 # Combining energies for AQA GCSE Chemistry 1. Bond energies

The breaking of bonds is endothermic, while the formation of bonds is exothermic. Combining these two processes can yield both endothermic and exothermic reactions depending on whether more energy is required to break the bonds or is given when new ones are created.

The energy transferred in a chemical reaction can be calculated by subtracting total bond enthalpies of products from the total bond enthalpies of reactants. If multiple same bonds are broken or formed, multiples of the bond energies are used.

Mean bond enthalpy is and average of bond energies across various molecules. Mean bond enthalpy can be calculated by combining an energy change in a reaction and other bond energies, essentially working backwards to how energy change of a reaction is found. Mean bond enthalpies do not always match to a bond energy of an atom in a specific molecule or structure since factors such as bond strength and interactions with other particles can influence the individual values. # 1

Li has drawn a diagram that shows the bonds of both reactants and products of a reaction. Calculate the molar change of energy of the reaction by using the mean bond enthalpies provided below.

ΔH(H−H) = 436 kJ/mol
ΔH(O=O) = 495 kJ/mol
ΔH(N≡N) = 941 kJ/mol
ΔH(N−H) = 391 kJ/mol
ΔH(O−H) = 463 kJ/mol

The energy transferred in a chemical reaction can be calculated by subtracting total bond enthalpies of products from the total bond enthalpies of reactants. If multiple same bonds are broken or formed, multiples of the bond energies are used.

In this case two H−H, one O=O bonds are broken and four O−H bonds are made.
ΔH = 436 × 2 + 495 − 463 × 4
ΔH = 872 + 495 − 1852
ΔH = −485 kJ/mol

−485 kJ/mol # 2

How many bonds are broken during the reaction provided?

CH₄ + 2O₂ → CO₂ + 2H₂O

CH₄ has 4 C−H bonds and 2O₂ has 2 O=O bonds; in total 4 + 2 = 6.

6 # 3

Calculate the energy change per one mole of the reaction provided. Some of the mean bond enthalpies are provided below.

2CH₄ + Cl₂ → 2CH₃Cl + H₂

ΔH(C−O) = 358 kJ/mol
ΔH(C−H) = 413 kJ/mol
ΔH(Cl−Cl) = 242 kJ/mol
ΔH(C−Cl) = 328 kJ/mol
ΔH(O−H) = 463 kJ/mol
ΔH(H−H) = 436 kJ/mol

The energy transferred in a chemical reaction can be calculated by subtracting total bond enthalpies of products from the total bond enthalpies of reactants. If multiple same bonds are broken or formed, multiples of the bond energies are used.

In this case one Cl−Cl, two C−H bonds are broken and two C−Cl bonds with one H−H are made.
ΔH = 242 + 2 × 413 − (328 × 2 + 436)
ΔH = 242 + 826 − 1092
ΔH = −24 kJ/mol

−24 kJ/mol # 4

Find the energy change per one mole of the reaction provided. Some of the mean bond enthalpies are provided below.

N₂ + 3H₂ → 2NH₃

ΔH(H−H) = 436 kJ/mol
ΔH(O=O) = 495 kJ/mol
ΔH(N≡N) = 941 kJ/mol
ΔH(N−H) = 391 kJ/mol
ΔH(O−H) = 463 kJ/mol

The energy transferred in a chemical reaction can be calculated by subtracting total bond enthalpies of products from the total bond enthalpies of reactants. If multiple same bonds are broken or formed, multiples of the bond energies are used.

In this case three H−H, one N≡N bonds are broken and six N−H bonds are made.
ΔH = 3 × 436 + 941 − 6 × 391 = 1308 + 941 − 2346 = −97 kJ/mol

−97 kJ/mol # 5

Alice has drawn a diagram that shows the bonds of both reactants and products of a reaction. Calculate the molar change of energy of the reaction by using the mean bond enthalpies provided below.

ΔH(H−H) = 436 kJ/mol
ΔH(Br−Br) = 193 kJ/mol
ΔH(H−Br) = 366 kJ/mol

The energy transferred in a chemical reaction can be calculated by subtracting total bond enthalpies of products from the total bond enthalpies of reactants. If multiple same bonds are broken or formed, multiples of the bond energies are used.

In this case one H−H, one Br−Br bonds are broken and two H−Br bonds are made.
ΔH = 436 + 193 − 2 × 366 = 629 − 732 = −103 kJ/mol

−103 kJ/mol End of page