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

1 Answer
Dec 6, 2016

What, an example? Because all reactions with #DeltaG < 0# are spontaneous by definition.

#"HCl"# reacting with #"Al"(s)# is quite spontaneous:

#2"Al"(s) + 6"HCl"(l) -> 2"AlCl"_3(aq) + 3"H"_2(g)#

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, #color(green)(DeltaS_"rxn" > 0)#.

Furthermore, this reaction is known to release heat (you may or may not do it in lab someday), so #color(green)(DeltaH < 0)#, i.e. the reaction is exothermic.

Finally:

#DeltaG = DeltaH - TDeltaS#

So:

#DeltaG = (-) - T(+)#

But #color(green)(T > 0)# necessarily, by definition of it being in units of #"K"#. A negative minus any positive is another negative, so #bb(DeltaG < 0)# for this reaction at all temperatures.

Therefore, adding #HCl# to aluminum solid is spontaneous at at least room temperature.