Which reaction has been chosen as the 'standard' against which every other reaction is measured?

A. F2 + 2e ---> 2F–
B. O2 + 4H+ + 4e ---->2H2O
C. 2H+ + 2e---> H2
D. Li+ + e ---> Li
E. H2 + 2e --->2H

i got C is that right?

I guess C is correct, since there is no "none of these" as a choice. Technically, the H^+ is at 1 M activity and H2 gas is at 101.325 kPa (1 atmosphere) and convention is to show that when writing the cell notation. Technically, again, the problem asks which REACTION has been chosen and that doesn't require stating molarity and pressure. So C is correct.

its wrong!

To determine which reaction has been chosen as the 'standard' against which every other reaction is measured, we need to consider the concept of standard reduction potentials. Standard reduction potentials are a measure of the tendency of a species to gain electrons and get reduced. The standard reduction potential of a reaction is the potential difference measured when the species is reduced against the standard hydrogen electrode (SHE), which is assigned a standard reduction potential of 0 volts.

In the options provided, the reaction with the standard reduction potential closest to 0 volts is the one chosen as the 'standard' reaction. Let's evaluate the provided options:

A. F2 + 2e ---> 2F–: The standard reduction potential for this reaction is +2.87 volts.
B. O2 + 4H+ + 4e ---->2H2O: The standard reduction potential for this reaction is +1.23 volts.
C. 2H+ + 2e---> H2: The standard reduction potential for this reaction is 0 volts.
D. Li+ + e ---> Li: The standard reduction potential for this reaction is -3.05 volts.
E. H2 + 2e --->2H: The standard reduction potential for this reaction is 0 volts.

Based on the standard reduction potentials, the reaction with a standard reduction potential closest to 0 volts is option C: 2H+ + 2e---> H2. Therefore, your answer is correct.

Please note that the choice of which reaction is selected as the 'standard' can vary depending on the context and application. In this case, the H+/H2 reaction has been chosen as the reference for standard reduction potentials at standard conditions.