Endothermic reactions
5 marks · HardA student mixed 6.0 g of potassium nitrate (KNO₃, molar mass = 101 g/mol) in 50.0 g of water in a calorimeter. The temperature of the water fell by 3.2 °C. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹.
Calculate the number of moles of KNO₃ dissolved. (2 marks)
Calculate the amount of heat (q, in J) absorbed by the water during the dissolving process. (2 marks)
Calculate the enthalpy change, ΔH, for dissolving KNO₃ in kJ/mol. (1 mark)
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) 0.0594 mol · must show Shows use of n = mass / molar mass
(b) −669 J · must show q = m x c x ΔT (with negative q for endothermic, since temperature falls)
(c) +11.3 kJ/mol · must show q converted to kJ, correct sign, correct division by moles
Common mistakes: Wrong ΔT sign or unit conversion in final answer; Wrong number of moles; Wrong units or sign for ΔH
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Writes or uses
A1 (a) · 1 mark · Correct answer for moles of KNO₃ (0.0594 mol, accept 0.059–0.060)
M2 (b) · 1 mark · Writes or uses and chooses correct sign for temperature decrease
A2 (b) · 1 mark · Correct answer for heat change (–669 J, accept –666 to –672 J)
A3 (c) · 1 mark · Correct answer for ΔH given as +11.3 kJ/mol, accept +11.1 to +11.5 kJ/mol, correct sign. (follow-through)
Worked solution
(a) Moles of KNO₃:
(b) Heat absorbed by water (q):
(c) Enthalpy change (per mole):