AP® Practice Exam Free-Response Questions Notes

Lead Acid Battery

• Invented in 1859, it's the oldest type of rechargeable battery.
• Cells consist of lead metal (Pb) and lead(IV) oxide ($PbO<em>2$) plates immersed in sulfuric acid ($H</em>2SO_4$) electrolyte.

Overall Reaction

• The overall reaction equation for the lead acid battery is not provided in the initial text, thus it cannnot be stated whether Lead is oxidized, reduced, or both.

Half-Reactions and Standard Potentials

• Half-reactions and standard potentials (E in volts) are required to determine the voltage under standard conditions, but these are not provided in the initial text.

Effect of Sulfuric Acid Concentration

• The sulfuric acid concentration is much higher than $1 M$ in a lead-acid battery.
• The high concentration of sulfuric acid affects the actual measured voltage compared to the voltage calculated under standard conditions.

Lead(II) Sulfate Solubility

• Lead(II) sulfate ($PbSO<em>4$) has a $K</em>{sp}$ value of values are not provided in initial text at $25^\circ C$.

• Balanced Chemical Equation for Dissolution:

  $PbSO<em>4(s) \rightleftharpoons Pb^{2+}(aq) + SO</em>4^{2-}(aq)$

• Molar solubility of $PbSO<em>4$ can be calculated using the $K</em>{sp}$ value.

• The solubility of $PbSO_4$ would be lower in the solution within a lead storage battery due to the common ion effect from the presence of sulfate ions.

Valve-Regulated Lead-Acid (VRLA) Batteries

• During recharging, water is electrolyzed into hydrogen ($H<em>2$) gas and oxygen ($O</em>2$) gas, causing the battery to