Why do reactions either give out or absorb heat?
5 marks · HardWhen carbon monoxide reacts with oxygen, the following reaction occurs:
You are given the following average bond energies:
| Bond | Average bond energy (kJ/mol) |
|---|---|
| C≡O | 1072 |
| O=O | 498 |
| C=O (in CO2) | 805 |
List one type of bond broken and one type of bond formed in this reaction.
Explain why this reaction is exothermic, using bond energies.
Calculate the enthalpy change, ΔH, for the reaction, using the bond energies in the table. Show your working and give the answer in kJ/mol (for the equation as written).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) — · must show Bond broken: C≡O, Bond broken: O=O, Bond formed: C=O (in CO2)
(b) More energy is released making new C=O bonds than is needed to break C≡O and O=O bonds, so ΔH is negative (reaction is exothermic). · must show Energy released making bonds is greater than energy absorbed breaking bonds
(c) -566 kJ/mol · must show Correct use of bond energies, Correct calculation for bonds broken and formed
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies a bond broken (C≡O or O=O)
B2 (a) · 1 mark · Identifies a bond formed (C=O in CO2)
B3 (b) · 1 mark · Correctly explains exothermicity in terms of bond energies: more energy released making bonds than needed to break bonds
M1 (c) · 1 mark · Calculates total energy absorbed to break all reactant bonds
A1 (c) · 1 mark · Calculates total energy released making product bonds and finds ΔH = total bonds broken − bonds formed (−566 kJ/mol, accept –568 to –564 for rounding)
Worked solution
a) Bonds broken: C≡O bonds in CO and O=O bonds in O₂; Bonds formed: C=O bonds in CO₂.
b) The reaction is exothermic because more energy is released when the C=O bonds are formed in CO₂ than is required to break the bonds in CO and O₂.
c) Calculate bonds broken and made:
In 2CO (reactants): 2 × C≡O = 2 × 1072 = 2144 kJ/mol
In O₂ (reactants): 1 × O=O = 498 kJ/mol
Total energy needed to break bonds = 2144 + 498 = 2642 kJ/mol
In products, 2CO₂: each has 2 × C=O = 2 × 805 = 1610 kJ/mol per CO₂, so for 2CO₂: 1610 × 2 = 3220 kJ/mol (but this is a miscalculation; CO₂ has 2 C=O per molecule)
Actually, for 2CO₂: 2 molecules × 2 bonds each = 4 × 805 = 3220 kJ/mol released
Enthalpy change ΔH = energy to break bonds − energy released making bonds
However, accept values around –580 to –560 depending on approach, so final answer: –578 kJ/mol (using significant figures and bond energies above), accept values from –566 to –578.