Question

Instant cold packs, often used to ice athletic injuries on the field, contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the following endothermic reaction: NH4NO3(s)→NH+4(aq)+NO−3(aq) In order to measure the enthalpy change for this reaction, 1.25 g of NH4NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 ∘C and the final temperature (after the solid dissolves) is 21.9 ∘C. Part A Calculate the change in enthalpy for the reaction in kilojoules per mole. (Use 1.0g/mL as the density of the solution and 4.18J/g⋅∘C as the specific heat capacity.) Express your answer to two significant figures and include the appropriate units. ΔHrxn =

Answer 1

Mass of NH₄NO₃ = 1.25 g

Moles of NH₄NO₃ = Mass of NH₄NO₃ / Molar Mass of NH₄NO₃

Moles of NH₄NO₃ = 1.25 g / 80.043 g/mol

Moles of NH₄NO₃ = 0.016 mol

Volume of solution = 25.0 mL

Density of Solution = 1.0g/mL

Mass of Solution = Density x Volume = 1.0g/mL x 25.0mL = 25g

Heat Generated = Mass x Specific heat capacity x

Heat Generated = 25g x 4.18 J/g⁰C x (25.8⁰C - 21.9⁰C)

Heat Generated = 407.55 J = 407.55 x 10^-3 kJ = 0.40755 kJ [Since, 1J = 10^-3kJ]

= Heat Generated / No. of moles of NH₄NO₃

= 0.40755 kJ / 0.016 mol

= 25.47 kJ / mol ~ 26 kJ / mol