Study of the relationships between electricity and chemical reactions

CHAPTER 20:

ELECTROCHEMISTRY

Study of the relationships between electricity and

chemical reactions

Oxidation States

REVIEW: How do we determine the oxidation state?What is the oxidation state of the boldfaced

element:○ P2O5

○ NaH○ Cr2O7

-2

○ SnBr4

Redox Reactions

How do we determine if a reaction is a redox reaction?

How would we define the following vocabulary words:Oxidizing agent (oxidant): agent aquiring

electrons from another substance (reduced)Reducing agent (reductant): agent giving up

electrons (oxidized)

Identifying Oxidizing and Reducing Agents

Cd (s) + NiO2 (s) + 2 H2O (l) Cd(OH)2 (s) + Ni(OH)2 (s)

2 H2O (l) + Al (s) + MnO4-(aq) Al(OH)4

- (aq) + MnO2 (s)

Balancing Redox Reactions How is balancing a redox reaction

different than balancing a normal equation?

How do we split a redox reaction?

Why would using half reactions make the process easier?

Balancing equations: acidic solutions In balancing reactions, we use “skeleton”

ionic equation.Predict why this is called the skeleton ionic

equation. Step-by-step procedure:

STEP 1: BALANCE EQUATION○ Balance elements other than H and O○ Balance O atoms by adding H2O as needed

○ Balance H atoms by adding H+ as needed○ Finally balance charge by adding e- as needed

Balancing equations: acidic solution cont’

STEP 2: Multiply half-reactions by integers as needed to make the number of electrons lost in the oxidation half-reaction equal the number of electrons gained in the reduction half-reaction

STEP 3: Add half reactions (simplify if possible by canceling species found on both sides)

STEP 4: Check to make sure atoms and charges are balanced

ಅಭ್ಯಾ��ಸ (Kannada)

Given the skeleton equation:MnO4

- (aq) + C2O42- (aq) Mn2+ (aq) + CO2 (g)

April 10th, 2013

DO NOW:Predict how balancing a redox reaction

in a basic solution is different than in an acidic solution.

Basic Solutions

For Basic solutions:Follow same process except for after

balancing H+, add OH- to both reactants and products side to “neutralize” the H+ ion.

행하다 (Korean)

See Worksheet

Voltaic Cells

Energy is released when using a voltaic cell, how is it being used?

How can we define voltaic cell?

Draw a diagram of a voltaic cell in your notebook.

Anodes and Cathodes!

How do we differentiate between an anode and a cathode?

Predict how the voltaic cell conducts electricity.

Why is using a salt bridge necessary?

Predict which direction the anions flow.

Describing a voltaic cell Cr2O7

2- (aq) + 14 H+ (aq) + 6 I- (aq) 2 Cr3+ (aq) + 3 I2 (s) + 7 H2O (l)

Is a spontaneous reaction in a voltaic cell. A solution containing K2Cr2O7 and a solution containing H2SO4 are poured into 1 beaker and a solution containing KI is poured into a separate beaker. 2 pieces of metal that won’t react with either are suspended into both solutions. Indicate the following:

○ Reaction at anode○ Reaction at cathode○ Direction of Electron flow○ Direction of ion flow○ Signs of electrodes

Electricity Flow

Describe the flow of water in a waterfall.

Why?

Why do electrons flow spontaneously from an anode to a cathode?

How do we measure energy between the two?

Potential Energy

Potential Energy difference between the two electrodes is called cell potential.Also known as: Electromotive force (EMF)

○ Pushes e- through circuitAssume 25°C standard conditionsStandard Cell Potential: E°cell

Depends on particular anode and cathode half cells

How might we measure E°cell

Reference Cell Why is it necessary to have a reference

half reaction?Allows for measure of other cell potentials

directly. Reference cell: SHE (standard hydrogen

electrode)

Calculating E°cell

How do we calculate E°cell from the SHE?

IE: oxidation of zinc and reduction of hydrogen ion

How would we represent this standard reduction potential?

Reactions

How do we determine how likely a reaction is to occur?

Given that electrons must flow spontaneously, what assumption can we make about the values of the E°red of both the anode and the cathode?

April 11th, 2013

Do Now:Thermodyanmics Quiz.

○ Have out Calculator and Reference Sheet.

Spontaneity of Voltaic Cells How can we use our equation for standard cell

potentials as a generalized equation?

Based on this equation and your knowledge of thermodynamics, predict how we determine if the reaction is spotaneous.

NOTE: E = EMF @ nonstandard conditions

E°= EMF @ standard conditions

Practice: Is this reaction spontaneous:Cu (s) + 2 H+ (aq) Cu2+ (aq) + H2 (g)

Reactivity of Metals

How are the standard reduction potentials related to the activity series of metals?

Which is a stronger reducing agent: silver or nickel? Why?

Gibbs Free Energy (it never leaves you alone) How can we related Gibbs free energy to EMF?

Predict the sign of the Gibbs free energy if you have a positive EMF. Why is this to be expected?

Rewrite the equation for a reaction in “standard conditions”

How can we relate E° to K?

April 12th, 2013

DO NOW: Using standard reduction potentials,

calculate standard free energy change and k @ 298K for:

4 Ag (s) + O2 (g) + 4H+ (aq) 4 Ag+ (aq) + 2 H2O (l)

Suppose the reaction is halved, calculate the change in gibbs free energy at standard conditions, EMF at standard conditions, and k.

Non standard Conditions

As a cell is discharged (released all possible electricity) reactants are consumed and products are generated the concentration changes until EMF drops to 0 (dead cell)

We want to answer: How is EMF generated under non-standard conditions

Concentration is a changin! Walther Nernst (1864-1941)

Established many theoretical foundations of electrochemistry

Effect of concentration on cell EMF can be obtained from effect of concentration on Gibbs Free Energy.

Which equation would we want to use to relate effect of concentration on Gibbs Free Energy. Why?

Practice… again.

Calculate the EMF @ 298K generated by the voltaic cell in which the reaction is

Cr2O72- (aq) + 14H+ (aq) + 6 I- (aq) 2 Cr3+ (aq) + 3 I2 (s) + 7 H2O (l)

[Cr2O72-] = 2.0 M

[H+] = 1.0 M

[I-]= 1.0 M

[Cr3+ ] = 1.0 x 10-5 M

One More

If the potential of a Zn-H+ cell is at 0.45 V at 25°C when [Zn2+] = 1.0 M and PH2 = 1.0 atm, what is the H+ concentration?

Concentration cells

Voltaic cells constructed by the same species in both half-cells with different concentrations

Ni2+ - Ni reaction Write the two half reactions and the overall

reaction.

How can we calculate the EMF of this cell?

Determine the EMF.

If E° is 0, what produces the driving force?

**NOTE: when concentrations become the same: Q = 1, E = 0

pH meters Generating a potential by a concentration

difference

Example: A voltaic cell with 2 hydrogen electrodes: Electrode 1: PH2 = 1.0 atm and unknown concentration of H+ ions. Electrode 2 is a standard Hydrogen electrode. At 298K, E° = 0.211 V, and the electrical current is observed to flow from electrode 2 to electrode 1. What is the pH of the unknown solution?

Electrolysis (Electrolytic cells) Non-spontaneous redox reactions driven

by outside electrical energyIE: decomposition of molten NaCl to Na

and Cl2 2 electrodes immersed in either molten

salt of solutionProcess still similar to voltaic cellsBattery acts as electron pump

Electroplating

How is electroplating different than general electrolysis?Uses electrolysis to

deposit a thin layer of metal on another metal to improve beauty or resistance to corrosion

Uses active electrodes instead of inert

Quantitative Electrolysis How many moles of electrons are needed to create

the following from their ions?SodiumCopperAluminum

Amount of substance reduced or oxidized in electrolytic cell is proportional to number of electrons passed through cell

Quantity of charge is measured in CoulombsCharge on 1 mol = 96485 C.

○ Coulomb = quantity of charge passing a point in a circuit in 1 s when the current is 1 A (ampere) Coulombs = amperes x seconds

Practissimo!

Calculate the number of grams of aluminum produced in 1.00 hour by the electrolysis of molten AlCl3 if the electrical current is 10.0 A.