Unit 5 – Part 2: Redox Reactions and Electrochemistry

Unit 5 – Part 2: Redox Reactions and Electrochemistry

Oxidation Numbers

Oxidizing and Reducing Agents

Balancing Redox Reactions Acidic solutions Basic solutions

Galvanic Cells

Nernst Equation

Redox Reactions

Oxidation-Reduction Reactions (Redox Reactions) reactions that involve the transfer of

electrons between two reactants

an element in one reactant is oxidized while an element in another reactant is reduced

Redox Reactions

Many practical or everyday examples of redox reactions:

Corrosion of iron (rust formation) Forest fire Charcoal grill Natural gas burning Batteries Production of Al metal from Al2O3

Metabolic processes

combustion

Redox Reactions

Many different types of redox reactions:

Oxidation by Molecular Oxygen4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)

Oxidation of Metals by Acids or Water

Zn (s) + 2 H+ (aq) Zn2+ (aq) + H2 (g)

2 Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g)

Metal Displacement:Fe (s) + Ni2+ (aq) Fe2+ (aq) + Ni (s)

Redox Reactions

Oxidation: the loss of electrons oxidation number increases the gain of oxygen the loss of hydrogen

Reduction: the gain of electrons oxidation number decreases the gain of hydrogen the loss of oxygen

Redox Reactions

GER

LEO

LEO says GER

LEO: Lose Electrons

Oxidation

GER: Gain Electrons

Reduction

Mg (s) Mg2+ (aq) + 2 e-

2 H+ (aq) + 2 e- H2 (g)

Redox Reactions

Electrons are not explicitly shown in chemical equations.

Oxidation Numbers are used to keep track of electrons gained and lost during redox reactions.

Oxidation number a hypothetical number assigned to an

individual atom present in a compound using a set of rules.

May be positive, negative, or zero

Rules for Oxidation Numbers

Oxidation numbers are always reported for individual atoms or ions not groups of atoms or ions!!!!!!!!!!!

For an atom in its elemental form, the oxidation number is always zero.

H2: oxidation # = 0 for each H atom Cu: oxidation number = 0 Cl2: oxidation # = 0 for each Cl atom

Rules for Oxidation Numbers

For any monoatomic ion, oxidation # = charge of the ion

K+ oxidation # = +1Cl- oxidation # = -1S2- oxidation # = -2

Rules for Oxidation Numbers

Group 1A Metal Cations: Always +1

Group 2A Metal Cations: Always +2

Hydrogen (H) +1 when bonded to nonmetals -1 when bonded to metals

Rules for Oxidation Numbers

Oxygen (O) -1 in peroxides (O2

2-) -2 in all other compounds

Fluorine (F) always -1

Rules for Oxidation Numbers

The sum of the oxidation numbers of all atoms in any chemical species (ion or neutral compound) is equal to the charge on that chemical species

H2O: 1 + 1 + -2 = 0

MgCl2: 2 + -1 + -1 = 0

MnO4-: 7 + -2 + -2 + -2 + -2 = -1

Oxidation Numbers

For many compounds, you will be able to directly apply the rules to determine the oxidation number of all atoms except for one. Use the last rule to determine the

oxidation number of that last element.

Oxidation Numbers

Example: Determine the oxidation state of all elements in SO3.

Oxidation Numbers

Example: Determine the oxidation state of all elements in Cr2O7

2-.

Oxidation Numbers

Example: Determine the oxidation state of all elements in Cu(NO3)2

Redox Reactions

Changes in oxidation number are used to determine if a redox reaction has occurred and to identify the elements oxidized and reduced. Reduction (GER)

gain of electronsoxidation number decreases (is reduced)

Oxidation (LEO)loss of electronsoxidation number increases

Redox Reactions

Example: Is the following a redox reaction? If so, which element is oxidized? Which element is reduced

2 C6H6 (l) + 9 O2 (g) 12 CO2 (g) + 6 H2O (l)

Redox Reactions

Oxidizing Agent (oxidant): the reactant that causes another reactant

to be oxidized

the reactant that contains the element that is reduced

Reducing Agent (reductant): the reactant that causes another

substance to be reduced

the reactant that contains the element that is oxidized

Redox Reactions

Example: Did a redox reaction occur in the following reaction? If so, what is the oxidizing agent? What is the reducing agent?

2 Na (s) + 2 H2O (l) 2 NaOH (s) + H2 (g)

Redox Reactions

When writing the equation for a redox reaction, you must balance the atoms on both sides balance the loss and gain of electrons

For “simple” redox reactions, the loss and gain of electrons is “automatically” balanced when you balance the atoms

Zn (s) + 2 H+ (aq) Zn2+ (aq) + H2 (g)

Redox Reactions Most redox reactions are more complex

to balance.

Sn2+ (aq) + Fe3+ (aq) Sn4+ (aq) + Fe2+

(aq)

Sn2+(aq) + 2 Fe3+ (aq) Sn4+ (aq) + 2 Fe2+

(aq)

Redox Reactions Redox reactions can be broken up into

2 half-reactions: a reaction that shows either

oxidation or reduction alone Overall reaction:

Oxidation half reaction:

Reduction half reaction:

Zn (s) Zn2+ (aq) + 2 e-

2 H+ (aq) + 2 e- H2 (g)

Zn (s) + 2 H+ (aq) Zn2+ (aq) + H2 (g)

Redox Reactions

Notice that electrons lost = electrons gained in a balanced redox reaction:

Zn (s) Zn2+ (aq) + 2 e-

2 H+ (aq) + 2 e- H2 (g)

Zn (s) + 2 H+ (aq) Zn2+ (aq) + H2 (g)

Balancing Redox Reactions

Procedure for Acidic Solutions: Divide the equation into 2 incomplete

half reactionsone for oxidationone for reduction

Balance each half-reaction:balance all elements except H and Obalance O atoms by adding H2Obalance H atoms by adding H+

balance charge by adding e- to side with more positive overall charge

Balancing Redox Equations

Multiply each half reaction by an integer so that# e- lost = # e- gained

Add the half reactions together.Simplify where possible by canceling species appearing on both sides of equation

Check the equation# of atomstotal charge on each side

Balancing Redox Equations

Example: Balance the following redox reaction:

Cr2O72- + Fe2+ Cr3+ + Fe3+ (acidic soln)

Balancing Redox Equations

Balancing Redox Equations

Balancing Redox Reactions

Balancing Redox Reactions Procedure for Basic Solutions:

Divide the equation into 2 incomplete half reactionsone for oxidationone for reduction

Balancing Redox Reactions Balance each half-reaction:

balance all elements except H and Obalance O atoms by adding H2Obalance H atoms by adding H+

add 1 OH- to both sides for every H+

addedcombine H+ and OH- on same side to make H2O

cancel the same # of H2O from each sidebalance charge by adding e- to side with the more positive overall charge

different

Balancing Redox Equations

Multiply each half reaction by an integer so that# e- lost = # e- gained

Add the half reactions together.Simplify where possible by canceling species appearing on both sides of equation

Check the equation# of atomstotal charge on each side

Balancing Redox Reactions

Example: Balance the following redox reaction.

NH3 + ClO- Cl2 + N2H4 (basic soln)

Balancing Redox Reactions

Balancing Redox Reactions

Balancing Redox Reactions

Balancing Redox Reactions

Example: Balance the following redox reaction which takes place in acidic solution.

NO2- + Cr2O7

2- NO3- + Cr3+

Balancing Redox Reactions

Example: Balance this redox reaction which occurs under basic conditions.

Pb(OH)42- + ClO- PbO2 + Cl-