Bond Energy (Edexcel IGCSE Chemistry (Modular))
Revision Note
Bond energies
During a chemical reaction energy must be taken in to break bonds
Because energy is being taken in, bond breaking is an endothermic process
During a chemical reaction, energy is released when new bonds are formed
Because energy is released, bond making is an exothermic process
Whether a reaction is endothermic or exothermic overall depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed
Exothermic reactions
In an exothermic reaction:
The energy released when new bonds are formed is greater than the energy taken in to break bonds
The change in energy is negative since the reactants have more energy than the products
Therefore an exothermic reaction has a negative ΔH value
Making new bonds gives off heat from the reaction to the surroundings
Endothermic reactions
In an endothermic reaction:
The energy needed to break existing bonds is greater than the energy released when new bonds are formed
The change in energy is positive since the products have more energy than the reactants
Therefore an endothermic reaction has a positive ΔH value
Examiner Tip
Remember bond breaking is ENDothermic and results in the END of the bond.
Bond energy calculations
Each chemical bond has a specific bond energy associated with it
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed
This energy can be used to calculate how much heat would be released or absorbed in a reaction
To do this it is necessary to know the bonds present in both the reactants and products
We can calculate the total change in enthalpy for a reaction if we know the bond energies of all the species involved
Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
Add together the bond energies for all the bonds in the products – this is the ‘energy out’
Calculate the enthalpy change using the equation:
Enthalpy change (ΔH) = Energy taken in - Energy given out
Worked Example
Hydrogen and chlorine react to form hydrogen chloride gas:
H–H + Cl–Cl ⟶ H–Cl H–Cl
The bond energies are given in the table below.
Bond | Energy (kJ) |
|---|---|
H–H | 436 |
Cl–Cl | 242 |
H–Cl | 431 |
Calculate the overall energy change for this reaction and use this value to explain whether the reaction is exothermic or endothermic.
Answer:
Calculate the energy in
436 + 242 = 678 (kJ)
Calculate the energy out
2 x 431 = 862 (kJ)
Calculate the energy change
678 - 862 = –184 (kJ)
Since the energy change is a negative number, energy is being released (to the surroundings)
Therefore, the reaction is exothermic
Worked Example
Hydrogen bromide decomposes to form hydrogen and bromine:
2HBr ⟶ H2 + Br2
The overall energy change for this reaction is +103 kJ.
The relevant bond energies are shown in the table below.
Bond | Energy (kJ) |
|---|---|
H–Br | 366 |
Br–Br |
|
H–H | 436 |
Calculate the bond energy of the Br–Br bond.
Answer:
Calculate the energy in
2 x 366 = 732 (kJ)
State the energy out
436 + Br–Br
Overall energy change = energy in - energy out
+103 = 732 - (436 + Br–Br)
+103 = 732 - 436 - Br–Br
Calculate the bond energy of the Br–Br bond
Br–Br = 732 - 436 - 103
Br–Br = +193 (kJ)
Examiner Tip
For bond energy questions, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes. Don't forget to take into account the balancing numbers when working out how many of each type of bond is being broken/formed.
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