Cells as Chemical Probes

A chemical reaction that can occur within one half-cell win reach equilibrium and is assumed to remain at equilibrium. Such a reaction is not the net cell reaction. 

Question Why can we assume that the concentrations of acetic acid and acetate ion are equal to their initial (formal) concentrations  

But what is the reaction in the left half-cell The only element we find in two oxidation states is hydrogen. We see that H2(g) bubbles into the cell, and we also realize that every aqueous solution contains H h. Therefore, hydrogen is present in two oxidation states, and the halfreaction can be written as 

The net cell reaction is not at equilibrium, because the measured voltage is 0.503 V, not 0 V. The Nemst equation for the cell reaction is 

When we put in all the known quantities, we discover that the only unknown is [H+]. The measured voltage therefore allows us to find [El+] in the left half-cell  

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