Question

Determine the amount of energy produced, in kcal, when 4.75 grams of NaCl are produced

Answer 1

For ionic compounds like NaCl, generally we measure the Lattice energy.

Now lattice energy is the amount of energy required to break an ionic compound completely into its constituent ions in gaseous state.

i.e NaCl(s) -----------> Na+(g) + Cl-(g)

Now for this conversion, the amount of heat required = 787 kJ mol^-1.

Thus for preparation of NaCl, we cann take the reverse i.e combining Na+ and Cl- to for NaCl. Here heat will be released and the amount of heat released = -787 kJ mol^-1.

(here the value is same . the -ve sign only indicates that heat is released. thus we can also take the value of heat as 787 kJ mol^-1.)

i.e Na+(g) + Cl-(g) -----------> NaCl(s) here energy produced = 787 kJ mol^-1.

= 188.098 kcal/mol

Now given NaCl = 4.75 grams

We know the molar mass of NaCl = 58.44 g/mol

Thus the given moles of NaCl = mass/ molar mass

= 4.75 grams/ 58.44 g/mol

= 0.0813 moles

Hence amount of heat released during formation of 0.0813 moles of NaCl = 0.0813 moles * 188.098 kcal/mol

= 15.28 kcal