Energetics
5 marks · EasyA student investigates the energy change when potassium nitrate dissolves in water.
State whether this process is exothermic or endothermic.
The student adds 5.0 g of potassium nitrate ( KNO_3 ) to 50.0 g of water in a polystyrene cup. The temperature drops from 20.0°C to 15.5°C. Calculate the heat energy change (Q) for the water. (Specific heat capacity of water = 4.18 J/g°C)
Calculate the number of moles of potassium nitrate dissolved. (M_r of KNO_3 = 101)
Calculate the molar enthalpy change (ΔH) for this process in kJ/mol. Give your answer to 2 significant figures.
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) Endothermic. The temperature drops so heat is absorbed.
(b) -940 J (give sign to indicate heat absorbed) J
(c) 0.0495 mol
(d) -19 kJ/mol
Common mistakes: Incorrect sign for enthalpy change; Wrong use of units
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states process is endothermic and links to temperature drop.
M1 (b) · 1 mark · Uses Q = mcΔT with correct values, including sign.
A1 (b) · 1 mark · Correct calculation of heat energy change, negative sign for heat absorbed.
M1 (c) · 1 mark · Correct calculation of moles (n = mass/Mr) of potassium nitrate.
A1 (d) · 1 mark · Correct use of values to find ΔH in kJ/mol, answer to 2 significant figures, correct sign. (follow-through)
Worked solution
a) The process is endothermic because the temperature falls, meaning heat is absorbed from the surroundings.
b) Heat energy change Q = -940 J (negative because temperature decreased).
d) The molar enthalpy change is -19 kJ/mol (to 2 significant figures).