# E° cell equation for MCAT/OAT/DAT

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#### tbearOD

##### Full Member
2+ Year Member
i was wondering when figuring out the value of E° cell which equation do i use?

1. cathode - anode

or

2. cathode + anode

the OAT Kaplan book is gave the second equation but the Kaplan MCAT book is giving me the first equation

E° cell = E° cathode - E° anode

Moving to MCAT Q&A forum.

The first equation is correct.

E° cell = E° cathode - E° anode

then do you know why the DAT bootcamp Equation sheet has
°=°+°
°=°ℎ+°

am i missing something?

A question from the ADA opted OAT sample test uses: °=°ℎ+°

Given the following half-cell reactions:

Cl2 (g) + 2e– 2Cl– (aq) E° = +1.36v
Cu2+ (aq)+ 2e– Cu(s) E °=+0.34v
what is the value of E° for the following reaction?
Cu2+ (aq) + 2Cl– (aq) Cu(s) + Cl2 (g)

A.–2.38v
B. –1.70v
C. –1.02v
D. +1.02v
E. +1.70v

whereas another question uses: °=°ℎ - °

what is the standard cell potential
Zn2+ + 2e- --> Zn(s) E° = -0.763
Cu2+ + 2e- --> Cu(s) E° = +0.337

A.–1.10v
B. –0.430v
C. +0.430v
D. +1.10v
E. +1.78v

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Is this an MCAT question or a DAT question? The sign difference depends on how the anode or cathode half-reactions are written.

The quoted information above is a great source. I would go there to get a better grasp on electrochem.

For a quick answer (but you should read that page) is that it is possible a mistake in their writings.

Both of those answers are correct.

For question 1 - in the way the equation is written you are forced to calculate the E standard in the forward direction. Therefore, cathode - anode is 0.34 - (+1.36)

For question 2 - it does not provide you with an equation so you are forced to find the most positive E standard. 0.337 - (-0.763)

My advice is understand the equation is Cathode - Anode. Keep your signs how they are presented and plug it into the equation. Maybe they have the addition to skip a step under certain circumstances? Anyways, read the quoted text above.

Good luck