How do entropy and enthalpy affect the reversibility of a reaction (Equilibrium)

[frog0101]

Hi,
From the new syllabus:
Analyse examples of non-equilibrium systems in terms of the effect of entropy and enthalpy, for example:

• Combustion reactions
• Photosynthesis

I am not sure that I understand how entropy and enthalpy affect/determine the reversibility of a reaction, I know that their is a relationship between Gibbs free energy and equilibrium (if it is very close to zero in a closed system, equilibrium should occur, as both forward and reverse reactions are predicted). Aren't all closed system reactions (theoretically) capable of being reversed (although it may require significant amounts of energy).

Any help would be great, thanks.