Chem 104

Quiz #2 Spring '03 name__________________________

1. (a) For the reaction A + B <=> C + D

at 0o C, DSrxn = 200 J/K ^. mol and DH = 10.0 kJ/mol.

Will this reaction spontaneously occur? Prove it.

This reaction is endothermic, not energetically favorable (-DH) but favored by entropy (+DSrxn) so a calculation is necessary to determine the sign of DG and the spontaneity of the reaction.

DG = DH - TDS

= 10.0 kJ/mol - (298 K)(200x 10^-3 kJ/molK)

= -44.6 kJ/mol

Since DG is negative, the reaction is spontaneous.

(b) If the temperature is increased to 100o C, will the driving force increase or decrease for the reaction of A + B to form C + D? Explain your answer in one or two sentences.

Increasing the temperature will make the second term in the equation

DG = DH - TDS

larger and the overall free energy value more negative which is means that the driving force has increased. If you don’t see this – do the same calculation as in (a) using T = 373 K.

2. A reaction must be spontaneous when:

a) DSsys is positive

b) DSsurr is positive

c) both (a) and (b)

d) none of the above

S_total = DSsys + DSsurr

The second law says that for spontaneous reactions, S_total >0 and if DSsys and DSsys are both positive then S_total must be positive and the reaction will be spontaneous.

3. In the reaction: Ti(s) + O2 (g) <=>TiO2 (s) entropy is expected to:

a) increase

b) decrease

c) remain unchanged

d) cannot tell from information given

The formation of products leads to 1 mole of solid which has lower entropy than the reactants that are composed of 1 mole of solid and 1 mole of gas.

S_gas > S_liquid > S_solid