Endothermic reactions
2 marks · NormalWhen 0.10 mol of solid ammonium nitrate, , dissolves in water, the temperature of the water decreases as 3.40 kJ of heat is absorbed from the surroundings.
Calculate the molar enthalpy change, , for the dissolution of ammonium nitrate in kJ/mol. Include the correct sign.
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-34 kJ/mol (±1)
Also accept: +34 kJ/mol (if sign explained as endothermic/absorbed); -34.0 kJ/mol
Common mistakes: Using wrong sign for endothermic process; Not scaling to per mole
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Marking points
M1 · 1 mark · Correct method: divides energy by moles to find molar enthalpy change. Accept correct rearrangement or unit conversion.
A1 · 1 mark · Correct final value and sign: –34 kJ/mol (or equivalent, with sign explained for endothermic direction, allow rounding). Unit required for full mark.
Worked solution
Heat absorbed, kJ; moles of ammonium nitrate = 0.10 mol.
Molar enthalpy change:
As heat is absorbed from the surroundings (temperature decreases), the process is endothermic. So, kJ/mol or –34 kJ/mol, depending on convention (usually shown as positive for endothermic, negative for exothermic).