Unit 9: Topic 9 - Cell Potential Under Nonstandard Conditions Flashcards
Consider the electrochemical reaction Cu (2+) + Zn –> Zn(2+) + Cu. The concentration of Zn 2+ is decreased. How does this affect the cell potential of the reaction?
This decrease in the concentration of a product causes a large decrease in Q. A decrease in Q will make the reaction more spontaneous and therefore drive it to the right. This causes a much more negative ∆G, which will increase ∆E. Similarly, if Q was increased, the cell potential ∆E would decrease. Therefore, concentrations of reactants and products have an effect on the cell potential.
Why is Le Chatelier’s principles not effective when explaining electrochemical systems?
Electrochemical systems are not in equilibrium. Le Chatelier’s principles explain what occurs when a system in equilibrium is disturbed, but this is not the case for an electrochemical system.
Explain how the standard potential is related to the reaction quotient and what occurs if the reaction quotient is altered.
For a cell under standard conditions, Q = 1, and E = 0. This is because the concentrations of the system under standard conditions are equal, and the reaction quotient equals 1. If Q < K, then the reaction proceeds towards the products to reach equilibrium, which causes E to increase as the reaction is driven toward the right. If Q > K, then E decreases because the reaction is driven toward the left and toward the reactants. The sign of E also determines the spontaneity: a positive E gives a spontaneous reaction.
Define what a concentration cell is. How can you determine the direction of spontaneous electron flow, and how can you determine the cell potential?
A concentration cell is an electrolytic cell where both cells contain the same electrode but in differing concentrations. The concentration cell dilutes the more concentrated solution and concentrates the more dilute solution, eventually reaching equilibrium and creating a voltage. The direction of spontaneous electron flow is from the lower concentration cell to the higher concentration cell, and the cell potential can be determined from Nernst Equation (discussed below).
Does the Nernst Equation qualititatively make sense?
E = Eº - RT/nF ln(Q). If Q is decreased, we should expect an increase in E, and we do from this equation, since a decrease in Q results in a higher E. Similarly, an increase in Q causes a decrease in E. The RT/nF ln(Q) term accounts for all possible changes to concentration, and makes sense intuitively.
