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Chapter 12

Electrochemistry

12.1 Daniel / Galvanic Cell versus Electrolytic Cell

• Zn is lower than Cu on the

Electrochemical Series

• Zn + Cu2+

� Zn2+

+ Cu

E° = 0.76 + 0.34 = 1.1 V > 0

• Natural process

• -ive terminal to Zn and +ive terminal to Cu

• Zn2+

+ Cu � Zn + Cu2+

• E° = - 0.76 - 0.34 = - 1.1 V < 0

• Artificial process

• If Zn2+

solution is dilute, than trace H+ from H2O becomes H2

(cathodic reduction)

• If Cu2+

solution is dilute, than trace OH- from H2O becomes O2

(anodic oxidation)

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12.2 Charge Based Calculations

• Q = It = nF

12.3 Acid / Base Dissociation

• NaOH � Na+ + OH

- Kb = [Na

+][OH

-] / [NaOH]

pKb = -log ([Na+] / [NaOH]) + pOH

pOH = pKb + log ([Na+] / [NaOH])

Since pH + pOH = 14 = pKw (for water and all aqueous solutions),

14 - pH = pKb + log ([Na+] / [NaOH])

pH = 14 - pKb - log ([Na+] / [NaOH])

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• Strong acid / base calculation:

• Weak acid / base calculation:

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• Acid / base strength

- C2H5 etc also electron donating (increasing ion stability)

- C6H5 , halogens, OH etc are electron withdrawing (decreasing ion stability)

(in this case, more electron drawing power decreases ion stability and

therefore decreases base strength)

**So how about for ethanoic acid dissociation in water?

Where R is CH3

(in this case, more electron drawing power INcreases ion

stability and therefore INcreases base strength)

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12.4 Titration Indicators are Weak Acids / Bases

• Most indicators change colour across a pH range of 2 or concentration

range of 102

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12.5 Titration Curves

• Equivalence point shift

pH = 14 - pKb - log ([Na+] / [NaOH])

(Curve can actually be plotted)

• Titrating sodium carbonate (use which indicator?)

HCl + Na2CO3 � NaCl + NaHCO3

HCl + NaHCO3 � NaCl + H2O + CO2

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• Titrating phosphoric acid H3PO4 (again, use which indicator?)

12.6 Buffer Solutions

• Weak acid or weak base buffer solutions (must have salt present)

•

pH = 14 - pKb - log ([Na+] / [NH4OH])

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• If asked to calculate pH change when more acid or base is added, just

plug in new [acid] or [base] into the equation for calculation. This is to

determine buffer capacity i.e. how much more acid or base to upset the

buffered pH.

pH = 14 - pKb - log ([Na+] / [NH4OH])

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• Biological importance

• If asked to calculate pH change when more acid or base is added, just

plug in new [acid] or [base] into the equation for calculation. This is to

determine buffer capacity i.e. how much more acid or base to upset the

buffered pH.

• Complexometric titration with edta4-

or Y4-

(alkaline buffered)

H2PO4- � H

+ + HPO4

2- � H

+ + PO4

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pH = 14 - pKb - log ([Na+] / [NH4OH])

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- used in Mg2+

and Ca2+

concentration determination for hard water

analysis etc.

12.7 Standard Hydrogen Electrode (SHE)