MadSci Network: Chemistry
Query:

Re: How do acids function in a battery?

Date: Tue Jan 15 13:22:58 2002
Posted By: Elsa Lee, Grad student, Molecular Cell Biology, University of California, Berkeley
Area of science: Chemistry
ID: 1006638836.Ch
Message:

Dear Sara,

I found a very helpful tutorial on a website called How stuffworks.com.
Here is the link: 
http://www.howstuffworks.com/battery.htm


According to their website, an electrochemical reaction between the type 
of metal and acid used in a battery occurs in order for electrons to 
move from one pole to another.  The acid eats away at the metal used 
to liberate the electrons. The actual metals and electrolytes used 
control the voltage of the battery -- each different reaction has a 
characteristic voltage. 

Here an except from their website:

Battery Reactions
Probably the simplest battery you can create is called a zinc/carbon 
battery. By understanding the chemical reaction going on inside this 
battery you can understand how batteries work in general. 

Imagine that you have a jar of sulfuric acid (H2SO4). Stick a zinc rod in 
it, and the acid will immediately start to eat away at the zinc. You will 
see hydrogen gas bubbles forming on the zinc, and the rod and acid will 
start to heat up. Here's what is happening: 

The acid molecules break up into three ions: two H+ ions and one SO4-- 
ion. 

The zinc atoms on the surface of the zinc rod lose two electrons (2e-) to 
become Zn++ ions. 

The Zn++ ions combine with the SO4-- ion to create ZnSO4, which dissolves 
in the acid. 

The electrons from the zinc atoms combine with the hydrogen ions in the 
acid to create H2 molecules (hydrogen gas). We see the hydrogen gas as 
bubbles forming on the zinc rod. 

If you now stick a carbon rod in the acid, the acid does nothing to it. 
But if you connect a wire between the zinc rod and the carbon rod, two 
things change: 

The electrons flow through the wire and combine with hydrogen on the 
carbon rod, so hydrogen gas begins bubbling off the carbon rod. 
There is less heat. You can power a light bulb or similar load using the 
electrons flowing through the wire, and you can measure a voltage and 
current in the wire. Some of the heat energy is turned into electron 
motion. 

The electrons go to the trouble to move to the carbon rod because they 
find it easier to combine with hydrogen there. There is a characteristic 
voltage in the cell of 0.76 volts. Eventually, the zinc rod dissolves 
completely or the hydrogen ions in the acid get used up and the 
battery "dies." 

Modern batteries use a variety of chemicals to power their reactions. 
Typical battery chemistries include: 

Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon 
chemistry is used in all inexpensive AA, C and D dry-cell batteries. The 
electrodes are zinc and carbon, with an acidic paste between them that 
serves as the electrolyte. 

Alkaline battery - Used in common Duracell and Energizer batteries, the 
electrodes are zinc and manganese-oxide, with an alkaline electrolyte. 
Lithium photo battery - Lithium, lithium-iodide and lead-iodide are used 
in cameras because of their ability to supply power surges. 

Lead-acid battery - Used in automobiles, the electrodes are made of lead 
and lead-oxide with a strong acidic electrolyte (rechargeable). 

Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, 
with potassium-hydroxide as the electrolyte (rechargeable). 
Nickel-metal hydride battery - This battery is rapidly replacing nickel-
cadmium because it does not suffer from the "memory effect" that nickel-
cadmiums do (rechargeable). 

Lithium-ion battery - With a very good power-to-weight ratio, this is 
often found in high-end laptop computers and cell phones (rechargeable).
 
Zinc-air battery - This battery is lightweight and rechargeable. 

Zinc-mercury oxide battery - This is often used in hearing-aids. 
Silver-zinc battery - This is used in aeronautical applications because 
the power-to-weight ratio is good. 

Metal-chloride battery - This is used in electric vehicles. 

The tutorial is very interesting. I think you will find it very 
educational.

Have fun with science!
Elsa





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