Chapter 4: Balancing Equations and Stoichiometry
Jan 14, 2016
Chapter 4:
Balancing Equations and Stoichiometry
Key Terms & Concepts
Stoichiometry Chemical Equations
reactants and products balancing chemical equations
Chemical Calculations Limiting Reactant Theoretical and Percent Yield
Stoichiometry
Stoichiometry is the study of the quantitative nature of chemical formulas and chemical reactions.
Stoichiometry is one the the most essential tools in chemistry It allows to quantify everything from
global warming to drug manufacturing
Chemical Equations Chemical reactions are represented
in a concise manner by chemical equation
For example, when H2 burns in O2, H2O is formed. The chemical equation for this reaction
is:2 H2 + O2 2H2O
Chemical Equations
2 H2 + O2 2 H2O The compounds on the left of the
arrow are called “reactants” The compounds on the right of the
arrow are called “products” H2 and O2 are reactants, H2O is the
product
Chemical Equations
2 H2 + O2 2 H2O Notice that the number of atoms or each
element is equal on both sides of the equation 4 H, 2 0
All chemical equations must meet this requirement Chemical equations must be balanced!!
We balance equations by changing coefficients, not chemical formulas
Chemical Equations
Chemical Equations Consider the following chemical
equationCH4 + O2 CO2 + H2O
unbalanced Start with elements that only appear
in one compound on either side of the equation C and H are only in one compound on
either side C is balanced
Chemical Equations
CH4 + O2 CO2 + H2O
unbalanced 4 H’s in reactants, 2 H’s in products
Put coefficient of 2 in front of H2O 4 H’s in reactants, 4 H’s in products
CH4 + O2 CO2 + 2 H2O
unbalanced
Chemical Equations
CH4 + O2 CO2 + 2 H2O
unbalanced 2 O’s in reactants, 4 O’s in products
Put coefficient of 2 in front of O2
4 O’s in reactants, 4 O’s in products
CH4 + 2 O2 CO2 + 2 H2O
balanced
Chemical Equations
Chemical Equations
Consider this equation:C3H8 + O2 CO2 + H2O
3C + 8H + 2O 1C + 2H + 3O
Chemical Equations
Balance C and HC3H8 + O2 3CO2 + 4H2O
3C + 8H + 2O 3C + 8H + 10O
Chemical Equations
Balance OC3H8 + 5O2 3CO2 + 4H2O
3C + 8H + 10O 3C + 8H + 10O
Chemical Equations
Example 4.1Balance the following chemical equations(1) Mg + HCl MgCl2 + H2
(2) K + H2O KOH + H2
(3) CaCl2 + Na3PO4 Ca3(PO4)2 + NaCl
(4) NaN3 Na + N2
(5) C8H18 + O2 CO2 + H2O
Chemical Equations Example 4.1Balance the following chemical equations(1) Mg + 2 HCl MgCl2 + H2
(2) 2 K + 2 H2O 2 KOH + H2
(3) 3 CaCl2 + 2 Na3PO4 Ca3(PO4)2 + 6 NaCl
(4) 2 NaN3 2 Na + 3 N2
(5) 2 C8H18 + 25 O2 16 CO2 + 18 H2O
Chemical Equations Example 4.2
Write a balanced chemical equation for the following reactions
(1)ammonium nitrate decomposes to nitrogen gas, oxygen gas, and water
(2)iron reacts with oxygen gas and water to form iron(II) hydroxide
(3)ammonia reacts with oxygen gas to produce nitrogen monoxide and water
Chemical Equations
Example 4.2Write a balanced chemical equation for the following reactions
(1) 2 NH4NO3 2 N2 + O2 + 4 H2O
(2) 2 Fe + O2 + 2 H2O 2 Fe(OH)2
(3) 4NH3 + 5 O2 4 NO + 6 H2O
Chemical Calculations
2 H2 + O2 2 H2O
2 molecules 1 molecule 2 molecules
2(6.022x1023) molecules 6.022x1023 molecules 2(6.022x1023) molecules
2 mol 1 mol 2 mol
Stoichiometric coefficients can be interpreted as either number of molecules or number of moles.
Chemical Calculations
Example 4.3How many moles of water can be produced from 5.25 mol O2?
OH mol 10.5O mol 1
OH mol 2O mol 5.25 2
2
22
Chemical Calculations
Example 4.4How many moles of oxygen are required to completely react with 8.50 moles of butane, C4H10?
OH 10CO 8O 13HC 2 222104
2104
2104 O mol 55.3
HC mol 2
O mol 13HC mol 8.50
Chemical Calculations
We can’t directly measure moles. We can measure mass.
We can use the stoichiometric coefficients of a reaction to determine the mass relationships.
However, we must always convert mass to moles. We cannot directly compare the masses of
reactants and products. We can only compare the moles of
reactants and products.
Chemical Calculations
The general scheme is:
Chemical Calculations
Example 4.5Geranyl formate is used as a synthetic rose essence in cosmetics. The compound is prepared from formic acid and geraniol:
HCOOH + C10H18O C11H18O2 + H2O
A chemist needs to make some geranyl formate for a batch of perfume. How many grams of geranyl formate can a chemist make from 375g of geraniol?
Chemical Calculations
g 443mol 1
g 182.3
OHC mol 1
OHC mol 1
g 154.2
mol 1g 375
g/mol 182.3OHC
g/mol 154.2 OHC
1810
21811
21811
1810
Chemical Calculations
Example 4.6Solid lithium hydroxide is used in space vehicles to remove exhaled carbon dioxide. The lithium hydroxide reacts with gaseous carbon dioxide to form solid lithium carbonate and liquid water. How many grams of carbon dioxide can be absorbed by each 1.00 g of lithium hydroxide?2 LiOH (s) + CO2 (g) Li2CO3 (g) + H2O
(l)
Chemical Calculations
Example 4.62 LiOH (s) + CO2 (g) Li2CO3 (g) + H2O (l)
22
2 CO g 0.919CO mol 1
g 44.01
LiOH mol 2
CO mol 1
g 23.95
LiOH mol 1LiOH g 1.00
Theoretical and Percent Yield
The amount of product that can be produced from a given amount of reactants is the theoretical yield.
However, no reaction goes to actual completion. The amount of products that is actually produced from a given amount of reactants is the actual yield.•Some reactants may not react•Reactants may react in an undesired way
(side reactions) •May be difficult to remove products from
pot
Theoretical and Percent Yield
The extent of the desired reaction is typically reported as the percent yield.
100%Yield lTheoretica
Yield Actual YieldPercent
Theoretical and Percent Yield
Example 4.7Look back at Example 4.5. If the chemist starts with 375g of geraniol and collects 417g of purified product, what is the percent yield of the synthesis?
%1.94%100g 443
g 417
Theoretical and Percent Yield
Example 4.825.0 g of sodium metal is burned in an excess of chlorine gas. What is the theoretical yield of sodium chloride? If 54.8 g of sodium chloride is actually produced, what is the percent yield of the reaction?
2 Na + Cl2 2 NaCl
Theoretical and Percent Yield
yield al theoretic theis This
NaCl g 63.5NaCl mol 1
g 58.4
Na mol 2
NaCl mol 2
g 23.0
Na mol 1Na g 25.0
Theoretical and Percent Yield
yieldpercent theis This
86.3% 100%NaCl g 63.5
NaCl g 54.8
Theoretical and Percent Yield
Example 4.9Titanium is a strong, lightweight, corrosion-resistant metal that is used in aeronautics and bicycle frames. It is prepared by the reaction of titanium (IV) chloride with molten magnesium between 950C and 1150C.TiCl4(g) + 2 Mg(l) Ti(s) + 2 MgCl2(l)
In a certain industrial process 3.54x107 g of TiCl4 are reacted with 1.13x107 g of Mg. (a)Calculate the theoretical yield of Ti in grams. (b)Calculate the percent yield if 7.91x106 g of Ti are actually produced.
Theoretical and Percent Yield
yield al theoretic theis Ti g 108.94 and
reactant limiting theis TiCl
Ti g 101.11Ti mol 1
g 47.9
Mg mol 2
Ti mol 1
g 24.3
Mg mol 1Mg g 101.13
Ti g 108.94Ti mol 1
g 47.9
TiCl mol 1
Ti mol 1
g 189.7
TiCl mol 1TiCl g 103.54
6
4
7
7
6
4
44
7
Theoretical and Percent Yield
yieldpercent theis This
88.5%100%Ti g 108.94
Ti g 107.916
6
Limiting Reactants
Most reactions do not occur with stoichiometric equivalent amounts of each reactant.•One reactant is used up first•This reactant is the limiting reactant
because it limits the amount of products that can be formed
Limiting Reactants
Consider the “ham sandwich” example•one sandwich is made from one slice of
ham, one slice of cheese and two slices of bread
•How many ham sandwiches can be made from six slices of ham, seven slices of cheese and 14 slices of bread?
•What is the limiting reactant?
Limiting Reactants
Limiting Reactants
If a problem gives specific amounts of two or more reactants it is a limiting reactant problem.
Determine the amount of product that can be formed from each reactant•The reactant which produces the smallest
amount of product is the limiting reactant•The remaining reactants are said to be in
excess
Limiting Reactants
Example 4.10How many moles of water can be formed when 10.0 moles of H2 reacts with 4.50 moles of O2? What is the limiting reactant?
Limiting Reactants
Example 4.10
2 H2 + O2 2 H2O
formed becan OH of mol 9.00 and
reactant limiting theis O
OH mol 9.00O mol 1
OH mol 2O mol 4.50
OH mol 10.0H mol 2
OH mol 2H mol 10.0
2
2
22
22
22
22
Limiting Reactants
Example 4.11Solutions of sulfuric acid and lead (II) acetate react to form solid lead (II) sulfate and aqueous acetic acid. If 15.0 g of sulfuric acid and 15.0 g of lead (II) acetate are mixed, calculate the number of grams of lead (II) sulfate that can be produced. Also calculate the number of grams of the excess reagent remaining after the reaction is completed.
Limiting Reactants
formed becan PbSO of g 14.0 and
reactant limiting theis COO)Pb(CH
PbSO g 0.41PbSO mol 1
g 303.3
COO)Pb(CH mol 1
PbSO mol 1
g 25.33
COO)Pb(CH mol 1COO)Pb(CH g 15.0
PbSO g 46.4PbSO mol 1
g 303.3
SOH mol 1
PbSO mol 1
g 1.98
SOH mol 1SOH g 15.0
4
23
4423
4
2323
4442
4
4242
H2SO4 + Pb(CH3COO)2 PbSO4 + 2 CH3COOH
Limiting Reactants
excess.in SOH of g 10.5 g 4.52 - g 15.0
leavingreaction in the used are SOH of g 4.52
SOH g 52.4SOH mol 1
g 98.1
COO)Pb(CH mol 1
SOH mol 1
g 25.33
COO)Pb(CH mol 1COO)Pb(CH g 15.0
42
42
424223
42
2323
End-of-Chapter Exercises
Suggested End-of-Chapter Exercises5, 9, 10, 14, 17, 20, 22, 28