Electrochemistry (E4)

Question 1

In electrochemical cells, how do electrons from the oxidation reaction reach the reduction reaction?

Answer 1

run through external circuit (wire)

Question 2

In electrochemical cells, what leaves the reduction side and how (through what)? Why is this necessary?

Answer 2

spectator ions move through salt bridge. this is necessary because electrons are flowing in so the spectator ions need to go to the oxidation side to maintain a balanced charge on both sides

Question 3

what connects the wire to each half of the electrochemical cell?

Answer 3

electrode

Question 4

where does the chemistry take place in the electrochemical cell?

Answer 4

on the surface of the electrode

Question 5

what is the electrode on the oxidation side called?

Answer 5

anode

Question 6

what is the electrode on the reduction side called?

Answer 6

cathode

Question 7

what is another name for the reduction side?

Answer 7

cathode side

Question 8

what is another name for the oxidation side?

Answer 8

anode side

Question 9

a chemical reaction can be spontaneous in _____ direction(s)

Answer 9

one

Question 10

what is voltaic cell? what are other names for “voltaic cell”?

Answer 10

a cell in electrochemistry where the reaction occurs spontaneously in one direction. oxidation will occur spontaneously at the anode and the reduction will occur spontaneously at the cathode. i.e. e- pushed from anodic (oxidation) side across wire to cathodic (reduction) side

other names: galvanic cell or battery

Question 11

what is an electrolytic cell?

Answer 11

requires an external power source to drive the chemistry in a non-spontaneous direction

Question 12

how can we change the direction of a chemical reaction?

Answer 12

use external voltage to adjust free energy (lowering free energy in products or reactants)

Question 13

what kind of reaction occurs in an electrochemical cell?

Answer 13

redox reaction

Question 14

oxidation reactions _______ electrons

Answer 14

lose/”produce”

Question 15

reduction reactions _______ electrons

Answer 15

gain/”consume”

Question 16

are electrodes parts of the reaction?

Answer 16

sometimes, but sometimes they simply serve as a means to “deliver/collect” electrons

Question 17

Which way do ions flow across the salt bridge?

Answer 17

depends. positive ions can flow from the anode side to the cathode side or negative ions can flow from the cathode side to the anode side

Question 18

what is specially required for reactions with no metal reactants and no metal products in an electrochemical cell

Answer 18

an inert (non-reactive) metal electrode

Question 19

are ions in the salt bridge part of the chemical reaction?

Answer 19

it depends. they can be, but it could be any strong electrolyte. they don’t have to participate in the chemistry

Question 20

how do you write the short hand notation for an electrochemical cell?

Answer 20

atomic symbol for solid anode | aqueous components of anodic side || reactant of cathodic side | product of cathodic side | cathode (refer to notes)

Question 21

What is SHE? What are the values?

Answer 21

standard hydrogen electrode
2H+(aq) + 2e- —> H2(g)
H+ = 1 M
H2 = 1 atm

Question 22

For half reactions, how do we measure their potential (free) energy? What do we compare it to?

Answer 22

We use the hydrogen electrode as the standard, so the potential energy of a half reaction can be greater or less than SHE. The hydrogen electrode’s standard energy is 0??

We put the hydrogen electrode on one side of the cell and put the other half reaction on the other side of the cell to determine the unknown’s potential energy. We then measure the oxidation/reduction and then determine the reduction potential for the unknown half reaction.

Question 23

Half reactions with positive potential (free energy) are strong ______?

Answer 23

Oxidizing agents -> They are reduced and gain electrons. The more positive the potential, the more they want to take electrons.

Question 24

Half reactions with negative potential (free energy) are strong _______?

Answer 24

Reducing agents -> They are oxidized and lose electrons. The more negative the potential, the more forceful they are in giving electrons.

Question 25

The strongest oxidizing agent is

Answer 25

F2 (Fluorine)

Question 26

The strong reducing agent is

Answer 26

Li (Lithium)

Question 27

Why did we choose the hydrogen electrode as the standard?

Answer 27

I don’t know. We could choose whatever we want, but this is what most people chose. Remember the standard COULD be anything, but then everything must be related to that standard. Eg. Since SHE is often used, F2 will always be +3___ and Li -#___ because hydrogen is used as the “standard” where H+ (aq) is 1M and H2 (g) is 1 atm.

Question 28

For this course, the standard potentials on the table are? The LHS of the equation _____ in oxidizing agent strength as you go _____. The RHS of the equation ______ in reducing agent strength as you go ______.

Answer 28

• reduction potential
• increases, up
• decreases, down

Question 29

oxidizing agents want to be ______. Strong oxidizing agents will ______ electrons

Answer 29

• reduced

- forcefully take

Question 30

reducing agents want to be ______. Strong reducing agents will ______ electrons

Answer 30

• oxidized

- forcefully give

Question 31

lower (eg negative) standard potential energy means

Answer 31

the RHS of the equation is a good reducing agent. non-spontaneous??? reactions goes backwards??? the more negative, the harder it is to reduce that half reaction (remember table is written in reduction half reactions). likewise, the easier it is to oxidize.

Question 32

higher (eg positive) standard potential energy means

Answer 32

the LHS of the equation is a good oxidizing agent. spontaneous??? reaction goes forward??? the more positive, the easier it is to reduce that half reaction (remember the table is written in reduction half reactions). likewise, the harder it is to oxidize.

Question 33

The LHS of the standard reduction table are the half-reactions that are going to be _______. The RHS of the standard reduction table are the half-reactions that are going to be _____.

Answer 33

• reduced

- oxidized

Question 34

Will Ag+ oxidize Fe? Why or why not?

Table link:
http://ch301.cm.utexas.edu/data/standard-potentials.php

Answer 34

Yes. reduction potential of Ag+ > reduction potential of Fe

Ag+ on LHS and Fe on RHS

Question 35

Standard Potential equation for a Cell

Where do we get these values?

Answer 35

E standard of cell = E standard of cathode - E standard of anode

OR (since cell should be written anode || cathode | electrode? eg Pt)

E = right - left

Get values from table of standard reduction potentials

Question 36

Is potential energy dependent on number of electrons?

Answer 36

NO. potential (free energy) is simply the energy difference between two half reactions.

Question 37

the standard potential for a voltaic cell is ____

Answer 37

positive

Question 38

how are standard potential energy and standard free energy related? what does this mean?

Answer 38

negative free energy = spontaneous = positive standard potential energy

likewise

positive free energy = non spontaneous = negative standard potential energy

Question 39

how can we measure free energy difference in electrochemical cells?

Answer 39

measure the difference in electrical potential

Question 40

how do electrons flow in voltaic cells? electrolytic cells?

Answer 40

in both, electrons flow from the anode to the cathode via a wire even though voltaic cells are spontaneous (electrons are driven through the external circuit by the free energy of the spontaneous redox reactions occuring in each half cell) and electrolytic cells are nonspontaneous

Question 41

what is the sign on a cathode for a voltaic cell? why? what is the sign on a cathode for an electrolytic cell? why?

Answer 41

voltaic:
+
drawing in electrons to “feed” reduction, so the cathode must be positively charged

because reaction is not spontaneous, so there’s nothing there to pull/push electrons. so electrons are FORCED onto the cathode, making it negative

Question 42

what is the sign on an anode for a voltaic cell? why? what is the sign on a anode for an electrolytic cell? why?

Answer 42

pushing electrons out, so must be a negative charge

electrolytic:
+
nonspontaneous. electrons are forcefully REMOVED from the anode, making it positive

Question 43

how do we force a reaction to occur in an electrolytic cell?

Answer 43

attach external power supply to overcome uphill in free energy. voltage must be significant enough to overcome the negative potential of the cell itself (which is spontaneous in the reverse direction)

Question 44

the standard potential for an electrolytic cell is _____. its free energy is ______

Answer 44

negative, positive (nonspontaneous)

Question 45

other names for “electrolytic cell”?

Answer 45

electroplating, electrodeposition

Question 46

why would an electrolytic cell be used?

Answer 46

to generate unstable (high free energy) compounds or elements from stable (low free energy) compounds

Question 47

what must we separate in voltaic cells? why? electrolytic cells?

Answer 47

reactants because they could react with one another since voltaic cells are spontaneous

we don’t have to separate reactants in electrolytic cells because electrolytic cells are nonspontaneous, but we DO have to separate the products because they will spontaneously recombine

Question 48

current

Answer 48

the amount of electrical charge that flows during a period of time (charge/time)
eg 1 Amp = 1 Coulomb/sec

Question 49

Why do we use Faraday’s constant? What is it?

Answer 49

to convert charge to numbers of electrons
charge in Coulombs of 1 mole of electrons
= 96485 C/mole. derived from charge on one electron divided by Avogadro’s number (1.602E-19/6.022E23).

formal def’n: the charge in Coulombs carried by one mole of electrons

Question 50

Faraday’s Law of Electrolysis

Answer 50

the amount of product formed or reactant consumed by an electric current is stoichiometrically equivalent to the amount of electrons supplied

Question 51

in electrochemistry, how can the “amount of chemistry” by quantified? any special cases?

Answer 51

measuring current over time. current might change over long periods of time, but it will likely remain constant for short periods.