Which of the reactions are spontaneous (favorable)?
A. 2Mg(s) + O2(g) rightarrow 2MgO (s) delta G = - 1137 kJ / mol
B. NH3(g) + HCl(g ) rightarrow NH4Cl(s) delta G = - 91.1 kJ / mol
C. AgCl(s) rightarrow Ag+ (aq) + Cl-[aq) delta G = 55.6 kJ / mol
D. 2H2(g) + O2(g) rightarrow 2H2O(g) delta G= 456 kJ / mol
E. C(s) + H2O(t) rightarrow CO(g) + H2(g) delta G = 90.8 kJ / mol
F. CH4(g) + 2O2(g) rightarrow CO2(g) + 2H2O(t) delta G = -820 kJ / mol
A. Reaction A.
B. Reaction B.
C. Reaction C.
D. Reaction D.
E. Reaction E.
F. Reaction F.

Explanation: Gibbs Free Energy, symbol G, combines Enthalpy (sum of internal energy) and Entropy (measure of how much the energy of atoms and molecules become spread out in a process).

The change in free energy (ΔG) is the sum of enthalpy and the product of temperature and entropy of the system.

With free energy, it is possible to determine if a reaction is spontaneous, i.e., happens wthout external energy, or non-spontaneous.

If ΔG is negative, reaction is spontaneous. If ΔG is positive, reaction is non-spontaneous.

Analysing each ΔG of the reactions above, reactions A, B and F's Gibbs Free energy are negative, so these 3 are spontaneous.