When #1.0# #g# of potassium chloride, #KCl#, is dissolved in #25# #mL# (=#25# #g#) of water, it causes the temperature to drop from #24.33^o# #C# to #22.12^o# #C#. What is the molar enthalpy of dissolution for #KCl#?

Question

Noah Aldrich · Accepted Answer

The energy absorbed from the water by this endothermic reaction is approximately #Delta H=17.3# #kJmol^-1#.

The response under consideration is: #KCl_"(s)" to K^"+"(aq) + Cl^"-" (aq)# The reaction is endothermic, so the value of #Delta H# we calculate at the end will be positive. First, though, we need to calculate the amount of energy absorbed from the water, which will be negative: #DeltaH=mCDeltaT=mC(T_2-T_1)# #=25xx4.186xx(22.12-24.33)=-232.05# #J# This is the amount of energy absorbed from the water by the process of dissolving the #KCl#. This is the same as the energy gained by the 'system' of dissolving #KCl#. That means energy gained of #232.05# #J# for the #1.0# #g# sample. What we want to know, though, is the total energy absorbed when 1 mole of #KCl# dissolves. One gram of KCl is: #n=m/M=1/74.55=0.0134# #mol# That means the total energy gain if we had dissolved an entire mole, #74.55# #g#, of #KCl#, is 74.55 times as great, which gives: #Delta H=74.55# #mol^-1xx232=17295.6# #Jmol^-1 ~~ 17.3# #kJmol^-1#

Avery Adams · Answer

undefined The molar enthalpy of dissolution for KCl can be calculated using the formula:

ΔH = q / n

where:
ΔH is the molar enthalpy of dissolution,
q is the heat absorbed or released by the solution (in joules),
n is the number of moles of solute.

First, calculate the heat absorbed or released (q) using the formula:

q = mcΔT

where:
m is the mass of the solvent (in grams),
c is the specific heat capacity of the solvent (for water, it is approximately 4.18 J/g°C),
ΔT is the change in temperature (in °C).

Then, calculate the number of moles of KCl using its molar mass (74.55 g/mol) and the mass of KCl used (1.0 g).

Finally, plug the values into the formula for ΔH to find the molar enthalpy of dissolution.