The voltage of an electrochemical cell is also related to the Gibbs free energy
ID: 1001335 • Letter: T
Question
The voltage of an electrochemical cell is also related to the Gibbs free energy of its chemical reaction through the relationship of G = -nFE, where n is the number of electrons transferred in the balanced equation, E is the cell's voltage, and F is the Faraday constant (96,485 C/mol). (a) In order for a reaction to occur (be spontaneous), what must be the sign (+/-) for G? (b) Under spontaneous circumstances, then, what must be the sign (+/-) for E (your voltage)? (c) Knowing the answers to (a) and (b) above, how can one deduce the direction of spontaneity for a particular voltaic cell reaction by using only a voltmeter without doing any calculations
Explanation / Answer
We know the spontaneity criteria for the given reaction as,
i) If, G < 0 i.e. G = – ve then reaction is spontaneous in given direction.
ii) If G > 0 i.e. G = +ve then reaction is non-spontaneous in given direction.
iii) If G = 0 Reaction said to be at equilibrium.
The voltage of an electrochemical cell (E) , Gibbs free energy change (G) are related to each other as,
G = – nFE
Where,
n is the number of electrons transferred in the balanced equation and is +ve always
F is the Faraday constant (96,485 C/mol) and is also +ve always.
E is the cell potential which can be either +ve/–ve
From above equation its clear that,
i) If, E > 0 i.e. E = +ve then G = – ve
ii) If E < 0 i.e. E = –ve then G = + ve
Hence we can now have spontaneity criteria in terms of E,
A) If, E > 0 i.e. E = +ve then G = – ve and spontaneous will be the reaction.
B) If E < 0 i.e. E = –ve then G = + ve and non-spontaneous will be the reaction.
Based on this let us answer the questions.
Q.(a) In order for a reaction to occur (be spontaneous), what must be the sign (+/-) for G?
Answer: G must have –ve sign for a spontaneous reaction.
Q.(b) Under spontaneous circumstances, then, what must be the sign (+/-) for E (your voltage)?
Answer: E must have +ve sign for spontaneous reaction.
Q.(c) Reading of Voltameter:
Answer:
1) A voltmeter is used to measure the potential of an electrochemical cell.
2) Voltmeters have a +ve and –ve terminal and, by convention, a red wire is connected to +ve terminal and a black wire is connected to the –ve terminal.
3)The sign of the reading on the voltmeter tells us the spontaneous direction of the electron flow. Spontaneous redox reaction corresponds to flow of electrons from Anode to the cathode. If the black wire is connected to the Anode and the red wire is connected to the cathode, then the voltmeter will show a positive value.
I.e. For spontaneous reaction Voltameter shows +ve value.
=========================XXXXXXXXXXXXXXXXXX===========================================
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.