Question involving voltage and galvanic batteries

In summary: I'm stumped.In summary, the student is struggling with a physics question and believes that it may be related to the Kaplan tests. The student is not taking any expensive study courses and believes that the answer may be found through reversing one equation and adding it to the other.
  • #1
dungy
2
0
I'm currently studying for the MCAT and most of the physics questions I get wrong, the explanations adequately help me. But, one question has me stumped.

"Question:"

What is the standard emf for the galvanic cell in which the following overall reaction occurs?

2Na(s) + Cl2(g) -> 2Na+(aq) + 2Cl–(aq)

Half-reaction (V) E° red
Na+(aq) + e– -> Na(s) –2.71
Cl2(g) + 2e– -> 2Cl–(aq) +1.36

A) –1.35 V
B) +1.35 V
C) +4.07 V
D) +6.78 V

"end of question"

The answer they provide is:
According to the table, the oxidation of Na has a potential of +2.71 V and the reduction of Cl2 has a potential of +1.36 V. The standard emf for the galvanic cell is therefore +2.71 V + 1.36 V = +4.07 V. Thus, C is the best answer.

My understanding was, to calculate the emf, I would have needed to double the charge during the reduction of sodium because 2 moles are used in the cell. But the Kaplan test clearly claims that the reduction of sodium is only counted once. Why is this? I chose D) +6.78 V

I'm tired of getting 12 every time I write the physics section practice, it's my weakest. I want better.

No, I'm not taking any silly expensive study courses to help me.
 
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  • #2
I think you're right, it's like chemistry thermodynamics exercises, you reverse one equation, and add it to the other after multiplying (this time by two to the first half equation) to get the target equation and the voltage term.
 
  • #3
That's what I thought...

I would have chalked it up to the test being wrong, but this is the second time it would have made exactly the same mistake. These are questions from the official Kaplan tests and I haven't seen any other answers that made no sense to me, so it seems like an awfully large coincidence.
 

What is voltage?

Voltage is a measure of the electric potential difference or the amount of electrical energy that is carried by an electric current.

How is voltage measured?

Voltage is typically measured using a voltmeter, which is a device that can measure the potential difference between two points in an electrical circuit.

What is a galvanic battery?

A galvanic battery is a type of electrochemical cell that converts chemical energy into electrical energy. It consists of two electrodes, an anode and a cathode, and an electrolyte solution that allows for the transfer of ions between the electrodes.

How does a galvanic battery produce electricity?

In a galvanic battery, a chemical reaction occurs between the anode and the cathode, causing a flow of electrons from the anode to the cathode. This flow of electrons creates an electrical current that can be used to power devices.

What factors affect the voltage of a galvanic battery?

The voltage of a galvanic battery is affected by the type of materials used for the electrodes, the concentration and type of electrolyte solution, and the temperature of the battery. These factors can impact the rate of the chemical reaction and the flow of electrons within the battery.

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