A student dissolves 14.6 g of ammonium chloride (NHĻCI) in 200. g of water in a well-insulated open cup. She then observes the temperature of the water fall from 20.0 °C to 15.1 °C over the course of 8.3 minutes. Use this data, and any information you need from the ALEKS Data resource, to answer the questions below about this reaction:

NH4+ (aq) + Cl- (aq) > NH4CI(s)

calculate the amount of heat that was released or absorbed by the reaction in this case kJ per mole of NH4CI

q = 1.6 x 10⁴ kJ/ mol NH₄CI  

Explanation:

In this question we are given the change in temperature for the dissolution of 14.6 g NH₄Cl. We know from calorimetry that the change in temperature for a mass, m , of water is given by the expression:

q = m x c x Δt

where q is the heat absorbed or released upon dissolution, and c is the specific heat of water.

We have all that is needed to compute q for the dissolution of ammonium chloride, but please note we are being asked the q for the reverse reaction. Thus we would compute q reverse its sign and divide it by the number of moles in 14.6 g NH₄Cl to give the answer this question.

c for H₂O = 4.186 J/g ºC

ΔT = T final - Tinicial = 15.1 ºC - 20.0 ºC =  - 5.10 ºC

q = 200 g x 4.186 J/gºC x  - 5.1 ºC

  =  -4270 J = - 4.3 x 10³ kJ

Thus for the reverse reaction:

NH₄+ (aq) + Cl- (aq) > NH₄CI(s)

q = - ( - 4.3 x 10³ kJ ) = 4.3 x 10³ kJ

The molecular weight of  NH₄CI is 53.49 g /mol.

⇒ mol NH₄CI = 14.6 g /53.49 g/mol = 0.27 mol

and the heat absorbed per mol of NH₄CI  , endothermic reaction, is

q = 4.3 x 10³ kJ / 0.27 mol = 1.6 x 10⁴ kJ/ mol NH₄CI