aqueous reactions chapter 20 “oxidation-reduction reactions” leo says ger
TRANSCRIPT
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CHAPTER 20CHAPTER 20
““Oxidation-Reduction ReactionsOxidation-Reduction Reactions””
LEO SAYS GER
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Oxidation-Reduction Reactions
• An oxidation occurs when an atom or ion loses electrons.
• A reduction occurs when an atom or ion gains electrons.
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Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)
Redox currently says that electrons are transferred between reactants
Mg + S → Mg2+ + S2-
•The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg2+
•The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S2-
(MgS)
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Oxidation-Reduction Reactions
One cannot occur without the other.
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Problems
• 4.45 a,b
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Oxidation Numbers
To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.
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Oxidation Numbers
• Elements in their elemental form have an oxidation number of 0. ( Al)
• The oxidation number of an ion is the same as its charge. B+2
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Oxidation Numbers
• Nonmetals have negative oxidation numbers based on the ions they form, although some are positive in certain compounds or ions.Oxygen has an oxidation number of −2,
except in the peroxide ion in which it has an oxidation number of −1.
Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.
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Oxidation Numbers
• Nonmetals tend to have negative oxidation numbers, Fluorine always has an oxidation number of −1.The other halogens have an oxidation
number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.
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Oxidation Numbers
• The sum of the oxidation numbers in a neutral compound is 0.
• The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.
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Problems
• 4.49
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Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)11
2
00
22
ClNaClNa
Each sodium atom loses one electron:
Each chlorine atom gains one electron:
eNaNa10
10 CleCl
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LEO says GER :LEO says GER :
eNaNa10
Lose Electrons = Oxidation
Sodium is oxidized
Gain Electrons = Reduction
10 CleCl Chlorine is reduced
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LEO says GER :LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent.
Mg(s) + S(s) → MgS(s)
Mg is oxidized: loses e-, becomes a Mg2+ ion
S is reduced: gains e- = S2- ion
Mg is the reducing
agent
S is the oxidizing agent
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Problems
• 4.51 a,b,c,d
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Displacement Reactions
In this reaction,
silver ions oxidize
copper metal.
Cu (s) + 2 Ag+ (aq) Cu2+ (aq) + 2 Ag (s)
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Displacement Reactions
The reverse reaction,
however, does not
occur.
Cu2+ (aq) + 2 Ag (s) Cu (s) + 2 Ag+ (aq) x
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Activity Series
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Using Molarities inStoichiometric Calculations
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TitrationThe analytical technique in which one can calculate the concentration of a solute in a solution.
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Titration