Honors Chemistry
Name: ______Date: ______Mods: ______
Unit 16: Electrochemistry and Redox Reactions
A - Oxidation States
· Redox rxns occur when ______are transferred between reactants
► Oxidation is a process which occurs when an atom, ion, or molecule ______ electrons (LEO)
► Reduction is a process which occurs when an atom, ion, or molecule ______ electrons (GER)
LEO the lion says GER!!!
► Oxidation and reduction go hand in hand à one cannot occur without the other … because every electron that is lost must be transferred and gained by something!
· Oxidation Numbers (aka: Oxidation States)
► To determine if a redox rxn has occurred, oxidation numbers are assigned to each ______in a compound or ion
► These #s keep track of electrons lost and gained during a reaction
► The oxidation # of some atoms will ______in a redox rxn (one # on reactant side, but different # on product side)
o Oxidation (LEO) = ______in oxidation #
o Reduction (GER) = ______in oxidation #
· Rules for Assigning Oxidation Numbers
(1) Elements in their elemental form (a single element or diatomic molecule) have an oxidation # of zero
► ex: Mg (s), Fe (s), O2 (g), H2 (g)
(2) The oxidation number of a monatomic ion is the same as its charge
► ex: alkali metal ions = +1; alkali earth ions = +2; S2- = -2; Al3+ = +3; etc
(3) Nonmetals usually have negative oxidation #s (though they can be positive in certain compounds or ions)
► Oxygen (typically assign oxygen’s oxidation # first)
o -2 almost always
o -1 when in the peroxide ion (O22-) (ex: CaO2 à calcium peroxide)
► Hydrogen
o -1 when bonded to a metal (b/c metals are usually positive cations)
o +1 when bonded to a nonmetal (b/c nonmetals are usually negative anions) (ex: HCl, H2S)
► Halogens
o Fluorine: -1 always
o Other Halogens: -1 (except if combined with oxygen à ex: ClO-, ClO2-, ClO3-, ClO4- … then halogens have positive oxidation #s)
(4) In a neutral compound the sum of the oxidation #s of the atoms equals zero
► MgCl2 à 1(Mg) + 2(Cl) = 0
(5) In a polyatomic ion the sum of the oxidation numbers of the atoms equals the charge on the ion
► PO43- à 1(P) + 4(O) = -3
· Systematic Approach to Assigning Oxidation Numbers
► Is the substance elemental? àif yes, oxid.# = 0
► Is the substance ionic?
o If so, does it contain monatomic ions? à alkali metals, alkali earth, Al3+ ???
o If yes, the oxid.# on the monatomic ions = charge of ion
► Does it contain elements which have specific rules? à H, O, halogens
► Use Steps 4 & 5 to figure out oxid.#s for remaining elements without specific rules
· Examples - Assigning Oxidation Numbers:
Substance: Show your WORK in this box for full credit / Oxidation Numbers:A / Na2SO4 / Na = / S = / O =
B / CO32- / C = / O = / ------
C / H2S / H = / S = / ------
D / Co(ClO)2 / Co = / Cl = / O =
E / NO3- / N = / O = / ------
F / BaO2 / Ba = / O = / ------
G / P2O5 / P = / O = / ------
H / Cr2O72- / Cr = / O = / ------
I / SCl2 / S = / Cl = / ------
· Redox Reactions
► Many redox rxns are ______rxns
► A + BX à AX + B
► This obviously has a relationship to the activity series of metals
o those metals which are easier to oxidize are higher up on the series
· Redox Reactions: Determining Oxidation & Reduction
Example: Use oxidation numbers to determine what is being oxidized and what is reduced.
2 Al (s) + 3 CuCl2 (aq) à 2 AlCl3 (aq) + 3 Cu (s)
Net Ionic:
______is being oxidized; ______is being reduced
______is the oxidizing agent; ______is the reducing agent
(Note: the reactant which includes the oxidized atom is known as the “reducing agent” or “reducer” and the reactant which includes the reduced atom is known as the “oxidizing agent” or “oxidizer”)
Practice: Use oxidation numbers to determine what is being oxidized and what is reduced.
Mg (s) + CoSO4 (aq) à MgSO4 (aq) + Co (s)
Net Ionic:
______is being oxidized; ______is being reduced
______is the oxidizing agent; ______is the reducing agent
B - Balancing Redox Equations
· When balancing any reaction, we must have equal types and #s of atoms on both side of the equation to obey the law of conservation of matter…
► With redox rxns. There is an additional requirement: the gain and loss of ______must be balanced
► Perhaps the easiest way to balance a redox rxn is via the ______-______method
· Balancing Equations by Half-Reaction Method in ACIDIC solutions:
► On paper, the oxidation half and reduction half of the rxn are treated as two separate processes
o First, balance each of these half reactions, and then combine them to attain the balanced equation for the overall reaction
(1) Assign oxidation #s to determine what is being oxidized and what is reduced
(2) Write the oxidation and reduction half reactions
(3) Balance each half-reaction:
a. Balance elements other than H and O first
b. Balance O by adding H2O
c. Balance H by adding H+ (because this is an acidic solution we add H+ ions)
d. Balance charge by adding electrons
(4) Multiply the half-reactions by integers so that the electrons gained and lost are the same in both the oxidation-half and the reduction-half
(5) Add the half-reactions, canceling things that appear on both sides
(6) Make sure the equation is balanced according to mass (equal atoms)
(7) Make sure the equation is balanced according to charge (equal electrons)
· Example – Balancing Redox Reactions in ACIDIC Solutions
MnO4- (aq) + C2O42- (aq) à Mn2+ (aq) + CO2 (g)
Reduction-Half:
Oxidation-Half:
______
· Consider the incomplete half-reactions below.
(a) Use oxidation #s to identify the reactions below as oxidation or reduction
(b) Place the correct # of electrons on the appropriate side to complete the half-reaction
1. I2 (s) à 2 I– (aq)
2. 2 Cr3+ (aq) + 7 H2O (l) à Cr2O72– (aq) + 14 H+ (aq)
C - Voltaic Cells
· Avoltaic cell, or______cell, is an electrochemicalcellthat derives electrical energy from spontaneous redox reactions. Abatteryis a type of device consisting of two or moreelectrochemical cellsthat convert stored chemical energy into electrical energy
· Components of a Voltaic Cell:
o Cathode Half-Cell:
§ contains the cathode (positive terminal) of the cell; ______occurs at the cathode (Red Cat)
§ this half-cell includes an aqueous solution made with ______of the cathode
o Anode Half-Cell:
§ contains the anode (negative terminal) of the cell; ______occurs at the anode (An Ox)
§ this half-cell includes an aqueous solution made with ______of the anode
o Salt Bridge:
§ Contains ______which allow “spectator” ions (which do not take part in the actual redox reaction) to move between the anode-half cell and the cathode-half cell; the purpose of the salt bridge is for ions to pass from one cell to the other as e- are transferred in order to maintain charge ______in each half-cell
§ the negative ions in the salt bridge will migrate into the ______half-cell where e- are lost, while positive ions migrate to the ______half-cell.
§ The salt bridge is necessary because once even one electron flows from the one half-cell to the other, the charges in each half-cell would become unbalanced and the flow of electrons would stop.
o Wire/Switch:
§ The wire/switch connects the anode and the cathode to one another
§ When the wire is connected (or the switch is turned on) the circuit is complete and electrons can flow from the ______to the ______.
§ The current that flows through this wire can be measured by a voltmeter
Diagram 1: Voltaic Cell BEFORE the Redox Reaction Occurs
1) Label which electrode acts as the cathode and which acts as the anode in Diagram 1.
o Use the activity series to determine which metal is more likely to be oxidized; this metal will act as the ______while the other metal will act as the ______.
2) Represent this voltaic cell in cell notation.
3) There appears to be two Zn2+ and two Cu2+ ions present in each half-cell before the redox reaction starts. How does the number of NO3– ions in each half-cell relate to this?
Diagram 2: Voltaic Cell AFTER the Redox Reaction Occurs
1) In Diagram 2, draw in arrows (< or >) to depict the direction in which electrons flow after the wire is connected and the redox reaction takes place
2) Compare the solutions in Diagram 1 to Diagram 2. What happened to the number of aqueous Zn2+ and Cu2+ ions present in the half-cell solutions before the wire is connected compared to after wire is connected?
3) Explain how the anode half-cell in Diagram 2 remains electrically neutral (no charge) after the wire is connected, electrons are lost, and the number of Zn2+ ions changes.
4) Explain how the cathode half-cell in Diagram 2 remains electrically neutral (no charge) after the wire is connected, electrons are gained, and the number of Cu2+ ions changes.
5) After the wires are connected and the oxidation and reduction reactions are allowed to occur, what happens to the masses of the Zn electrode (anode) and the Cu electrode (cathode)? Label this mass change in Diagram 2.
a. Explain, in terms of Zn atoms and Zn2+ ions, what happened to the mass of the Zn electrode (anode).
b. Write out the balanced half-reaction which occurs at the anode.
c. Explain, in terms of Cu atoms and Cu2+ ions, what happened to the mass of the Cu electrode (cathode).
d. Write out the balanced half-reaction which occurs at the cathode.