[Rutherford]

Why many reactions don't take place although ΔG<0?
I don't have the answer. I thought maybe because of Ea. Is it so?

[ramboacid]

That's an explanation. E_a is related to the rate constant by the Arrhenius Equation. Just because a process is spontaneous doesn't mean it will necessarily be fast. Benzene is technically thermodynamically unstable if you look at the thermodynamic data, but it's not going to suddenly decompose back into hydrogen and carbon, or at least not at an appreciable rate.

Also, if you're talking about ΔG° instead of ΔG, the concentrations of the reactants and products will drive the reaction in the direction approaching equilibirum.

[Rutherford]

Ok, thanks.

[Darren]

Why many reactions don't take place although ΔG<0?
I don't have the answer. I thought maybe because of Ea. Is it so?

Also, the reaction may be feasible but the electrode potential for the cell may not be very positive so as to drive the reaction forward. This means that the reaction is not pushed much towards the products that you want to obtain.