unit 5 part 2: redox reactions and electrochemistry unit5c sp14.pdfredox reactions the battery in...

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

The battery in your car provides electrical energy to start your car using the following reaction:

This is a redox reaction.

PbO2 (s) + Pb (s) + 2 HSO4- (aq) + 2 H+ (aq) 2 PbSO4 (s) + 2 H2O (l)

Corrosion of the battery terminal in your car is caused by a complex reaction between the metal terminal, oxygen, and the battery acid, H2SO4.

This reaction is also a redox reaction.

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

Cu2+ (aq) + Mg (s) Cu (s) + Mg2+ (aq) oxidized reduced

Redox Reactions

Oxidation: the loss of electrons

chemical species becomes more positively charged

the gain of oxygen

An increase in oxidation number

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

Redox Reactions

Reduction: the gain of electrons

the chemical species becomes more negatively charged

the gain of hydrogen

A decrease in oxidation number

Cu2+ (aq) + 2 e- Cu (s)

Redox Reactions

LEO: Lose Electrons Oxidation

GER: Gain Electrons Reduction

GER

LEO

LEO says GER

Redox Reactions

Oil Rig

Oil : Oxidation Involves

Loss of e-

Rig : Reduction Involves

Gain of e-

Redox Reactions

Many practical or everyday examples of redox reactions: Corrosion of iron (rust formation)

4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)

Combustion CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Batteries 4 Al (s) + 3 O2 (g) + 6 H2O (l) 4 Al(OH)3 (s)

(aluminum-oxygen batteries)

Redox Reactions

Additional examples of redox reactions: Metabolic processes

Pyruvic acid lactic acid Pyruvic acid acetaldehyde ethanol

Oxidation of Metals by Acids, Water, or Metal Salts

2Al (s) + 3H2SO4 (aq) Al2(SO4)3(aq) + 3H2 (g) 2 Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g)

Redox Reactions

The electrons gained and lost during a redox reaction are not explicitly shown in chemical equations.

How do you know if a redox reaction has occurred? You must examine the oxidation number of each of the elements present in the reactants and products.

Redox Reactions

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


For any monoatomic ion, oxidation # = charge of the ion K+ oxidation # = +1 Cl- oxidation # = -1 S2- oxidation # = -2

Group 1A Metal Cations are always +1

Group 2A Metal Cations are always +2


Hydrogen (H) in a compound +1 when bonded to nonmetals -1 when bonded to metals or B

Oxygen (O) in a compound -1 in peroxides (O2

2-) -2 in all other compounds

Fluorine (F) in a compound always -1


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 + 4(-2) = -1

This last rule is used to determine the oxidation number of an element that has no specific “rule.”

Oxidation Numbers

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

Oxidation Numbers

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

Oxidation Numbers

Example: Determine the oxidation number of P in NaH2PO4

Oxidation Numbers

Example: Determine the oxidation number of all elements in Cr2O7

2-.

Redox Reactions

To determine if a redox reaction has occurred, compare the oxidation number of identical elements on the reactants and products side of the equation. If the oxidation number changes, then a redox reaction has occurred. Oxidation:

Loss of electrons Increase in oxidation number

Reduction Gain of electrons Decrease in oxidation number.

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

Example: Identify the element that has been oxidized and the one that has been reduced.

4 Al (s) + 3 O2 (g) + 6 H2O (l) 4 Al(OH)3 (s)

PbO2 (s) + Pb (s) + 2 HSO4- (aq) + 2 H+ (aq) 2 PbSO4 (s) + 2 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: Identify the oxidizing and reducing agents in the following reactions. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

4 Al (s) + 3 O2 (g) + 6 H2O (l) 4 Al(OH)3 (s)

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 reactions one for oxidation one for reduction

Balance each half-reaction: balance all elements except H and O balance O atoms by adding H2O balance 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 atoms total charge on each side


Example: Balance the following redox reaction: Cr2O7

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


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

ClO3- + CH3OH Cl2 + H2CO


Procedure for Basic Solutions: Divide the equation into 2 incomplete half reactions one for oxidation one for reduction


Balance each half-reaction: balance all elements except H and O balance O atoms by adding H2O balance H atoms by adding H+

add 1 OH- to both sides for every H+

added combine H+ and OH- on same side to make H2O

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

different


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 atoms total charge on each side


Example: Balance the following redox reaction.

NH3 + ClO- Cl2 + N2H4 (basic soln)


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

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

-

unit 5 part 2: redox reactions and electrochemistry unit5c sp14.pdfredox reactions the battery in...

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