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
Rules for Oxidation Numbers
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
Rules for Oxidation Numbers
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
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 + 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
Balancing Redox Equations
Example: Balance the following redox reaction: Cr2O7
2- + Fe2+ Cr3+ + Fe3+ (acidic soln)
Balancing Redox Reactions
Example: Balance the following redox reaction which takes place in acidic solution.
ClO3- + CH3OH Cl2 + H2CO
Balancing Redox Reactions
Procedure for Basic Solutions: Divide the equation into 2 incomplete half reactions one for oxidation one for reduction
Balancing Redox Reactions
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
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
Balancing Redox Reactions
Example: Balance the following redox reaction.
NH3 + ClO- Cl2 + N2H4 (basic soln)
Balancing Redox Reactions
Example: Balance this redox reaction which occurs under basic conditions.
Pb(OH)42- + ClO- PbO2 + Cl
-