Question. Write the expression for Kc, and give its units, for each equilibrium.(a) 2SO2(g) + O2(g) ⇌ 2SO3(g) (2)
(b) PCl5(g) ⇌ PCl3(g) + Cl2(g) (2)
(c) 2HI(g) ⇌ H2(g) + I2(g) (2)Model answer.(a) Kc = [SO3]2 / [SO2]2[O2]
Units: (mol dm−3)2 / (mol dm−3)3 = (mol dm−3)−1 = dm3 mol−1(b) Kc = [PCl3][Cl2] / [PCl5]
Units: (mol dm−3)2 / mol dm−3 = mol dm−3(c) Kc = [H2][I2] / [HI]2
Units: (mol dm−3)2 / (mol dm−3)2, which cancel, so no units.Examiner's comments.• Use square brackets. Round brackets or amounts in moles lose the mark.
• Products go on the top. In (c) the product is H2 and I2, so this expression is the inverse of the one for H2 + I2 ⇌ 2HI.
• Balancing numbers become powers, not multipliers: write [SO3]2, not 2[SO3].
• A quick check for units: the power of (mol dm−3) is (moles of gas on the right) minus (moles of gas on the left). For (a) this is 2 − 3 = −1, giving dm3 mol−1.
• If the question says 'no units', write that; do not leave the line blank.
(b) PCl5(g) ⇌ PCl3(g) + Cl2(g) (2)
(c) 2HI(g) ⇌ H2(g) + I2(g) (2)Model answer.(a) Kc = [SO3]2 / [SO2]2[O2]
Units: (mol dm−3)2 / (mol dm−3)3 = (mol dm−3)−1 = dm3 mol−1(b) Kc = [PCl3][Cl2] / [PCl5]
Units: (mol dm−3)2 / mol dm−3 = mol dm−3(c) Kc = [H2][I2] / [HI]2
Units: (mol dm−3)2 / (mol dm−3)2, which cancel, so no units.Examiner's comments.• Use square brackets. Round brackets or amounts in moles lose the mark.
• Products go on the top. In (c) the product is H2 and I2, so this expression is the inverse of the one for H2 + I2 ⇌ 2HI.
• Balancing numbers become powers, not multipliers: write [SO3]2, not 2[SO3].
• A quick check for units: the power of (mol dm−3) is (moles of gas on the right) minus (moles of gas on the left). For (a) this is 2 − 3 = −1, giving dm3 mol−1.
• If the question says 'no units', write that; do not leave the line blank.