Upload
kevyn
View
52
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Chapter 10 Quantity Relationships in Chemical Reactions. Conversion Factors from Equations. The coefficients in a chemical equation give us the conversion factors to convert from the number of moles of one substance, to the number of moles of other substance in a reaction. - PowerPoint PPT Presentation
Citation preview
www.cengage.com/chemistry/cracolice
Mark S. CracoliceEdward I. Peters
Mark S. Cracolice • The University of Montana
Chapter 10Quantity Relationshipsin Chemical Reactions
Conversion Factors from EquationsThe coefficients in a chemical equation give us the conversion factors to convert from the number of moles of one substance,
to the number of moles of other substance in a reaction.
4 NH3(g) + 5 O2 (g) 4 NO(g) + 6 H2O (g)
2
3Omol5
NHmol4
NOmol4NHmol4 3
O23
Hmol6NHmol4•
NOmol4Omol5 2
OHmol6Omol5
2
2
OHmol6NOmol4
2
Conversion Factors from EquationsExample:How many moles of water are formed by the reaction of 3.20
moles of ammonia NH3 with oxygen.
Step 1: Write and balance the chemical equation:
Conversion Factors from Equations4 NH3(g) + 5 O2 (g) 4 NO(g) + 6 H2O (g)
Step2 Find what is given : 3.20 mol NH3
Step 3 Find what is to be found: ? mol H2O Step 4 Find the path : mol NH3 mol H2O Step 5 : Find the right conversion factor
3
2NHmol4Hmol6 O
Conversion Factors from Equations
4 NH3(g) + 5 O2 (g) 4 NO(g) + 6 H2O (g)
Step 6 Set up the calculation:
3.20 mol NH3 x (6 mol H2O / 4 mol NH3) = 4.80 mol H2O
Note that 6 and 4 are exact numbers; they do not affect the significant figures in the final answer.
Conversion Factors from EquationsExample 2 How many moles of oxygen are required to burn
2.40 moles of ethane, C2H6?Step 1: Write the chemical equation
2 C2H6(g) + 7 O2 (g) 4 CO2(g) + 6 H2O (l) • conversion conversion factor • Number of moles C2H6 number of moles O2: 7 mol O2/ 2 mol C2H6
• Number of moles C2H6 number of moles H2O : 6 mol H2O / 2 mol C2H6
• Number of moles C2H6 number of moles CO2 : 4 mol CO2 / 2 mol C2H6
• Number of moles O2 number of moles CO2 : 4 mol CO2/ 7 mol O2
• Number of moles O2 number of moles H2O : 6 mol H2O / 7 mol O2 • Number of moles CO2 number of moles H2O 6 mol H2O / 4 mol CO2:
Conversion Factors from Equations2 C2H6(g) + 7 O2 (g) 4 CO2(g) + 6 H2O (l)
Step2 Find what is given : 2.40 mol C2H6
Step 3 Find what is to be found: ? mol O2
Step 4 Find the path : mol C2H6 mol O2
Step 5 : Find the right conversion factor 7 mol O2/ 2 mol C2H6
Conversion Factors from Equations
2 C2H6(g) + 7 O2 (g) 4 CO2(g) + 6 H2O (l)
Step 6 Set up the calculation:
2.40 mol C2H6 x (7 mol O2/ 2 mol C2H6) = 8.40 mol O2
Note that 7 and 2 are exact numbers; they do not affect the significant figures in the final answer.
Conversion Factors from EquationsExample 3 Ammonia is formed from its elements. How many
moles of hydrogen are needed to produce 4.20 moles NH3. Chemical equation :
N2 + 3 H2 2 NH3
Conversion Factors from Equations
N2 + 3 H2 2 NH3
Given 4.20 moles NH3
Wanted mol H2
Path 4.20 moles NH3 mol H2
Factor 3 mol H2/ 2 moles NH3
Calculation 4.20 moles NH3 x (3 mol H2/ 2 moles NH3) = 6.30 mol NH3
Mass–Mass StoichiometryHow to Solve a Stoichiometry Problem:
The Stoichiometry Path
Step 1: Change the mass of the given species to moles.
Step 2: Change the moles of the given species to the moles of the wanted species.
Step 3: Change the moles of the wanted species to mass.
Mass–Mass StoichiometryMass-to-Mass Stoichiometry Path
Mass of Moles of Moles of Mass ofGiven Given Wanted Wanted
Molar Mass Equation Molar Mass coefficients
Mass Given × × ×
Mass–Mass StoichiometryExample 1 Calculate the number of grams of carbon dioxide
produced by burning 66.0 g of heptanes C7 H16 (l) Step 1 Chemical reaction :
C7H16(l) + 11 O2 (g) 7 CO2(g) + 8 H2O (l) Step2 Find what is given : 66.0 g C7H16
Step 3 Find what is to be found: ? g of CO2
Mass–Mass StoichiometryC7H16(l) + 11 O2 (g) 7 CO2(g) + 8 H2O (l)
Step 4 Find the path : g of C7H16 mol C7H16 mol CO2 g of CO2
Step 5 : Find the right conversion factors
g of C7H16 mol C7H16 : 1 mol C2H6/ 100.20 g C7H16
mol C7H16 mol CO2 7 mol CO2/ 1 mol C7H16
mol CO2 g of CO2 44.01 g CO2/ 1 mol CO2
Mass–Mass StoichiometryC7H16(l) + 11 O2 (g) 7 CO2(g) + 8 H2O (l)
Step 6 Set up the calculation:
66.0 g C7H16 x (1 mol C7H16/ 100.20 g C7H16) x (7 mol CO2/ 1 mol C7H16) x (44.01 g CO2/ 1mol CO2) =
203 g of CO2
Percent YieldThe actual yield of a chemical reaction is
usually less than the ideal yieldpredicted by a stoichiometry calculation because:
• reactants may be impure
• the reaction may not go to completion
• other reactions may occur
Actual yield is experimentally determined.
Percent YieldPercent yield expresses the ratio of
actual yield to ideal yield:
% yield = × 100%
Percent YieldExample Calculate the theoretical yield of carbon dioxide and the
percent yield when burning of 66.0 grams of C7 H16 produced 181 grams of CO2 .
C7H16(l) + 11 O2 (g) 7 CO2(g) + 8 H2O (l)
Theoretical yield of CO2 66.0 g x ( 1mol C7H16/ 100.20 g C7H16) x
( 7 mol CO2 / 1 mol. C7H16) x ( 44.01 CO2 / 1 mol. CO2 ) =
= 203 g CO2
Percent yield = ( 181g/203g) x 100 = 89.2 %
Percent YieldExample:Consider the reaction of hydrogen and nitrogen that forms
ammonia with a 92.2% yield. What quantity of ammonia will be produced by reacting 125 g of hydrogen with excess nitrogen?
Solution:Use 92.2% yield as a PER expression:
3 H2 + N2 2 NH3
125 g H2 × × × ×
= 649 g NH3
Limiting Reactants
Three atoms of carbon react with two molecules of oxygen:C(g) + O2(g) CO2(g)
Limiting ReactantsThe reaction will stop when two molecules
of oxygen are used up.Two carbon dioxide molecules will form;One carbon atom will remain unreacted:
C + O2 CO2
Start 3 2 0
Used (+) orProduced (–) – 2 – 2 + 2
Finish 1 0 2
Limiting ReactantsLimiting Reactant
The reactant that is completely used up.(Oxygen)
Excess ReactantThe reactant initially present in excess.
(some will remain unreacted)(Carbon)
Limiting Reactants• The reactant that is completely used up by the reaction, is
called the limiting reactant. Other reactants have some excess which will remain unreacted.
• There are two approaches to solving limiting reactant problems: the comparison of moles method and the smaller amount method.
Limiting Reactants: Compare MolesComparison-of-Moles Method
How to Solve a Limiting Reactant Problem:
1. Convert the number of grams of each reactant to moles.2. Identify the limiting reactant by comparing the theoretical
mole ratio of reactants to the actual mole ratio.3. Calculate the number of moles of each species that reacts or
is produced using the limiting reactant number of moles.4. Calculate the number of moles of each species that remains
after the reaction.5. Change the number of moles of each species to grams.
Limiting Reactants: Compare MolesExample Calculate the mass of antimony(III) iodide that can be
produced by the reaction of 129 g antimony, Sb (Z=51), and 381 g iodine. Also find the number of grams of the element that will be left.
2 Sb + 3 I2 2 SbI3
Calculate the number of moles:
Moles of Sb = 129 g Sb x (1 mol Sb/ 121.8 g Sb) = 1.06 mol Sb
Moles of I2 = 381 g I2 x (1 mol I2 / 253.8 g I2 ) = 1.50 mol I2
Limiting Reactants: Compare Moles2 Sb + 3 I2 2 SbI3
Theoretical mole ratio:mol of I2 / mol of Sb = 3 mol of I2 / 2 mol of Sb =
1.5 mol of I2 / 1 mol of Sb Actual mole ratio:mol of I2 / mol of Sb = 1.5 mol of I2 / 1.06 mol of Sb
=1.42 mol of I2 / 1 mol of Sb
Actual mole ratio of iodine over antimony is smaller than theoretical mole ratio, so iodine is the limiting reactant and antimony is in excess.
Limiting Reactants: Compare Moles2 Sb + 3 I2 2 SbI3
The number of mole Sb used = 1.50 mol of I2 x ( 2 mol of Sb/ 3 mol of I2 ) = 1.00 mol of Sb
mol of Sb in excess = (1.06 - 1.00) mol of Sb = 0.06 mol of Sb
mass of Sb in excess = 0.06 mol of Sb x (121.8 g Sb/mol Sb)= 7 g Sb
Limiting Reactants: Compare Moles2 Sb + 3 I2 2 SbI3
The number of mole SbI3 produced =1.50 mol of I2 x ( 2 mol of SbI3 / 3 mol of I2 ) = 1.00 mol of SbI3
mass of SbI3 produced = 1.00 mol of SbI3 x (502.5 g SbI3 /mol
SbI3)= 502.5 g SbI3
Limiting Reactants: Smaller AmountHow to Solve a Limiting Reactant Problem by Smaller
Amount Method
1. Calculate the amount of product that can be formed by the initial amount of each reactant.a) The reactant that yields the smaller amount of product is
the limiting reactant.b) The smaller amount of product is the amount that will be
formed when all of the limiting reactant is used up.2. Calculate the amount of excess reactant that is used by the
total amount of limiting reactant.3. Subtract from the amount of excess reactant present initially
the amount that is used by all of the limiting reactant. The difference is the amount of excess reactant that is left.
Limiting Reactants: Smaller AmountExample Calculate the mass of antimony(III) iodide that can be
produced by the reaction of 129 g antimony, Sb (Z=51), and 381 g iodine. Also find the number of grams of the element that will be left.
2 Sb + 3 I2 2 SbI3
Solution:Step 1 is to calculate the amount of product that can be formed
by the initial amount of each reactant.
Limiting Reactants: Smaller Amount
2 Sb + 3 I2 2 SbI3
First, assume that antimony is the limiting reactant. Grams of SbI3 produced =
0.129g Sb x(1 mol Sb/121.8 gSb) x (2mol SbI3/2mol Sb) x
(502.5 g SbI3 /molSbI3) =
= 532 g SbI3
Limiting Reactants: Smaller Amount2 Sb + 3 I2 2 SbI3
Second, assume that iodine is the limiting reactant.
grams of SbCl3 produced =
381g I2 x ( mol I2 / 253.8 g I2 ) x (2mol SbI3/3mol I2) x (502.5 g SbI3 / mol SbI3) =
• = 503 g SbI3
• The reactant iodine, which yields the smaller amount (503 g) of SbI3 , is the limiting reactant.
• The amount of antimony(III) iodide produced is 503 g.
Limiting Reactants: Smaller Amount2 Sb + 3 I2 2 SbI3
The amount of antimony required is calculated as follows Grams of antimony required =
381 g I2 x (1 mol I2 /253.8 g I2 ) x (2 mol Sb/ 3 mol I2) x (121.8 g Sb/mol Sb) = 122 g Sb
• The amount of antimony in excess = 129 g (initial) – 122g (used) = 7 g Sb (left)
EnergyEnergy
The ability to do work or transfer heat.
SI (derived) energy unit: Joule
Joule (J):The amount of energy exerted when a force of one newton (force
required to cause a mass of 1 kg to accelerate at a rate of1 m/s2) is applied over a displacement of one meter:
1 joule = 1 newton × 1 meter
1 J = 1 kg • m2/sec2
EnergyNon-SI metric energy unit: Calorie
calorie (historical definition):Amount of energy required to raise the
temperature of 1 g of water by 1°C.
calorie (modern definition)1 cal 4.184 J
A food Calorie (Cal) is a thermochemical kilocalorie.In scientific writing it is capitalized;
in everyday writing, interpret the context.
1 kcal = 4.184 kJ
Thermochemical EquationsEnthalpy
H = E + PVWhere E is the internal energy of the system, P is the pressure of
the system, and V is the volume of the system. P, V, E, H which depend only on the present state of the system, and do not depend on the system’s past or future, are called state functions.
∆H is the enthalpy of reaction.
∆ means “change in”: It is determined by subtracting theinitial quantity from the final quantity
Thermochemical EquationsEnthalpy of Reaction, ∆H
It can be demonstrated that for a reaction studied at constant pressure the heat of reaction is a measure of the change in enthalpy1 for the system.
Heat of reaction = H(products) - H (reactants ) = Δ H
When a system gives off heat (reaction is exothermic)enthalpy of the system goes down and Δ H has a negative value.
When a reaction absorbs heat (reaction is endothermic) enthalpy
increases and Δ H is positive..
Thermochemical EquationsThere are two ways to write a thermochemical EquationMethod 1 The ΔH of the reaction is written to the right of the equation
2 C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H2O (g) ΔH = -2855 kJ Method 2Energy is included in the thermochemical equation as if it were a
reactant or product. 2 C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H2O (g) + 2855 kJ
Thermochemical Stoichiometry
Since there is a proportional relationship between moles of different substances and heat of reaction, conversion factors can be written between kilojoules and moles of any substance.
These factors are used in solving thermochemical stoichiometry problems.
Thermochemical StoichiometryExample : How many kilojoules of heat are released when 73.0 g C2H6 (g) burn Equation :2 C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H2O (l) + 3119 kJ
Thermochemical Stoichiometry2 C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H2O (l) + 3119 kJ
Given 73.0 g C2H6 (g) Wanted : kJ
Path g C2H6 (g) mol C2H6 (g) kJ Factors
(1 mol C2H6 / 30.07 g C2H6 )
(3119 kJ/ 2 mol C2H6 (g))
Thermochemical Stoichiometry
2 C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H2O (l) + 3119 kJ
Solve the problem: 73.0 g C2H6 (g) x (1 mol C2H6 / 30.07 g C2H6) x
(3119 kJ/ 2 mol C2H6 (l) ) = = 3.79 x 103 kJ
Thermochemical StoichiometryExample:When propane, C3H8(g), is burned to form gaseous carbon
dioxide and liquid water, 2.22 × 103 kJ of heat is released for every mole of propane burned. What quantity of heat is released when 1.00 × 102 g of propane is burned?
Solution:First, write and balance the thermochemical equation to
determine the stoichiometric relationships.
C3H8 (g) + 5 O2 (g)3 CO2 (g) + 4 H2O (l) ∆H = –2.22x103 kJ
Thermochemical StoichiometryC3H8 (g) + 5 O2 (g)3 CO2 (g) + 4 H2O (l) ∆H = –2.22 × 103 kJ
GIVEN: 1.00 × 102 g C3H8
1 mol C3H8/44.09 g C3H8
PATH: g C3H8 mol C3H8
2.22 × 103 kJ/1 mol C3H8
kJ
1.00 × 102 g C3H8 × × = 5.04 × 103 kJ
HOMEWORKHomework: 1, 7, 23, 35, 45, 47, 67.