Chapter 18 Oxidation-­‐
Reduction Reactions & Electrochemistry Oxidation-­‐Reduction Reactions “Redox” LEO SAYS GER
Oxidation-­‐Reduction Reactions • Also known as redox or electron transfer reac-ons • One or more elements change oxida-on number • All single displacement and combus-on reac-ons • Some synthesis and decomposi-on reac-ons How can I tell if it is redox? •  If there is an element in the reactants or products you know it must be redox. •  In a redox reac-on, electrons are lost and gained. •  The element that loses electrons is oxidized •  The element that gains electrons is reduced •  LEO says GER (OIL RIG) •  If oxida-on numbers change during the reac-on, it must be redox. Oxidation and Reduction (Redox) q  Electrons are transferred
q  Spontaneous redox rxns can transfer
energy
q  Electrons (electricity)
q  Heat
q  Non-spontaneous redox rxns can be
made to happen with electricity
q  So what are oxidation numbers?
What are oxidation numbers? • A tool to help determine which element is oxidized and which is reduced. • For ionic compounds, the oxida-on number is the element’s charge Rules for Assigning Oxidation Numbers Rules 1 & 2 1.  The oxidation number of any uncombined
element is zero
2. The oxidation number of a monatomic ion
equals its charge
0
0
+1
−1
2 Na + Cl 2 → 2 Na Cl
Oxidation Reduction Reactions (Redox) 0
0
+1
−1
2 Na + Cl 2 → 2 Na Cl
Each sodium atom loses one electron:
0
+1
−
Na → Na + e
Each chlorine atom gains one electron:
0
−1
Cl + e → Cl
−
LEO says GER : Lose Electrons = Oxidation
0
+1
Na → Na + e
−
Sodium is oxidized
Gain Electrons = Reduction
0
−1
Cl + e → Cl
−
Chlorine is reduced
Reducing Agents and Oxidizing Agents q  The substance reduced is the oxidizing agent
q  The substance oxidized is the reducing agent
0
+1
−
Na → Na + e
Sodium is oxidized – it is the reducing agent
0
−1
−
Cl + e → Cl
Chlorine is reduced – it is the oxidizing agent
Rules for Assigning Oxidation Numbers Rules 3 & 4 3. The oxidation number of oxygen in
compounds is -2
4. The oxidation number of hydrogen in
compounds is +1
+1
−2
H2O
Rules for Assigning Oxidation Number Rule 5 5. The sum of the oxidation numbers in
the formula of a compound is zero (0)
+1
−2
H2O
2(+1) + (-2) = 0
H
O
+2
−2 +1
Ca (O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation Numbers Rule 6 6. The sum of the oxidation numbers in the
formula of a polyatomic ion is equal to
its charge
? −2
N O3
−
? −2
S O4
2−
X + 3(-2) = -1
N
O
X + 4(-2) = -2
S
O
∴ X = +5
∴ X = +6
The Oxidation Number Rules -­‐ SIMPLIFIED 1. The sum of the oxidation numbers in
ANYTHING is equal to its charge
2. Hydrogen in compounds is +1
3. Oxygen in compounds is -2
4. Charge on uncombined element is
zero
Try a few… •  ClO2
•  ClO•  N2O
•  H2S
Oxidized or Reduced? •  2Sr + O2 à 2SrO
•  2Li + S à Li2S
•  2Cs + Br2 à 2CsBr
•  3Mg + N2 à Mg3N2
Half Reactions •  Fe2+ + MnO4- à Fe3+ + Mn2+
•  Sn2+ + IO3- à Sn4+ + I•  S2- + NO3- à S + NO
•  NH3 + NO2 à N2 + H2O
Not All Reactions are Redox Reactions Reactions in which there has been no change
in oxidation number are not redox rxns.
Examples:
+1 +5 −2
+1
−1
+1
−1
+1 +5 −2
Ag N O3 (aq) + Na Cl (aq) → Ag Cl ( s ) + Na N O 3 (aq )
+1 −2 +1
+1
+6 −2
+1
+6 −2
+1
−2
2 Na O H (aq) + H 2 S O 4 (aq) → + Na 2 S O 4 (aq ) + H 2 O(l )
Identifying Electron Transfer in Redox-­‐Reactions 4Al(s) + 3O2(g) → 2Al2O3(s)
Each Al atom loses 3 electrons (for at total of 12 electrons) Each O atom gains 2 electrons (for a total of 12 electrons) Oxidation-­‐Reduction Reactions (cont.) • Always have both oxida-on and reduc-on • LEO goes GER • Lose e-­‐ = oxidized, Gain e-­‐ = reduced • Split reac-on into oxida-on half-­‐
reac-on and a reduc-on half-­‐
reac-on • Half-­‐reac-ons include electrons Oxidation •  OxidaDon is the process that occurs when: 1.  Oxida-on number of an element increases 2.  Element loses electrons 3.  Compound adds oxygen 4.  Compound loses hydrogen 5.  Half-­‐reac-on has electrons as products Reduction •  ReducDon is the process that occurs when: 1.  Oxida-on number of an element decreases 2.  Element gains electrons 3.  Compound loses oxygen 4.  Compound gains hydrogen 5.  Half-­‐reac-ons have electrons as reactants Predicting Products of Metal + Nonmetal Reactions •  Metal + nonmetal → ionic compound •  Ionic compounds always solids unless dissolved in water •  In the ionic compound the metal is now a ca-on. •  In the ionic compound the nonmetal is now an anion. Predicting Products of Metal + Nonmetal Reactions (cont.) • To predict direct synthesis of metal + nonmetal: 1.  Determine the charges on the ca-on and anion from their posi-ons on the periodic table 2.  Balance the charges to get the formula of the compound 3.  Balance the equa-on Balancing Oxidation-­‐Reduction Reactions: the Half-­‐Reaction Method • One method is to write separate reac-ons for the oxida-on and reduc-on processes. • Called half-­‐reacDons • Mul-ply the half-­‐reac-ons so the number of electrons lost in the oxida-on and gained in the reduc-on is equal. • Add the half-­‐reac-ons together and cancel the electrons.