Download Limiting Reactants and Percent Yield

Chapter 9
Chemical Quantities
Chapter Objectives:
 To understand the molecular and mass information given in a balanced equation.
 To learn to use a balanced equations to determine relationships between moles of reactants and
moles of products.
 To learn to relate masses of reactants and products in a chemical reaction.
 To carry out mass calculations that involve scientific notation.
 To understand the concept of limiting reactants.
 To learn to recognize the limiting reactant in the reaction.
 To learn to use the limiting reactant to do stoichiometric calculations.
Information Given by Chemical Equations
Chemical change involves:
1. _____________________________ of atom groupings.
2. one or more substances changing to ______________________________
The equation for a chemical reaction gives you the relative numbers of ___________________ and
_______________ molecules.
Gaseous carbon monoxide and hydrogen react to produce liquid methanol
(CH 3OH)
Write an unbalance chemical equation for this reaction:
________________________
+ ______________________
→_________________________
+ ______________________
→_________________________
Now, balance the equation
________________________
Write three ways the information given in the balanced chemical equation can be interpreted:
________________________
+
______________________
→ _________________________
________________________
+
______________________
→
_________________________
________________________
+
______________________
→
_________________________
Gaseous propane (C3 H8 ) reacts with oxygen gas to produce carbon dioxide and water
Write a balanced chemical equation:
→
Write down the same reaction in terms of molecules.
→
Write down the same reaction in terms of moles.
→
Mole-Mole relationships
A sandwich requires 2 slices of bread and 3 slices of meat. Write an equation to represent this.
____B + ____M  _____B2M3
10 B + 15 M  __?_ B2M3
Write an equation to represent
__________________+
_________________

_________________
__________________+
_________________

_________________
The ratio of moles of one substance to moles of another substance in a balanced chemical equation is called
the __________________________________.
How many moles of water can be produced with 2.9 moles of oxygen gas?
O2 (g)  2H 2 (g)  2H 2 O(l)
How many moles of oxygen gas are there compared to moles of water in the balanced equation?
2.9 mol O2 x ___________  _____ mol H 2 O
How many moles of methanol can be produced with 5.67 moles of hydrogen gas?
CO(g)  2H 2 (g)  CH 3OH(l)
5.67 mole H2 x
(----------------------------) = _____mole CH OH
Homework: Chapter 9 Assessment (p. 310, #2,3,6,8,9,10)
3
Mass Calculations
C3H8(g) + O2(g)  CO2(g) + H2O(g)
What mass of oxygen will be required to react with exactly 44.1 g or propane?
What do we know?
-- C3H8(g) + O2(g)  CO2(g) + H2O(g)
-- 44.1 g of propane
How do we solve this problem?
Balance the equation:
____C3H8(g) + ____O2(g)  ____CO2(g) + ____H2O(g)
What next?
1. Convert grams of propane to moles of propane
(
44.1 g C3H8(g) X --------------------------------------------
)
= __________mole C3H8(g)
2. Determine the number of moles of oxygen using the coefficients in the balanced chemical equation:
__________ mole C3H8(g) reacts with ____________ O2(g)
(
__________mole C3H8(g) X ------------------------------------------
) = ____________mole O
3. Use the molar mass of oxygen to calculate the grams of oxygen needed:
(
_________ mole O2(g) X ------------------------------------------
) = ____________g O
2(g)
2(g)
Steps to Calculate the Mass of Reactants and Products in a Chemical Reaction
Step 1________________________________________________________________________________
Step 2________________________________________________________________________________
Step 3________________________________________________________________________________
Step 4________________________________________________________________________________
Step 5________________________________________________________________________________
What is Stoichiometry?
Stoichiometry - _________________________________________________________________________
__________________________________________________________________________
Practice:
HF(aq) + SiO2(s)  SiF4(g) + H2O(l)
A. Calculate the mass of hydrogen fluoride needed to react with 5.68 g of silica (SiO2).
B. Calculate the mass of water vapor produced in the reaction.
List the steps required for solving A.
1. ____________________________________
2. Convert the mass of silica to _____________________.
3. Use balanced equation to set up the mole ratio between __________________ and
____________________.
4. Use mole ratio to calculate number of moles of _______________________.
5. Convert from ________________ HF to ___________________ HF.
Solve part B below. Show all work.
Homework: Chapter 9 Assessment (p. 310, #11,13a-d,14a-d,15a-b,16a-b,17,18,21,24)
Limiting Reactants and Percent Yield
Making a sandwich requires 2 pieces of bread, 3 slices of meat and one slice of cheese. Write a chemical
equation to represent making one sandwich:
___B + ___M + ___C  ____S
How many sandwiches could you make if you had:
20 slices of bread
24 slices of meat
12 slices of cheese
Determine how many sandwiches can be made with each of the following ingredients:
20 slices of bread x
24 slices of meat x
1 sandwich
 ______ sandwiches
2 slices of bread
1 sandwich
 ______ sandwiches
3 slices of meat
12 slices of cheese x
1 sandwich
 ______ sandwiches
1 slice of cheese
How many sandwiches can be made? _________
What will be left over?
___slices of bread
___slices of meat
___slices of cheese
The ratio of N2 to H2 necessary to produce NH3 is ______N2 and ____H2 which results in the production of
_______NH3.
Draw in the NH3 molecules that will result from the reaction of hydrogen and nitrogen in the above
container. How many NH3 molecules will be formed? _______________________
What will be left over?__________________________
To determine how much product will be formed from a given mixture of reactants, we have to look for the
reactant that is ____________________________ -- the one that runs out __________________ and thus
limits the amount of product that can form.
The reactant that runs out first and thus limits the amount of products that can form is called the
_________________________________________________________.
Sample problems:
CH4(g) + H2O(g)  H2(g) + CO(g)
What mass of water is required to react exactly with 249 g of methane?
What do we know?
1. balanced chemical equation:
2. ______g CH4(g)
How do we solve this?
Mass of CH4(g)  moles of CH4(g)  moles of H2O (g)  mass of H2O (g)
Step 1. Balance the equation:
____CH4(g) + ____H2O(g)  ____H2(g) + ____CO(g)
Step 2. Convert mass of methane to moles of methane:
249 g CH 4 x
______mol CH 4
 ______ g CH 4
_______g CH 4
Step 3. Using balanced equation determine the ratio between water and methane:
______mol CH 4 x
______mol H 2 O
 ______ mol H 2 O
_______mol CH 4
Step 4. Determine the mass of oxygen required by converting moles of oxygen to grams.
______mol H 2 O x
______g H 2 O
 ______ g H 2 O
_______mol H 2 O
Practice Problem:
Lithium nitride, an ionic compound containing Li+ and N3- ions, is prepared by the reaction of lithium metal
and nitrogen gas. Calculate the mass of lithium nitride formed from 56.0 g of nitrogen gas and 56.0 g of
lithium metal in the unbalanced reaction:
Li(s) + N2(g)  Li3N(s)
Step 1. Balance the chemical equation
_____Li(s) + _____N2(g)  _____Li3N(s)
Step 2. Convert known masses of reactants to moles.
56.0 g Li x
______mol Li
 ______ g Li
_______g Li
56.0 g N 2 x
______mol N 2
 ______ g N 2
_______g N 2
Step 3. Using the numbers of moles of reactants and the appropriate mole ratios, determine which reactant is
limiting.
______mol Li
mol Li x
 ______ moles of N 2 required
_______mol N 2
Which is greater moles of N2 available (step 2) or moles of N2 required (step 3)?
What is the limiting reactant? ______________________
Step 4. Using the amount of the limiting reactant and the appropriate mole ratios, compute the number of
moles of the desired product.
______ mol Li x
______mol N 2
 ______ mol N 2
_______mol Li
Step 5. Convert from moles of product to grams of product using the molar mass.
mol N 2 x
______mol N 2
 ______ g N 2
_______g N 2
Homework: Chapter 9 Assessment (p. 310, #30, 31, 35, 37, 38)
Percent Yield
_______________________________ the maximum amount of a given product that can be formed when the
limiting reactant is completely consumed.
________________________________the actual yield of a product as the percentage of the theoretical
yield.
How do you calculate Percent Yield?
Homework: Chapter 9 Assessment (p. 310, #40, 42, 44, 45)