# Question Video: Identifying the Overall Equation for the Reaction That Occurs When Lead–Acid Batteries Discharge Chemistry

What is the overall reaction equation for the spontaneous redox reaction that occurs when lead–acid batteries discharge?

03:09

### 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.

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