Question

Which statement about the change in bond energy of this reaction is correct?

Answer 1

Step 1

The equation to calculate ΔH (change of enthalpy) is:

`\Delta H=\text{ }\Sigma(bonds\text{ broken\rparen-}\Sigma(bonds\text{ formed\rparen}`

(In the absence of standard formation enthalpies)

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Step 2

The reaction provided:

CH4 + 4 Cl2 => CCl4 + 4 HCl

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Step 3

Procedure: from step 1,

ΔH = [H(4H-C) + H(Cl-Cl)] - [H(4C-Cl) + H(4H-Cl)]

ΔH = [(4x411 + 4x242) - (4x327 + 4x427)] kJ/mol = - 404

ΔH = - 404 kJ/mol < 0 => Exothermic

Answer: The reaction is exothermic because the total bond energy of the reactants is less than the total bond energy of the products.