Which compound is an unsaturated hydrocarbon?
(1) hexanal (3) hexanoic acid
(2) hexane (4) hexyne

The answer is (4) hexyne. The definition of unsaturated hydrocarbon is that there is one or more double bonds between carbon atoms. In hexanal and hexanoic acid, there is also double bond but it is not between carbon atoms. So they are not unsaturated hydrocarbon.

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.