Covalent Bonding
5 marks · EasyHydrogen (H) and carbon dioxide (CO) are both covalent molecules. Calculate the relative molecular mass (M) of each molecule. (Relative atomic masses: H = 1, C = 12, O = 16)
Which of these molecules would you expect to have a higher boiling point? Explain your answer in terms of molecular mass and intermolecular forces.
State the type of bonding present in both hydrogen and carbon dioxide molecules.
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(a) H: 2 × 1 = 2; CO: 12 + 2 × 16 = 44 · must show H2 calculation: 2, CO2 calculation: 44
(b) CO has a higher molecular mass, so its intermolecular forces are stronger, meaning it will have a higher boiling point than H.
(c) Covalent.
Common mistakes: Confusing relative atomic mass and molecular mass; Mistaking intermolecular for covalent bonds
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Marking points
M1 (a) · 1 mark · Calculates M_r of H2 correctly as 2.
A1 (a) · 1 mark · Calculates M_r of CO2 correctly as 44.
B1 (b) · 1 mark · Identifies CO2 will have higher boiling point due to greater molecular mass.
A2 (b) · 1 mark · Explains higher boiling point is due to stronger intermolecular forces caused by larger molecular mass.
B2 (c) · 1 mark · States 'covalent' as the type of bond in both molecules.
Worked solution
a) H2: 2 × 1 = 2; CO2: 12 + (2 × 16) = 12 + 32 = 44
b) Carbon dioxide (CO2) would have a higher boiling point because it has a higher relative molecular mass. Higher molecular mass leads to stronger intermolecular forces, so more energy is needed to boil CO2.
c) Both molecules contain covalent bonds.