Question

What is the energy change if 84.0 g of calcium oxide (CaO) reacts with excess water in the following reaction?

CaO (s) + H2O(l) yields Ca(OH) 2 (s) deltaH = -65.2 kJ/mol
65.2 kJ of heat is absorbed by the reaction.
98 kJ of heat is absorbed by the reaction.
65.2 kJ of heat is released by the reaction.
98 kJ of heat is released by the reaction.

Answer 1

The energy change if 84.0 g of CaO react with excess water is 98KJ of heat is released.

calculation
heat = number of moles x delta H

delta H = - 65.2 Kj/mol

first find the number of moles of CaO reacted

moles = mass/molar mass
the molar mass of CaO = 40 + 16= 56 g/mol
mass = 84 g
moles therefore = 84 g/56 g/mol =1.5 moles

Heat is therefore = 1.5 moles x -65.2 = - 97.8 Kj = -98 Kj

since sign is negative the energy is released