Presentation on theme: "Study of the relationships between electricity and chemical reactions."— Presentation transcript:
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: ○ P 2 O 5 ○ NaH ○ Cr 2 O 7 -2 ○ SnBr 4
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) + NiO 2 (s) + 2 H 2 O (l) Cd(OH) 2 (s) + Ni(OH) 2 (s) 2 H 2 O (l) + Al (s) + MnO 4 - (aq) Al(OH) 4 - (aq) + MnO 2 (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 H 2 O 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
ಅಭ್ಯಾಸ (Kannada) Given the skeleton equation: MnO 4 - (aq) + C 2 O 4 2- (aq) Mn 2+ (aq) + CO 2 (g)
April 10 th, 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.
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 Cr 2 O 7 2- (aq) + 14 H + (aq) + 6 I - (aq) 2 Cr 3+ (aq) + 3 I 2 (s) + 7 H 2 O (l) Is a spontaneous reaction in a voltaic cell. A solution containing K 2 Cr 2 O 7 and a solution containing H 2 SO 4 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 circuit Assume 25°C standard conditions Standard 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 11 th, 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) Cu 2+ (aq) + H 2 (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 12 th, 2013 DO NOW: Using standard reduction potentials, calculate standard free energy change and k @ 298K for: 4 Ag (s) + O 2 (g) + 4H + (aq) 4 Ag + (aq) + 2 H 2 O (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 Cr 2 O 7 2- (aq) + 14H + (aq) + 6 I - (aq) 2 Cr 3+ (aq) + 3 I 2 (s) + 7 H 2 O (l) [Cr 2 O 7 2- ] = 2.0 M [H + ] = 1.0 M [I - ]= 1.0 M [Cr 3+ ] = 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 [Zn 2+ ] = 1.0 M and P H 2 = 1.0 atm, what is the H+ concentration?
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: P H 2 = 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 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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