Calorimetry experiments for determining the enthalpy changes of reactions
3 marks · NormalIn a calorimetry experiment, 1.25 g of magnesium reacts completely with excess hydrochloric acid. The temperature of the 100 g acid solution rises by 9.8°C. The specific heat capacity of the solution is 4.18 J/g°C. Calculate the molar enthalpy change (ΔH) of this reaction in kJ/mol. The relative atomic mass of magnesium, Mg, is 24.
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-41 kJ/mol (±1)
Also accept: -41.8 kJ/mol; -42 kJ/mol
Common mistakes: not converting J to kJ; not using correct molar mass
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Marking points
M1 · 1 mark · Calculates heat released: Q = 100 × 4.18 × 9.8 = 4096 J
M2 · 1 mark · Converts Q to kJ and calculates the number of moles of Mg: n = 1.25 / 24 = 0.0521 mol
A1 · 1 mark · Calculates ΔH = Q/n = 4.096/0.0521 = 78.6; answer must be negative, so ΔH = -79 kJ/mol (accept answers in range -41 to -42 if correct division and sign). (follow-through)
Worked solution
Q = mcΔT = 100 × 4.18 × 9.8 = 4096 J = 4.096 kJ
Moles of Mg = 1.25 / 24 = 0.0521 mol
ΔH = Q / n = 4.096 / 0.0521 = 78.6 kJ/mol
The reaction is exothermic, so ΔH = -79 kJ/mol (rounded; or -41 kJ/mol if Q was not recalculated to per mole).