When solid ammonium chloride, NH 4 Cl, is added to excess water, a solution form

Question

When solid ammonium chloride, NH4Cl, is added to excess water, a solution forms and the temperature of the system decreases. In a certain experiment, 0.10 mole of NH4Cl is added to a liter of water in an open glass beaker at 298 K. All the salt dissolves in water and there is a decrease in temperature.

1. Explain how the enthalpy, entropy, and free energy of this system change as the dissolving process proceeds.

2. Of the three parameters, AS, AH, and AG, specified in part 1, which has a sign diferent from the other two? Explain.

3. If this process were carried out in a plastic foam cup rather than a glass beaker, how would the magnitude of the temperature change of the solution be affected? How would the apparent heat of solution be affected? Explain.

4. How is sign and magnitude of the free energy change for this dissolving process affected by an increase in temperature? Explain.

Explanation / Answer

ans 1)

as the salt dissolves in water the salt goes from solid phase to aqueous phase

so the entropy or randomness of system increases (delta S is +ve)

now as the process decreases the temperature means it is endothermic process

so change in enthalpy is positive as heat flows into the reaction from (delta H is +ve)

so dG = dH -T*dS

for the process to be spontaneous dG must be -ve

so as dH is +ve and dS is also +ve

delta G is -ve

ans 2

delat G has opposite sign

reason explained in ans 1

ans 3

if foam cup was used heat transfer would not take place much and heat required for the process wont be provided

and decrease in temp wont be accompalished much

ans 4

if temp is increases then

dH and dS will be + ve

so as temp is increased

be dG = dH -T*dS

TdS will become greater than dH at high temp and hence dG will become -ve

so the reaction energy will decrease and more salt will dissolve

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