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
This reaction is also a redox reaction. Redox Reactions The battery in your car provides electrical energy to start your car using the following reaction: PbO 2 (s) + Pb (s) + 2 HSO 4 - (aq) + 2 H + (aq) 2 PbSO4 (s) + 2 H 2 O (l) This is a redox reaction. Corrosion of the battery terminal in your car is caused by a complex reaction between the metal terminal, oxygen, and the battery acid, H 2 SO 4.
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 Cu 2+ (aq) + Mg (s) Cu (s) + Mg 2+ (aq) reduced oxidized
Redox Reactions Oxidation: the loss of electrons chemical species becomes more positively charged Mg (s) Mg 2+ (aq) + 2 e - the gain of oxygen An increase in oxidation number
Redox Reactions Reduction: the gain of electrons the chemical species becomes more negatively charged Cu 2+ (aq) + 2 e - Cu (s) the gain of hydrogen A decrease in oxidation number
Redox Reactions LEO: GER Lose Electrons Oxidation GER: LEO LEO says GER Gain Electrons Reduction
Redox Reactions Oil : Oxidation Involves Loss of e - Oil Rig 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 O 2 (g) 2 Fe 2 O 3 (s) Combustion CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Batteries 4 Al (s) + 3 O 2 (g) + 6 H 2 O (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) + 3H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) 2 Na (s) + 2 H 2 O (l) 2 NaOH (aq) + H 2 (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. H 2 : oxidation # = 0 for each H atom Cu: oxidation number = 0 Cl 2 : 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 S 2- 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 (O 2 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 H 2 O: 1 + 1 + -2 = 0 MgCl 2 : 2 + -1 + -1 = 0 MnO 4- : 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 SO 3.
Oxidation Numbers Example: Determine the oxidation number of all elements in Cu(NO 3 ) 2
Oxidation Numbers Example: Determine the oxidation number of P in NaH 2 PO 4
Oxidation Numbers Example: Determine the oxidation number of all elements in Cr 2 O 7 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 C 6 H 6 (l) + 9 O 2 (g) 12 CO 2 (g) + 6 H 2 O (l)
Redox Reactions Example: Identify the element that has been oxidized and the one that has been reduced. 4 Al (s) + 3 O 2 (g) + 6 H 2 O (l) 4 Al(OH) 3 (s) PbO 2 (s) + Pb (s) + 2 HSO 4 - (aq) + 2 H + (aq) 2 PbSO4 (s) + 2 H 2 O (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. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) 4 Al (s) + 3 O 2 (g) + 6 H 2 O (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) Zn 2+ (aq) + H 2 (g)
Redox Reactions Most redox reactions are more complex to balance. Sn 2 + (aq) + Fe 3+ (aq) Sn 4+ (aq) + Fe 2+ (aq) Sn 2 + (aq) + 2 Fe 3+ (aq) Sn 4+ (aq) + 2 Fe 2+ (aq)
Redox Reactions Redox reactions can be broken up into 2 half-reactions: a reaction that shows either oxidation or reduction alone Overall reaction: Zn (s) + 2 H + (aq) Oxidation half reaction: Zn (s) Zn 2+ (aq) + 2 e - Reduction half reaction: Zn 2+ (aq) + H 2 (g) 2 H + (aq) + 2 e - H 2 (g)
Redox Reactions Notice that electrons lost = electrons gained in a balanced redox reaction: Zn (s) Zn 2+ (aq) + 2 e - 2 H + (aq) + 2 e - H 2 (g) Zn (s) + 2 H + (aq) Zn 2+ (aq) + H 2 (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 H 2 O 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: Cr 2 O 7 2- + Fe 2+ Cr 3+ + Fe 3+ (acidic soln)
Balancing Redox Reactions Example: Balance the following redox reaction which takes place in acidic solution. ClO 3 - + CH 3 OH Cl 2 + H 2 CO
Balancing Redox Reactions Procedure for Basic Solutions: Divide the equation into 2 incomplete half reactions one for oxidation one for reduction
different Balancing Redox Reactions Balance each half-reaction: balance all elements except H and O balance O atoms by adding H 2 O 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 H 2 O cancel the same # of H 2 O from each side balance charge by adding e - to side with the 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 Reactions Example: Balance the following redox reaction. NH 3 + ClO - Cl 2 + N 2 H 4 (basic soln)
Balancing Redox Reactions Example: Balance this redox reaction which occurs under basic conditions. Pb(OH) 4 2- + ClO - PbO 2 + Cl -