Question

1) For which of the following processes does entropy decrease?

A. H2(g) + Cl2(g)   →   2HCl(g)

B. N2(g) + 3H2(g)   → 2NH3(g)

C. Making amorphous glass from crystalline SiO2 (quartz)

D. CH3OH(l) → CH3OH(aq)

2) Use the values provided in the table to calculate the standard free energy change for the following reaction at 298 K:  

2H2O2(g)  → 2H2O(g) + O2(g)  

Substance	∆Gf° (kJ/mol) at 298 K
H2O(g)	−228
H2O2(g)	−105

A

−666 kJ

B

+666 kJ

C

+246 kJ

D

−246 kJ

E	The standard free energy of formation of O2(g) is needed because its value is not equal to zero.

3. Consider the 2nd Law of Thermodynamics.  For a spontaneous process, if the entropy change of the system is negative, what can we conclude about the entropy change of the surroundings?

A	The entropy change of the surroundings must also be negative because the entropy of the universe increases for a spontaneous process.

B	The entropy change of the surroundings must also be negative because the entropy of the universe decreases for a spontaneous process.

C	The entropy change of the surroundings must be largely positive because the entropy of the universe increasesfor a spontaneous process.

D

The entropy change of the surroundings must be largely positive because the entropy of the universe decreasesfor a spontaneous process.

4 Which of the following indicates that a chemical or physical change is spontaneous, specifically when starting with reactants and products in standard state conditions?

A)

DG > 0

B)

DG < 0

C)

DG° > 0

D)

DG° < 0    

5. The equation DG= DH−TDS is derived from the 2nd Law of Thermodynamics.  If a chemical reaction is notspontaneous at any temperature under standard state conditions, we can conclude that:

A)	DH° < 0 and  DS° < 0

B)	DH° < 0 and  DS° > 0

C)	DH° > 0 and  DS° < 0

D)	DH° > 0 and  DS° > 0

Answer 1

1)

entropy of gas > entropy of aqueous > entropy of liquid > entropy of solid

in A there are equal number of gases on both sides.

So, its not possible to predict whether the entropy is increasing or decreasing

in B, there are more gases on reactant than product.

So, entropy of product is less than reactant.

Entropy is decreasing.

in C, amorphous solid are more disordered. So, amorphous solids have more entropy. Hence entropy is increasing

Answer: B

2)

Given:

Gof(H2O2(g)) = -105.0 KJ/mol

Gof(H2O(g)) = -228.0 KJ/mol

Gof(O2(g)) = 0.0 KJ/mol

Balanced chemical equation is:

2 H2O2(g) ---> 2 H2O(g) + O2(g)

ΔGo rxn = 2*Gof(H2O(g)) + 1*Gof(O2(g)) - 2*Gof( H2O2(g))

ΔGo rxn = 2*(-228.0) + 1*(0.0) - 2*(-105.0)

ΔGo rxn = -246 KJ

Answer: -246 KJ

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