Section 9.3 - Limiting Reagent and Percent Yield

Limiting Reagent In preparing cup cakes, there may be one ingredient which determines how many cup cakes you can make.

The ingredient which limits the number of cup cakes which can be made is the limiting ingredient. When you run out of it, you can’t make any more.

When two or more chemicals react, there may be one chemical which determine the amount of product formed.

When one reactant is used up, the reaction will stop. Once the reaction stops, no more products will form.

The substance that limits the amount of product that can form is called the limiting reagent.

The balanced equation tell us the mole ratio of reactants to products. Using this mole ratio, we can determine the amount of product produced from the amount of reactant used.

In the equation C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) 1 mole of propane gas will combine with 5 moles of oxygen gas to form 3 moles of carbon dioxide gas and 4 moles of water vapor.

If we react 1 mole of C3H8 with only 4 moles of O2, we will not get 3 moles of CO2 produced or 4 moles of H20.

The O2 is the limiting reagent. The amount of O2 used determines how much CO2 and H2O will be produced.

Under these conditions, the reaction will only occur until the oxygen is used up.

The amount of product(s) formed is determined (limited) by the amount of the limiting reagent.

How many blocks of each color will remain? A set of blocks contains 5 red blocks, 4 green blocks, and 3 blue blocks. If we have 1000 blocks of each color, how many sets of blocks can we make? How many blocks of each color will remain?

Example: Hydrogen reacts with chlorine to form hydrogen chloride according to the equation: H2(g) + Cl2(g)  2 HCl(g)

How many moles of HCl will be produced? Example: If you begin with 2.5 moles of H2 and 3.25 moles of Cl2, what is the limiting reagent? Explain. How many moles of HCl will be produced?

Theoretical Yield The amount of product(s) formed if all of the limiting reagent is consumed is called the theoretical yield of product(s).

In order to determine the theoretical yield, you must first determine the limiting reagent.

In order to determine the limiting reagent, you must calculate the amount of product formed if each reactant is assumed to be completely consumed.

The other reactant is said to be “in excess”. The reactant that produces the least amount of product is the limiting reagent. The other reactant is said to be “in excess”.

The least amount of product formed from the limiting reagent reacting with an excess of the other reactant is the theoretical yield.

Example: Supposed you were asked to make sandwiches for the church picnic. Each sandwich requires 2 slices of bread and one slice of ham.

Example: You are given 100 slices of bread and 60 slices of ham. What is the limiting reagent? What is the theoretical yield?

This is not always the case. Percent Yield In calculating the theoretical yield, you assume that all of the limiting reagent is transformed into product. This is not always the case.

Sometimes all of a limiting reagent is not converted to product Sometimes all of a limiting reagent is not converted to product. Sometimes part of the product is lost in the recovery process.

As a result, the actual yield is not always equal to the theoretical yield.

The actual yield is what you get under your specific laboratory conditions. The theoretical yield is what you would get under ideal laboratory conditions.

We often compare the actual yield to the theoretical yield in terms of the percent yield.

The percent yield is a ratio of actual yield to theoretical yield expressed as a percent.

Example: Silicon reacts with oxygen to form silicon dioxide Si(s) + O2(g)  SiO2(s) How many grams of silicon dioxide will form when 20.0 g silicon are mixed with 20.0 g of oxygen?

If the actual yield of SiO2 is 32.5 g, calculate the percent yield. Example: Silicon reacts with oxygen to form silicon dioxide Si(s) + O2(g)  SiO2(s) If the actual yield of SiO2 is 32.5 g, calculate the percent yield.

42.8 g SiO2 is produced if Si is the limiting reagent.

37.6 g SiO2 is produced if O2 is the limiting reagent.

O2 is the limiting reagent.