A coffee cup calorimeter can be used to investigate the \"cold pack reaction\",

Question

A coffee cup calorimeter can be used to investigate the "cold pack reaction", the process that occurs when solid ammonium nitrate dissolves in water: NH_4NO_3(s) rightarrow NH_4^+ (aq) + NO_3^- (aq) Suppose 25.0 g of solid NH_4NO_3 at 23.0 degree c is added to 250 mL H_2O at the same temperature. After all of the solid dissolves, the temperature is measured to be 15.6 degree C. A) calculate the reaction enthalpy (in kJ/mol) for the cold pack reaction (assume the specific heat capacity of the solution is the same as for the water, 4.184 J/g* degree C). The density of water is 1 g/mL. Just give the number to 2 significant figures. Is the reaction endothermic or exothermic?

Explanation / Answer

m = 25 g of solid NH4NO3

V = 250 mL = 250 g of h2o

dT = 15.6°C - 23°C = -7.4

a)

HRxn = Q/n

n = mol = mass/MW = 25/80.043 = 0.3123 mol of NH4NO3

so..

Q = m*C*(Tf-T)

Q = (250+25)*4.184*(15.6-23) = -8514.44 J

HRxn =8514.44 / 0.3123 = 27263.656 J/mol = 27.263 kJ/mol

must be positive, since this is ENDOTHERMIC, dT is negative

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