2.5 – Fractional Conversion Yield Selectivity Excess

2.5.0 – Learning Objectives

By the end of this section you should be able to:

Understand the concepts of fractional conversion, yield, selectivity, and percent excess.

2.5.1 – Introduction

Fractional conversion, yield, selectivity, and percent excess are all important methods of describing how a chemical reaction progresses. They are useful values to solve material balances.

2.5.2 – Fractional Conversion

Fractional conversion is defined as the number of moles of a compound that reacted divided by the amount of the moles that were fed.

$F r a c t i o n a l < t e x t > c o n v e r s i o n = \frac{{\dot{n}}_{c o n s u m e d}}{{\dot{n}}_{i n p u t}}$

Another useful form of writing fractional conversion is

$F r a c t i o n a l < t e x t > c o n v e r s i o n = \frac{{\dot{n}}_{i n p u t} - {\dot{n}}_{o u t p u t}}{{\dot{n}}_{i n p u t}}$

2.5.3 – Yield (Fractional Yield)

Yield is defined as the moles of product formed divided by the ideal number of moles formed.

 $Y i e l d = \frac{{\dot{n}}_{f o r m e d}}{{\dot{n}}_{i d e a l}}$

The ideal number of moles formed is assuming that the limiting reactant reacts fully and that there are no side reactions.

2.5.4 – Selectivity

Selectivity is defined as the moles of desired products formed divided by the moles of undesired products formed.

 $S e l e c t i v i t y = \frac{{\dot{n}}_{p r o d u c t < t e x t > d e s i r e d}}{{\dot{n}}_{p r o d u c t < t e x t > u n d e s i r e d}}$

== 2.5.5 – Percent Excess

Percent excess is defined as the number of moles that are unreacted divided by the moles fed. $P e r c e n t < t e x t > e x c e s s = \frac{{\dot{n}}_{i n p u t} - {\dot{n}}_{c o n s u m e d < t e x t > t h e o r e t i c a l l y}}{{\dot{n}}_{c o n s u m e d < t e x t > t h e o r e t i c a l l y}}$

Note: any of these terms above can be expressed as a percent (by multiplying 100 to the values)