Video Transcript
What is the overall reaction
equation for the spontaneous redox reaction that occurs when lead–acid batteries
discharge? (A) Pb solid plus four PbSO4
aqueous plus two H2O liquid forms two PbSO4 solid plus PbO2 solid plus two H2SO4
aqueous. (B) Pb solid plus PbO2 solid plus
two H2SO4 aqueous forms two PbSO4 solid plus two H2O liquid. (C) PbO2 solid plus two PbSO4
aqueous plus two H2 forms two PbSO4 solid plus Pb solid plus two H2O liquid. (D) Two PbSO4 solid plus two H2O
liquid forms two PbO solid plus two H2SO4 aqueous. Or (E) two PbSO4 solid plus two H2O
liquid forms Pb solid plus PbO2 solid plus two H2SO4 aqueous.
A lead–acid battery is an example
of a secondary galvanic cell. All secondary galvanic cells have
the capacity to discharge and recharge in two distinct phases. If a cell is operating as a
galvanic cell, it is discharging. And if it is operating as an
electrolytic cell, it is recharging. We can understand the chemistry of
electrochemical cells by examining the half equations that occur at each
electrode.
To answer our question, we will
focus only on the reactions that occur during the discharge phase of a lead–acid
battery. At the anode, oxidation occurs
during discharge and can be represented by the following equation. Pb solid plus H2SO4 aqueous forms
PbSO4 solid plus two H+ aqueous plus two e−. At the cathode, reduction of the
lead(IV) dioxide occurs during discharge. PbO2 solid plus H2SO4 aqueous plus
two H+ aqueous plus two e− forms PbSO4 solid plus two H2O liquid.
Now, we can combine these two half
equations to represent the overall cell equation for a lead–acid battery acting as a
galvanic cell during discharge. Pb solid plus PbO2 solid plus two
H2SO4 aqueous forms two PbSO4 solid plus two H2O liquid. As we can see, the only answer
choice that represents this overall equation is answer choice (B).
Therefore, answer choice (B) is our
correct answer. Pb solid plus PbO2 solid plus two
H2SO4 aqueous forms two PbSO4 solid plus two H2O liquid.
