What is the final temperature of the solution when 10.0 g of ammonium nitrate from the previous question dissolves in 100.0 ml of room temperature water (22.0°c)?

(a) ΔHproducts>ΔHreactants; (b) endothermic; (c) ionic bonds between NH+4 and NO-3.

(a) Enthalpy of formation of reactants and products

We have the reaction

Mr/g⋅mol-1mmmll80.04
NH4NO3(s)⇌NH+4(aq)+NO-3(aq)
ΔfH∘/kJ⋅mol-1m-365.1-132.8mmm-206.6

ΔfH∘reactants=-365.1 kJ.mol

ΔfH∘products=(-132.8 - 206.6) kJ=-339.4 kJ/mol

Moles of NH4NO3=10.0g NH4NO3×1 mol NH4NO380.04g NH4NO3=0.1249 mol NH4NO3=0.1249 mol reaction

ΔHreactants=0.1249mol reaction×-365.1 kJ1mol reaction=-45.6 kJ

ΔHproducts=0.1249mol reaction×-339.4 kJ1mol reaction=-42.4 kJ

∴ ΔHproducts>ΔHreactants

(b) Exothermic or endothermic?

The temperature decreased when the ammonium nitrate dissolved in the water.

Energy left the system, so the reaction is endothermic.

(c) Bonds broken

The ionic bonds between the NH+4 ions and NO-3 ions were broken when the ammonium nitrate dissolved in the water.