# What is a reaction with a negative DeltaG and occurs spontaneously?

Dec 6, 2016

What, an example? Because all reactions with $\Delta G < 0$ are spontaneous by definition.

$\text{HCl}$ reacting with $\text{Al} \left(s\right)$ is quite spontaneous:

$2 {\text{Al"(s) + 6"HCl"(l) -> 2"AlCl"_3(aq) + 3"H}}_{2} \left(g\right)$

As you form a gas and an aqueous solution from a solid and a concentrated aqueous solution, you increase the entropy of the system after the reaction (because gases move around much more than liquids and solids).

So, qualitatively, $\textcolor{g r e e n}{\Delta {S}_{\text{rxn}} > 0}$.

Furthermore, this reaction is known to release heat (you may or may not do it in lab someday), so $\textcolor{g r e e n}{\Delta H < 0}$, i.e. the reaction is exothermic.

Finally:

$\Delta G = \Delta H - T \Delta S$

So:

$\Delta G = \left(-\right) - T \left(+\right)$

But $\textcolor{g r e e n}{T > 0}$ necessarily, by definition of it being in units of $\text{K}$. A negative minus any positive is another negative, so $\boldsymbol{\Delta G < 0}$ for this reaction at all temperatures.

Therefore, adding $H C l$ to aluminum solid is spontaneous at at least room temperature.