Instant cold packs contain ammonium nitrate and water separated by a thin plasti
ID: 924911 • Letter: I
Question
Instant cold packs contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves to the endothermic reactions: HH_4NO_3(s) rightarrow NH^+_4(eq) + NO^-_3(eq) In order to measure the enthalpy change for this reaction, 1.25 g of NH_4NO_3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 degree C and the final temperature (after the solid dissolves) is 21.9 degree C. Calculate the change in enthalpy for the reaction in J. Use 1.0 g/mL as the density of the solution and 4.18 J/g degree C as the specific heat capacity.Explanation / Answer
NH4NO3 --> NH4+ and NO3-
m = 1.25 g of NH4NO3
V = 25 ml
Ti = 25.8°C
Tf = 21.9 °C
Q = m*¨Cp*(Tf-Ti)
Q = 25*4.18*(21.9-25.8) = -407.55 J
Hrxn = -Q/n
n = mass/MW = 1.25/80.0434 = 0.0156165
Hrxn = 407.55/0.0156165 = 26097.396984 J/mol = 26.9 kJ/mol
nearest answer is
2.61*10^4 kJ (must be positive since it is endothermic solution)