Chemical Mass Balances

6. Chemical Reaction Stoichiometry

A material balance can be complicated when a chemical reaction occurs in the process. Relative amounts of reactants and products in the input and output stream can be constraint by the stoichiometric equation of the reaction. A material balance on a reactive substance does not have the simple input = output, but must contain the generation or consumption term.

a.    Stoichiometry

Stoichiometry is the theory of the proportions in which chemical species combine with one another. The stoichiometry equation of a chemical reaction is a statement of the relative number of molecules or moles of reactants and products that participate in the reaction. A valid stoichiometry equation must be balanced; that is that the number of atoms on each atomic species be the same on each side.

The stoichiometry ration of two molecular species participating in a reaction is the ratio if their stoichiometric coefficients in the balanced reaction equation. The amount of a particular reactant that was consumed can be determined by using the stoichiometry ratio as a conversion factor.

2SO2 + O2  ® 2SO3

Stoichiometric ratios:

(2 mol SO3 generated)/(1 mol O2 consumed)

If it is known that 1600 kg/h of SO3 is to be produced, calculate the amount of oxygen produced

(1600 kg/h SO3 × 1kmol SO3 × 1kmol O2 consumed)/(80 kg SO3 × 2kmol SO3 generated) = 10 kmol O2/ h

b.    Limiting and excess Reactants

Stoichiometric proportion is when two reactants, A and B, has a ration equals the stoichiometric ratio obtained from the balanced reaction equation.

The limiting reactant is the reactant that would run out if the reaction proceeded to completion and the other reactants is the excess reactants. The stoichiometric requirement is the amount needed to react completely with the limiting reactant. The fractional excess of a reactant is the ratio of the excess to the stoichiometric requirements.

The percentage excess of A is 100 times the fractional excess.

The fractional conversion of a reactant is the ratio

f = moles reacted / moles fed