5.5.1.3 The energy change of reactions
(Bond energy calculations)
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During a chemical reaction,
bonds are broken in the reactants and bonds are formed in the products.
Energy must be supplied to break the bonds in the reactants.
Bond breaking is an endothermic process.
Energy is transferred to the surroundings when bonds in products are formed.
Bond making is an exothermic process.
Consider the reaction between hydrogen and chlorine react to form hydrogen chloride gas.
H2 + Cl2 ⟶ 2HCl
Working out overall energy change:
Overall energy change = breaking bonds total energy – making bonds total energy
Worked example 1:
Hydrogen and chlorine react to form hydrogen chloride gas.
Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic. (6)
Word equation: hydrogen + chlorine --> hydrogen chloride
Symbol equation: H2 + Cl2 ⟶ 2HCl
H – H + Cl – Cl --> 2 H – Cl
The table below shows the bond energies.
Step 1. Calculate energy taken in when breaking bonds (1)
(1 x H - H) + (1 x Cl - Cl)
436 + 243 = 679 KJ/mol
Step 2. Calculate energy released from making bonds (1)
(2 x H - Cl)
2 x 432 = 864 KJ/mol
Step 3. Calculate total energy change (1)
679 – 864 = - 185 KJ/mol
Step 4. Explain in terms of energy transfer if this reaction is endothermic or exothermic. (3)
Think: 679 < 864
So, energy taken in when breaking bonds < energy released from making bonds
Answer:
1. the energy released from making bonds is higher than energy taken in when breaking bonds
2. so, overall more energy has been lost to the surrounding
3. the reaction is exothermic.
(Hint: When energy change is a negative number, the reaction is exothermic.)
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