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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 9

Lesson 10: Electrolysis and Faraday's law

# Quantitative electrolysis

Given the amount of electrical charge that passes through an electrolytic cell, we can calculate the quantity of substances consumed or produced during electrolysis (or vice versa). The total charge is related to the magnitude of the current and the time it runs by the equation I = Q/t, where I is the current in amperes, Q is the charge in coulombs, and t is the time in seconds. Created by Jay.

## Want to join the conversation?

• When we calculate amperes over a certain period of time, is that the average amperes? Because we learned that instantaneous E changes as reaction gets closer to equilibrium, and since the concentration of reactants decrease over time, I'd imagine that the reaction gets slower, impacting amperes. If that's the case, then is it average?
(1 vote) • If this were a voltaic cell, then yes the current would decrease as the reaction progressed. However, here we have an electrolytic cell where an external power source is supplying a constant voltage (and therefore a constant current). So the current is constant here because the voltage isn’t originating from the reaction itself, but a battery.

Hope that helps.
• Ok, so I am putting this question here because it is generally about Faraday's constant and all that. Let's say your calculating your change in free energy(delta G). You have the equation delta G=-nFE where n is the moles of electrons, F is the faraday constant, and E is the cell potential(positive for galvanic cells). Does that negative sign apply to the whole equation? And if so, why does the faraday constant never have negative sign if the negative sign is applied to the whole equation? Thanks so much for whoever is gonna help me in advance!
(1 vote) • Before I answer the question I just want to point out that the equation you listed only applies for standard free energy change, ΔG°. Which means that the cell potential is also a standard cell potential, E°. The Nernst equation is how we would relate equilibrium to cell potential and is analogous to the nonstandard Gibbs free energy equation.

Using the equation ΔG° = -nFE°, we can think of the negative sign as multiplying the other three terms by -1. So we could also write it as: ΔG° = -1*nFE°. The negative sign only applies to the -1 and not the other three terms. The only reason it’s there is that it makes positive cell potentials correlate to negative changes in free energy since both represent spontaneous reactions.

Mathematical constants, like Faraday’s constant, are always listed as positive numbers. This eliminates any confusion when using constants as to whether we have to use a positive number or a negative number in formulae.

Hope that helps.
(1 vote)