Why is Reaction Spontaneousity Not Occuring at 298K?

[LordVoldemort]

Homework Statement

Sorry - this was on a test I took today, so I don't remember it exactly. A reaction (I believe a combustion reaction) is known to be spontaneous at 298 K. When the reactants are placed are placed into a container, no reaction occurs. Why? Info: The reactant side had 15 moles of gas while the products had only 9 (see next section).

2. The attempt at a solution
I had to guess, so I reasoned that a reaction is spontaneous iff the Gibbs free energy is lowered. The fact that it is not spontaneous means that the Gibbs energy change was positive. I recalled that enthalpy assumes constant pressure, and in a container, a change from 15 moles of gas to 9 moles would mean a large pressure change, so I guessed that this would cause the enthalpy (which was negative) to decrease in magnitude.

 

[Ygggdrasil]

If no reaction has occurred, can you conclude that the reaction is not spontaneous? Are there other requirements for a chemical reaction to occur? Gasoline and oxygen combine spontaneously at room temperature to form carbon dioxide and water. Yet, people's cars are not bursting into flames. Why is this?