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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) - PowerPoint PPT Presentation
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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-