Bond Energy Calculations Exam Questions

Bonds broken:

• 2 x H-H = 2 x 436 = 872 kJ
• 1 x O=O = 498 kJ
• Total energy in = 872 + 498 = 1370 kJ [1]

Bonds formed:

• 4 x O-H = 4 x 464 = 1856 kJ [1]
• (2 molecules of H2O, each with 2 O-H bonds)

Delta H = energy to break bonds - energy to form bonds [1] = 1370 - 1856 = -486 kJ [1]

Examiner note: Count bonds carefully: 2H2O has FOUR O-H bonds, not two. The negative sign means exothermic. Formula: delta H = sum of bonds broken - sum of bonds formed.

Bonds broken:

• 4 x C-H = 4 x 413 = 1652 kJ (but only 2 C-H bonds are actually broken — see note)

Correct approach — consider only bonds that change:

• 2 x C-H = 826 kJ
• 2 x Cl-Cl = 486 kJ
• Total broken = 1312 kJ [1]

Bonds formed:

• 2 x C-Cl = 692 kJ
• 2 x H-Cl = 864 kJ
• Total formed = 1556 kJ [1]

Delta H = 1312 - 1556 [1] = -244 kJ [1]

Alternative (breaking ALL bonds in reactants and forming ALL bonds in products): Broken: 4(C-H) + 2(Cl-Cl) = 1652 + 486 = 2138 kJ Formed: 2(C-H) + 2(C-Cl) + 2(H-Cl) = 826 + 692 + 864 = 2382 kJ Delta H = 2138 - 2382 = -244 kJ (same answer)

Examiner note: Both methods give the same answer. The safer approach at IGCSE level is to break ALL bonds in reactants and form ALL bonds in products. This avoids errors in deciding which bonds change.

Bonds broken:

• 4 x C-H = 1652 kJ
• 2 x O=O = 996 kJ
• Total = 2648 kJ [1]

Bonds formed:

• 2 x C=O = 1610 kJ
• 4 x O-H = 1856 kJ
• Total = 3466 kJ [1]

Delta H = 2648 - 3466 [1] = -818 kJ [1]

Examiner note: CO2 has two C=O double bonds. H2O has two O-H bonds. Count all bonds in each molecule, then multiply by the number of molecules. The answer is negative (exothermic), consistent with combustion.

(a) Delta H = 1200 - 950 = +250 kJ [1]

(b) Endothermic [1]

(c) Products drawn higher than reactants, with an upward arrow labelled +250 kJ [1]

Examiner note: When bonds broken > bonds formed in energy terms, the reaction absorbs energy overall. Delta H is positive = endothermic. More energy is needed to break the old bonds than is released by forming the new ones.

• Combustion involves burning a fuel in oxygen, breaking bonds in the fuel and O2 [1]
• New bonds are formed in the products (CO2 and H2O), which are very strong bonds (C=O and O-H) [1]
• The energy released by forming these strong bonds is greater than the energy needed to break the bonds in the reactants, so the overall process releases energy [1]

Examiner note: The key insight is that CO2 and H2O have particularly strong bonds (C=O = 805 kJ/mol, O-H = 464 kJ/mol). This is why combustion products are very stable and combustion is always exothermic.

(a) Bonds broken: 1 x N≡N + 3 x H-H = 945 + 1308 = 2253 kJ [1] Bonds formed: 6 x N-H = 6 x 391 = 2346 kJ [1] Delta H = 2253 - 2346 = -93 kJ [1]

(b) Exothermic [1]

Examiner note: 2NH3 has a total of 6 N-H bonds (3 per molecule x 2 molecules). The N≡N triple bond is very strong (945 kJ/mol), which is why this reaction has a high activation energy and needs an iron catalyst and high temperature industrially (the Haber process).

• The student is correct that C=C (614) is stronger than C-C (346), and C=O (805) is stronger than C-O (358) [1]
• However, a C=C double bond is not twice as strong as a C-C single bond (2 x 346 = 692, but C=C = 614) [1]
• This means the second bond in a double bond is weaker than the first, which is why double bonds are more reactive / easier to break open for addition reactions [1]

Examiner note: This is an evaluation question testing deeper understanding. The fact that C=C < 2 x C-C means the pi bond is weaker than the sigma bond. This explains the reactivity of alkenes compared to alkanes.