Chemistry Lab 2005 Coaches Clinic Hammond, IN October 1st & 2nd, 2004 Patti Smith

Dont Forget….. Safety Requirements! Description of the Competition!

2005 Topics Oxidation / Reduction (2nd year) Chemical Bonding (new)

Oxidation Reduction Students must be able to write oxidation and reduction half reactions, assign oxidation numbers, balance redox reactions in neutral, acidic, and basic solutions, and calculate standard cell potentials using a table of standard reduction potentials. ChemTeam Chemistry Coach

Oxidation / Reduction Tasks Use a sequence of redox reactions to construct an activity series Construct a simple voltaic cell and measure its potential Stoichiometry and electrochemical process Construct simple electrolytic cells (State & National level: fuel cells) State & National level: knowledge of Nernst equation & common storage batteries may be required

Definition of Oxidation - Reduction Oxidation-reduction reactions An oxidation reduction reaction is one in which electrons are transferred between atoms. Oxidation is the loss of electrons Reduction is the gain of electrons Oxidizing agents are substances which take electrons away from other atoms.( ex. oxygen & chlorine ) An oxidizing agent is reduced when it oxidizes another atom Oxygen and chlorine are commonly used, strong oxidizing agents. Reducing agents are substances which donate electrons to other substances. (ex. hydrogen and carbon ) A reducing agent is oxidized in the process of reducing another atom Group 1 and 2 metals easily donate electrons.

Balancing Redox Reactions Using Oxidation Numbers United Streaming Quicktime

Balancing Redox Reactions Using the Half- Reaction Method Assign oxidation numbers Identify substance oxidized Identify substance reduced Write 1/2 reactions (eliminate spectator ions, if necessary, adding appropriate number of electrons to balance charge Balance electrons gained / lost Add 1/2 reactions to get overall reaction Add spectator ions (if necessary) and finish balancing the equation

Balancing Redox Reactions Using the Half- Reaction Method Al(aq) + HCl(aq) = AlCl 3 (aq) + H 2 (g) Oxidation: ( Al(aq) = Al 3+ (aq) + 3e - ) x 2 = 6 e - Reduction:( 2H 1+ (aq) + 2e = H 2 (g) ) x 3 = 6 e - Net ionic: 2Al(aq) + 6H 1+ (aq) = 2 Al 3+ (aq) + 3 H 2 (g) (note: I would balance this by inspection;) )

Acidic Solutions Use H 2 O to balance O, and H + to balance H..then e - to balance charge. CuS(s) + NO 3 - (aq) = Cu 2+ (aq) + SO 4 2- (aq) + NO(aq) (4 H 2 O + CuS(s) = Cu 2+ (aq) + SO 4 2- (aq) + 8H + (aq) + 8 e - ) x 3 = 24 e - ( 3 e H + (aq) + NO 3 - (aq) = NO(aq) + 2 H 2 O ) x 8 = 24 e - 12 H 2 O + 3 CuS(s) = 3 Cu 2+ (aq) + 3 SO 4 2- (aq) + 24 H + (aq) 32 H + (aq) +8 NO 3 - (aq) = 8 NO(aq) + 16 H 2 O Overall Reaction….. Hip hip hooray! 3 CuS(s) +8 NO 3 - (aq) + 8 H + (aq) = 3 Cu 2+ (aq) + 3 SO 4 2- (aq) + 8 NO(aq) + 4 H 2 O

Basic Solutions Balance as in acidic solution….at the end, add an OH - (aq) to both sides for every H + (aq) present, combining to yield water on the H + side. 3 Cu + 8 NO H + = 8 NO + 3 Cu SO H 2 O + 8 OH OH - 3 Cu + 8 NO H 2 O = 8 NO + 3 Cu SO H 2 O + 8 OH - 3 Cu + 8 NO H 2 O = 8 NO + 3 Cu SO OH -

Calculate Standard Cell Potentials Zn(s) + Cu 2+ (aq) = Zn 2+ (aq) + Cu(s) Zn(s) = Zn 2+ (aq) + 2 e - E o (ox) = -(-0.76 V) = 0.76 V Cu 2+ (aq) + 2 e - = Cu(s) E o (red) = 0.34 V E o = E o (red) + E o (ox) = = 1.10 V The units of half-cell potentials are volts, not volts per mole or volts per electron. All we do when combining half-reactions is add the two half-cell potentials. We do not multiply these potentials by the integers used to balance the number of electrons transferred in the reaction.

Cell Notation

Stoichiometry of Electrolysis

Use a sequence of redox reactions to construct an activity series Most single replacement reactions to fewest Most double replacement reactions to fewest

Fuel Cell central electrolyte layer is sandwiched between two catalyst layers. hydrogen atom splits into a proton and an electron when it contacts the negative anode proton passes through the central electrolyte layer, while the electron passes through an external circuit. circuit returns the electrons to the positive side of the electrolyte layer where they bond again with the protons and join with an oxygen molecule, creating water in the positive cathode catalyst layer.

Nernst Equation

Oxidation Reduction websites Standard Reduction Potential (calculate standard cell potentials): Stoichiometry & electrochemical process: Electrolytic Cells: Fuel Cells The Nernst Equation:

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