Learning Goals
0 Identify the limiting reactant in a chemical
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equation.
Identify the excess reactant, and calculate the
amount remaining after the reaction is complete.
Calculate the mass of a product when the
amounts of more than one reactant are given.
Calculate the theoretical yield of a chemical
reaction from data.
Determine the percent yield for a chemical
reaction.
Why do Reactions Stop?
0 Reactions proceed until one of the
reactants is used up and one is left in
excess.
0 The limiting reactant limits the extent of
the reaction and, thereby, determines the
amount of product formed.
0 The excess reactants are all the leftover
unused reactants.
Why do Reactions Stop?
0 Determining the limiting reactant
is important because the amount
of the product formed depends on
this reactant.
Calculating Products
0 If 200.0g of sulfur reacts with
100.0g of chlorine, what mass of
disulfur dichloride is produced?
S8(l) + 4Cl2(g) → 4S2Cl2(l)
Calculating Products
1.
Determine the moles of reactants.
Calculating Products
2. Determine whether the two reactants
are in the correct mole ratio, as given in
the balanced chemical equation.
Only 1.808 mol of chlorine is available for every 1 mol
sulfur, instead of the 4mol of chlorine required by the
balanced chemical equation, thus chlorine is the limiting
reactant.
Calculating Products
3. Calculate the amount of product formed.
Calculating Products
0 Now that you have determined the limiting
reactant and the amount of product
formed, what about the excess reactant,
sulfur? How much of it reacted?
Calculating Products
1.
You need to make a mole-to-mass calculation
to determine the mass of sulfur needed to
react completely with 100.0 g of chlorine.
Calculating Products
2. Next, obtain the mass of sulfur needed.
Calculating Products
3.
Knowing that 200.0g of sulfur is available and
only 90.44g is needed, you can calculate the
amount of sulfur left unreacted when the
reaction ends.
Calculating Products
0 Using an excess reactant can
speed up the reaction.
0 Using an excess reactant can drive
a reaction to completion.
Practice
0 The reaction between solid white
phosphorus (P4) and oxygen produces
solid tetraphosphorus decoxide.
0 Determine the mass of P4O10 formed if 25.0 g
of P4 and 50.0 g of oxygen are combined.
0 How much of the excess reactant remains
after the reaction stops?
Percent Yield
0 Laboratory reactions do not
always produce the calculated
amount of products.
0 Reactants stick to containers.
0 Competing reactions form other
products.
Percent Yield
0 The theoretical yield is the
maximum amount of product that
can be produced from a given
amount of reactant.
0 The actual yield is the amount of
product actually produced when
the chemical reaction is carried out
in an experiment.
Percent Yield
0 The percent yield of a product is
the ratio of the actual yield
expressed as a percent.
Practice
0 Solid silver chromate (Ag2CrO4) forms when
potassium chromate is added to a solution
containing 0.500g of silver nitrate. Determine
the theoretical yield of silver chromate.
Calculate the percent yield if the reaction
produces 0.455 g of silver chromate.