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Ox alwaysalways change during redox reactions: Increase Ox = OxidationOxidation Decrease Ox = ReductionReduction
It’s a redox reaction if: element → compoundor: compound → element
S8(s) + 8 O2(g) → 8 SO2(g)
Ox(S) : 0 → +4 (oxidized: lost e- ; increased Ox)Ox(O) : 0 → -2 (reduced: gained e- ; decreased Ox)
Oxidation Numbers & Redox Reactions
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A more complex example:
Cu(s)+4H+(aq)+2NO3-(aq) → Cu2+(aq)+2NO2(g)+2H2O(ℓ)
00 +2+2+1+1 +1 -2+1 -2+4 -2+4 -2+5 -2+5 -2
Cu is oxidized (Ox ↑; loss of e-).
H is unchanged.
O is unchanged.
N is reduced (Ox ↓; gain of e-).
Oxidation Numbers & Redox Reactions
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Pb(NO3)2(aq) + 2 Kl(aq) 2 KNO3(aq) + Pbl2(s)
Exchange reactions are not redox – no change in oxidation state occurs.
+2+2 +2+2+1+1 +1+1-1-1 -1-1
Oxidation Numbers & Redox Reactions
NO3-: Ox(O) = -2; Ox(N) = +5
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Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) 0 +2 0 +2 +2 +2 0 0
Cu(s) + 2 AgNO3(aq) → Cu(NO3)2(aq) + 2 Ag(s) 0 +1 0 +1 +2+2 0 0
Not all metals can displace another from its salts:Cu(s) + ZnSO4(aq) no reactionno reaction
An activity seriesactivity series was developed…
Displacement Reactions
Redox:
A XZ AZ X+ +
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Displace H2 from steam or acid
Displace H2 from H2O (ℓ), steam or acid
Displace H2 from acid
No reaction with H2O, steam or acid
Activity Series of Metals
SbCuHgAgPdPtAu
LiK
BaSrCaNa
MgAl
MnZnCr
FeNiSnPb
H2
Ease ofoxidationdecreases
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Powerful reducing agents at the top.
Higher elements displace lower ones:
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
Metals at the bottom are unreactive.• Coinage metals• Their ionsions are powerful oxidizing agents.
SbCu:
Ag:
Au
LiK::
Na
Mg:
ZnCr
Fe:
Pb
H2
Displacement Reactions
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Displacement Reactions
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Displacement Reactions2 Na(s) + 2 H2O(ℓ) → 2 NaOH(aq) + H2(g)
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pH Scale• In pure water, a few molecules
ionize to form H3O+ and OH–
H2O + H2O OH– + H3O+
• In acidic and basic solutions, these concentrations are not equalacidic: [H3O+] > [OH–]basic: [OH–] > [H3O+]neutral: [H3O+] = [OH–]
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pH Scale• Measure how much H3O+ is in a
solution using pH• pH < 7.0 = acidic• pH > 7.0 = basic• pH = 7.0 = neutral• Measure of H3O+ and OH–
concentration (moles per liter) in a solution
• As acidity increases, pH decreases
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pH Scale• The pH scale is logarithmic:The pH scale is logarithmic:
100100 101022 log(10log(1022) = 2) = 21010 101011 log(10log(1011) = 1) = 111 101000 log(10log(1000) = 0) = 00.10.1 1010–1–1 log(10log(10–1–1) = –1) = –10.010.01 1010–2–2 log(10log(10–2–2) = –2) = –2
• pHpH = –log [H= –log [H33OO++]]
• pH if [HpH if [H33OO++] = 10] = 10–5–5? 10? 10–9–9? Acidic ? Acidic or basic?or basic?
• pH if [HpH if [H33OO++] = 0.000057 M?] = 0.000057 M?
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Finding [H[H33OO++] from pH] from pH
[H[H33OO++] = 10] = 10-pH-pH
What is [HWhat is [H33OO++] if pH = 8.9?] if pH = 8.9?
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pH: Quantitative Measure of Acidity
• Acidity is related to concentration of H+ (or H3O+)
• pH = -log[H3O+]