Bond energy is the energy to break one mole of a bond
Definition
Bond energy
The energy needed to break one mole of a particular bond, measured in kJ mol-1.
- Each type of bond has its own value, measured in kJ mol−1\text{kJ mol}^{-1}kJ mol−1.
- A larger value means a stronger bond that takes more energy to break.
- The same value is released when that bond is formed.
- Values are always supplied in a question, since they are measured quantities.
- Every bond in the molecule counts, so CH4\text{CH}_4CH4 contains four carbon to hydrogen bonds.
Key Idea
One value serves both directions: energy in to break the bond, the same energy out to make it.
The calculation is bonds broken minus bonds made
- Count every bond in the reactants and add their bond energies together.
- Count every bond in the products and add their bond energies together.
- The energy change is the difference between the two totals: energy change=energy in to break bonds−energy out in making bonds\text{energy change} = \text{energy in to break bonds} - \text{energy out in making bonds}energy change=energy in to break bonds−energy out in making bonds
- The balanced equation fixes how many of each molecule to count.
- The answer carries the unit kJ mol−1\text{kJ mol}^{-1}kJ mol−1.
Common Mistake
The subtraction runs broken minus made, and reversing it reverses the sign of the answer.
A worked example: hydrogen and chlorine
- The equation is: H2+Cl2→2HCl\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}H2+Cl2→2HCl
- The bond energies given are 436436436 for H−H\text{H}-\text{H}H−H, 242242242 for Cl−Cl\text{Cl}-\text{Cl}Cl−Cl and 431431431 for H−Cl\text{H}-\text{Cl}H−Cl.
- The bonds broken are one of each in the reactants: 436+242=678 kJ mol−1436 + 242 = 678\ \text{kJ mol}^{-1}436+242=678 kJ mol−1
- The bonds made are two H−Cl\text{H}-\text{Cl}H−Cl bonds: 2×431=862 kJ mol−12 \times 431 = 862\ \text{kJ mol}^{-1}2×431=862 kJ mol−1
- Subtracting gives the energy change: 678−862=−184 kJ mol−1678 - 862 = -184\ \text{kJ mol}^{-1}678−862=−184 kJ mol−1
Example
The balancing number matters: 2HCl2\text{HCl}2HCl means the H−Cl\text{H}-\text{Cl}H−Cl value is counted twice.
Reading the sign of the answer
- A negative answer means more energy was released than taken in, so the reaction is exothermic.
- A positive answer means more energy was taken in than released, so it is endothermic.
- The example above gives −184 kJ mol−1-184\ \text{kJ mol}^{-1}−184 kJ mol−1, so that reaction is exothermic.
- The size of the number says how much energy moved, and the sign says which way.
- A conclusion states the direction as well as the figure.
Note
The sign is part of the answer, so an exothermic value is written with its minus sign.
Where these calculations go wrong
- Forgetting a balancing number undercounts the bonds on one side.
- Missing a bond inside a molecule, such as the four bonds in methane, does the same.
- Subtracting the wrong way round gives an answer with the wrong sign.
- Leaving the unit off, or leaving the sign off, loses part of the answer.
- Checking that an exothermic reaction came out negative catches most of these.
Exam technique
- Listing the bonds on each side before any arithmetic makes a miscount easy to spot.
- The two totals are written down separately, so the subtraction can be checked.
- A final answer carries a number, a sign and a unit.
Self review
- What does a bond energy of 436 kJ mol−1436\ \text{kJ mol}^{-1}436 kJ mol−1 mean?
- Write the expression used to calculate the energy change of a reaction.
- How many carbon to hydrogen bonds are broken in one molecule of methane?
- An energy change comes out as +92 kJ mol−1+92\ \text{kJ mol}^{-1}+92 kJ mol−1. Is the reaction exothermic or endothermic?
- Using 436436436, 242242242 and 431431431, calculate the energy change for H2+Cl2→2HCl\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}H2+Cl2→2HCl.
