Question

Match the reaction (under standard conditions) with the temperature range at which it is expected to be spontaneous.

	1) H2 (g) + Br2 (l) --> 2 HBr (g) the heat of formation for HBr is -36 kJ/mol
	2) H2 (g) + I2 (s) --> 2 HI (g) the heat of formation for HI is +26 kJ/mol
	3) N2 (g) + 2 O2 (g) --> 2 NO2 (g) the heat of formation for NO2 is +34 kJ/mol
	4) N2 (g) +  3 H2 (g) --> 2 NH3 (g) the heat of formation for NH3 is -46 kJ/mol

A.	spontaneous at all temperatures
B.	spontaneous at high temperatures
C.	spontaneous at low temperatures
D.	nonspontaneous at all temperatures

Answer 1

For a reaciton to be spontanous

G < 0

and since G = H - TS

then

H - TS < 0

1)

Entropy increases since 1 mol of gas converts to 2 mol of gas; then + entropy,T is always + and H is negative

G = (-) - (+)(+) = always negative; therefore ALWAYS spontanous

2)

H = posotive, S = positive, since more gas in products, T = positve always

therefore

G = H - TS

G = + - (+)(+) = + -

H < TS

T must be high in order to be spontanous

3)

S = negative since 3 mol f gas turn to 2 mol of gas

H = +34

T = always positive so

G =H - TS < 0

+34 - (+)(-) < 0

NEVER will be spontanous since products are always positive

4)

S = negative since decrease in mol

H = negative

T = postive

then

G = H - TS <0

(-) - (+)(-)

T must be high in order to be spontanous