What chemical reaction occurs in a lead-acid battery when discharging?

In a lead-acid battery during discharging, chemical energy is converted to electrical energy. This process involves the electrochemical reactions between the lead dioxide (PbO2), sponge lead (Pb), and sulfuric acid (H2SO4) in the electrolyte. When the battery discharges, these reactions generate electrical energy that can be used to power electronic devices or engines.

Specifically, the reaction can be described as lead dioxide reacting with sulfuric acid to produce lead sulfate (PbSO4) and water (H2O), while simultaneously releasing electrical energy. This fundamental principle is what allows lead-acid batteries to serve effectively as power sources in various applications, such as in vehicles or as backup power systems.

The other choices relate to different forms of energy conversion. For example, the conversion of chemical energy to mechanical energy could occur in systems where chemical reactions drive physical movement, but that's not the case in the context of a lead-acid battery's function. Similarly, conversion of electrical energy to thermal energy or mechanical energy doesn't accurately describe the discharge process of a lead-acid battery. Thus, the understanding of the discharge process clearly identifies that chemical reactions lead to the generation of electrical energy.