Bond Making & Breaking (Oxford AQA IGCSE Chemistry): Revision Note
Exam code: 9202
Bond Making & Breaking
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
In an exothermic reaction:
The energy released when new bonds are formed is greater than the energy taken in to break bonds
In an endothermic reaction:
The energy needed to break existing bonds is greater than the energy released when new bonds are formed
The energy level diagram below is for the reaction to produce ammonia from hydrogen and nitrogen
The energy needed to break the bonds is 2253 kJ
The energy released when new bonds are formed is 2346 kJ
The overall reaction is therefore exothermic because more energy was released when new bonds are formed
The overall energy change for the reaction is -93 kJ
Energy Level Diagram showing bond breaking and making
Bond Dissociation 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 energy 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 energy change using the equation:
Energy change = Energy taken in - Energy given out
Worked Example
Hydrogen reacts with iodine to form hydrogen iodide.
H2 + I2 ⟶ 2HI
The relevant bond energies are shown in the table below.
Bond | Bond Dissociation Energy (kJ) |
|---|---|
H–I | 295 |
H–H | 436 |
I–I | 151 |
Calculate the overall energy change for this reaction and use this value to explain why the reaction is endothermic.
Answer:
Calculate the energy in
436 + 151 = 587 (kJ)
Calculate the energy out
2 x 295 = 590 (kJ)
Calculate the energy change
587 - 590 = -3 (kJ)
The reaction is exothermic because:
More energy is released than taken in
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 | Bond Dissociation 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
Examiner Tips and Tricks
Remember: Exothermic reactions will have a negative overall energy change, endothermic reactions will have a positive overall energy change.
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