Limiting Reagent and Percent Yield

1
Limiting Reagent and Percent Yield
2
Limiting Agent

• The limiting reagent limits or determines the
  amount of product that can be formed in a
  reaction.
• The reaction occurs only until the limiting
  reagent is used up.

3
Limiting Agent

• The reactant that is not completely used up in a
  reaction is called the excess reagent.
• The amount of product formed in a reaction can be
  determined from the given amount of limiting
  reagent.

4
Determining the Limiting Reagent in a Reaction

• What is the limiting reagent when 80g Cu reacts
  with 25g S?
• 2Cu(s) S(s) ? Cu2S(s)
• Step 1 convert to moles
• 80g Cu x (1 mol Cu/63.5g Cu) 1.26 mol Cu
• 25g S x (1 mol S/32.1g S) 0.779 mol S

5
Determining the Limiting Reagent in a Reaction

• Step 2 Look at the mole ratio from the balanced
  equation
• There is 2 moles of Cu for 1 mole of S.
• Step 3 Find the amount of moles in the equation.
• 1 mol of S .779 mol S
• 2 moles of Cu 1.26 mol Cu x (1 mol S/2 mol Cu)
  0.630 S
• Since the amount of sulfur needed is 0.630 moles
  and the given amount is 0.779 moles, this
  indicates that sulfur is in excess. Thus copper
  is the limiting reagent.

6
Determining the Limiting Reagent in a Reaction -
Practice

• 23. If 2.70 mol C2H4 is reacted with 6.30 mol O2,
  identify the limiting reagent.

7
Determining the Limiting Reagent in a Reaction -
Practice

• 24. Identify the limiting reagent when 6.00g HCl
  reacts with 5.00g Mg.
• Mg(s) 2HCl(aq) ? MgCl2(aq) H2(g)

8
Using a Limiting Reagent to Find the Quantity of
a Product

• What is the maximum number of grams of Cu2S that
  can be formed from
• 2Cu(s) S(s) ? Cu2S(s)
• Step 1 Use moles of limiting reagent to
  calculate moles of what you are looking for.
• 1.26 mol Cu (from previous problem)
• 1.26 mol Cu x (1 mol Cu2S/2 mol Cu) 0.63 mol
  Cu2S

9
Using a Limiting Reagent to Find the Quantity of
a Product

• Step 2 turn moles into grams
• 0.63 mol Cu2S x (159g Cu2S/1 mol Cu2S) 100.17 g

10
Using a Limiting Reagent to Find the Quantity of
a Product

• 25. How many grams of water can be produced by
  the reaction of 2.40 mol C2H2 with 7.4 mol O2?

11
Using a Limiting Reagent to Find the Quantity of
a Product

• 26. If 2.70 mol C2H4 is reacted with 6.30 mol O2?
• C2H4(g) 2O2(g) ? 2CO(g) 2H2O(g)
• A. identify the limiting reagent.
• B. Calculate the moles of water produced.

12
Percent Yield

• When an equation is used to calculate the amount
  of product that will form during a reaction, a
  value representing the theoretical yield is
  obtained.
• The theoretical yield is the max amount of
  product that could be formed from given amounts
  of reactants.

13
Percent Yield

• The amount of product that actually forms when
  the reaction is carried out in the laboratory is
  called the actual yield.
• The actual yield is often less than the
  theoretical yield.

14
Percent Yield

• The percent yield is the ratio of the actual
  yield to the theoretical yield expressed as a
  percent.
• Percent yield (actual yield/theoretical yield)
  x 100

15
Percent Yield

• In calculating the ratio of the actual yield to
  the theoretical yield, the percent yield is a
  measure of the efficiency of the reaction.
• A percent yield should not normally be larger
  than 100, but can be lower due to reactions not
  going to completion.

16
Calculating the Theoretical Yield of a Reaction

• What is the theoretical yield of CaO if 24.8g
  CaCO3 is heated?
• CaCO3(s) ? CaO(s) CO2(g)
• Use the 3 Steps
• g CaCO3? mol CaCO3? mol CaO? g CaO

17
Calculating the Theoretical Yield of a Reaction

• 24.8g CaCO3 x (1 mol CaCO3/100.1g CaCO3)
• .248 mol CaCO3
• .248 mol CaCO3 x (1 mol CaO/1 mol CaCO3)
• .248 mol CaO
• .248 mol CaO x (56.1g CaO/1 mol CaO)
• 13.91 g CaO

18
Calculating the Theoretical Yield of a Reaction

• 27. When 84.8 g of iron(III) oxide reacts with an
  excess of carbon monoxide, iron is produced. What
  is the theoretical yield of this reaction?
• Fe2O3(s) 3CO(g) ? 2Fe(s) 3CO2(g)

19
Calculating the Theoretical Yield of a Reaction

• 28. When 5g of copper reacts with excess silver
  nitrate, silver metal and copper nitrate are
  produced. What is the theoretical yield of silver
  in this reaction?

20
Calculating the Percent Yield of a Reaction

• What is the percent yield of the reaction if the
  theoretical yeild of CaO is 13.9g and 13.1 g CaO
  is produced?
• yield (actual yield/theoretical yield) x 100
• (13.1g/13.9g) x 100 94.2

21
Calculating the Percent Yield of a Reaction

• 29. if 50g of silicon dioxide is heated with an
  excess of carbon, 27.9g of silicon carbide is
  produced. What is the percent ield of this
  reaction?

22
Calculating the Percent Yield of a Reaction

• If 15g of nitrogen reacts with 15g of hydrogen,
  10.5g of ammonia is produced. What is the percent
  yield of this reaction?