Question

With secondary applications ranging from sedative to fire retardant, calcium bromide is used primarily in concentrated solution as an industrial drilling fluid.
(a) Use Table 13.4 and the lattice energy (2132 kJ/mol) to find the heat of solution (kJ/mol) of calcium bromide.

We are given, or can look up, the individual enthalpy components for a salt dissolving in water and have to determine their signs to calculate the overall heat of solution (ΔHsoln). The components are the lattice energy (the heat absorbed when the solid separates into gaseous ions) and the heat of hydration for each ion (the heat released when the ion becomes hydrated). The lattice energy is always positive, so ΔHlattice = 2132 kJ/mol. Heats of hydration are always negative, so from Table 13.4, ΔHhydr of Ca2+ = −1591 kJ/mol and ΔHhydr of Br− = −284 kJ/mol. We use Equation 13.2, noting that there are 2 mol of Br−, to obtain ΔHsoln.

Solution
ΔHsoln = ΔHlattice + ΔHhydr of the ions
= ΔHlattice + ΔHhydr of Ca2+ + 2(ΔHhydr of Br−)
= 2132 kJ/mol + (−1591 kJ/mol) + 2(−284 kJ/mol)
= −27 kJ/mol

Why are 2 mol of Br- being used?

Answer 1

Calcium Bromide dissociate to form one mole of calcium ion and 2 moles of Bromide Ion.

therefore, while calculating hydration energy, 2 moles of Bromide Ions are used.