|S with Oxygen|
Sulfur is a nonmetal with a characteristic yellow color. It is flammable and when heated above 200°C it combusts in air with a very pale bluish flame, as can be seen on the video during the ignition step. When lighted sulfur is inserted into a flask containing pure oxygen gas, an intensive beautiful blue flame is observed. Oxygen concentration in air is about 21 vol%, so sulfur combustion is much slower in air than in pure oxygen. This is an example of how reactant concentration can affect reaction rate.
The main product of sulfur combustion is sulfur(IV) oxide, according to the chemical equation:
A very small amount of sulfur(VI) oxide is formed besides sulfur(IV) oxide:
However, even if sulfur combustion is carried out in pure oxygen, sulfur trioxide formation is not significant. Namely, the reaction of sulfur(VI) oxide formation is exothermic enough (enthalpy of the formation is – 395 kJ/mol) to causes the initially formed sulfur(VI) oxide to immediately decomposes to sulfur(IV) oxide and oxygen, according to the following equation:
When sulfur dioxide formed during the combustion is dissolved in water, a very weak sulfurous acid is formed:
Sulfur dioxide is released in our atmosphere from the smokestacks of coal-burning power plants and other industrial sources. In a several step process SO2 is converted into sulfuric acid, H2SO4. The acid may be carried to the ground in rain, which is then called acid rain. Acid rains are more acidic than ordinary rains, the pH value being close to 4.5 and sometimes as low as 2.5 (like a lemon juice). Acid rain has harmful effect on plants and fishes, and can damage concrete and steel infrastructure.