The heat of reaction for a chemical reaction can be calculated by finding the su

Question

The heat of reaction for a chemical reaction can be calculated by finding the sum of the bond energies of the products and subtracting that from the sum of the bond energies of the reactants:

Heat of reaction= Sum of the energy for the bonds broken Sum of the energy for the bonds formed

= Sum of reactant bond energies Sum of product bond energies

When calculating the sum of the bond energies, each bond in the reaction must be accounted for. For example, CH4 is a reagent with a coefficient of 1 in the reaction. There are four CH bonds in methane and one methane molecule per reaction, for a total of four CH bonds on the reactant side. All four bonds must be accounted for when finding the sum of the bond energies for the reactants.

Calculate the heat of reaction using the average bond dissociation energies given in the introduction and your answer to Part B for the reaction

CH4 + 2O2 CO2 + 2H2O

Express your answer in kilojoules per mole to three significant figures.

Explanation / Answer

As mentioned in the question,
Sum of the energy for the bonds broken Sum of the energy for the bonds formed
                       = Sum of reactant bond energies Sum of product bond energies

So heat of the reaction will be equal to sum of reactant bond energies sum of product bond energies

Energy of bonds: C—H 357.98 kJ/mol
O2 494.04 kJ/mol
H2O 460,2 kJ/mol
CO2 732 kJ/mol

So heat of the reaction is
= (4x357.98 + 2x494.04) - (2x460.2 + 732 ) = 767.6 kJ/mol

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