Unit 15: Electrochemistry

Unit 15: Electrochemistry

Oxidation-Reduction reactions Unit 15.1

Oxidation and Reduction (Redox) Electrons are transferred Spontaneous redox rxns can transfer energy Electrons (electricity) Heat Non-spontaneous redox rxns can be made to happen with electricity

Redox Reactions Oxidation : loses electrons (gains charge) Reduction: gains electrons (loses charge) LEO says GER If something is oxidized, something else is reduced

2 Oxidation Reduction Reactions (Redox) 0 Na 0 2 + 1 1 + Cl 2 Na Cl Each sodium atom loses one electron: Each chlorine atom gains one electron: 0 Na + 1 Na 0 1 Cl + e Cl + e

LEO says GER : Lose Electrons = Oxidation 0 Na + 1 Na + e Sodium is oxidized Gain Electrons = Reduction 0 1 Cl + e Cl Chlorine is reduced

Oxidation Numbers Oxidation numbers (Oxidation states): Indicates the general location of electrons in a compound. It is not necessarily the charge of an atom. For ionic compounds the oxidation number is the charge of the ion Covalent compounds have no charges so the oxidation number isn t an actual charge Oxidation numbers are always for an individual atom (not a group of atoms)

Rules for Assigning Oxidation Numbers Rules 1 & 2 1. The oxidation number of any uncombined element is zero 2. The oxidation number of a monatomic ion equals its charge (any ionic compound has ions in it so the charge of the ion is the oxidation number) 2 0 Na 0 2 + 1 1 + Cl 2 Na Cl

Rules for Assigning Oxidation Numbers Rules 3 & 4 3. To find the oxidation number in a covalent compound, find the charge of the last atom written if it were an ion. 4. The oxidation number of hydrogen in compounds is +1, unless it is with a metal and then it is -1 +1 2 H O 2

Rules for Assigning Oxidation Number Rule 5 5. The sum of the oxidation numbers in the formula of a compound is 0 +1 2 O 2 + 2 2 + 1 H Ca( O H ) 2 2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation Numbers Rule 6 6. The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its charge remember first find the charge of the second atom written if it were an ion and use that to find the oxidation number of the X + 3(-2) = -1 N O first atom? 2 N O 3? 2 S O X + 4(-2) = -2 S O 2 4 X = +5 X = +6

Find the oxidation numbers for the following UF6 H2SO4 ClO3 - HCl SO4 2- O2 Ca 2+ Na N2O5 P4O10 KH Answers +6, -1 +1, +6, -2 +5.-2 +1, -1 +6, -2 0 +2 0 +5,-2 +5,-2 +1, -1

Not All Reactions are Redox Reactions Reactions in which there has been no change in oxidation number are not redox rxns. Examples: + 1 + 5 2 + 1 1 + 1 1 + 1 + 5 2 Ag N O3 aq + NaCl aq Ag Cl s + Na N O3 aq ( ) ( ) ( ) ( ) + 1 2 + 1 + 1 + 6 2 + 1 + 6 2 + 1 2 2 NaO H ( aq) + H 2 S O4( aq) + Na2 S O4( aq) + H 2O( l)

Redox reactions Are the following reactions redox? SO2 + H2O H2SO3 (NOT REDOX) H2 + Cl2 2HCl (REDOX) What is oxidized? What is reduced? Hydrogen is oxidized (oxidation number increased) Chlorine is reduced (oxidation number went down)

electrochemistry unit 15.2

Spontaneous reactions Metals are more reactive to the left and down on the periodic table Nonmetals are more reactive to the right and up on the periodic table Active metals: Lose electrons easily Are easily oxidized Active nonmetals: Gain electrons easily Are easily reduced Use reduction potentials to determine if a reaction is spontaneous

Will the following reactions occur? Cl2 + 2Br - Cl - + Br2 Yes, chlorine is reduced and is higher than bromine on the reduction potential chart Magnesium + hydrochloric acid rxn: Mg +2 HCl MgCl2 + H2 yes, hydrogen is reduced and is higher than magnesium on the reduction potential chart

Voltaic Cells In spontaneous oxidation-reduction (redox) reactions, electrons are transferred and energy is released. If the reactants are in direct contact, heat is produced

Voltaic Cells We can use that energy to do work if we make the electrons flow through an external device. We call such a setup a voltaic cell.

Voltaic Cells A typical cell looks like this. The oxidation occurs at the anode. The reduction occurs at the cathode.

Voltaic Cells Once even one electron flows from the anode to the cathode, the charges in each beaker would not be balanced and the flow of electrons would stop.

Voltaic Cells Therefore, we use a salt bridge, usually a U-shaped tube that contains a salt solution, to keep the charges balanced. Cations move toward the cathode. Anions move toward the anode.

In the cell, then, electrons leave the anode and flow through the wire to the cathode. As the electrons leave the anode, the cations formed dissolve into the solution in the anode compartment. Voltaic Cells

As the electrons reach the cathode, cations in the cathode are attracted to the now negative cathode. The electrons are taken by the cation, and the neutral metal is deposited on the cathode. Voltaic Cells

Standard Reduction Potentials Reduction potentials for many electrodes have been measured and tabulated.

Standard Cell Potentials The cell potential (voltage) at standard conditions can be found through this equation: E cell = E red (cathode) E red (anode)

Cell Potentials For the oxidation in this cell, E red = 0.76 V For the reduction, E red = +0.34 V

Cell Potentials E cell = E red (cathode) E red (anode) = +0.34 V ( 0.76 V) = +1.10 V