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

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

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

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+ +

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

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

Displacement Reactions

Displacement Reactions2 Na(s) + 2 H2O(ℓ) → 2 NaOH(aq) + H2(g)

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–]

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

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?

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?

pH: Quantitative Measure of Acidity

• Acidity is related to concentration of H+ (or H3O+)

• pH = -log[H3O+]