[aerosam]

Hi everyone

Here is a rather trivial question, but i have no idea why certain reactions are feasible and certain reactions are not.
Suppose i have H2 + 1/2 O2 = H20
We all know that it occurs.
How would we prove that such a reaction is feasible at a particular temperature, by looking at quantities like enthalpy of formation, Gibbs free energy etc?
We also know that it requires a spark to initiate the reaction. How would we determine that as well?

My aim is the following.
Given a generic reaction, how do i know whether it will occur at a particular temperature or not and if it needs heating etc?

Thanks
Sam

[ramboacid]

How would we prove that such a reaction is feasible at a particular temperature, by looking at quantities like enthalpy of formation, Gibbs free energy etc?

More or less.

The change in the gibbs free energy of a reaction determines whether a reaction is spontaneous or nonspontaneous. Negative values of  indicate spontaneity, and positive values indicate nonspontaneity.

For a reaction to occur, the gibbs free energy must be negative at the specified conditions. You can find the molar gibbs free energies of substances in the standard state (25 C, 1 bar) from a table, usually found in the appendix of every quality chem textbook, and then find the change in gibb's free energy using Hess's Law.

There is a nice little equation relating values of gibbs free energy at different temperatures:
 :delta: G_rxn =  :delta: G^o_rxn + RTln(Q)

If you know the concentrations of the reactants and products, and the  :delta: G^o of the reaction, you can determine the spontaneity of the reaction. Q is the reaction quotient.

If you can't obtain values for the gibbs free energy, you could always calculate them from the enthalpy and entropy changes using

 :delta: G =  :delta: H - T :delta: S

As for the spark, I would assume it raises the temperature so that the reactants can pass the activation energy barrier, but I am unsure.

[aerosam]

Thanks for the explanation.
So let's try it on a simple reaction such as
KOH + HCl = KCl + H20
\delta G (KOH)= -379 KJ/mol
\delta G (HCl)= -95
\delta G (KCl)= -408
\delta G (H2O)= -237
So \delta G reaction = -408 -237 -(-95 -379) = -171 KJ/mol
So i guess the above reaction occurs spontaneously at room temperature(as expected)

Now let's take the reaction
H2 + 1/2 O2 = H20
\delta G (H2)= 0
\delta G (O2)= 0
So \delta G reaction = -237
But we know that it requires a spark and that the concentration of H2 in O2 should be sufficiently high.
If i just light a match in air, i don't think i will start making water. So what am i missing?

Can we conclude that \delta G < 0 for a reaction is a necessary but not sufficient condition for a reaction to  occur? What else do we need?

[sjb]

Thanks for the explanation.
So let's try it on a simple reaction such as
KOH + HCl = KCl + H20
\delta G (KOH)= -379 KJ/mol
\delta G (HCl)= -95
\delta G (KCl)= -408
\delta G (H2O)= -237
So \delta G reaction = -408 -237 -(-95 -379) = -171 KJ/mol
So i guess the above reaction occurs spontaneously at room temperature(as expected)

Now let's take the reaction
H2 + 1/2 O2 = H20
\delta G (H2)= 0
\delta G (O2)= 0
So \delta G reaction = -237
But we know that it requires a spark and that the concentration of H2 in O2 should be sufficiently high.
If i just light a match in air, i don't think i will start making water. So what am i missing?

Can we conclude that \delta G < 0 for a reaction is a necessary but not sufficient condition for a reaction to  occur? What else do we need?

How much air would you need to generate, say 1 ml of water?

[aerosam]

Yes, the matchstick was probably the wrong example, given the low concentration of Hydrogen in air.
My point is how do we determine if a park or ignition source is necessary to start a reaction?