.
Endothermic reaction
A reaction where more energy is required to break bonds than is released when making new bonds, resulting in a positive overall energy change.
Bond energy
The mean amount of energy required to break one mole of a particular covalent bond.
Enthalpy change (ΔH)
The overall change in energy of a reaction, representing the difference between the energy taken in and the energy given out.