Chemistry Chapter 8: Redox Reactions and Electrochemistry Flashcards
A concentration cell is a type of
galvanic cell
Definition of oxidation state
The oxidation state of an atom is a hypothetical charge it would have if all bonds were considered to be 100% ionic.
Measure of electron distribution in a chemical species
Pure element’s oxidation state:
0
Monoatomic ions oxidation state:
Same as charge. Ie Fe2+ is +2
Sum of oxidation states
= charge of molecule
Oxidation state of fluorine
-1
Oxidation state of halogens
-1 unless bonded to more electronegative halogen or N or O (could be +1, 3, 5, 7)
Standard Reduction Potential
Measure of how much a species likes to be REDUCED. The standard cell potential is the potential difference between the cathode and anode.
Measured in volts
E°
All standard reduction potentials are measured relative to the
hydrogen electrode (0V)
The more positive the reduction potential
the more the species likes to be reduced (spontaneous)
the more negative the reduction potential
the more the species doesn’t to be reduced (nonspontaneous)
which species will be reduced and which will be oxidized? What do you do to the oxidized species?
These are reduction potentials so Ag will be reduced.
Make sure to reverse the oxidized species
Do you need to balance reduction and oxidization potentials?
NO. Reduction and oxidization potentials are not affected by stiochiometric constants. IE if you double the products, you still get the same voltages for reduction potential
What reaction takes place at the cathode? (regardless if it’s a galvanic or electrolytic cell)
Reduction
red cat
What reaction takes place at the anode? (regardless if it’s a galvanic or electrolytic cell)
oxidation
an ox
Galvanic Cell
spontaneous reduction reaction is used to generate a POSITIVE potential difference that drives a current
In galvanic cells, E°cell must always be
POSITIVE because spontaneous
How to find reaction direction in a galvanic cell
Method 1: Determine reaction at cathode and anode. Reverse the half-reaction at the anode and flip RP sign. Add potentials.
Method 2: use E°cell = E°cathode - E°anode (USE ONLY REDUCTION potentials)
Daniell Cell - why does it have a salt bridge?
common set up for a galvanic cell
the anode comes more positive and the cathode becomes more negative
so this charge imbalance would grow that electrons are repelled from cathode and therefore reaction would stop
what does this shorthand mean
Concentration Cell conditions
1) Electrodes must be the same material
2) two regions must have concentration difference
type of galvanic cell
Electrolytic cell
Electrical energy is used to drive nonspontaneous redox reactions
Electrolytic cells always have __________ E°cell values
NEGATIVE because they are non spontaneous
true or false
Electrodes in electrolytic cells are conductive but won’t engage in unwanted side reactions
true
Cathode in electrolytic cell is (positive/negative?)
negative
Anode in electrolytic cell is (positive/negative?)
Positive
Regardless of cell type, electrons always move from
Anode to Cathode
Electroplating
Nonspontaneous reduction of metal ions in solution. Results in deposition of solid metal onto the cathode
Equation for mols of metal plated
n = mol e- transferred and F = Faraday’s constant = 1 x 10^5C/mol e-.
remember that Current can also be thought of as C/s
When a battery is discharging it’s acting as a
galvanic cell
When a battery is charging it’s acting as a
electrolytic cell
Examples of rechargeable batteries
Lead Acid and Nickel Cadmium
Why use the nernst equation?
helps account for how electrical potential of a cell is affected by temperature and concentration of reactions
what is the nernst equation
The Nernst equation is used to calculate the electrode potential (E) of an electrochemical cell under non-standard conditions, where ion concentrations are different from 1 M and/or the temperature is not 25°C.
Under standard conditions (25C) how can you simplify the nernst equation?
Calculating free energy of a redox reaction in an electrochemical cell
If delta G is’t given, what do you do for chemical cells
Spontaneous reaction has what delta G, Ecell and Keq?
Non-spontaneous reaction has what delta G, Ecell and Keq?
How to solve this question
1) figure out what is oxidized and what is reduced at the anode/cathode for a galvanic cell. IE Ni2+ +2e- –> Ni is reduced at the cathode
2) REVERSE this for the electrolytic cell (recharging). ie Cd2+ + 2e- –> Cd is reduced at the cathode. Therefore, Cd is deposited at the cathode
Do you need a salt bridge for an electrolytic cell?
NO In an electrolytic cell, an external power source drives the reaction non-spontaneously.
Since the external voltage forces electrons to move, there’s no need for a salt bridge to maintain charge neutrality.
Instead, ions in the same electrolyte solution (or sometimes a porous membrane) help complete the circuit.
Why do you need a salt bridge for galvanic cells?
Because there is a build up of negative charge on the cathode (since the electrons flow from anode to cathode).
the circuit grinds to a halt if there’s a build up negative charge because cathode becomes LESS positive (and we need
therefore the salt bridge lets positive ions to travel to the cathode to balance out negative charge
Difference between which charge to follow for circuits vs chemical cells
Follow electrons for chemical reactions.
In galvanic cells, electrons flow from the negative anode to the positive cathode.
Follow current (positive charge) for circuits.
Conventional current flows from the positive cathode to the negative anode.
When NAD+ gets reduced, why does a H+ get added (since H+ is positive)
NAD+ gains 2 electrons (from oxidized species)
