Question

The following is a table of bond energies.

bond	Bond energy/ kj per moles
C-C	347
C-H	413
C=O	805
O=O	498
O-H	464

Use the values given to calculate the heat of combustion of propane. The equation for combustion of propane is ${{C}_{3}}{{H}_{8}}(g)+5{{O}_{2}}(g)\to 3C{{O}_{2}}(g)+4{{H}_{2}}O(g)$

Answer 1

Hint: The bond energy or the bond enthalpy is visualized as the average amount of energies required or produced when a bond in the chemical compound is broken or formed during a reaction.

Complete step by step solution:
-Bond energy gives us an idea about the strength of a chemical bond and therefore it tells us about the stability of the compound which means that more energy is required to break the bond it means that the compound is more stable and similarly if less energy is required to break the bond it means that the compound is less stable.
-Given in the question: ${{C}_{3}}{{H}_{8}}(g)+5{{O}_{2}}(g)\to 3C{{O}_{2}}(g)+4{{H}_{2}}O(g)$
In the reactant side, in propene there are 2 C-C and 8 C-H bonds and in the oxygen there are 5 O=O present. The value of C-C , C-H and O=O is mentioned in the given table as 347, 413 and 498 respectively. The bond energy required to break the bond in the reactant side = $2(347)+8(413)+5(498)=+6488$ kj/mol
-In the product side the energy is released after the bond breaking, in the product side there are 6 C=O bond present and 8 O-H bonds are present. The value of C=O, O-H is mentioned in the table as 805 and 464 respectively
-The bond energy released in the product side will be =$-6(805)-8(464)=-8536$ kj/mol
The heat of combustion will be = $-8536+6488=-2048$ kj/mol

Note: When the bond in a chemical compound is broken then there is a need for energy hence the bond energy will have a positive value and when the bond in a chemical compound is formed then there will be release in energy hence the bond energy will have a negative value.