How Batteries Work: The Lead Acid Accumulator ReactionsHow Batteries Work: The Lead Acid Accumulator Reactions To understand how a battery backup solar array provides power throughout the night as well as the day we need to know how batteries work to both store and discharge that power. The chemistry of the standard lead-acid accumulator battery, which is better known to us as the car battery, is not simple but it is worth the effort to understand the working of this marvel of engineering. Not only will this understanding give us insight into the workings of one of the devices we rely on in daily life, but it will also inform us as to how best to maintain these batteries to maximize their lifespans. How Batteries Work: A Single Cell of the Battery The term battery means repeating identical units grouped together. In terms of our lead-acid accumulator or car battery, the units that are grouped together are called cells. There are dozens of cells in each lead-acid battery. By understanding the chemical reactions occurring in one of these cells we will have a good view of the operation of the whole battery. We will start our exploration of this battery by assuming that it is fully charged and ready to release electricity. The Lead-Acid battery is based on just those two chemicals, lead and acid. Lead in the Battery The lead is in two forms, which are pure Lead (Pb) and Lead Dioxide (PbO2). Both of these versions of Lead are in solid form. If we look at the oxidation number rules for these two versions of Lead, we can see that pure Lead has an oxidation number of zero whereas the Lead Dioxide has an oxidation number of +4. This is important as it is movement away from these starting oxidation numbers that results in the generation of electricity in the battery. These two solid forms of lead form the two electrodes of the battery cell. Acid in the Battery The liquid in the middle is Sulfuric Acid which has the chemical formula H2SO4 and is mixed with water. This acid is composed of two protons (H+) and the sulfate ion (SO4 2-). In the battery it is in the ionized form of H+ and HSO4-. This is because Sulfuric Acid easily loses one of its two protons in water and only loses the second when the acid reacts with something such as the lead metal.
We can see that the two versions of Lead are not connected in this picture. This represents the situation where the battery is not in use; the different metals in the cell are not in contact (no complete circuit) and so no reactions occur. The inspiring part of the lead-acid battery is what happens when the circuit is completed. Both the Lead and the Lead Dioxide form into Lead Sulfate, PbSO4. This is a solid that coats both metal plates as it forms. This is important, as it is the key to being able to recharge the battery. Let's have a look at each of these reactions in turn. Pure Lead to Lead Sulfate It is best if we track this reaction through the the standard balancing redox reactions process. First, we line up the reactants (lead and the HSO4- version of the acid) and then the product, the Lead Sulfate. Again, the oxidation numbers are included on top of the main chemicals.
Here we can see that the Lead has been oxidized as its oxidation number has increased from 0 to +2. The electrons released will travel through a circuit and do some work before they come to the other terminal of the cell where the lead oxide waits to undergo this reaction: Lead Oxide to Lead Sulfate
Once again only the Lead has been affected in terms of its oxidation number. In this case the Lead has gone from +4 to +2 meaning it has been reduced. The Lead Oxide has absorbed the electrons given off by the pure Lead. How Batteries Work: The Overall Reaction If we combine these two half reactions, again following the rules for balancing redox reactions, we get the following. Note that the different steps simply result from crossing out things that are the same on both sides of the arrow.
So that is the reaction that is occurring in each cell of the battery. For each atom of lead converted to PbSO4 two electrons are produced. These are absorbed by one piece of PbO2 becoming PbSO4. How Batteries Work: The Reverse Reaction: Recharging the Battery To obtain the reverse reactions all we need to do is turn the arrows to face the opposite direction. If we do this we can see that the PbSO4 is converted back into Pb on one plate and PbO2 on the other. This is a forced reaction that can only occur if extra electrons are pushed into the battery.
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