Chapter 20: Electrochemistry

Chapter 20: Electrochemistry

Study of the relationships between electricity and chemical reactions CHAPTER 20: ELECTROCH 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 halfreaction 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 (Kannada)Kannada)

Given the skeleton equation: MnO4- (aq) + C2O42- (aq) Mn2+ (aq) + CO2 (g) April 10 , 2013 th 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. (Kannada)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 Cr2O72- (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 wont 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 circuit Assume 25C standard conditions Standard Cell Potential: Ecell Depends on particular anode and cathode half cells How might we measure Ecell 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 Ecell

How do we calculate Ecell 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 Ered of both the anode and the cathode? April 11 , 2013 th

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 (Kannada)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 12 , 2013 th 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 25C when [Zn2+] = 1.0 M and PH = 1.0 atm, what is the H+ concentration?

2 Concentration cells Voltaic cells constructed by the same species in both half-cells with different concentrations Ni 2+ - 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: PH = 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? 2 Electrolysis (Kannada)Electrolytic cells)

Non-spontaneous redox reactions driven by outside electrical energy IE: decomposition of molten NaCl to Na and Cl2 2 electrodes immersed in either molten salt of solution Process still similar to voltaic cells Battery 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? Sodium Copper Aluminum Amount of substance reduced or oxidized in

electrolytic cell is proportional to number of electrons passed through cell Quantity of charge is measured in Coulombs Charge 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.

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