Video Transcript
Mechanics use a hydrometer to check
the relative density of sulfuric acid in five different lead–acid car batteries. The following table shows their
results. Which battery or batteries need to
be recharged? (A) Only battery 3. (B) Only battery 5. (C) Batteries 2 and 5. (D) Only battery 1. (E) Batteries 3 and 4.
First of all, a hydrometer is an
instrument used to measure the relative density of a liquid or solution. Another term for relative density
is specific gravity. Relative density is the ratio of
the density of a liquid or solution to the density of pure water at a specific
temperature and pressure.
In this problem, we’ve been given
the relative density measurements of sulfuric acid in five different lead–acid car
batteries. When the relative density of
sulfuric acid in a battery drops below a certain value, a mechanic knows it is time
to recharge the battery. Let’s clear some space to discuss
why this happens.
A lead–acid car battery is
classified as a secondary galvanic cell. This is a type of electrochemical
cell that can function as both a galvanic cell and an electrolytic cell depending on
which processes are occurring. At the anode in the galvanic cell,
oxidation of lead metal occurs. This reaction uses up sulfuric
acid, or H2SO4 aqueous. At the cathode of the galvanic
cell, the reduction of lead(IV) oxide occurs. This process also uses up sulfuric
acid. As the concentration of sulfuric
acid in the cell decreases, the relative density also decreases.
The redox reactions that occur in
the galvanic cell are used to power an external device, and this process is referred
to as discharging. The sulfuric acid used up during
discharging can be regenerated by recharging the battery. To recharge the battery, an
external battery charger is used. During recharging, the lead–acid
battery functions as an electrolytic cell.
We can see here that one of the
products of the redox reactions that occur during recharging is sulfuric acid. So, how do mechanics know when
recharging is necessary? If the relative density of sulfuric
acid in the battery drops below 1.20 grams per cubic centimeter, then it’s time for
the battery to be recharged. We can see that the relative
density is less than this value in batteries 3 and 4. Therefore, batteries 3 and 4 need
to be recharged.
In conclusion, based on the given
relative density values for sulfuric acid, the batteries that need to be recharged
are batteries 3 and 4, or answer choice (E).
