Quizlet

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• Next Wednesday is Test 3!!

Clicker 1

• Audio 0:03:32.431301
• The titration of 80.0 mL of an unknown concentration of H3PO4 solution requires 126 mL of .218 M KOH solution. What is the concentration of H3PO4 solution (in M)
  • A) 1.03 M
  • B) 0.343 M
  • C) 0.114 M
  • D) 0.138 M
  • E) 0.0461 M

Oxidation–Reduction Reactions

• Audio 0:09:22.846460
• The reactions in which electrons are transferred from one reactant to the other are called oxidation–reduction reactions.
  • These are also called redox reactions.
  • Many redox reactions involve the reaction of a substance with oxygen.
  • Examples:
  • 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g) (combustion)
  • 2 H2(g) + O2(g) → 2 H2O(g)
  • 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s) (rusting)

Oxidation and Reduction (Redox)

• Audio 0:11:42.911701
• To convert a free element into an ion, the atoms must gain or lose electrons.
  • NOTE: If one atom loses electrons, another must accept them.
• Atoms that lose electrons are being oxidized.
  • The reaction is referred to as oxidation.
• Atoms that gain electrons are being reduced.
  • The reaction is referred to as reduction.
• Audio 0:14:01.108735
• Example: 2 Na(s) + Cl2(g) → 2 NaCl(s)
  • Audio 0:14:42.939831
  • Na → Na+ + 1 e– (oxidation; sodium lost electrons)
  • Cl2 + 2 e– → 2 Cl– (reduction; chlorine atom gained electrons)

Redox Reactions

• Audio 0:15:20.876721
• Oxidation and reduction must occur simultaneously.
  • If an atom loses electrons, another atom must take them.
• The reactant that reduces an element in another reactant is called the reducing agent.
  • The reducing agent contains the element that is oxidized.
• The reactant that oxidizes an element in another reactant is called the oxidizing agent.
  • The oxidizing agent contains the element that is reduced.
• Example: 2 Na(s) + Cl2(g) → 2 NaCl(s)
  • Na is oxidized, while Cl is reduced.
  • Na is the reducing agent, and Cl2 is the oxidizing agent.

Oxidation States: “Electron Book Keeping”

• Audio 0:19:09.006032
• For reactions that are not metal + nonmetal, or do not involve O2, a method for determining how the electrons are transferred is required to determine which element is being oxidized and which element is being reduced.
• Chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reaction.
  • Audio 0:20:40.459510
  • Even though they look like them, oxidation states are not ion charges!
    • Oxidation states are imaginary charges assigned based on a set of rules.
    • Ion charges are real, measurable charges.

Rules for Assigning Oxidation States

• Audio 0:23:41.166326
• The following rules are in order of priority:
  1. Free elements have an oxidation state = 0.
     • Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)
  2. Monatomic ions have an oxidation state equal to their charge.
     • Na = +1 and Cl = −1 in NaCl
  3. (a) The sum of the oxidation states of all the atoms in a compound is 0.
     • Na = +1 and Cl = −1 in NaCl, (+1) + (−1) = 0

Rules for Assigning Oxidation States Continued

• Audio 0:25:29.324643
  1. (b) The sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion.
     • N = +5 and O = −2 in NO3 –, (+5) + 3(−2) = −1
  2. (a) Group I metals have an oxidation state of +1 in all their compounds.
     • Na = +1 in NaCl
  3. (b) Group II metals have an oxidation state of +2 in all their compounds.
     • Mg = +2 in MgCl2
  4. In their compounds, nonmetals have oxidation states according to the following table.
     • Nonmetals higher on the table take priority.

Rules for Assigning Oxidation States

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Practice Problem: Assigning Oxidation States

• Audio 0:28:14.744310
• Assign an oxidation state to each atom in each element, ion or compound (a) Cl2 (b) Na+ (c) KF (d) CO2 (e) SO4^2- (f) K2O2

Clicker 2

• Audio 0:35:08.680784
• Determine the oxidation state of nitrogen in NO2
  • A) +5
  • B) +3
  • C) 0
  • D) +2
  • E) +4

E

Identifying Redox Reactions

• Audio 0:36:40.373032
• Oxidation: An increase in oxidation state
• Reduction: A decrease in oxidation state
• 
  • Carbon changes from an oxidation state of 0 to an oxidation state of +4.
    • Carbon loses electrons and is oxidized.
  • Sulfur changes from an oxidation state of 0 to an oxidation state of –2.
    • Sulfur gains electrons and is reduced.

Redox Reaction

• Audio 0:37:34.617890
• NOTE:
• The transfer of electrons does not need to be a complete transfer (as occurs in the formation of an ionic compound) for the reaction to qualify as oxidation–reduction.
  • For example, consider the reaction between hydrogen gas and chlorine gas:
    • H2(g) + Cl2(g) → 2 HCl(g)
• When hydrogen bonds to chlorine, the electrons are unevenly shared, resulting in:
  • An increase of electron density (reduction) for chlorine
  • A decrease in electron density (oxidation) for hydrogen

Combustion as Redox

• 2 H2(g) + O2(g) → 2 H2O(g)
• Hydrogen and Oxygen reacts to form water in the gaseous state (steam).
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Practice Problem: Oxidation States and Identifying

• Audio 0:39:01.686365
• What Is Being Oxidized and What Is Being Reduced Use oxidation state to identify the element that is oxidized and the element that is reduced:
• Mg(s) +2 H2O(l) à Mg(OH)2(aq) + H2(g)

Clicker 3

• Audio 0:39:32.867693
• What element is undergoing reduction (if any) in the following reaction?
  • Zn(s) + 2AgNO3(aq) -> Zn(NO3)2(aq) + 2 Ag(s)
  • A) Zn
  • B) N
  • C) O
  • D) Ag
  • E) This is not an oxidation-reduction reaction

D

Redox without Combustion

• 2 Na(s) + Cl2(g) → 2 NaCl(s)
• Sodium metal transfers electrons to chlorine gas to form solid sodium chloride. The sodium metal is reduced and chlorine is oxidized.
• 

Practice Problem: Oxidation States and Identifying

• What Is Being Oxidized and What Is Being Reduced
• Determine whether or not each reaction is an oxidation-reduction reaction. For each oxidation-reduction reaction, identify the oxidizing agent and the reducing agent.
  • a) 2 Mg(s) + O2(g) à 2 MgO(s)
  • b) 2HBr(aq) + Ca(OH)2(aq) à 2 H2O(l) + CaBr2(aq)
  • c) Zn(s) + Fe2+(aq) à Zn2+(aq) + Fe(s)

Vocab

Term	Definition
titration	when a substance in a solution of known concentration is reacted with another substance in a solution of unknown concentration
equivalence point	when exactly enough solution has been added the reactants are in their stoichiometric ratio
indicator	chemical that changes color when the solution undergoes large changes in acidity/alkalinity
endpoint	when the indicator changes color during titration
equivalence point	when the number of moles of H+ equals the number of moles of OH– in an acid-base titration
oxidation	when atoms lose electrons
reduction	when atoms gain electrons
reducing agent	contains the element that is oxidized in a redox reaction
oxidizing agent	the reactant that oxidizes an element in another reactant in a redox reaction
oxidation–reduction reactions	reactions in which electrons are transferred from one reactant to the other