Electrochemistry

potential

(AKA: voltage & EMF) a measure of the free energy of the reaction -difference in free energy btw reactant and products directly corresponds to electrical potential of cell

Faraday's Constant

(F) is charge in coulombs (C) of 1 mole of electrons. Approximately 96485 C/mol -can be found by multiplying charge of electron (1.602E-19) by Avogadro's number (6.022E23)

The Nerst equation contains the reaction quotient Q. For a voltaic cell made using the half cells Cr3+ (aq) + 3e- → Cr (s) E˚ = -0.74 V Cr3+ (aq) + e- → Cr2+ (aq) E˚ = -0.41 V the reaction quotient match which of the following? Notation: each chemical species in the answer choices is labelled as being on the anode side (a) or the cathode side (c) of the electrochemical cell.

([Cr2+]^3 [Cr3+]a) / ([Cr3+]^3c)

The standard reduction potentials at 25˚C for two electrodes of an electrochemical cell are Cd2+ + 2e- → Cd E˚ = -0.40 V Fe2+ + 2e- → Fe E˚ = -0.44 V You construct a electrochemical cell with these species so that it has positive cell potential of _____ volts. In your electrochemical cell, the Fe electrode would be the ______.

+0.04; anode

Consider the reaction: 2Ag+ (aq) + Cu (s) → Cu2+ (aq) + 2Ag (s) If the standard reduction potentials of Ag+ and Cu2+ into Ag and Cu are +0.80 V and +0.34 V, respectively, calculate the value of Eo for the given reaction.

+0.46 V

If the standard free energy change for combustion of 1 mole of CH4 (g) is -818 kJ, calculate the standard voltage that could be obtained from a fuel cell using this reaction. Hint: First write out the combustion of 1 mole of CH4 (g). Then, write out the half-reactions from this overall reaction and balance assuming acidic conditions.

+1.06 V; DeltaG = -nFE, -818000 J = -(8)(96485)E; E = 1.06 v

In a fuel cell the following redox reaction takes place: 2H2 (g) + O2 (g) → 2H2O (l) If you ran one mole of the reaction above, assuming 100% efficiency, you generate -474,706 joules of work (the negative sign means work is done by your fuel cell). If work is equal to ∆G˚, what it the E˚ of the fuel cell?

+1.23 V

Dry cell battery

-1st portable battery that didn't "spill" -electrolyte contained in a paste

The standard potential of the cell Pt (s) | Fe2+ (aq), Fe3+ (aq) || Ag+ (aq) | Ag (s) is +0.029 V at 25oC. Calculate ∆G˚ for the reaction: Fe2+ (aq) + Ag+ ⇌ Fe3+ (aq) + Ag (s)

-2.8 kJ molrxn-1; DeltaG = -nFE = (-1)(96485)(0.029) = -2798, divide. by 1000 to get to kJ

Fuel Cells

-active redox materials "refilled" as they get depleted

Redox reactions

-always involve oxidation and reduction -one species is always oxidized and one is always reduced -number of electrons must be balanced

Current in Voltaic Cells

-chemistry is spontaneous -can be quantified by measuring the current over time -current may change over long periods of time but for short periods it is typically constant

Delta G

-corresponds to max reversible work we can extract from this chemical reaction `

alkaline. hydrogen/oxygen fuel cell

-environmentally cleanest fuel cell -produces water as its product

Lead Acid Cells

-ex: car battery (heavy, robust, rechargeable) -Lead has 3 diff oxidation states (+4 for PbO2, 0 for metallic Pb)

reduction

-gain of reduction -if reduced, oxidation #goes down

Electrochemical cells

-in electrochemical cells we physically separate oxidation and reduction chemistry in different "compartments" -electrons from oxidation run through an external circuit before being used in a reduction reaction -each half cell has an electrode

oxidation

-loss of electrons -if oxidized, oxidation # goes up

alkaline cells

-most familiar battery -typically have 1.5 V -come in various shapes and sizes (usually letters)

Secondary Cells

-rechargeable -reversible -can be used multiple times -most cost effective -popular secondary batteries: -NiCD or nicad -Pb-acid, Pb-storage, car-battery -Lithium Ion

Battery

-ultimate portable voltaic cell -self contained power factory -electric power production due to considerable free energy drop of reactants as they convert to products -myriad potentials found in batteries are matched equally by their variety of shapes and sizes

Primary Cells

-voltaic cells (battery) made up to be a one time use battery -non-rechargeable -chem rxns only go in one direction (non-reversible) -popular primary cells: -dry cells, Zn-carbon, Le Clanche Cell, Heavy duty -alkaline cell -silver oxide -lithium cells

Calculate the potential for the cell at 25˚C: Cu | Cu2+ (0.0025 M) || Cu2+ (0.75 M) | Cu

0.0733 V

Consider this electrochemical cell: Fe (s) | Fe2+ (10-3 M) || Pb2+ (10-5 M) | Pb (s) Pb2+ + 2e- → Pb (s) E˚ = -0.126 V Fe2+ + 2e- → Fe (s) E˚ = -0.440 V The potential for this cell at 25˚C is _______ The cell at the nonstandard conditions described above is _______spontaneous than the cell at standard conditions. (Hint: Remember that the shorthand notation indicates the anode and cathode compartments).

0.255 V; less

How many moles of metallic aluminum (Al) could be produced from Al3+ at a current of 0.09 amperes for 964,853 seconds?

0.3 mol Al; .09 x 964853, divide by F constant, divide by 3

3 fundamental types of batteries

1. Primary Cells 2. Secondary Cells 3. Fuel Cells

rules to oxidation numbers

1. oxidation number of atom in neutral elemental state is 0 2. oxidation number for monatomic ion is its charge 3. sum of all oxidation numbers in chem species must equal charge on chem species 4. in compounds, H is given +1, with metals, given -1 5. Most electronegative element is assigned its charge as an ion 6. oxygen is given -2

In an alkaline battery the overall chemical reaction is 2MnO2 (s) + Zn (s) → Mn2O3 (s) + ZnO (s) The standard cell potential is +1.43 V. What is the potential of a working battery?

1.43 V until one of the solid reactants is completely consumed

The reaction 2Ag+ (aq) + Fe (s) → Fe2+ (aq) + 2Ag (s) takes place in a battery generates a current of 2 amp. How much solid iron is consumed in 1 hour?

2.08 g; ((2)(1 x 60 x 60)) divide by F constant, divide by 2, multiply by Fe mass

What current is required to produce 91.6 g of chromium metal from chromium (VI) oxide in 12.4 hours?

22.8 A; divide 91.6 g by chromium mass, multiply by 6, multiply by F constant, divide by (12.4 x 60 x 60)

standard reaction. used to measure potential

2H+ (aq) + 2e- -> H2 (g)

standard reduction potentials are established by comparison to the potential of which half reaction?

2H+ + 2e- -> H2

Ampere

Ampere = A = Coulomb/second = C/s

Consider the following half-reactions: Ag+ (aq) + e- → Ag (s) E˚ = 0.80 V Fe3+ (aq) + e- → Fe2+ (aq) E˚ = 0.77 V Cu2+ (aq) + 2e- → Cu (s) E˚ = 0.34 V Which species is the strongest reducing agent?

Cu (most positive on products)

Nernst Equation

E = E° - (0.05916/n)(logQ) *Q = [Anode]/[Cathode] -can be used to calculate potential of an electrochemical cell under any condition

working with equilibrium constant (K)

E° = (0.05916/n)(logK)

Standard Cell Potential

E°cell = E°red + E°ox E°cell = E*cathode - E°anode

What is the cathode in: Ag (s) | Ag+ (aq) || Fe2+ (aq) | Fe (s) Ag+ + e- → Ag E˚ = +0.80 V Fe2+ + 2e-→ Fe E˚ = -0.44 V and what type of cell is it?

Fe; an electrolytic cell

what is the electrolytic substance in a lead-acid battery?

H2SO4 (aq)

Ksp from E°

K = e^(nFE°/RT)

In a fuel cell the following redox reaction takes place: 2H2 (g) + O2 (g) → 2H2O (l) The substance being reduced is ____ and the substance being oxidized is ____.

O2 : H2

Consider the following half-reactions: Mn2+ + 2e- → Mn E˚ = -1.029 V Ga3+ + 3e- → Ga E˚ = -0.560 V Fe2+ + 2e- → Fe E˚ = -0.409 V Sn2+ + 2e- → Sn E˚ = -0.136 V Of the species listed, the strongest oxidizing agent is

Sn2+ (most positive on reactants side)

Concentration Cells

Step 1: build a cell w/ identical half reactions (ex: Zn | Zn2+ || Zn2+ | Zn) Step 2: Change conc of aqueous species to make them different (ex: Zn 2+ (anode) = 0.001 M, Zn2+ (cathode) = 1 M; so Q = 0.001) Step 3: calculate potential using Nernst Equation

The overall reaction for an alkaline battery is: 2MnO2 (s) + Zn (s) → Mn2O3 (s) + ZnO (s) Which species is oxidized as the battery is used?

Zn(s)

In a fuel cell the following redox reaction takes place: 2H2 (g) + O2 (g) → 2H2O (l) Between the anode and cathode compartments there is a proton exchange membrane (PEM) which allows H+ ions to flow between the anode and cathode. In a PEM fuel cell, the PEM acts as:

a salt bridge

salt bridge

allows migration of spectator ions to balance flow of electrons -only spontaneous in one direction

voltaic cell

an electrochemical cell in which a spontaneous chemical reaction will drive the electrons through an external circuit. The overall process is a conversion of chemical energy into electrical energy -oxidation will occur spontaneously at anode(-) and reduction at cathode (+)

electrolytic cell

an electrolytic cell in which external electric energy (power) is used to drive a non-spontaneous chemical reaction forward -standard potential is negative -cathode is negative and anode is positive

shorthand notation

anode | anode solution || cathode solution | cathode

The key difference between primary cell batteries and the secondary cell batteries is that secondary cell batteries __________.

can be recharged

Current

current = charge/time -the amount of electrical charge that flows during a period of time -measured in Amps or Amperes (A)

Match the terms with correct units Current Charge Time (for calculations with Current)

current: Amperes (A) or (C/s) Charge: coulomb (C) Time: seconds (s)

A ______ battery contains the electrolyte in a paste

dry cell

a car battery could be classified as:

either a galvanic or an electrolytic cell depending on whether it's charging or in use

ΔG° less than 0 matches K _____ 1, which matches E° ________ 0 and they all mean that the process is "_____" as written and the ________ are favored. ΔG° > 0 matches K ________ 1, which matches E° ______ 0 and they all mean that the process is "______" as written and the ______ are favored

greater than, greater than, spontaneous, products; less than, less than, non-spontaneous, reactants

What is the purpose of the graphite powder in an alkaline cell?

it is an extended electrode surface in contact with the active cathode material

on which side are the substances that are going to be reduced?

left

A fuel cell is ____

like a battery to which you can add more reactants

electrolytic cell

non spontaneous

The equilibrium constant for the reaction 2Hg (l) + 2Cl- (aq) + 2Ni2+ (aq) → Ni (s) + Hg2Cl2 (s) is 5.6 x 10-20 at 25oC. Without calculations, we know this reaction is _________ at 25°C. With calculations, we know that the value of E˚ for a cell utilizing this reaction is _______.

non-spontaneous; -0.57 V E° = (0.05916/n)(logK) = (0.05916/2)(log (5.6E-20)) = -0.57 V

oxidation numbers

number of electrons that belong to an element compared to number of valence electrons

For a given set of conditions, the chemistry is spontaneous in ________ . A spontaneous, working electrochemical cell is called ________ cell or a galvanic cell or a battery. In a galvanic cell the cathode is given a _______ sign. A non-spontaneous electrochemical cell is called __________ cell and requires an external power source to operate. In an electrolytic cell, the cathode is given a _________ sign. In all electrochemical cells oxidation occurs at the _____ and reduction occurs at the _________ . The external voltage changes the direction of a chemical reaction by changing the ________ .

one direction; a voltaic; positive; an electrolytic; negative; anode; cathode; free energy

anode

oxidation side "An Ox"

molecular chlorine, bromine and iodine are good

oxidizing agents

cathode

reduction side "RED CAT"

on which side are the substances that are goings to be oxidized?

right

the lead-acid battery in your car is a ____

secondary cell

voltaic cells

spontaneous

The standard potential of the cell Pt (s) | Fe2+ (aq), Fe3+ (aq) || Ag+ (aq) | Ag (s) is +0.029 V at 25°C. Without calculating anything, we know that ______ at 25°C. The equilibrium constant for the reaction at 25° is _______.

spontaneous, 3.1 E° = (0.05916/n)(logK) 0.029 = (0.05916/1)(logK) K = 3.1

balancing redox reactions

study half rxn method

oxidizing agents

substances that are easiest to reduce

reducing agents

substances that want to lose their. electrons (oxidize)

oxidation state

the charge based on oxidation number

1 Faraday is....

the charge in coulombs carried by one mole of electrons

electrical work

w = q x E (charge times potential) total work: change that is run through the cell (coulombs, C) times the potential (volts, V) * 1 CV = 1 J work = DeltaG = -nFE

standard hydrogen electrode

where the concentration of the H+ is 1 M and the pressure of the H2 gas is 1 atm