Chapter 20 - Electrochemistry

Question 1

Balancing Redox Reactions (Acidic)

Answer 1

1. Divide equation into half reactions
2. Balance Half reactions (Add H2O to balance Os and add H+ to balance Hs.
3. Multiply each side by an integer to make sure electrons are equal
4. Combine half reactions to form final redox reaction and cancel species

Question 2

Balancing Redox Reactions (Basic)

Answer 2

1.Divide equation into half reactions
2. Balance Half reactions (Add H2O to balance Os and add H+ to balance Hs.
3. For every H+, add an OH- to each side of the equation. For every H+ and OH- pair, simplify it to H2O
4. Multiply each side by an integer to make sure electrons are equal
5. Combine half reactions to form final redox reaction and cancel species

Question 3

Voltaic Cell

Answer 3

a device where the flow of electrons goes through an external circuit instead of directly between reactants in the same reaction vessel

Question 4

Anode

Answer 4

(-), the electrode at which oxidation occurs

Question 5

Cathode

Answer 5

(+), the electrode at which reduction occurs

Question 6

Electron direction

Answer 6

Electrons always flow from Anode (where oxidation occurs) to Cathode (where reduction occurs) in spontaneous reactions

Question 7

Salt Bridge

Answer 7

allows for ion migration between two substances to maintain neutrality of charge

Question 8

Volt is equal to…

Answer 8

J/C
(Joules/coulombs)

Question 9

Electron value

Answer 9

1.6 x 10^-16 C
1 mol = 96485 C

Question 10

Cell potential

Answer 10

Written as Ecell, measures the potential difference between the two electrodes of a voltaic cell. It is measured in volts. This value should always be positive.

Question 11

Standard Cell Potential

Answer 11

Written as E°cell, this measures the potential difference between the two electrodes of a voltaic cell in standard conditions (1atm, 0° C, 1 M).
The standard Cell Potential is the same as the Standard Reduction potentials, meaning that it can be used to find the values of the standard reduction potentials of the half reactions that make up the whole reaction, using the equation
E°cell = E°red.(cathode) - E°red.(anode)

Question 12

Electrical Potential

Answer 12

Measures the potential energy per electrical charge. Changing the mole value of the equation does not change the potential energy value, as the ratio would remain the same.
Ex:
Zn + 2e- -> Zn E°red = -0.76 V
10Zn + 20e- -> 10Zn E°red = -0.76 V

Question 13

Half Reaction Tables

Answer 13

Tables list half reaction with an assigned E°red value.
The higher the value, the half-reaction will have a higher tendency to become reduced but lower tendency to become oxidized.
Alternatively, the lower the value, the half-reaction will have a lower tendency to become reduced but a higher tendency to become oxidized.

Knowing this, we can also apply our knowledge of oxidizing and reducing agents to this. Those who get reduced are considered oxidizing agents while those who get oxidized are considered reducing agents. Looking at the table, since half-reactions with a high E°red value will be easiest to reduce but most difficult to oxidize, thus meaning that they are potent oxidizing agents. Alternatively, half-reactions with a low E°red value will be hardest to reduce but easiest to oxidize, thus meaning that they are potent reducing agents.
Keep in mind that the oxidizing agents will be on the reactants side of the equation (the one with the electrons) while the reducing agents will be on the products side of the equation.

Question 14

Oxidizing Agents

Answer 14

Halogens (F, Cl, Br, I, At, Ts)
Oxygen (O2)
Oxyanions (anions containing oxygen that follows the format of AxOyZ-)
These species will have positive values of E°red and thus easily undergo reduction

Question 15

Reducing Agents

Answer 15

H2
Alkali metals (Li, Na, K, Rb, Cs, Fr)
Alkaline-Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)
These species have negative values of E°red and thus can easily undergo oxidation

Question 16

Spontaneity of a reaction

Answer 16

E° = E°red(red. process) - E°red(ox. process)
Equation is generalized to determine spontaneity. A negative value means that is it non spontaneous while a positive value indicates the the reaction is spontaneous.

Question 17

Free energy and spontaneity

Answer 17

∆G = -nFE
n = number of moles of electrons transferred
F = 96485 J/V-mol
units is in J/mol
Occurs at constant temperature and pressure

Question 18

Standard State of Free Energy

Answer 18

∆G° = -nFE°

Question 19

E°

Answer 19

E° = ∆G/-nF = -RTlnK/-nF = RT/nF (lnK)

Question 20

Nernst Equation

Answer 20

E = E° - RT/nF (lnQ) = E° - 2.303RT/nF (logQ)

When T = 298:
E = E° - 0.0592V/n (logQ)
units in volts

Question 21

Concentration cells

Answer 21

Same as voltaic cells except it only uses one type of substance. The current is driven by a difference in concentration between the two cells. The concentration cell will attempt to reach an equilibrium of concentrations by transferring electrons from the higher concentration the the cell with lower concentration.

Question 22

Electrolysis

Answer 22

Nonspontaneous reactions that are driven by an external power source to occur. They take place in electrolytic cells.

Question 23

Coulombs

Answer 23

1C = A x S
A = Amperes (charge of current)
S = seconds
This equation is used to find the number of coulombs passing through a cell during a given time frame.

Question 24

Electroplating

Answer 24

Always occurs at the cathode where metal is placed upon it. By using the equation It = nF, we can find the amount of charge passing through the cell.
I = current in amperes or coulombs/sec
t = seconds
n = moles of electrons
F = 96485 C/mol-e