12. Electrochemistry DONE

Question 1

What is the EMF (electron motive force)?

How is it related to ∆G?

Answer 1

EMF: Corresponds to the voltage or electrical potential difference of the cell.

EMF is also noted as E cell

if the EMF is positive, the cell is able to release energy, (∆G <0) so it’s spontaneous
If the EMF is negative, the cell must absorb energy (∆G >0), which means it is non-spontaneous.

Question 2

How is EMF (E°_cell) calculated?

Answer 2

The EMF is calculated by
EMF (E°_cell)= E°_{red cathode} - E°_{red anode}

If EMF/E°_cell is +, it is spontaneous
If EMF/E°_cell is -, it is not spontaneous (electrolytic cell)

Question 3

What is the difference between a galvanic cell and a electrolytic cell?

Answer 3

Galvanic Cell: spontaneous (+ EMF, - ∆G)
Anode is (-), Cathode is (+)

Electrolytic cell: NOT spontaneous (- EMF, + ∆G), require electricity
Anode is (+), Cathode is (-)

Galvanic cells and electrolytic cells are the opposite.

EMF= E°_cell

Question 4

In electrochemical cells, what way do the electrons run and what way does the current run?

Answer 4

They run in OPPOSITE directions.

Electrons always runs from anode to cathode, regardless whether it is a electrolytic cell or galvanic cell.

Current always runs in the opposite direction of the flow of electrons, from cathode to anode

Question 5

Where does oxidation and reduction occur in:
Galvanic Cell
Electrolytic cell

Answer 5

Galvanic Cell: oxidation (anode) Reduction (cathode) AN OX REDCAT
Electrolytic cell: oxidation (anode) reduction (cathode)

Regardless of cell type, reduction ALWAYS occurs in the cathode, and reduction ALWAYS occurs in the anode

Question 6

Between two electrochemical cells, what does the salt bridge do?

Answer 6

It releases anions to neutralize the solution, otherwise each solution would become too charged, and the circuit would not function.

Book: The purpose of the salt bridge is to exchange anions and cations to balance, or dissipate, newly generated charges.

Question 7

What is Faraday’s law?

What is the equation?

Answer 7

It states that the liberation of gas and deposition of elements on electrodes is direction proportional to the number of electrons being transferred during oxidation reduction reactions (i.e. it has stoichiometric properties)

Question 8

What is the surge current?

Answer 8

It is an above-average current transiently released at the beginning of the discharge phase; it wanes rapidly until a stable current is achieved.

Question 9

In a Galvanic cell, decide whether the answer is an anode or cathode.

Site of Oxidation:
Electrons flows toward it:
Current flows towards it:
Has (-) designation:
Attracts cations:

Answer 9

Site of Oxidation: Anode (AnOX, RedCAT)
Electrons flows toward it: Cathode
Current flows towards it: Anode (opposite of electron charge)
Has (-) designation: anode (repels (in essence) the electrons) (Because the anode of a galvanic cell is the source of electrons, it is considered the negative electrode)
Attracts cations: Cathode (because it is so negative)

Question 10

What type of cell (galvanic or electrolytic) has a +∆G
What type has a positive E°_cell?

Answer 10

+∆G= electrolytic (non spontaneous). If ∆G were negative, it would be galvanic.

E°_cell= if it is positive, it is spontaneous. If it is negative, it is not spontaneous, so would be a galvanic cell.

Question 11

If you wanted to solve this problem, what would you do?

How much current is required to produce .23 kg Na from a molten NaCl electrolytic cell that runs for 30 hours? Assume 100% efficiency.

Answer 11

1. Convert .23kg to grams, and then find moles.
2. Convert 30 hours to seconds.
3. Moles of electrons per each mole of NaCl= 1
4. Remember Faradays constant (~97,000 C)

Use the equation:

Answer = ~9A

Question 12

In an electrochemical cell, what is a electrode?

Answer 12

They are strips of metal or other conductive material placed in a electrolyte solution.

Question 13

What factor can influence batteries?

Answer 13

Temperature

Question 14

How do you write out a cell diagram for an electrochemical (galvonic) cell?

Answer 14

1. The reactants and products are always listed from left to right in this form: anode / anode solution // Cathode solution/ Cathode
2. A single vertical line represents a phase boundary.
3. A double vertical line indicates the presence of a salt bridge or some other type of barrier.

Question 15

What is the the equivalent of Faraday’s Constant (i.e. what is it)

Answer 15

97,000 C

Question 16

What type of cell is a neuron? (thinking of this with electrochemistry)? What does this mean?

Answer 16

Neurons are a type of concentration cells (a type of Galvanic cell).

In this type of cell, the electrodes are chemically identical. Therefore, the current is generated as a function of a concentration gradient between the two solutions surrounding the electrodes.

Question 17

Is the anode considered positive or negative in electrolytic cell? In the galvonic cell?

Answer 17

Galvonic: anode (-), cathode (+)
Electrolytic: anode (+), cathode (-)

Reduction/Oxidation does NOT change

Question 18

In an electrolytic cell, where does reduction take place? What about in a galvonic cell?

Answer 18

Oxidation ALWAYS takes place in the anode, and reduction always takes place in the Cathode

Question 19

Where will anions and cations be attracted to (anode vs cathode)?

(electrochemistry)

Answer 19

Anions will always be attracted to the anode.
Cations will always be attracted to the cathode.

In this, the cations in the solution are attracted to the cathode, and form a salt on the cathode)

This is true regardless of the type of cell (galvanic/electrolytic)

Question 20

What occurs where in an electrolytic cell?

Site of Oxidation:
Electrons flows toward it:
Current flows towards it:
Has (-) designation:
Attracts cations:

Answer 20

Site of Oxidation: Anode
Electrons flows toward it: Cathode
Current flows towards it: anode
Has (-) designation: Cathode
Attracts cations: Cathode

Question 21

What is the reduction potential, and how does it relate to understanding redox equations and batteries?

Answer 21

The reduction potential quantifies the tendency for a species to gain electrons and be reduced.

The higher the reduction potential (+), the more likely that it is to be reduced (be the cathode)

The reduction potential is given in books.

Question 22

What is the standard electromotive force, and how does this relate to the Standard reduction potential?

This assumes standard conditions (298 K, 1 atm, and 1M concentrations)

What is the equation?

Answer 22

Standard EMF (E°cell) is the difference between the reduction potential of two half reactions in the battery.

If the difference is Positive, it will occur spontaneously (galvanic).

If the difference is negative, it will not be spontaneous and require energy (electrolytic)

Question 23

How are standard reduction potentials measured?

Answer 23

A sample is measured by setting up a cell relative to a standard hydrogen electrode, which is given a reduction potential of 0 V by convention

Question 24

If a cell’s electromotive force (emf) is denoted as a positive value, what does that mean? What if it is negative?

Answer 24

Positive EMF: is a galvanic reaction, occurs spontaneously
Negative EMF: is a electrolytic reaction, does not occur spontaneously

Question 25

Answer 25

E°_cell = -0.59 V= electrolytic
E°_cell = +.25 V= galvanic

** remember, if E°_cellis (+), it will occur spontaneously (galvanic)

Question 26

-

Answer 26

1. =0.534V – (– 0.036 V)= .57 V
2. Iodine has the higher standard reduction potential, so it’s the cathode.

Remember, it is Cathode- Anode (minus a negative number is addition)

Question 27

How do you use the reduction potentials to determine what is the cathode and what is the anode?

Difference between electrolytic cells and galvanic cells:

Answer 27

Galvanic cells:
The more positive one is reduced (i.e. is the cathode)
The more negative one is oxidized (i.e. the anode)

NEED TO VERIFY THIS WITH THE BOOK
Electrolytic cells:
The more positive one is oxidized (the anode)
The more negative one is reduced (the cathode)

The electrode with the more positive reduction potential is forced by the external source to be oxidized, and is therefore the anode.
The electrode with the less positive reduction potential is forced to be reduced, and is therefore, the cathode

Question 28

Answer 28

Question 29

True or false:
The potential of each electrode does not depend on the size of the electrode (the amount of material), but rather on the identity.

Answer 29

True!

The standard reduction potential of an electrode will not change unless the chemical identity of that electrode is changed.

Question 30

What is the equation that describes the relationship between ∆G° and EMF (electromotor force) ?

Assuming standard conditions.

Answer 30

∆G°= -nFE°cell

∆G°= standard change in free energy
n= number of moles of electrons
F= faraday constant= 1F= 96,485 C
E°_cell= EMF of the cell

Important= if Faraday Constant is expressed in coulombs, then ∆G must be expressed in J, not kJ

Question 31

If ∆G° is positive, what is E°_cell?
iF ∆G° is negative, what is E°_cell?
iF o ∆G° is 0, what is E°_cell?

Answer 31

If ∆G° is positive, what is E°_cell? = negative (non-spontaneous reaction)
iF ∆G° is negative, what is E°_cell? = positive (spontaneous reaction
iF o ∆G° is 0, what is E°_cell? (occurs in concentration cells

Question 32

What is the Nernst equation, and what is it used for?

Answer 32

describes the relationship between the concentration of species in a solution under non-standard conditions and the electromotive force.

NON-STANDARD conditions is the key here.

E_cell=EMF under nonstandard conditions
E°_cell= emf of the cell under standard conditions
R= ideal gas constant
T= temperature, in Kelvin
n= number of moles
F= farady constant= 1F= 96,485 C
Q= reaction quotient for the reaction at a given point in time (remember, don’t include solids or pure liquids

Question 33

What is the relationship between K_eq and E°_cell?

Answer 33

Remember, the K_eq is an equilibrium constant. When it is less than one, it means that there are more products on the bottom than on the top, therefore, it will not happen spontaneously. If it does not happen spontaneously, the E_cell is negative.

When K_eq (the ratio of products’ concentrations at equilibrium over reactants’ raise to their stoichiometric coefficients) is greater than 1, E°_cell is positive
When K_eq is less than 1, E°_cell is negative
When K_eqis equal to 1, E°_cell is 0