Download Limiting Reactant

Chapter 8
Sections 5 & 6
Limiting Reactant, Theoretical Yield,
and Percent Yield
Limiting Reactants
• In a laboratory, a reaction is rarely carried out with
exactly the required amounts of each of the
reactants. In a most cases, one or more reactants is
present in excess.
• Once one of the reactants is used up, no more
products can be formed.
• The substance completely used up first in a reaction
is called the limiting reactant.
Limiting Reactant
• Limiting Reactant -The reactant that limits the
amounts of the other reactants that can be
combined and the amount of product that can
form in a chemical reaction.
• Excess Reactant -The substance that is not
used up completely in a reaction.
Bicycle Example:
If I have:
12 frames, 10 handlebars, & 16 wheels
• How many bicycles can I build?
• What limits the # of bicycles?
• After I build all the bikes, what excess parts
and how many of each remain?
Limiting Reactant Problem #1
• Silicon dioxide (quartz) is usually quite
unreactive, but reacts readily with hydrogen
fluoride according to the following equation:
• SiO2 + 4HF → SiF4 + 2H2O
• If 2.0 mol of HF are exposed to 4.5 mol of SiO2,
which is the limiting reactant?
Solution
2.0 mol HF x 1 mol SiO2
4 mol HF
• = 0.5 mol of SiO2, needed
• We have 4.5 mol of SiO2
• AND, 4.5 – 0.5 = 4.0 mol excess SiO2
• Therefore, HF is the limiting reactant
Problem #2
3Fe + 4H2O → Fe3O4 + 4H2
a. When 36.0g of H2O is mixed with 167g Fe,
which is the limiting reactant?
b. What mass in grams of black iron oxide
(Fe3O4) is produced?
c. What mass in grams of excess reactant
remains when the reaction is completed?
Solution (part a):
36.0g H2O x 1 mol H2O x 3 mol Fe
18.016g H2O
4 mol H2O
= 83.7g Fe, needed
Have 167g
Therefore, H2O is the limiting reactant
x
55.85g Fe
1 mol Fe
Solution (part b): What mass in grams of black iron oxide (Fe O ) is
3 4
produced?
• Start with the limiting reactant!!
36.0g H2O x 1 mol H2O x 1 mol Fe3O4 x 231.55g Fe3O4
18.016g H2O 4 mol H2O
1 mol Fe3O4
= 116g Fe3O4
Solution (part c): What mass in grams of excess reactant remains when
the reaction is completed?
167g Fe - 83.7g Fe
= 83.3 g Fe
Theoretical Yield
• The maximum amount of product that can be
produced from a given amount of reactant
• The amount calculated in stoichiometry
problems is the theoretical yield.
*In most cases, the amount actually produced in
a chemical reaction is less than the theoretical
yield.
Homework
• Read p. 257-264
• Do p. 276 #46, 48, & 52
Actual Yield
• The amount of a product obtained from a
reaction
• To determine the efficiency of a reaction,
chemists calculate the percent yield – the
ratio of the actual yield to the theoretical
yield, multiplied by 100
% Yield = actual yield
x 100
theoretical yield
Example 1:
• The theoretical yield of a reaction is 25.0g, but
when doing the reaction the student was only
able to collect 22.8g. What is the percent
yield?
22.8g x 100
25.0g
= 91.2%
Example 2:
C6H6 (l) + Cl2 (g) → C6H5Cl (s) + HCl (g)
• When 36.8g C6H6 of react with an excess of Cl2, the
actual yield of C6H5Cl is 38.8g. What is the Percent
Yield of C6H5Cl?
36.8g C6H6 x 1 mol C6H6 x 1 mol C6H5Cl x 112.55g C6H5Cl
78.108g C6H6
1 mol C6H6
1 mol C6H5Cl
= 53.03g C6H5Cl , theoretical yield
% Yield = 38.8g x 100
53.03g
= 73.2%
Homework
• Read pages 257-264
• Answer question p. 276 #60, 62 & 64