Chemistry of a Lead Acid Battery Chemistry Composition

Chemistry of a Lead Acid Battery Chemistry

Composition of a Lead Acid Battery • Positive Plate: Lead Oxide (Pb. O) • Negative Plate: Lead (Pb) • Electrolyte: sulphuric acid (H 2 SO 4)

Review of Acid Terminology • Strong vs. Weak Acids – Strong acids completely ionize in water • EX: HCl H+ + Cl- (HCl + H 2 O H 3 O+ + Cl-) – Weak acids partially ionize in water • EX: HC 2 H 3 O 2 ⇌ H+ + C 2 H 3 O 2 - (note ⇌ versus ) • Monoprotic vs. Diprotic Acids – Monoprotic acid has one H+ (EX: HCl) – Diprotic acid has two H+ (EX: H 2 SO 4)

The Electrolyte: H 2 SO 4 • H 2 SO 4 is classified as a strong acid (completely ionizes) but that is not exactly true – The first ionization of H 2 SO 4 is complete and occurs instantly when in water • EX: H 2 SO 4 H+ + HSO 4 - – But! The second ionization is extremely partial • EX: HSO 4 - ⇌ H+ + SO 4 -2 (Ka = 0. 012!) • This is very important in understanding the chemistry of a lead acid battery!

ü for understanding • In general, is sulfuric acid considered a strong or weak acid? Strong • Why is sulfuric acid considered to be strong? It completely ionizes • What ion related to sulfuric acid is weak? HSO 4 • Why is the bisulfate ion, HSO 4 considered weak? It partially ionizes

Lead Acid Batteries are rechargable • This leads to two different sets of redox reactions – The discharging phase – The charging phase

Chemistry of Discharge Phase • Negative plate reaction (oxidation): – Pb + HSO 4 Pb. SO 4 + H+ + 2 e • Positive plate reaction (reduction): – Pb. O 2 + HSO 4 + 3 H+ + 2 e- Pb. SO 4 + 2 H 2 O • Overall discharge redox reaction: – Pb + Pb. O 2 + 2 HSO 4 + 2 H+ 2 Pb. SO 4 + 2 H 2 O or…Pb + Pb. O 2 + 2 H 2 SO 4 2 Pb. SO 4 + 2 H 2 O Note: states are removed for simplicity ØPb, Pb. O 2, and Pb. SO 4 are always solids ØHSO 4 -, H+, and H 2 SO 4 are always aqueous ØH 2 O is always a liquid

The flow of electrons = ELECTRICITY!

Chemistry of Charge Phase • Negative plate reaction (reduction): – Pb. SO 4 + H+ + 2 e- Pb + HSO 4 • Positive plate reaction (oxidation): – Pb. SO 4 + 2 H 2 O Pb. O 2 + HSO 4 + 3 H+ + 2 e- • Overall charge redox reaction: – 2 Pb. SO 4 + 2 H 2 O Pb + Pb. O 2 + 2 HSO 4 + 2 H+ or… 2 Pb. SO 4 + 2 H 2 O Pb + Pb. O 2 + 2 H 2 SO 4

ü for understanding • What metal is always at the negative plate? Pb • What compound is always at the positive plate? Pb. O 2 • What is the electrolyte in a battery? H 2 SO 4 or HSO 4 • What compound is the “waste” product of discharge and has to be removed in the charging process? (think!) Pb. SO 4

Why do batteries “die”? • When the battery discharges solid Pb. SO 4 is formed. • In a new battery, the Pb. SO 4 is in a spongy form which can easily be converted back to Pb and Pb. O 2. • Over time the Pb. SO 4 will tend to crystallize. • The crystallized Pb. SO 4 cannot be converted back to Pb and Pb. O 2 so there is not only less reactant material but it also coats the surface of the Pb and Pb. O 2. New Battery Dead Battery