is a reducing agent in the overall reaction for photosynthesis?

Hydrogen ion [H]

Explanation:

overall reaction of photosynthesis is given in this two steps equation . 

Firstly, oxygen in water is oxidized by the light energy:

            2H2O          →           O2 + 4 [H.]                   

 [H.] reduces the carbon in CO2:

            4 [H.] + CO2          →           (CH2O) + H2O                                    

This reaction occurs together, to yeild overall reaction

CO2 + H2O       →      (CH2O) +O2

Where [H.], the overall reducing agent

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.