ELECTROCHEMISTRY Flashcards
What is oxidation?
Oxidation is the loss of electrons, which makes the species more positive
What is an oxidizing agent?
Is low in free energy and wants to gain electrons; drives oxidation of another species
What is reduction?
Reduction if the gain of electrons , which makes the species more negative
What is a reducing agent?
Is high in free energy and wants to lose electrons; drives reduction of another species
If something is oxidized, what happens to the oxidation number?
The oxidation number increases
If something is reduced, what happens to the oxidation number?
The oxidation number decreases
REVIEW QUESTION: What does OIL RIG stand for?
Oxidation is loss; Reduction is gain
What are the rules for assigning oxidation numbers?
- Atoms in their neutral state have an oxidation number of zero
- The oxidation number of monoatomic ions is its charge (e.g. iron (III) is 3+)
- The sum of the oxidation numbers in a chemical species much equal the charge of that chemical species
- When in a compound, hydrogen has an oxidation number of +1
- When bonded to a metal, hydrogen has an oxidation number of -1 –> NOT ON EXAM
- The most electronegative element is assigned its charge as an ion (e.g. in MgBr2, Br is -1 and Mg is 2+)
- When bonded to another oxygen, oxygen has an oxidation number of -1 –> NOT ON EXAM
- When bonded, group 1 is +1, group 2 is +2… group 7 is usually -1
What are the rules for balancing a redox reaction in acidic conditions?
- Identify species for oxidation and reduction
- Split each into their half reactions
- Balance the atoms
- Balance oxygen using H2O
- Balance hydrogen using H+
- Balance the electrons
- Create the overall reaction
What are the rules for balancing a redox reaction in basic conditions?
- Identify species for oxidation and reduction
- Split each into their half reactions
- Balance the atoms
- Balance oxygen using H2O
- Balance hydrogen using H+ AND OH-
- NOTE: OH- is added to both sides of the reaction
- Balance the electrons 7. Create the overall reaction
What is an electrochemical cell?
An electrochemical cell physically separates the oxidation and reduction reactions
What are the components of an electrochemical cell?
- Anode
- Anodic solution
- Cathode
- Cathodic solution
- Salt bridge
What is the purpose of the anode?
The anode serves as the location for oxidation, where electrons flow out into the external circuit
What is the purpose of the cathode?
The cathode serves as the location for reduction, where electrons flow in from the external circuit
* grows a breard
What is the purpose of the salt bridge?
The salt bridge allows the migration of spectator ions to balance the flow of electrons
Which direction do electrons flow in an electrochemical cell?
Electrons always flow from the anode to the cathode
What direction does current flow?
Current flows from cathode to anode (opposite of electrons)
What are the shorthand notation rules for an electrochemical cell?
- A salt bridge is represented by | |
- If the reagents are of the same phase, they are separated by a comma; if the reagents undergo a phase change, they are separated by a |
- The anode is always listed on the left, the cathode on the right
- List all known concentrations
What symbol is used to separate reagents of the same phase?
,
What symbol is used to separate reagents which undergo a phase change?
|
When does an inert electrode need to be added to the equation?
When a species is either aqueous or gaseous
e.g. Ag(s) | Ag+ (aq) | | Fe3+ (aq), Fe2+ (aq) | Pt
What is standard concentration for an electrochemical cell?
1.0 M, which is very concentrated
What is the purpose of a standard hydrogen electrode (SHE)?
This reaction was selected and assigned a zero potential at standard conditions:
2H+ + 2e- → H2(g)
E° = 0 V
Now we can compare everything to this
What are the 2 types of electrochemical cells?
- Galvanic (voltaic) cells
- Electrolytic cells
What is the sign convention for voltaic cells?
The anode (anion) is negative and the cathode (cation) is positive
* signs are as you would expect them to be
What is the sign convention for electrolytic cells?
The anode (anion) is positive and the cathode (cation) is negative
* signs are the reverse as you would expect them to be
What is ΔG, K, and E° for a voltaic cell?
ΔG < 0, K > 1, and E° > 0
What is ΔG, K, and E° for an electrolytic cell?
ΔG > 0, K < 1, and E° < 0
Standard reduction potential chart
Top of chart is strongest reduction - strongest oxidizing agent
Bottom of chart is strongest oxidation (weakest reduction) - strongest reducing agent
What are the equations for standard cell potentials?
- E°cell = E°cathode - E°anode
- Cathode (PULL) - value straight from table
- Anode (PUSH) - value straight from table
- E°cell = E°oxidation + E°reduction
- Oxidation (PUSH) - reverse value from table
- Reduction (PULL) - value straight from table
What is the formula for current?
Current = charge/time
What are the units for current?
Amps (A) = Coulomb/second
What is Faraday’s Constant (F)?
Faraday’s Constant is the change in Coulombs (C) of 1 mole of electron (96,485 C/mole e-)
How do you calculate Faraday’s Constant?
Multiple the charge of an electron (1.602 x 10^-19) by Avagadro’s Number (6.022 x 10^23)
What is the formula for moles of reaction?
Moles created = I * t / n * F = charge applied / charge per mole rxn
* this can be used in replacement of stoichiometry
How do you calculate the charge of an electrochemical cell?
Multiply the current by the given time
How do you calculate the moles of electrons?
Divide the charge of the electrochemical cell by Faraday’s constant
How do you calculate the moles of a species?
Divide the calculated moles of electrons by the coefficient in front of the electrons in the balanced reaction
How do you calculate the mass of the species?
Multiply the moles of that species by its molar mass
What is the formula for work (free energy) at a certain set of conditions?
work = G = -nFE
n is the number of electrons per mole reaction
F is Faraday’s Constant
E is the potential (voltage)
What is the Nernst equation (equation for non-standard cell potential)?
E = E° - (0.0591/n) log Q
…where Q = [product/reactant]
How to solve for Q?
Q = products/reactants
Solids and liquids are not in the expression, only gaseous and aqeous species
Raise to the power of their coefficients (need balanced equation)
How does Q affect voltage?
Increase in Q (add reactant, remove products) = drop in voltage
Decrease in Q (remove reactant, add products) = increase in voltage
What is the Nernst equation at equilibrium (dead battery)?
E° = (0.0591/n) log K
This is because:
E = 0 and,
K = Q
How to calculate Ksp from electrical potential?
If rxn is the Ksp reaction, then
E° = (0.0591/n) log K becomes
E° = (0.0591/n) log Ksp
* make an electrolytic cell where Ksp <<< 1 (E° < 0)
Which equation is flipped in a voltaic cell?
The equation with the most negative E°
Which equation is flipped in an electrolytic cell?
The equation with the most positive E°
What is a battery?
A battery is the ultimate portable voltaic cell
What are the 3 fundamental types of batteries?
- Primary cells
- Non-rechargable
- Non-Reversible
- Secondary cells
- Rechargable
- Reversible
- Fuel Cells
- Re-fillable
What is a primary cell?
A primary cells is a vltaic cell (battery) that is made to be a one-time use battery, meaning that it is a non-chargable cell and only flows in the one direction
They all have the same rxn and thus the same standard potential
They differ in size and this differ in current
What are examples of a primary cell?
Alkaline batteries (AAA, AA, D batteries) with a voltage of 1.5 V and varying currents
What is a secondary cell?
A secondary cell can be recharged over and over again during its lifetime
What are examples of a secondary cell?
Car battery (lead acid battery - a specifi type that involves lead in 3 solid oxidized states (Pb(s), PbSO4(s), PbO2(s))
- Pb is the most reduced form (oxidation); PbO2 is the most oxidized form (reduction)
- Both discharge into PbSO4
- Recharge into PbO2 (oxidation) and Pb (reduction)
What is a fuel cell?
A fuel cell is designed to have the active redox materials “refilled” as they get depleted, meaning they behave like how your car works: you don’t buy a new gas tank when you run out of gas, you buy more gasoline
2H2 + O2 → 2H2O
Half Rxns:
[H2 → 2H+ + 2e-] x2
O2 + 4H+ + 4e- → 2H2O
What are examples of a fuel cell?
Rockets
What is potential (E)?
Potential is essential the voltage of a battery, or rather the amount of work (free energy) needed to move charge
What is the relationship of potential to the battery?
The larger the potential (voltage), the more useful the battery
What is capacity (Q)?
Capacity (total energy) is essentially the amount of reactants, or rather the charge extracted from the reactants
What is the formula for capacity?
Q = I * t
What is the unit for capacity?
Coulombs (C)
What does capacity tell you about a battery?
The capacity of a battery tells us how much total charge a battery can generate
What is current (I)?
Current it charge per time, or rather how quickly the electrons move
What is power (P)?
Power is the potential times the current, or rather how much energy is transfered AND how quickly
What is the formula for power?
P = V * Q / t = V * I
What is the unit for power?
Watts = J/second
How is power related to current?
The higher the power, the higher the current
How is power related to potential?
The higher the power, the higher the potential (voltage)
What does cycle life of a battery indicate?
Cycle life indicates how well the battery recharges
What are the ideal qualities for a battery?
- Light
- Small
- Solid phase
What is a dry cell?
A dry cell is essentially a battery without any liquids (the electrolyte was contained in a paste); voltage is help constant and often have very low currents
Are dry cells reversible?
No
Are dry cells rechargable?
No
How fast are the reactions of dry cells?
They are very slow reactions
How does a concentrated cell produce a voltage?
Voltage is produced when electrons travel to the more concentrated cell because it has a lower free energy - use differences in G
What is E° of a concentrated cell?
E°cell = 0
What is K for a concentracted cell?
K = 1
What does low concentration say about the anode?
Low concentration indicated a low entropy (S) and a high free energy (G)
What does high concentration say about the cathode?
High concentration indicates mixing, high entropy (S), and low free energy (G)
Common equation for electrolysis in 1 beaker:
2NaCl → 2Na + Cl2
Na+ + e- → Na
2Cl- → Cl2 + 2e-
How do you solve for Ksp/K/Q?
Ksp/K/Q = 10 ^ (n)(E°)/0.0591
