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