chapter 8 quantities in chemical reactions
DESCRIPTION
Chapter 8 Quantities in Chemical Reactions. Products are carbon dioxide and w a t e r. Octane in gas tank. Octane mixes with oxygen. 2006, Prentice Hall. CHAPTER OUTLINE. Global Warming: Too Much Carbon Dioxide. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 8 Quantities in Chemical Reactions
2006, Prentice Hall
Octane in gas tank
Octane mixeswith oxygen
Products arecarbon dioxideand water
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CHAPTER OUTLINE
Stoichiometry Molar Ratios Mole-Mole Calculations Mass-Mole Calculations Mass-Mass Calculations Limiting Reactant Percent Yield
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Global Warming: Too Much Carbon Dioxide
• The combustion of fossil fuels such as octane (shown here) produces water and carbon dioxide as products.
• Carbon dioxide is a greenhouse gas that is believed to be responsible for global warming.
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The greenhouse effect
• Greenhouse gases act like glass in a greenhouse, allowing visible-light energy to enter the atmosphere but preventing heat energy from escaping.
• Outgoing heat is trapped by greenhouse gases.
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Combustion of fossil fuels produces CO2.
• Consider the combustion of octane (C8H18), a component of gasoline:
2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
• The balanced chemical equation shows that 16 mol of CO2 are produced for every 2 mol of octane burned.
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Global Warming
• scientists have measured an average 0.6°C rise in atmospheric temperature since 1860
• during the same period atmospheric CO2 levels have risen 25%
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The Source of Increased CO2
• the primary source of the increased CO2 levels are combustion reactions of fossil fuels we use to get energy (methane and octane)– 1860 corresponds to the beginning of the
Industrial Revolution in the US and Europe
(g)(g)(g)(g) OH 2 CO O 2 CH 2224 (g)(g)(g)(l) OH 18 CO 16 O 25 HC 2 222188
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STOICHIOMETRY
Stoichiometry is the quantitative relationship between the reactants and products in a balanced chemical equation.
A balanced chemical equation provides several important information about the reactants and products in a chemical reaction.
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MOLARRATIOS
For example:
1 N2 (g) + 3 H2 (g) 2 NH3 (g)
1 molecule 3 molecules 2 molecules
100 molecules 300 molecules 200 molecules
106 molecules 3x106 molecules 2x106 molecules
1 mole 3 moles 2 moles
This is the molar ratios between
the reactants and products
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Examples:
2
2
mol O=
mol CO 13
8
4 10
2
mol C H=
mol H O
Determine each mole ratio below based on the reaction shown:
2 C4H10 + 13 O2 8 CO2 + 10 H2O
2
10
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Stoichiometry - Allows us to predict products that form in a reaction based on amount of reactants.
Stoichiometry
The amount of each element must be the same throughout the overall reaction.
For example, the amount of element H or O on the reactant side mustequal the amount of element H or O on the product side.
2H2 + O2 2H2O
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STOICHIOMETRICCALCULATIONS
Stoichiometric calculations can be classified as one of the following:
MOLES of compound A
MOLES of compound B
molar ratio
Mole-mole calculations
MASS of compound A
MM
Mass-mole calculations
MASS of compound B
MM
Mass-mass calculations
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MOLE-MOLECALCULATIONS
Relates moles of reactants and products in a balanced chemical equation
MOLES of compound A
MOLES of compound B
molar ratio
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How many moles of ammonia can be produced from 32 moles of hydrogen? (Assume excess N2 present)
Example 1:
1 N2 (g) + 3 H2 (g) 2 NH3 (g)
32 mol H23
2
mol NHx
mol H23 = 21 mol NH3
Mole ratio
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In one experiment, 6.80 mol of ammonia are prepared. How many moles of hydrogen were used up in this experiment?
Example 2:
1 N2 (g) + 3 H2 (g) 2 NH3 (g)
6.80 mol NH32
3
mol Hx
mol NH32 = 10.2 mol H2
Mole ratio
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MASS-MOLECALCULATIONS
Relates moles and mass of reactants or products in a balanced chemical equation
MOLES of compound A
MOLES of compound B
molar ratio
MASS of compound A
MM
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How many moles of ammonia can be produced from the reaction of 125 g of nitrogen?
Example 1:
2
2
mol Nx
g N
1 N2 (g) + 3 H2 (g) 2 NH3 (g)
125 g N2
1
28.0= 8.93 mol NH3
3
2
mol NHx
mol N12
Molar mass Mole ratio
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MASS -MASSCALCULATIONS
Relates mass of reactants and products in a balanced chemical equation
MOLES of compound A
MOLES of compound B
molar ratio
MASS of compound A
MM
MASS of compound B
MM
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What mass of carbon dioxide will be produced from the reaction of 175 g of propane, as shown?
Example 1:
1 C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (g)
Mass of propane
Moles of propane
Moles of carbon dioxide
Mass of carbon dioxide
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Example 1:
3 8
3 8
mol C Hx
g C H2
3 8
mol COx
mol C H
1 C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (g)
175 g C3H8 44.1
1
= 524 g CO2
1
3
2
2
g COx
mol CO
Molar massMole ratio
44.01
Molar mass
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LIMITINGREACTANT
When 2 or more reactants are combined in non-stoichiometric ratios, the amount of product produced is limited by the reactant that is not in excess.
This reactant is referred to as limiting reactant. When doing stoichiometric problems of this
type, the limiting reactant must be determined first before proceeding with the calculations.
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LIMITING REACTANT ANALOGY
Consider the following recipe for a sundae:
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LIMITING REACTANT ANALOGY
How many sundaes can be prepared from the following ingredients:The number of sundaes possible is limited by the amount of syrup, the limiting reactant.
Limiting reactant
Excess reactants
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LIMITINGREACTANT
When solving limiting reactant problems, assume each reactant is limiting reactant, and calculate the desired quantity based on that assumption.
A + B C
A is LR
B is LR
Calculate amount of C
Calculate amount of C
Compare your answers for each assumption; the lower value is the correct assumption.Lower
value is correct
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A fuel mixture used in the early days of rocketry was a mixture of N2H4 and N2O4, as shown below. How many grams of N2 gas is produced when 100 g of N2H4 and 200 g of N2O4 are mixed?
Example 1:
2 N2H4 (l) + 1 N2O4 (l) 3 N2 (g) + 4 H2O (g)
Limiting reactant
Mass-mass calculations
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Example 1:
2 4
2 4
mol N Hx
g N H2
2 4
mol Nx =
mol N H100 g N2H4
32.04
4.68 mol N2
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2 N2H4 (l) + 1 N2O4 (l) 3 N2 (g) + 4 H2O (g)
Assume N2H4 is LR
1
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Example 1:
2 4
2 4
mol N Ox
g N O2
2 4
mol Nx =
mol N O200 g N2O4
92.00
6.52 mol N2
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2 N2H4 (l) + 1 N2O4 (l) 3 N2 (g) + 4 H2O (g)
Assume N2O4 is LR
1
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Example 1:
6.52 mol N2
2 N2H4 (l) + 1 N2O4 (l) 3 N2 (g) + 4 H2O (g)
Assume N2O4 is LR
Assume N2H4 is LR 4.68 mol N2
Correct amount
N2H4 is LR
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Example 1:
2
2
g Nx
mol N4.68 mol N2
28.0= 131 g N2
2 N2H4 (l) + 1 N2O4 (l) 3 N2 (g) + 4 H2O (g)
Calculate mass of N2
1
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How many grams of AgBr can be produced when 50.0 g of MgBr2 is mixed with 100.0 g of AgNO3, as shown below:
Example 2:
MgBr2 + 2 AgNO3 2 AgBr + Mg(NO3)2
Limiting Reactant
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Example 2:
2
2
mol MgBrx
g MgBr 2
mol AgBrx
mol MgBr50.0 g MgBr2 184.1 1
2
Assume MgBr2 is LR
1
MgBr2 + 2 AgNO3 2 AgBr + Mg(NO3)2
g AgBrx =
mol AgBr187.8
1 102 g AgBr
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Example 2:
3
3
mol AgNOx
g AgNO 3
mol AgBrx
mol AgNO100.0 g AgNO3 169.9 22
Assume AgNO3 is LR
1
MgBr2 + 2 AgNO3 2 AgBr + Mg(NO3)2
g AgBrx =
mol AgBr187.8
1 111 g AgBr
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Example 2:
111 g AgBrAssume AgNO3 is LR
Assume MgBr2 is LR 102 g AgBr
Correct amount
MgBr2 is LR
MgBr2 + 2 AgNO3 2 AgBr + Mg(NO3)2
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PERCENT YIELD
The amount of product calculated through stoichiometric ratios are the maximum amount product that can be produced during the reaction, and is thus called theoretical yield.
The actual yield of a product in a chemical reaction is the actual amount obtained from the reaction.
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PERCENT YIELD
The percent yield of a reaction is obtained as follows:
Actual yieldx100 = Percent yield
Theoretical yield
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In an experiment forming ethanol, the theoretical yield is 50.0 g and the actual yield is 46.8 g. What is the percent yield for this reaction?
Example 1:
Actual yield% yield = x100
Theoretical yield
46.8 g= x100 =
50.0 g 92.7 %
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Silicon carbide can be formed from the reaction of sand (SiO2) with carbon as shown below:
Example 2:
1 SiO2 (s) + 3 C (s) 1 SiC (s) + 2 CO (g)
When 100 g of sand are processed, 51.4g of SiC is produced. What is the percent yield of SiC in this reaction?
Actual yield
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Example 2:
2
2
mol SiOx
g SiO 2
mol SiCx
mol SiO100 g SiO2 60.1
66.7 g SiC
11
Calculate theoretical yield
1
1 SiO2 (s) + 3 C (s) 1 SiC (s) + 2 CO (g)
g SiCx =
mol SiC140.1
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Example 2:
Actual yield% yield = x100
Theoretical yield
51.4 g= x100 =
66.7 g 77.1 %
Calculate percent yield
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Theoretical and Actual Yield• In order to determine the theoretical yield, we
use reaction stoichiometry to determine the amount of product each of our reactants could make.
• The theoretical yield will always be the least possible amount of product.– The theoretical yield will always come from the
limiting reactant.
• Because of both controllable and uncontrollable factors, the actual yield of product will always be less than the theoretical yield.
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1. Limiting reactant - the reactant that limits the amount of product produced in a chemical reaction. The reactant that makes the least amount of product.
2. Theoretical yield - the amount of product that can be made in a chemical reaction based on the amount of limiting reactant.
3. Actual yield - the amount of product actually produced by a chemical reaction.
4. Percent yield - The percent of the theoretical yield that was actually obtained.
% yield = actual yield
theoretical yieldx 100
Chap. 8 terms you should know
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THE END