Question

Classify each of the following reactions as one of these four types:
• spontaneous at all temperatures
• not spontaneous at any temperature
• spontaneous below a certain temperature but not above
• spontaneous above a certain temperature but not below

(a) KClO3 (s) + S (s) ® KCl (s) + SO3 (g); ΔH = -435 kJ; ΔS = 164 J/K
(b) C2H5OH (g) ® C2H4 (g) + H2O (g); ΔH = 45.5 kJ; ΔS = 125.7 J/K
(c) 2 NO (g) + O2 (g) ® 2 NO2 (g); ΔH = -115.4 kJ; ΔS = -146.5 J/K
(d) PbS (s) + Mg (s) ® MgS (s) + Pb (s); ΔH = 66 kJ; ΔS = -9 J/K

Answer 1

a ) spontaneous reaction takes place

b) Non - spontaneous reaction takes place

c) reaction does not takes place

d) reaction does not takes place

because ,

here is a very easy way to think about this, and it goes back to the definitions of entropy (S) and enthalpy (H.)

Start with entropy, the disorder of a system. It takes energy to force order on any system. If you have to pump energy in, then the reaction isn't very spontaneous, is it? So, you want a positive entropy, which means that disorder is increasing.

Moving on to enthalpy (heat.) If a reaction evolves heat, then it is a favorable reaction. However, if you have to put heat into it for the reaction to go, then it wasn't very spontaneous, was it? Remember that if you have to add heat, the delta H is positive; if heat evolves on its own, the delta H is negative.

So, a reaction with positive S and a negative H is spontaneous.

This comes from Gibbs free energy relationship:

G = H -TS

If G is negative, then the reaction is spontaneous. To get a negative G, you would want the right side of the equation to be as negative as possible. H would ideally be negative, but S would be positive.