Teacher Demonstration Discussion and Introduction of Concept
The pH range over which the indicator changes from the acid color to the basic color for methyl orange is 3.2-4.4 (Lide, 2002). At pH > 4.4, the yellow basic form predominates. On the other hand, if methyl orange is in a solution of pH < 3.2, the red acidic form predominates.
The burning matches have produced a color change of the indicator from yellow to red. It indicates that something acidic dissolved in the indicator solution.
1. What acid caused the color change?
2. Where did this acid come from?
We may consider carbonic acid as the source of the acid as it is a product of the combustion of wood. However, the burning of the wooden splint did not cause a color change of the indicator. We have to conclude that the ignited match introduced something much more acidic than CO2. The acid must have come from something in the flammable head of the match.
The head of a match contains sulfur and potassium chlorate(V). The match can be ignited when the head comes in contact with phosphorus, generally coated on the side of the box. The chemical reaction when we strike a match is the oxidation of a 'fuel' by potassium chlorate(V). Sulfur in the match head acts as the fuel, while phosphorus only initiates the reaction (Emsley, 2001). SO2 is produced as a result of the oxidation of sulfur. The reaction of the burning of a safety match goes as follows:
3S(s) + 2KClO3(s) -----> 3SO2(g) + 2KCl(s)
SO2 is significantly more soluble in water than CO2, as indicated by its Henry's law constant of 1.2 molL-1atm-1 compared to 3.38 x 10-2 molL-1atm-1 for CO2 (Lide, 2002). The value of the acid dissociation constant of 1.7 x 10-2 for SO2 is more than four orders of magnitude higher than the value of 4.45 x 10-7 for CO2 (Lide, 2002). Thus, a small amount of dissolved SO2 is able to lower the pH of a solution considerably resulting in the observed color change of the indicator. Dissolved CO2 can also lower the pH of a solution, but to a much lesser extent that it is unable to cause a color change of the indicator.1
As atmospheric CO2 dissolves in water to form carbonic acid, rain is naturally acidic. Normal rain has a pH of about 5.6. It has been generally considered that rain whose pH is lower than 5.6 to be acid rain because this is the pH value of carbon dioxide in equilibrium with distilled water.
The main cause of acid rain is the presence of strong mineral acids, mainly sulfuric (H2SO4) and nitric (HNO3) acids, derived from the atmospheric oxidation of sulfur dioxide (SO2) and nitrogen oxides (NOX). These gases are emitted from both natural and anthropogenic sources.
Volcanic eruptions release gaseous SO2 and H2S that are eventually converted to sulfuric acid. Thus rain that falls far from inhabited or industrialized areas, through supposedly unpolluted air, has a pH lower than 5.6 - generally about 5.0.
The burning of fossil fuels is the most important anthropogenic source. Sulfur as pyrite is present as impurities in coal, gasoline and petroleum products and is oxidized to SO2 when these fuels are burned. Nitrogen oxides released from the burning of fossil fuels in automobiles and power plants come from the air itself rather than from impurities in fuels. Nitrogen and oxygen of the air can be converted to NO at high temperatures. The temperatures of combustion chambers of the internal combustion engine are effective in this conversion. NO reacts with oxygen to form NO2 which dissolves in water to from nitric acid.
Acid rain has a variety of ecologically damaging consequences, including the
following:
References:
1. Fong, Man Wai. “A Demonstration of Acid Rain.” Asia-Pacific Forum on Science Learning and Teaching, Volume 5, 1, Article 4 (Apr., 2004). http://www.ied.edu.hk/apfslt/v5_issue1/fongmw/index.htm#contents