Stoichiometry in Combustion

Stoichiometry is the study of the relationship between quantities of reactants and products in chemical equations. It is based on the principle of conservation of mass. Therefore, it involves balancing both sides of the equation by assuring there are the same number of atoms for all elements involved. 

In the field of mechanical engineering, stoichiometry is mostly used in the field of thermodynamics when studying combustion reactions. In this type of reactions, a fuel reacts chemically with oxygen in order to release energy and combustion products. When a combustion is complete, all the carbon present in the fuel is burned to carbon dioxide, all the hydrogen is burned to water, all the sulfur is burned to sulfur dioxide, and all other combustible elements are fully oxidize. If these conditions are not met, the combustion is said to be incomplete. Now, we will observe a couple of examples.

Example 1:

Balance the equation below for the complete combustion of methane.

Solution:

By applying the principle of conservation of mass, we can create equations to balance the number of atoms of the same elements on both sides in terms of the coefficients of each compound in the reaction.

By solving the system of four equations and four unknowns, the balanced chemical equation is:

Example 2:

For the balanced equation in the previous example, obtain the air-fuel ratio.

Solution:

The ratio of the amount of air in this reaction to the amount of fuel is about 17.24.

Bibliography:

Michael J. Moran, Howard N. Shapiro. Fundamentals of Engineering Thermodynamics. John Wiley & Sons, Inc., 2008.