How do you calculate enthalpy for a reaction at non-standard conditions?
Do you know how to calculate the change in enthalpy and entropy for a substance as you change the conditions (T, P, ...) in a thermodynamically reversible fashion? Now take the reactants at "standard" conditions, transform them to reactant(s) at the non-standard conditions, react them, and take the product(s) back to the standard condition.  Compare with the reaction occurring at standard conditions. Figure out the enthalpy and entropy differences between reacting at 'standard' and 'non-standard' conditions.  

As for your second question, ponder why some reactions do not go to completion.