Coffee Cup Calorimetry

Question

When 0.876 g of CaCl2(110.98 g/mol) is added to 95.0 gof water at 22.4°C in a ‘coffee-cup’ calorimeter, the temperature of the resulting solution increases to 29.7°C. The specific heat capacityof the solution is 4.184 J/°C·gand the heat capacity of the Styrofoam cup is negligible. A) Is this reaction endothermic or exothermic? How do you know? B) Calculate themolar heat of solution(∆Hrxn) of CaCl2in water.

Answer 1

Is the reaction endothermic or exothermic? Since the temperature of the surrounding increased, that means that heat was generated or given off by the reaction. This makes the reaction EXOTHERMIC. How you know is because the temperature increased

The molar heat of solution is the heat given off when 1 mole of CaCl₂ is dissolved in water. The experimental data are for 0.876 g CaCl₂, but we will convert to moles later:

heat = q = mC∆T

q = ?

m = mass of water = 95.0 g

C = specific heat of water = 4.184 J/g/deg

∆T = change in temperature = 29.7 - 22.4 = 7.3 degrees

q = (95.0 g)(4.184 J/g/deg)(7.3 deg) = 2902 J

As stated, this is the ∆H for dissolving 0.876 g CaCl₂. Now we convert to moles:

0.976 g CaCl₂ x 1 mole/110.98 g = 0.00879 moles.

Molar ∆H_rxn = 2902 J / 0.00879 moles = 330,148 J/mole = 330 kJ/mole (to 3 significant figures)