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Midterm 3: Chapter 20 Electrochemistry
Terms in this set (63)
What is a redox reaction?
A reaction in which certain atoms undergo changes in oxidation states
What is an oxidizing agent or oxidant?
A substance that receives electrons or causes
another substance to be oxidized
What is a reducing agent?
A substance that gives up electrons, thereby causing another substance to be reduced
What is a voltaic (galvanic) cell?
A device in which a spontaneous redox reaction occurs with the passage of electrons through an external circuit
What is an anode?
The electrode at which oxidation occurs
What is a cathode?
The electrode at which reduction occurs
An anode will:
Decrease in mass
A cathode will:
Increase in mass
What is the standard cell potential (also known as standard emf)?
The emf of a cell when all reagents are at standard conditions.
How is the standard cell potential (emf) denoted?
How can we relate the standard emf to standard reduction potentials of the reduction (cathode) and oxidation (anode) half-reactions
E°cell = E°red(cathode)−E°red(anode)
What is a standard reduction potential (also called a standard electrode potential)?
The potential of a reduction half-reaction under standard conditions, measured relative to the standard hydrogen electrode
What does the equation ΔG=−nFE, represent?
It should the change in free-energy depends on the negative sum of the positive number of electrons being transferred in moles but does not have units (n) multiplied by Faraday's constant (F) and the emf (E) of a reaction
What is Faraday's constant?
It is 1 mol of electrons' electric charge, 96,485 C/mol e^- or 96,485 J/V-mol
Because both n and F are positive numbers, if E is positive then G will be:
Because both n and F are positive numbers, if E is positive then the reaction will be:
Spontaneous because G will be negative
If we provided with E° we can find ΔG° by using the expression:
How are the reactions ΔG°=−nFE° and ΔG=−nFE different?
ΔG°=−nFE° uses the standard cell potential to find the change in standard free-energy, but ΔG=−nFE uses cell potential to find the change in free-energy
If we provided with ΔG° we can find K by using the expression:
If we provided with K we can find E° by using the expression:
If we provided with E° we can find K by using the expression:
If we provided with K we can find ΔG° by using the expression:
If we provided with ΔG° we can find E° by using the expression:
What is the Nernst equation at standard conditions?
E=E°−((0.0592 V)/(n))(log Q)
What does the Nernst equation allow us to find?
The emf (E) produced by a cell under nonstandard conditions or to determine the concentration of a reactant or product by measuring E for the cell
What is the Nernst equation at non-standard conditions?
What is a concentration cell?
A type of galvanic cell in which the species are the same, but in difference is concentrations
A substance in being reduced if it:
A substance in being oxidized if it:
When we assign an electrical potential to a half-reaction, we write it as a:
Why do standard reduction potentials not vary according to stoichiometry?
They are measured potential energies per electrical charge so they are intensive properties
What is a battery?
A portable, self-contained electrochemical power source that consists of one or more voltaic cells
What is a primary battery?
One that cannot be recharged
What is a secondary battery?
One that can be recharged
What is the balanced reduction reaction (cathode) that occurs in a lead battery?
PbO2(s) + HSO4^− (aq) + 3H^+ (aq) + 2e^− → PbSO4(s) + 2H2O(l)
What is the balanced oxidizing reaction (anode) that occurs in a lead battery?
Pb(s) + HSO4^− (aq)
→ PbSO4(s) + H^+ (aq) + 2e^−
The most common primary (non rechargeable) battery is the:
What is the balanced reduction reaction (cathode) for an alkaline battery?
2 MnO2(s) + 2H2O(l) + 2e^− → 2MnO(OH)(s) + 2OH^−(aq)
What is the balanced oxidizing reaction (anode) for an alkaline battery?
Zn(s) + 2OH^− (aq) → Zn(OH)2(s) + 2e^−
What is the balanced reduction reaction (cathode) for a nickel-cadmium, and nickel-metal hydride battery?
2NiO(OH)(s) + 2H2O(l) + 2e^− → 2Ni(OH)2(s) + 2OH^−(aq)
What is the balanced oxidizing reaction (anode) for a nickel-cadmium, and nickel-metal hydride battery?
Cd(s) + 2OH^−(aq) → Cd(OH)2(s) + 2e^−
Which type of battery has a greater specific energy density (the amount of energy stored per unit mass), a lithium ion battery or nickel-based batteries?
Li-ion batteries because they are extremely light
Why do Li-ion batteries produce higher voltages per cell compared to other batteries?
A lithium ion has a very large negative standard potential
A Li-ion battery produces a maximum voltage of ___ per cell
A nickel-cadmium and nickel-metal hydride battery generates a maximum voltage of ___ per cell
Why can a Li-ion battery deliver more power than other batteries of comparable size which is referred to as a higher volumetric energy density (the amount of energy stored per unit volume).
Li-ion batteries can produce a maximum voltage per cell that is roughly 3x larger than nickel batteries
What is an electrolytic cell?
use electrical energy from an external source to drive a non-spontaneous (ΔG > 0) cell reaction
What is electroplating?
Using electrolysis to coat the surface of one metal with another metal
What is quantitative electrolysis?
The flow of electricity is related to the stoichiometry, "yield", and rate of an electrolysis process (How much product is formed? How long will it take? etc.)
How is the quantity of charge passing through an electrical circuit, such as that in an electrolytic cell, generally measured?
What is a coulomb?
The quantity of charge passing a point in a circuit in 1 second when the current is 1 ampere (A) (C = amperes * seconds)
What is corrosion?
A spontaneous redox reactions where metals react with substances in their environment and are gradually destroyed
What is rusting?
The corrosion of iron or steel
What is the balanced reduction reaction (cathode) for a lithium battery?
Li(1-x)CoO2(s) + xLi^+ (aq) + xe^− → LiCoO2(s)
What is the balanced oxidation reaction (anode) for a lithium battery?
Li(x)C(graphite)(s) → xLi^+ (aq) + xe^-
How can rust be prevented?
With sacrificial anodes or galvanic coating
What is cathodic protection?
Coating a metal with a "sacrificial anode" that will more easily be oxidized
What is the balanced reduction reaction (cathode) for a hydrogen fuel cell?
O2(g) + 4H^+ (aq) + 4e^- → 2H2O(l)
What is the balanced oxidation reaction (anode) for a hydrogen fuel cell?
2H2(g) → 4H+(aq) + 4e^-
What is the Nernst Equation when expressed in terms of the base 10 logarithm
When the cell potential is negative in a voltaic cell, the cell reaction will not proceed spontaneously.
If ΔG° is positive, what can we assume?
It will not run until standard conditions
If ΔG is negative, what can we assume?
It will initially be spontaneous
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