always Ox always change during redox reactions: Oxidation Increase Ox = Oxidation Reduction Decrease Ox = Reduction It’s a redox reaction if:element → compound or: compound → element S 8 (s) + 8 O 2 (g) → 8 SO 2 (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) +2NO 3 - (aq) → Cu 2+ (aq) +2NO 2 (g) +2H 2 O (ℓ) 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(NO 3 ) 2 (aq) + 2 Kl(aq)  2 KNO 3 (aq) + Pbl 2 (s) Exchange reactions are not redox – no change in oxidation state occurs Oxidation Numbers & Redox Reactions NO 3 - : Ox(O) = -2; Ox(N) = +5

Fe(s) + CuSO 4 (aq) → FeSO 4 (aq) + Cu(s) Cu(s) + 2 AgNO 3 (aq)→ Cu(NO 3 ) 2 (aq) + 2 Ag(s) Not all metals can displace another from its salts: no reaction Cu(s) + ZnSO 4 (aq)no reaction activity series An activity series was developed… Displacement Reactions Redox: A XZ AZ X ++

Displace H 2 from steam or acid Displace H 2 from H 2 O (ℓ), steam or acid Displace H 2 from acid No reaction with H 2 O, steam or acid Activity Series of Metals Sb Cu Hg Ag Pd Pt Au Li K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2H2 Ease of oxidation decreases

Powerful reducing agents at the top. Higher elements displace lower ones: Zn(s) + CuSO 4 (aq) → ZnSO 4 (aq) + Cu(s) Mg(s) + 2 HCl(aq) → MgCl 2 (aq) + H 2 (g) Metals at the bottom are unreactive. Coinage metals ions Their ions are powerful oxidizing agents. Sb Cu : Ag : Au Li K : Na Mg : Zn Cr Fe : Pb H2H2 Displacement Reactions

2 Na(s) + 2 H 2 O(ℓ) → 2 NaOH(aq) + H 2 (g)

pH Scale In pure water, a few molecules ionize to form H 3 O + and OH – H 2 O + H 2 O  OH – + H 3 O + In acidic and basic solutions, these concentrations are not equal acidic: [H 3 O + ] > [OH – ] basic: [OH – ] > [H 3 O + ] neutral: [H 3 O + ] = [OH – ]

pH Scale Measure how much H 3 O + is in a solution using pH pH < 7.0 = acidic pH > 7.0 = basic pH = 7.0 = neutral Measure of H 3 O + and OH – concentration (moles per liter) in a solution As acidity increases, pH decreases

pH Scale The pH scale is logarithmic: log(10 2 ) = log(10 1 ) = log(10 0 ) = –1 log(10 –1 ) = – –2 log(10 –2 ) = –2 The pH scale is logarithmic: log(10 2 ) = log(10 1 ) = log(10 0 ) = –1 log(10 –1 ) = – –2 log(10 –2 ) = –2 pH= –log [H 3 O + ] pH= –log [H 3 O + ] pH if [H 3 O + ] = 10 –5 ? 10 –9 ? Acidic or basic? pH if [H 3 O + ] = 10 –5 ? 10 –9 ? Acidic or basic? pH if [H 3 O + ] = M? pH if [H 3 O + ] = M?

[H 3 O + ] from pH Finding [H 3 O + ] from pH [H 3 O + ] = 10 -pH What is [H 3 O + ] if pH = 8.9?

pH: Quantitative Measure of Acidity Acidity is related to concentration of H + (or H 3 O + ) pH = -log[H 3 O + ]