MCAT Chemistry Question — Standard State Cell Potential | Next Step Test Prep MCAT Chemistry Question — Standard State Cell Potential | Next Step Test Prep

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The standard reduction potentials for Zn and Cu are given below:

 

Zn2+(aq) + 2e → Zn(s) E°= -0.76 V

Cu+(aq) + e–  → Cu(s)  E°= 0.52 V

 

What is the standard state cell potential for the following reaction?

 

Zn(s) + 2Cu+(aq) → Zn2+(aq) + 2Cu(s)

 

  1. -1.8V
  2. -1.28V
  3. 0.28V
  4. 1.28V

 

Explanation

 

Given the standard reduction potentials:

 

Zn2+(aq) + 2e → Zn(s) E°= -0.76 V

Cu+(aq) + e–  → Cu(s)  E°= 0.52 V

 

The reduction potentials are used as is or multiplied by -1 if the reaction direction is reversed. Stoichiometry does not matter, the reduction potentials are summed for the standard cell potential.

 

Zn(s) + 2Cu+(aq) → Zn2+(aq) + 2Cu(s)

 

0.76V + 0.52V=1.28V

 

  1. -1.8V, incorrect, This uses Zinc in the wrong direction (it should be +0.76V), and copper is also in the wrong direction and multiplied twice. There should be no multiplying for stoichiometric coefficients.
  2. -1.28V, incorrect, This uses Zinc in the wrong direction and copper in the wrong direction.
  3. 0.28V, incorrect, This uses Zinc in the right direction and  Cu in the wrong direction.
  4. 1.28V, correct.

 

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