 Products of complete combustion (PCC) are useful for simple calculations.
 Can be considered an upper bound on the extent of reaction.
 Useful for defining heating values and providing a combustion basis.
 At high temperatures products dissociate to form $H_2$, $CO$, $H$, $O$, $OH$, etc.
Criteria for equilibrium

Consider a fixed mass at equilibrium

$dU = dQ + dW$
 $dW = PdV$
 at equilibrium differential changes are reversible, so $dQ = TdS$.

$dU = TdS  PdV$

Assume const $T$ and $P$, then we can move $T$ and $P$ inside the derivatives:
 $dU = d(TS)  d(PV)$.

Collect terms:
 $d(U+PVTS) = d(HTS)= dG = 0$.

At equilibrium $$dG_{T,P} = 0$$

Hence $G$ is a critical point.
 When not at equilibrium changes are not reversible and $dS > dQ/T$ so $dQ < TdS \rightarrow dG<0$,
 Hence $G$ is a minimum at equilibrium.

Note equilibrium is a state not a process.
 The $T,P$ in $dG_{T,P}=0$ does not mean that that the equilibrium applies to a constant $T$ and $P$ process.
 Rather, if you look in directions of constant $T$ and $P$, then the $G$ curve will be minimum at the equilibrium point.
Other equivalent equilibrium conditions
 Above, we had
$$dU = TdS  PdV$$
 For constant $T$, $P$, we have $dG_{T,P}=0$.
 For constant $S$, $V$, we have $dU_{S,V}=0$.
 For constant $S$, $P$, we have $dU=d(PV)\rightarrow dH_{S,P}=0$.
 For constant $T$, $V$, we have $dU=d(TS)\rightarrow dA_{T,V}=0$, where $A$ is Helmholz free energy.
 All of these four expressions are true of an equilibrium state at a given temperature and pressure.
 Similarly, all of these four expressions are true at a given $S$ and $V$, etc.
 Again, this is because equilibrium is a state, not a process.