Question

Using ΔH°f values found in Appendix D, calculate (a) the standard enthalpy change for the dissolution of AlCl3(s) in water, then (b) calculate the final temperature of the solution when 2.50 g of AlCl3(s) is dissolved in 52.50 g of water at 20.0°C. Use 4.18 J/g°C for the specific heat of the solution. Remember to write a complete BALANCED reaction.

Appendix D: AlCl3(s) = -705.6 kJ/mol

Answer 1

a)   AlCl3(s) + aq -----> Al^3+(aq) + 3Cl^-(aq)

   DH0dissolution = (DH0fAl+3(aq)+ 3*DH0fCl-(aq))-(DH0fAlCl3(s))

               = (-531 + 3*-167.159)-(-705.6)

             = -326.9 kj/mol

b) heat released(q) = m*s*DT

   m = mass of mixture = 2.5+52.5 = 55 g

   s = specific heat of mixture = 4.184 j/g.c

   DT = T-20

q = 326.9*(2.5/ 133.332) = 6.13 kj

(6.13*10^3) = 55*4.184*(T-20)

T = 46.64 c