Unit 27
Applications of Acids and Bases (Chapter 7)
•Acid Rain (7.8)
•Antacids (7.9)
•Acids/Bases in the House (7.10)
•Acids/Bases in Health (7.10)
Acid Rain – The Natural Background
There are three primary reactions that lead to the acidity of rain water, all
occurring to some extent naturally:
CO2 (g) +
H2O (l) →
H2CO3 (aq)
SO3 (g) +
H2O (l) →
H2SO4 (aq)
(The SO3 comes from oxygen reacting with SO2.)
3 NO2 (g) +
H2O (l) →
2 HNO3 (aq) + NO (g)
(The NO2 comes from the reaction of N2 with O2 from processes such as
lightning strikes.)
So, if these are natural processes, where do the manmade contributions come
into play?
Acid Rain – The Numbers
Consider the following table that shows the difference in concentration of the
three key components on the previous slide between natural sources and
manmade sources. The term ppm is parts per million – just like a percent
except instead of multiplying by 100 you multiply by one million (106).
Compound
Natural
Concentration
Source of Natural
Concentration
Concentration in
Urban
Environment
Source of Manmade
Contribution
CO2
0.05 ppm
Decomposition of
organic matter;
respiration
1 – 50 ppm
Industrial processes; fuel
combustion
SO2
0 to 0.01 ppm
Volcanic gases;
forest fires; bacterial
action
0.1 to 2 ppm
Industrial processes; fuel
combustion
NO
0.01 ppm
Lightning;
combustion of
organic matter
0.2 ppm
Internal combustion
engines
Reference: Table 18.2 in Chemistry: The Central Science, 12th Edition, T. L. Brown, H. E. LeMay, B. E. Bursten, C.
J. Murphy, P. M. Woodward, Pearson, 2012.
Acid Rain – The Effects
•Natural rainwater has a pH of about 5.6 due to the naturally occurring acidic
oxides listed on the previous page.
•Human activities nearly quadruple the concentration of SO2 in the air, in large
part due to sulfur containing coal and fossil fuels.
•The pH of acid rain can be as low as 4, and even lower in severe cases. That
may not sound like much, but remember the pH scale is based on powers of
10. For a drop in pH of two units, the concentration of H+ increases by a factor
of 102, or 100.
•Freshwater lakes populated with living organisms typically have a pH
between 6.5 and 8.5. The pH of freshwater does not match that of rainwater
for a variety of reasons, but recall that freshwater is in contact with all sorts of
rocks and minerals that can neutralize some of the acid nature. All biological
life suffers severely if the pH of a lake drops below 4.0.
Acid Rain – A Schematic
NOx refers to all of the nitrogen oxides.
http://www.howstuffworks.com
Acid Rain – The Measurements
This is a little dated, but
notice how the Northeast
has more acidic rain than
other parts of the country.
The coal in the east
typically has higher sulfur
content than that in the
west. The difference would
be even larger except the
western coal has a lower
energy content so you have
to burn more of it to get the
same amount of work done.
http://pubs.usgs.gov/gip/acidrain/2.html
Freshwater pH – The Measurements
Hydrogen ion concentration as pH from freshwater measurements made at the field laboratories,
2004. (Source: National Atmospheric Deposition Program/National Trends Network)
Improving Emissions
Chemistry can help reduce some of the problems related to sulfur
emissions. For example, the Clean Air Act was passed in 1970 with one
of the goals to reduce emissions of acid rain gases as well as others.
Making these changes required a combination of reducing high-sulfur
content coal and fuel as well as modifying processes to reduce the
emission of sulfur at the site – mainly power plants. If you want to see
the effects over a period of time, look at the website:
http://nadp.sws.uiuc.edu/data/animaps.aspx
If you click, for example, the Concentration Animation for SO4, it gives
an animation of the air concentration levels for about a twenty year
period from 1985 to about 2005. It is kind of interesting to watch. You
can also look at the Deposition Animation – the amount deposited in the
water and soil and look at nitrate and ammonium animations as well.
Visible Effects
http://weather-images.co.tv/images/mc/acid-rain-stone-erosion-of-statue-1.jpg
Statue over entrance to castle in Westphalia, Germany built in 1702.
The left picture was taken in 1908 and the right one in 1968.
More Visible Effects
Acid rain can have a severe impact on forests.
image from http://scienceclarified.com
Antacids
•The pH of the stomach is about 1 – it is extremely acidic.
•If the stomach becomes too acidic due to too much eating or perhaps
stress, one will usually resort to an antacid – just a fancy name for a base.
•Most antacids rely on one or more of a small list of bases to help neutralize
the excess acid. This list includes:
-
sodium bicarbonate
calcium carbonate
aluminum hydroxide
magnesium carbonate
magnesium hydroxide
•You are already familiar with the hydroxides as bases.
•The bicarbonate (HCO3- ) and carbonate (CO32-) ions also act as bases they are the conjugate bases of a couple of weak acids.
•A variety of other health effects can arise with overuse of any of these. For
example, over indulgence of sodium can cause high blood pressure
problems.
Other Acid-Base Uses and Functions
The most common acids you may find around the house include:
•
sulfuric acid – present in car batteries, some drain cleaners
•
hydrochloric acid (also known as muriatic acid) – used around
swimming pools, cleaning agents
•
acetic acid – vinegar is about 5% acetic acid
Acids and bases are an integral part of biological processes:
•
Strong acids and bases can denature (break down) proteins making
them unable to carry out their function
•
The blood pH must be within a very narrow range centered on about 7.4
or else severe consequences can follow.
End of Unit 27