Question

Mixture	ΔT
NaCl + H2O	-0.2
NaNO3 + H2O	-0.9
NaCH3COO + H2O	0.9

Which reactions are endothermic? Which are exothermic? What is the sign of enthalpy ΔH in each case?

What is the sign of change in Gibbs free energy ΔG for each process in the forward direction?

Could you predict the sign for entropy ΔS for these reactions? How could you tell the sign?

Answer 1

The dH = n*Cp*dT

For first mixture dT = -ve , So, dH is negative.

For second mixture dT = -ve , so, dH is negative.

For third mixture dT =+be , so, dH is positive.

The reaction with dH positive are endothermic reaction and with dH negative are exothermic reactions.

The third reaction is endothermic only. Other two are exothermic.

------------------------------

dG = dH - TdS

As dS does not change with temperature so , the sign of dG will be same as of dH above.

So, for first reaction dG is negative.

For second reaction dG is negative.

For third reaction dG is positive.

--------------------------

The above solutions formation takes place if and only if when the entropy increases. So, number of particles increase. That is randomness increases.

So, dS for all reaction is positive.