Exam 3 Pt. 4 - Electrochemistry

What is cell potential, or Ecell?

• The difference between cathode and anode

Ecell = Ecathode-Eanode

What half cell or element is used as a universal reference for cell potential? What is the voltage?

• H+; 0 V

What does + Ecell mean?

• Favored; spontaneous

opposite of -ΔG

What does - Ecell mean?

• Not favored; nonspontaneous 

opposite of +ΔG

When asked to predict spontaneity of a cathode and anode reaction at standard conditions, what steps do you take?

1. Assign oxidation state/numbers

2. Write half reactions based on oxidation/reduction

3. Find Ecell standard values (DO NOT CHANGE)

Do cathode and anode reactions need to be balanced at first?

• No

When should cathode and anode reaction be balanced?

• When trying to n = mols of electrons

When only given half reactions of two elements, how do you determine which one is the cathode and anode?

• Arrange them in such a way that they will produce a positive value (spontaneous)

so reaction will occur

What are the units of Ecell standard (two versions)?

• V or J/C

What does the unit C stand for? What constant is it used in?

• Coulombs; Faraday’s constant

What equation represents the relationship between E° and ΔG?

• ΔG = -nFE°cell 

What two forms of equation represent the relationship between E°cell and K?

• E°cell = (RT/nF)lnK

• E°cell =(0,0592V/n)logK

What equation represents the relationship between ΔG and K?

• ΔG° = -RTlnK

When given a voltaic cell reaction, how can you find E°cell, ΔG, and K?

• Use E°cell (cathode - anode); then use the triangle 

What does ΔG and E°cell values represent a very spontaneous reaction?

• ΔG; very large negative number

• E°cell; large positive number

What does it mean when the K value is very large versus very small?

• Large K; favored (producing more product and driven to near completion)

• Small K; not favored (producing more reactant)

What does it mean to comment on composition of equilibrium mixture and spontaneity?

• State whether spontaneous or not

• Explain what the K value means

When can you only use the second form?

• Only use when temp. is 25 degrees celsius 

How do you cancel out ln?

• e^()

How do you cancel out log?

• 10^()

What two forms of equations represent calculating Ecell at nonstandard conditions?

• Ecell = E°cell - (RT/nF)lnQ

• Ecell = E°cell - (0.0592/n)logQ

How do you write reaction quotient Q?

• Q = products^()/reactants^()

NO SOLIDS OR LIQUIDs write 1, coefficients 

How do you know if a voltaic cell equation is at standard or nonstandard conditions?

• Concentrations NOT at 1M

• Pressures NOT at 1 atm

What is a concentration cell?

• Cell made of same material/element but different concentration

When writing half reactions of a concentration cell what should you write to keep track of the different molarities?

• Write overall equation; know which concentration is reactant vs. product