Question

An instant cold pack consists of two bags, one containing water inside a bag containing ammonium nitrate. When the inner bag of water is broken by squeezing the package, ammonium nitrate dissolves and the solution becomes colder. Predict the sign for each of the following and provide a brief justification.

a.) ΔH

b.) ΔS

c.) ΔG

Answer 1

a) H will be positive. when ammonium nitrate is dissolved in water, there is a lowering of temperature. that means heat is flowing from surrounding to the system. that eventually means that energy required by the system to break the hydrogen bonding between water and energy required to break the lattice structure of ammonium nitrate is greater than the hydration energies for both the cation( ammonium) and the anion ( nitrate).

b) S will be positive in nature.

when the pack is squeezed and the packet is broken, ammonium nitrate gets free to mix with the molecules of water, in this process the randomness of ammonium nitrate molecule will increase. now the ammonium nitrate will get more volume to gets dissolved into, thus the entropy will increase because of randomization.

c) G will be negative in nature.

as the solvation is favored in case of ammonium nitrate, Gibbs free energy of the system after solvation will be less than that of the previously packed packet. the increase in entropy, in this case, dominates the enthalpy chnage in this process. thus according to the formula

G=H- TS

S is more positive than the H, thus G will become more and more negative.