Question

When 28 g of calcium chloride was dissolved in 100g water in a coffee-cup calorimeter, the temperature rose from 25* Celcius to 41.3. What is the enthalpy change for this process? Assume the solution is equal to the specific heat of water.

Answer 1

answer: Enthalpy of solution of CaCl₂ is H = Q KJ

Given initial temperature T₁ = 25 C =298 K   ; final temperature T₂ = 41.3 C = 314.3 K , T =( T₂ -T₁)

specific heat of water (s) = 4.2 KJ/Kg/K ,   mass of water (m) = 100g =0.1Kg, mass of CaCl₂ (W) = 28g

Heat change H =[ mx sx T ]KJ = 0.1 x 4.2 x (314.3 - 298 ) ======> H = 0.1 x 4.2 x 16.3 KJ

   H = 6.846 KJ

2nd part : molecular mass of CaCl₂ = (40+ 71) = 111g =====> mass of CaCl₂ taken = 28g

      Mass of CaCl₂ taken = W / 111 mole = 28 /111 mole = 0.25225 mole

amount of heat change when one mole of CaCl₂ is dissolved in water = 6.846 / 0.25225 = 27.139 KJ

By definition , enthalpy change of this process H = 27.139 KJ

NOTE: Enthalpy of solution is the amount of heat change taking place when one mole of solute isdissolved in large excess of water , such that no further haet change takes place upon dilution.