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Chemistry II
Chapter 5
Thermochemistry
Name ____________
1.) The study of energy and its transformations is known as
_________________________.
2.)____________________ considers the relationships between chemical
reactions and the energy changes involving heat.
Nature of Energy
3.)The magnitude of the kinetic energy, Ek, of an object depends on its
___________ (m) and _____________(v)
Formula for kinetic energy:
4.)Potential energy is the energy an object has by virtue of its
_______________ relative to other objects.
a. gravitational
b. elastic
c. chemical
d. electrical
5.)____________________________ results from the interactions
between charged particles.
6.)The formula for electrostatic force is
7.)k is the constant of proportionality (8.99 x 109 J m /C2)
- C is the _______________, the unit for electrical charge.
- Q1 and Q2 are the __________ on the two particles involved
- d is the distance between the particles
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8.)When the charges are the same, the electrostatic force, Ek, is
_________________________ and the charges _____________.
9.)When the charges are opposite the electrostic force, Ek, is
_____________________ and the charges _____________.
10.)The energy of a substance is due to
1.)
2.)
11.)The SI unit for energy is the _______________ (J) named in honor
of _____________________ ( 1818 -1889) – a British Scientist who
investigated heat and work.
12.) A ____________________ was originally defined as the amount of
energy required to raise the temperature of 1gram of water from 14.5 to
15.5 degrees Celsius
13.)Today it is defined in terms of the Joule
14.)When we use thermodynamics to analyze energy changes, we focus
our attention on a limited and well defined part of the universe.
a.)The portion we single out to look at is called the
___________________.
b.)Everything else is the ________________.
c.)A ________________________ can exchange energy
but not material with its surroundings.
Transferring Energy: Work and Heat
Energy is transferred in two ways:
1.)
2.)
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15.)The energy used to cause an object to move against a force is called
_________________.
Formula:
16.)The other way that energy is transferred is _____________- the
energy transferred by a hotter object to a colder one.
17.)________________ is the capacity to do work or to transfer heat.
Section 2:
The First Law of Thermodynamics
18.)The First Law of Thermodynamics –
19.)The _______________ _____________ of a system is the sum of all of
the kinetic and potential energy in the components of the system.
20.)We represent the internal energy with the symbol _______.
21.)We can never really know the internal energy of a system, E, but we
can find the _________________ in E (ΔE)
22.)A _______________ value for ΔE results when Efinal> Einitial which
means that energy has been put into the system ( ________________)
23.)A _______________ value for ΔE results when Efinal< Einitial which
means that energy has been lost to the surroundings (
________________)
Energy Diagrams
Exothermic
* Endothermic
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24.)When a system undergoes any chemical or physical change, the
change in internal energy ΔE is given by the heat, q, added to or
released by the system plus the work done on or by the system, w.
Formula:
25.)When heat, q, is added to a system q is positive
When work, w, is done on a system it has a positive value
Sample exercise 5.3
The hydrogen and oxygen gases in the cylinder illustrated in figure 5.3
are ignited. As the reaction occurs, the system loses 1150 J of heat to the
surroundings. The reaction also causes the pistion to rise as the hot
gases expand. The expanding gas does 480 J of work on the
surroundings as it pushes against the atmosphere. What is the change in
the internal energy of the system ?
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Endothermic –
Exothermic-
State Functions
Although we cannot know exactly what the internal energy, E,
of a system, it does have a definite value, Δ E can be
determined.
26.)Internal energy is a _____________ ____________.
A state function is a property of a system that is determined by
specifying the conditions of state.
27) The value of a state function depends only on its ___________
___________and not on the particular _______________ of the sample.
28.)B/c internal energy, E, is a state function Δ E depends only on the
__________________ and ______________ states and not the change
that occurred.
29.)Internal energy is a state function, but q+w is not a state function.
Section 3: Enthalpy
30.)Most commonly the only kind of work produced by chemical change
is ________________ work.
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31.)Usually reactions are carried out at ______________atmospheric
pressure.
For example: Consider the following equation
Zn + 2 HCl → ZnCl2 + H2
Net Ionic
Zn + 2H+ → Zn+2 + H2
32.) As the hydrogen gas is produced it does _______________ against
the atmosphere.
We can better see this work if the reaction takes place in a reaction
vessel like the one below.
33.) This kind of work is called __________ - ______________ work (PV Work).
Formula:
34. When the change in volume is positive the work done is
_________________.
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35. The thermodynamic function called _______________ accounts for
the heat flow in chemical changes occurring at constant pressure when
no forms of work are performed other than P-V work.
Enthalpy equals the internal energy plus the product of the pressure
and the change in volume of the system.
36.) The symbol for enthalpy is _________________.
Formula for enthalpy:
37.) Enthalpy is a ______________ function because internal energy, E,
pressure and volume are all state functions.
ΔH = ΔE + ΔPV
W = - PΔV
-W = P ΔV
ΔH = ΔE + ΔPV= q + w-w=q ( at constant pressure)
ΔH = qp
Change in enthalpy equals the heat gained or lost at constant pressure.
It is easier to measure the change in temperature than the change in
internal energy.
38.) When ΔH is ___________________ (qp is positive) the system has
gained heat from the surroundings.
39.) When ΔH is positive the reaction is ______________.
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40.) When ΔH is negative heat is ______________to the surroundings
and the reaction is _________________.
41. Because H is a state function, ΔH depends only on the ____________
and the ____________________ states of the system.
In section two we learned that q is not a state function. This is not a
contradiction of ΔH = qp . Enthalpy change, ΔH, only is equal to the
heat released or absorbed at ________________ pressure.
Section 4: Enthalpies of Reaction
ΔH= ΔHfinal + ΔHinitial
ΔH= ΔHproducts + ΔHreactants
42. The enthalpy change that accompanies a reaction is called the
_______________ _____ _______________ or heat of reaction (ΔHrxn)
(rxn is often used as an abbreviation for reaction)
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Consider the following reaction:
2H2 (g) + O2 (g) → 2H2O (g) ΔH= -483.6 KJ
43. When the above reaction is controlled so that exactly 2 moles of
hydrogen gas react with exactly one mole of oxygen gas, 483.6 kilojoules
of heat is released. Changes in enthalpy (ΔH) is negative so the reaction
is ________________.
44. Balanced chemical equations that show the associated enthalpy
changes are called _________________ equations.
45. The enthalpy change accompanying a reaction can also be
represented in an enthalpy __________________.
46. In an ____________________ reaction the enthalpy of the products
will be lower than the reactants.
47. In an _______________ reaction the enthalpy of the products will be
higher than the reactants.
The following guidelines are helpful when using thermochemical
equations and enthalpy diagrams.
48.)Enthalpy is an extensive property.
a.)___________________ properties depend on the amount
of substance present (i.e., mass, volume, amount of heat)
b.)__________________ properties do not depend on the
amount of substance present ( i.e., density, melting point,
boiling point)
49.) ΔH is directly dependent on the ______________ of reactant
consumed.
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For example,
If when 2 moles of hydrogen gas react with 1 mole of oxygen gas 493.6
KJ of heat is released.
2H2(g) + O2(g)  2H2O(g) ΔH= -493.6 KJ
Then when 4 moles of hydrogen gas react with 2 moles of oxygen gas
985.2 KJ are released.
Sample Example 5.5
How much heat is released when 4.5 g of methane gas is burned in a
constant pressure system?
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) ΔH= - 890 KJ
Practice Exercise
Hydrogen Peroxide can decompose to water and oxygen by the
following reaction:
2H2O2  2H2O(l) + O2(g) Δ H = -196 KJ
Calculate the value of q when 5.00 grams of H2O2(l) decomposes at
constant temperature.
50.) The enthalpy change for a reaction is equal in magnitude but
opposite in sign to ΔH for the reverse reaction.
For example:
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CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH= -890 KJ
CO2(g) + 2H2O(l) → CH4(g) + 2O2(g) ΔH= ______ KJ
51.)When we reverse a reaction, the roles of reactants and products are
reversed.
52.)The enthalpy change for a reaction depends on the state of the
reactants and products.
i.e., if the product of combustion of methane were gaseous H2O
instead of liquid H2O, the ΔH would be= -802 KJ instead of -890
KJ.
53.)It is very important to specify the _________________ of reactants
and products in thermochemical equations.
Definitions
54.) ______________________ (H) A quantity defined by the relationship
H = E + PV ; the enthalpy change, ΔH, for a reaction that occurs at
constant pressure is the heat evolved or absorbed in the reaction; ΔH=qp
55)_____________________ _____ _________________ (Hf)the enthalpy change that accompanies the formation of a substance from
the most stable forms of its component elements
56.)___________________ _____ __________________ (Hrxn)
The enthalpy change associated with a chemical reaction.
Using Enthalpy as Guide
57.) A process that is thermodynamically favored to happen is called a
______________________________ process.
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58.) By “spontaneous” we don’t mean that the reaction will form
products without intervention. Most often some amount of energy must
be imparted to get the process started.
59.)The enthalpy change of a reaction does give an indication as to
whether the reaction is likely to be ___________________.
60.)As a general rule, when the enthalpy change is _________ it is the
dominant factor in determining spontaneity.
61.)When ΔH is large and negative the reaction tends to be
____________________.
62.)Reactions for which ΔH is large and positive tend to be spontaneous in
the ___________________ direction
Section 5 : Calorimetry
63.)The measurement of heat flow is ___________________
64.)An apparatus used to measure heat flow is a _________________.
Heat Capacity and Specific Heat
65.)The _______________ ________________ of an object is the amount
of heat required to raise the temperature by 1K or 1⁰C. ( one degree Kelvin
is equal to one degree ________________) 1K = 1⁰C
66.)The greater the heat capacity the ________________ the heat
required to produce a given rise in temperature.
67.)The heat capacity of 1 mole of a substance is called its
___________________ _________________ _______________________.
68.)The heat capacity of 1 gram of a substance is called its
__________________ _________________ _____________________.
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69.)Formula for specific heat capacity:
70.)The specific heat capacity for water is much _________________ than
for other substances. Water can _______________ much heat with little
change in its _________________________.
71.)The high specific heat of water is essential for the maintenance of our
________________________
Sample exercise 5.6
• How much heat is needed to warm 250 g of water (about 1
cup) from 22ᵒC (about room temperature) to near its boiling
point, 98 ⁰C? The specific heat of water is 4.18 J/g-K.
• What is the molar heat capacity of water?
Practice Exercise
• Large beds of rock are used in solar heated homes to store
heat. Assume that the specific heat of the rocks is 0.82J/g-K.
Calculate the quantity of heat absorbed by 50 kg of rocks if
their temperature increases by 12 ᵒC.
• What temperature change would these rocks undergo if they
emitted 450 KJ of heat?
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Constant Pressure Calorimetry
72.)In calorimetry problems we will assume that the heat produced by the
reaction, qrxn is entirely _____________________ by the solution; it does
not escape the calorimeter.
73.)For an exothermic reaction, heat is __________________by the
reaction and _____________________ by the solution.
74.)For an endothermic reaction opposite is true- heat is
_________________ by the reactants and is ________________ by the
solution.
• qsoln = -qrxn
• qsoln = (specific heat of soln) x ( gram of soln)x ΔT)= -qrxn
75.) For dilute aqueous solutions, the specific heat of the solution will be
the same as water ( 4.18 J/g-k)
Sample Exercise 5.7
When a student mixes 50 mL of a 1.0 M HCl and 50mL of a 1.0M NaOH in a
coffee cup calorimeter, the temperature of the resultant solution increases
from 21.0 ⁰C to 27.5⁰C. Calculate the enthalpy change for the reaction,
assuming that the calorimeter loses only a negligible quantity of heat, that
the total volume of the solution is 100 mL , that its density is 1.0 g/mL
and that its specific heat is 4.18 J/g-K.
Practice Exercise
When 50 mL of a 0.100M AgNO3 and 50mL of a 0.100M HCl are mixed in a
constant –pressure calorimeter, the temperature of the mixture increases
from 22.30⁰C to 23.11⁰C. The temperature increase is caused by the
following reaction:
AgNO3(aq) + HCl(aq) → AgCl(s) + HNO3(aq)Calculate ΔH for this
reaction, assuming that the combined solution has a mass of 100 g and a
specific heat of 4.18 J/g-K
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Bomb Calorimetry: Constant –Volume Calorimetry
76.)One of the most important types of reactions studied using
calorimetry is _________________________ in which a compound reacts
completely with excess oxygen.
77.)Combustion reactions are most conveniently studied using a
_______________ _____________________.
78.)The substance to be studied is placed in a small cup within a sealed
vessel called a ________________ , which is designed to withstand high
pressures.
79.)_________________ is released when combustion occurs.
80.)The heat is absorbed by the calorimeter contents causing a rise in the
_____________________________ of the water.
81.)The temperature of the water is carefully measured _____________
and _______________ reaction.
82.)To calculate the ___________________ _____ ________________
from the measured temperature increase in the bomb calorimeter, we
must know the heat capacity of the calorimeter ,Ccal.
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Example Problem
The combustion of 1 gram of benzoic acid is known to produce 26.38 KJ of
heat. If exactly 1 gram of benzoic acid is combusted in a bomb calorimeter
and it increases the temperature by 4.857ᵒC, what is the heat capacity for
that calorimeter?
Ccal = 26.38 KJ / 4.857 ᵒC = 5.431 KJ/ᵒC
83.)To find the heat of a reaction, qrxn using a specific calorimeter:
Qrxn = -Ccal x ΔT
Sample exercise 5.8
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel. The
combustion of methylhydrazine with oxygen produces N2(g), CO2(g) and
H2O(l):
2 CH6N2(l) + 5O2(g) → 2N2(g) + 2 CO2(g) +6 H2O(l)
When 4.00 g of methylhydrazine is combusted in a bomb calorimeter, the
temperature of the calorimeter increases from 25.00ᵒC to 39.50 ᵒC. In a
separate experiment the heat capacity of the calorimeter is measured to
be 7.794KJ/g-ᵒC. What is the heat of reaction for the combustion of one
mole of CH6N2 in this calorimeter?
Practice exercise
A 0.58665g sample of lactic acid (HC3H5O3) is burned in a calorimeter
whose heat capacity is 4.812 KJ/ᵒC . The temperature increases from
23.10ᵒC to 24.95ᵒC. Calculate the heat of combustion of (a) lactic acid per
gram (b) per mole.
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84.Because reactions in a bomb calorimeter are carried out under
constant volume conditions, the heat transferred corresponds to a change
in ___________________ _____________________, ΔE, rather than
_________________________ ΔH. However both ΔE and ΔH are usually
very close in value.
Section 6 Hess’s Law
85.)It is possible to calculate ΔH for a reaction from the
______________________ ΔH values of other reactions.
If a particular reaction can be carried out in one step or in a series of steps
the sum of the enthalpy changes associated with the individual steps must
be the same as the enthalpy change associated with the one step process.
For example:
CH4(g) + 2O2(g) → CO2(g) +2 H2O(g)
2H2O(g) → 2H2O(l)
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
ΔH= -802 KJ
ΔH= -88KJ
ΔH= -890 KJ
To obtain the net equation
1.) The sum of the reactants of the two equations is placed on
the left side of the arrow.
2.) The sum of all products is placed on the product side
3.) Any substance that is on both sides of the equation in the
same phase can be canceled like in algebraic equations.
Hess’s Law:
If a reaction is carried out in a series of steps, ΔH for the reaction will
equal the sum of the enthalpy changes for the individual steps.
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The overall enthalpy change for the process is independent of the number
of steps or path in which the reaction occurs.
If a reaction is reversed, change the sign of Δ H
If a reaction is multiplied by a factor, multiply the enthalpy change by that
same factor.
85.)Hess’s Law provides a useful means of calculating _____________
_____________ that are difficult to measure directly.
The enthalpy of combustion of C to CO2 is -393.5kJ/mol carbon, and the enthalpy of
combustion of CO to CO2 is -283.0 kJ/mol CO:
C(s) + O (g) → CO (g)
ΔH= -393.5kJ
2
2
CO (g) + ½ O (g) → CO (g)
2
2
2
ΔH = -283.0 kJ
Using the above data , calculate the enthalpy of combustion of C to CO:
C(s) + ½ O → CO(g)
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Practice Exercise
Carbon occurs in two forms, graphite and diamond. The enthalpy of
combustion of graphite is -393.5KJ /mol and that of diamond is 395.4kJ/mol:
C(graphite) + O2(g) → CO2(g)
ΔH= -393.5 kJ
C(diamond) + O2(g) → CO2(g)
ΔH= -395.4 kJ
Calculate ΔH for the conversion of graphite to diamond:
C(graphite) → C(diamond)
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Sample Exercise 5.10
Calculate ΔH for the reaction
2C(s) + H2(g) → C2H2(g)
given the following reactions and their respective enthalpy changes:
C2H2(g) + 5/2 O2(g) →2CO2(g) + H2O(l)
C(s) + O2(g) → CO2(g)
H2(g) + ½ O2(g) → H2O(l)
ΔH=-1299.6 kJ
ΔH = -393.5 kJ
ΔH = -285.8 kJ
Calculate ΔH for the reaction
NO(g) + O(g) → NO (g)
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Given the following information:
NO(g) + O3(g) → NO2(g) + O2(g)
O3(g) → 3/2 O2(g)
O2(g) → 2 O(g)
ΔH= -198.9 kJ
ΔH= -142.3 kJ
ΔH= 495.0 kJ
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We always get the same value of ΔH for an overall reaction regardless of
how many steps we use to get to the final product.
We can use Hess’s Law to find the enthalpy change for a large number of
reactions from the tabulated ∆H values.
86.)Tables exist for the following:
a. Enthalpy of vaporizationb.) Enthalpy of fusionc.) Enthalpy of combustiond.) Enthalpy of formation
∆H is dependent on temperature and pressure; so, we define the conditions at
standard state.
87.)What is standard state for most enthalpies?
88.)What is the standard enthalpy of formation?
The stoichiometry is always arranged so that 1 mole of the product is
formed.
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Sample exercise 5.11
For which of the following reactions at 25◦C would the enthalpy change
represent a standard enthalpy of formation? For those where it does not,
what changes would need to be made in the reaction conditions?
a) 2Na(s) + ½ O2(g) → Na2O(s)
b) 2K(l) + Cl2(g) → 2KCl(s)
c) C6H12O6(s) → 6C(diamond) + 6H2(g) + 3O2(g)
Practice Exercise
Write the equation corresponding to the standard enthalpy of formation of
liquid CCl4 (Carbon tetrachloride.)
We can use ∆Hf◦ of different compounds to calculate ∆H for a reaction at
standard state.
Determine ∆H◦ for the following reaction.
C3H8 (g) + 5O2(g) → 3 CO2 (g) + 4 H2O(l)
On the bottom line:
∆Hrxn◦ = Σ n ∆Hf◦product - Σ m ∆Hf◦reactants
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Example 5.12
Calculate the standard enthalpy of change for the combustion of 1 mole of
benzene (C6H6) to CO2 (g) and H2O (l)
Practice
Given the following standard enthalpy of reaction, use the standard
enthalpies of formation in table 5.3 to calculate the standard enthalpy of
formation of CuO(s)
Food and Fuels
Section 8
89.)What is a fuel value?
The fuel value for any food or fuel can be measured by calorimetry.
Protein has 17KJ/g
Carbohydrates 17 KJ/g
Fats 38 KJ/g
90. Fats are the major energy storage molecules in the body for two reasons:
Read exercise 5.14 page 152
A plant such as celery contains carbohydrates in the form of starch and
cellulose. These two kinds of carbohydrates have essentially the same fuel
values when combusted in a bomb calorimeter. When we consume celery,
however, our bodies receive fuel values from the starch only. What can we
conclude about the starch stored as cellulose as foods?
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A 28 –g ( 1 oz) serving of a popular breakfast cereal served with 120 mL of
skim milk provides 8 g of protein, 26 g of carbohydrates and 2 g of fat. A)
Using the average fuel values of these kinds of substances, estimate the
amount of food energy in this serving. B.) A person of average weight uses
about 100 Cal/mile when running or jogging. How many servings of this
cereal provide the fuel value requirement for running 3 miles.
Practice Exercise
Dry red beans contain 62% carbohydrate, 22% protein, and 1.5% fat.
Estimate the fuel value of these beans. B.) Very light activity like reading or
watching television uses about 7 kJ/min. How many minutes of such activity
can be sustained by the energy provided by a serving of chicken noodle soup
containing 13 g or protein, 15 g carbohydrate and 5 g of fat.
Fuels:
The greater the % of carbon and hydrogen in a fuel the higher its fuel value.
Fossil FuelsNatural gasPetroleum –
Coal23
Compare the quantity of heat produced by the combustion of 1 gram of
propane compared to 1 gram of benzene.
Exercise 5.13
The standard enthalpy change for the reaction
CaCO3(s) → CaO(s) + CO2(g)
Is 178.1 KJ. From the values for the standard enthalpies of formation of
CaO(s) and CO2(g) given in table 5.3 calculate the enthalpy of formation of
CaCO3(s).
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