Arigato brought up a HCJC gas law qn from the prelim paper. I
thought I'd share another interesting qn that many JC students
struggle with.
8.4g of NaF is dissolved in 250g of water. Given lattice
enthalpy is -918 kJ/mol, hydration enthalpy of F- is -457 kJ/mol
and hydration enthalpy of Na+ is -390kJ/mol, what is the initial
temp of the water if final temp of soln is 20 deg C? Use 4.2 J g-1
K-1 for specific heat capacity of NaF(aq).
Hint : Draw a hess law diagram to calculate solution enthalpy
(note : lattice association enthalpy is exothermic, lattice
dissociation enthalpy is endothermic; students often confuse the
two). Next, use the formula : heat transferred = m c (delta)T.
(note : capital T represents temperature in Kelvins, small t in deg
C). Make (delta)T the subject, and hence find the initial
temperature. (note : 'delta' means 'change in').
