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Combining energies for Edexcel GCSE Chemistry

Combining energies

This page covers the following topics:

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.

Bond energies

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.

ฮ”H(Hโˆ’H) = 436 kJ/mol
ฮ”H(O=O) = 495 kJ/mol
ฮ”H(Nโ‰กN) = 941 kJ/mol
ฮ”H(Nโˆ’H) = 391 kJ/mol
ฮ”H(Oโˆ’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 Hโˆ’H, one O=O bonds are broken and four Oโˆ’H bonds are made.
ฮ”H = 436 ร— 2 + 495 โˆ’ 463 ร— 4
ฮ”H = 872 + 495 โˆ’ 1852
ฮ”H = โˆ’485 kJ/mol

โˆ’485 kJ/mol

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.

ฮ”H(Hโˆ’H) = 436 kJ/mol
ฮ”H(O=O) = 495 kJ/mol
ฮ”H(Nโ‰กN) = 941 kJ/mol
ฮ”H(Nโˆ’H) = 391 kJ/mol
ฮ”H(Oโˆ’H) = 463 kJ/mol

2

How many bonds are broken during the reaction provided?

CHโ‚„ + 2Oโ‚‚ โ†’ COโ‚‚ + 2Hโ‚‚O

CHโ‚„ has 4 Cโˆ’H bonds and 2Oโ‚‚ has 2 O=O bonds; in total 4 + 2 = 6.

6

How many bonds are broken during the reaction provided?

CHโ‚„ + 2Oโ‚‚ โ†’ COโ‚‚ + 2Hโ‚‚O

3

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

2CHโ‚„ + Clโ‚‚ โ†’ 2CHโ‚ƒCl + Hโ‚‚

ฮ”H(Cโˆ’O) = 358 kJ/mol
ฮ”H(Cโˆ’H) = 413 kJ/mol
ฮ”H(Clโˆ’Cl) = 242 kJ/mol
ฮ”H(Cโˆ’Cl) = 328 kJ/mol
ฮ”H(Oโˆ’H) = 463 kJ/mol
ฮ”H(Hโˆ’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 Clโˆ’Cl, two Cโˆ’H bonds are broken and two Cโˆ’Cl bonds with one Hโˆ’H are made.
ฮ”H = 242 + 2 ร— 413 โˆ’ (328 ร— 2 + 436)
ฮ”H = 242 + 826 โˆ’ 1092
ฮ”H = โˆ’24 kJ/mol

โˆ’24 kJ/mol

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

2CHโ‚„ + Clโ‚‚ โ†’ 2CHโ‚ƒCl + Hโ‚‚

ฮ”H(Cโˆ’O) = 358 kJ/mol
ฮ”H(Cโˆ’H) = 413 kJ/mol
ฮ”H(Clโˆ’Cl) = 242 kJ/mol
ฮ”H(Cโˆ’Cl) = 328 kJ/mol
ฮ”H(Oโˆ’H) = 463 kJ/mol
ฮ”H(Hโˆ’H) = 436 kJ/mol

4

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

Nโ‚‚ + 3Hโ‚‚ โ†’ 2NHโ‚ƒ

ฮ”H(Hโˆ’H) = 436 kJ/mol
ฮ”H(O=O) = 495 kJ/mol
ฮ”H(Nโ‰กN) = 941 kJ/mol
ฮ”H(Nโˆ’H) = 391 kJ/mol
ฮ”H(Oโˆ’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 Hโˆ’H, one Nโ‰กN bonds are broken and six Nโˆ’H bonds are made.
ฮ”H = 3 ร— 436 + 941 โˆ’ 6 ร— 391 = 1308 + 941 โˆ’ 2346 = โˆ’97 kJ/mol

โˆ’97 kJ/mol

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

Nโ‚‚ + 3Hโ‚‚ โ†’ 2NHโ‚ƒ

ฮ”H(Hโˆ’H) = 436 kJ/mol
ฮ”H(O=O) = 495 kJ/mol
ฮ”H(Nโ‰กN) = 941 kJ/mol
ฮ”H(Nโˆ’H) = 391 kJ/mol
ฮ”H(Oโˆ’H) = 463 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.

ฮ”H(Hโˆ’H) = 436 kJ/mol
ฮ”H(Brโˆ’Br) = 193 kJ/mol
ฮ”H(Hโˆ’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 Hโˆ’H, one Brโˆ’Br bonds are broken and two Hโˆ’Br bonds are made.
ฮ”H = 436 + 193 โˆ’ 2 ร— 366 = 629 โˆ’ 732 = โˆ’103 kJ/mol

โˆ’103 kJ/mol

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.

ฮ”H(Hโˆ’H) = 436 kJ/mol
ฮ”H(Brโˆ’Br) = 193 kJ/mol
ฮ”H(Hโˆ’Br) = 366 kJ/mol

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