Energetics: Bond Energies and Enthalpy Change

Calculate the change in bond enthalpy in kJ mol^-1 for this reaction, using the bond enthalpies below.

C-H 412 kJ/mol

C=C 612 kJ/mol

Br-Br 193 kJ/mol

C-C 348 kJ/mol

C-Br 276 kJ/mol

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking.

2(612) + 1(348) + 6(412) + 2(193) = 4430 kJ/mol

2. Calculate the total energy involved in bond making.

3(348) + 6(412) + 4(276) = 4620 kJ/mol

3. Calculate the change in energy by subtracting products from reactants.

ΔH = 4430 - 4620

ΔH = -190 kJ mol^-1

Calculate the change in bond enthalpy in kJ mol^-1 for this reaction, using the bond enthalpies below.

O-H 463 kJ/mol

O-O 143 kJ/mol

O=O 498 kJ/mol

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking.

4(463) + 2(143) = 2138 kJ/mol

2. Calculate the total energy involved in bond making.

4(463) + 498 = 2350 kJ/mol

3. Calculate the change in energy by subtracting products from reactants.

ΔH = 2138- 2350

ΔH = -212 kJ mol^-1

Calculate the change in bond enthalpy in kJ mol^-1 for this reaction, using the bond enthalpies below.

O-H 463 kJ/mol

O-O 143 kJ/mol

N-N 159 kJ/mol

N-H 391 kJ/mol

N≡N 945 kJ/mol

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking.

[4 x N-H = 4(391)] + [1 x N-N = 159] + [2 x O-O = 2(143)] + 4 x O-H = 4(463)] = 3861 kJ/mol

2. Calculate the total energy involved in bond making.

[1 x N≡N = 945] + 8 x O-H = 8(463)] = 4649 kJ/mol

3. Calculate the change in energy by subtracting products from reactants.

ΔH = 3861 - 4649

ΔH = -788 kJ mol^-1

Calculate the bond strength of the Cl-Cl bond using the bond enthalpies below.

H₂ + Cl₂ --> 2HCl ΔH = -184 kJ/mol

H-H 436 kJ/mol

H-Cl 431 kJ/mol

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking (endothermic, so positive).

2. Calculate the total energy involved in bond making (exothermic, so negative).

3. Calculate the change in energy by adding both numbers together.

You do not need to include a sign in your answer.

Answer explanation

1. Calculate the total energy involved in bond breaking (endothermic so positive number).

2. Calculate the total energy involved in bond making (exothermic so negative number).

3. Calculate the change in energy by adding both numbers together.

4. This is for four C-H bonds so remember to divide this number by four to get the average bond energy.

Calculate the bond strength of the enthalpy change of reaction using the bond enthalpies below.

H₂ + F₂ --> 2HF ΔH = ?

H-H 436 kJ/mol

F-F 158 kJ/mol

H-F 562 kJ/mol

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking (bond breaking is endothermic, so positive).

(436 + 158) = +594 kJ mol

2. Calculate the total energy involved in bond making (makes atoms more stable, exothermic because releases energy, so negative).

2 x (-562) = -1124 kJ mol^-1

3. Calculate the change in energy by adding both numbers together.

594 + (-1124) = -530 kJ mol^-1

Calculate the enthalpy change of reaction using the bond enthalpies in the table.

2H₂ + O₂ --> 2H₂O ΔH = ?

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking (bond breaking is endothermic, so positive).

2 x 432 + 1 x 495 = 1359 kJ mol^-1

2. Calculate the total energy involved in bond making (makes atoms more stable, exothermic because releases energy, so negative).

2 x 2 x 467 = -1868 kJ mol^-1

3. Calculate the change in energy by adding both numbers together.

1359 + (-1868) = -509 kJ mol^-1

Calculate the enthalpy change of reaction using the bond enthalpies in the table.

2H₂O --> 2H₂ + O₂ ΔH = ?

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking (bond breaking is endothermic, so positive).

2 x 2 x 467 = -1868 kJ mol^-1

2. Calculate the total energy involved in bond making (makes atoms more stable, exothermic because releases energy, so negative).

2 x 432 + 1 x 495 = 1359 kJ mol^-1

3. Calculate the change in energy by adding both numbers together.

-1359 + 1868 = +509 kJ mol^-1

Calculate the enthalpy change of reaction using the bond enthalpies in the table.

C-C 347 kJ/mol

C=C 614 kJ/mol

C-H 413 kJ/mol

C-Cl 339 kJ/mol

Cl-Cl 239 kJ/mol

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking (bond breaking is endothermic, so positive).

1 x C=C (1 x 614) + 4 x C-H (4 x 413) + 1 x Cl-Cl (1 x 239) = +2505 kJ/mol

2. Calculate the total energy involved in bond making (makes atoms more stable, exothermic because releases energy, so negative).

1 x C-C (1 x 347) + 4 x C-H (4 x 413) + 2 x Cl-Cl (2 x 339) = -2677 kJ/mol

3. Calculate the change in energy by adding both numbers together.

2505 + (-2677) = -172 kJ mol^-1

Calculate the enthalpy change of reaction using the bond enthalpies in the table.

CH₄ (g) + H₂O (g) --> C≡O (g) + 3H₂ (g)

Include a sign in your answer (+ or -), even if the answer is positive.

Answer explanation

1. Calculate the total energy involved in bond breaking (bond breaking is endothermic, so positive).

4 x C-H (4 x 413) + 2 x O-H (2 x 467) = +218 kJ/mol

2. Calculate the total energy involved in bond making (makes atoms more stable, exothermic because releases energy, so negative).

1 x C≡O (1 x 1072) + 3 x H-H ( 3 x 432) = -2368 kJ/mol

3. Calculate the change in energy by adding both numbers together.

2586 + (-2368) = +218 kJ mol^-1