Question

predict the signs of delta G, delta H, delta S, of the system for the following processes

a)Ammonia melts at -60 degrees Celcius

b) ammonia melts at -77 Celcius (normal melting point of Ammonia)

c) ammonia melts at 100 Celcius

Answer 1

melting is process where
solid converts to liquid-
NH3(s) -----> NH3(l)
here we need to add heat during melting
thus so melting is an endothermic process.For endothermic process
delta H is positive**
In melting solid NH3 changes to liquid ammoinia .In
liquid state particles have more freedom for random motion
or in otherwords liquid state is more disordered.When disorder
increases entropy increases.Hence sign of delta S will be positive
for melting of ammonia.***
we know delta G=delta H -T* delta S
dG =dH-TdS
for melting dH and dS are positive So dG will be negative only
at high temperature [above melting point] .dG is negative means reaction
is spontaneous. This happens above melting point.At melting
point phase change takes place ,thus dG=0.Also below melting
point dG will be positive

a) at -60 C =below melting
delta H = positive
delta S = positive
delta G = positive

b) at -77 C= at melting point

delta H = positive
delta S = positive
delta G = 0

c) at 100 C =above melting point
delta H = positive
delta S = positive
delta G =negative
*********
thank you