Endothermic reactions
4 marks · NormalThe table below shows the temperature change when different masses of potassium chloride (KCl) are dissolved in 100 g of water.
| Mass of KCl (g) | Temperature change (°C) |
|---|---|
| 1.0 | 0.7 |
| 2.0 | 1.5 |
| 3.0 | 2.3 |
| 4.0 | 3.2 |
(Specific heat capacity of water = 4.18 J g⁻¹ °C⁻¹)
Describe the pattern shown by the data in the table.
Calculate the heat absorbed by the solution when 4.0 g of KCl is dissolved.
Potassium chloride has a molar mass of 74.6 g/mol. Calculate the enthalpy change (ΔH) for dissolving KCl in kJ/mol for the entry with 4.0 g dissolved.
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) As the mass of KCl dissolved increases, the temperature change increases.
(b) 1337.6 J
(c) +24.9 kJ/mol
Common mistakes: not converting J to kJ; dividing by incorrect number of moles
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly describes pattern: as the mass of KCl increases, the temperature change increases.
M1 (b) · 1 mark · q = mcΔT calculation for 4.0 g KCl: 100 × 4.18 × 3.2
M2 (c) · 1 mark · Calculates moles of KCl: 4.0 g ÷ 74.6 g/mol
A1 (c) · 1 mark · Accurately calculates ΔH = q (in kJ) / n (mol), answer to suitable number of sig figs, correct sign. (follow-through)
Worked solution
a) The greater the mass of KCl dissolved, the greater the temperature change.
b) For 4.0 g KCl: q = mcΔT = 100 g × 4.18 J g⁻¹ °C⁻¹ × 3.2 °C = 1337.6 J
c) Moles KCl = 4.0 g / 74.6 g/mol = 0.0536 mol
q in kJ = 1337.6 J / 1000 = 1.3376 kJ
ΔH = q / n = 1.3376 kJ / 0.0536 mol = 24.96 kJ/mol
ΔH = +24.9 kJ/mol (rounded to 3 sig figs)