Endothermic reactions
4 marks · NormalA student dissolves 1.5 g of sodium carbonate (Na₂CO₃, molar mass = 106 g/mol) in 50 g of water in a polystyrene cup. The temperature rises by 1.8 °C. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹.
Calculate the heat energy transferred to the solution (q).
Calculate the number of moles of Na₂CO₃ dissolved.
Calculate the enthalpy change, ΔH, for dissolving Na₂CO₃ in kJ/mol. Give your answer to two decimal places.
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.
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(a) q = 376.2 J
(b) 0.0142 mol
(c) +26.50 kJ/mol
Also accept: +139.6 kJ/mol rounded appropriately
Common mistakes: not converting to kJ; using wrong mass for moles
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Calculates heat transferred: q = 50 × 4.18 × 1.8
M2 (b) · 1 mark · Calculates moles: 1.5 g / 106 g/mol
M3 (c) · 1 mark · Converts q to kJ and divides by moles for ΔH. (follow-through)
A1 (c) · 1 mark · Correct final value to two decimal places, correct sign and unit. (follow-through)
Worked solution
a) q = mcΔT = 50 g × 4.18 J g⁻¹ °C⁻¹ × 1.8 °C = 376.2 J
b) Number of moles = 1.5 g / 106 g/mol = 0.0142 mol
c) q in kJ = 376.2 J / 1000 = 0.3762 kJ
ΔH = q / n = 0.3762 kJ / 0.0142 mol = 26.50 kJ/mol (+ sign as temperature increases, exothermic)