Which of the following ionic compounds has negative enthalpy of solution?

Options

A. `NH_(4)NO_(3)`
B. `NH_(4)Cl`
C. `KCl`
D. `CaCl_(2)`

Answer

4

Explanation

Athletes often use instant cold packs and hot packs as first-aid devices to treat injuries. These devices operate by utilizing the enthalpy of solution concept.
A typical pack consists of a plastic bag containing a pounch of water and a dry chemical. Stricking the pack causes the pounch to break and the temperature of the pack will be either raised or lowered, depending on whether the enthalpy of solution of the chemical is exothermic or endothermic Generally, calcium chloride or magnesium sulphate is used in hot packs, and ammonium nitrate in cold packs. The reactions are
`CaCl_(2)(s)overset(H_(2)O)(rarr) Ca^(2+)(aq.)+2Cl^(-)(aq.), DeltaH_("soln")= -82.8 kJ`
`NH_(4)NO_(3)(s)overset(H_(2)O)(rarr) NH_(4)^(+)(aq.)+NO_(3)^(-)(aq.),DeltaH_("soln")= 26.2 kJ`
Experiments show that the addition of `40g` of `CaCl_(2)` (0.36 mole) to `100 mL` of `H_(2)O` raises the temperature from `20^(@)C` to `90^(@)C`.
Similarly, when `30 g NH_(4)NO_(3)` (0.3 mole) are dissolved in `100 mL` of water at `20^(@)C`, the temperature will be lowered to `0^(@)C`. A typical hot or cold pack works for about 20 min.