5-1
When you touch a motorcycle on a hot day, the metal feels even hotter than the fabric of the seat
because the metal is better at conducting heat to our hand.
5-2
The vinyl is better at conducting heat to our body and therefore feels hotter than a seat covered
by a towel or lambskin. The same property makes vinyl covered seats seem colder in winter than
seats covered with other fabrics.
5-3
The ice cubes will form more quickly in the metal trays. The metal is a good conductor of heat
and so it will act as a heat sink, cooling the water more quickly. The plastic tray will act as an
insulator and will slow down the formation of the ice cubes.
5-4
Temperature is a quantitative measure of whether an object can be labeled "hot" or
"cold." Heat is that which causes a change in temperature.
5-5
Heat capacity describes the amount of heat energy required to change the temperature of an
object by one temperature unit. Example units of measurement might be:
| Btu | Joule | Joule | cal |
| oF | oC | K | oC |
5-6
5-7
The "specific heat" of a substance is the heat energy in Joules required to raise the
temperature of one gram of a specified substance by one oC or one K. The
"molar heat capacity" of a substance is the heat energy in Joules required to raise the
temperature of one mol of a specified substance by one oC or one K.
5-8
Density mercury is 13.59 g/cm3
From the heat capacity of mercury we have
For an equal volume of water we have
Comparing the two liquids, water transfers 2.2 times the amount of heat as an equal volume of
mercury.
5-9
Heat gained by the water:
The molar heat capacity of copper
5-10
When a change in phase of matter occurs because a sample is heated or cooled, the temperature
of the sample does not change as long as two phases are simultaneously present. Consider the
example of boiling water where both the gas and liquid phases are present or freezing water
when both ice and liquid water are present.
5-11
The "latent heat of fusion" is the amount of heat required to transform a sample of
matter from the solid to the liquid state. The "latent heat of vaporization" is the
amount of heat required to transform a sample of matter from the liquid to the gaseous state. The
total heat change associated with the transformation depends upon the amount of matter present
in the sample. Therefore, these heats are extensive properties of a system.
5-12
The basis of the caloric theory was that "heat is a fluid that flows from a hot object to a
cold object." Heat is assumed to be conserved. In the kinetic-molecular theory, heat is no
longer a fluid that can be stored in an object. It is something that is transferred across the
boundary between a system and its surroundings. Heat is no longer assumed to be
conserved.
5-13
The heating of an object by friction is cited as an example where heat is not conserved. In
contrast, the melting and refreezing of a liquid might be used to illustrate conservation of
heat.
5-14
As a balloon filled with helium becomes warmer, the particles within become more agitated. This
induces a greater pressure on the walls, causing the balloon to expand.
5-15
The "system" is that part of the universe on which we focus our attention. The
"surroundings" are everything else. The "boundary" is the region which
separates the system from the surroundings. An ice-cube is a system separated from liquid water
surroundings in which it is floating by a boundary layer of water molecules. The ice-cube is a
system separated from gaseous surroundings of the atmosphere by a boundary layer of water
molecules and gas molecules. The liquid water is the system separated from the gaseous
surroundings of the atmosphere by a boundary layer of water molecules and gas molecules.
5-16
Hot and cold are manifestations of sensible heat. An object endowed with a large amount of
latent heat capacity may appear cool when compared with an object with a lesser latent heat
capacity when both are subjected to the same heat transfer.
5-17
Force is the push or pull exerted on/by an object. Work measures the result when the force acting
on an object moves the object over a distance.
5-18
Expansion work can be done on surroundings by reactions which generate products which
occupy more space than that of the reactants. The greatest amount of expansion work occurs for
reactions where the number of mols gaseous products is greater than the number mols gaseous
reactants.
(a) 
Same number of moles gaseous products and reactants. No expansion work.
(b) 
More moles gaseous product. Significant expansion work.
(c) 
Fewer moles gaseous product. Work done by surroundings on system.
(d) 
More moles gaseous products. Significant expansion work.
5-19
"The simplest way of distinguishing between heat and work is to ask, 'What would happen
if a thermal insulator, such as a blanket, was placed between the system and surroundings?' If the
thermal insulator would have no effect, the interaction between the system and its surroundings
involves work."
5-20
(a) The ice-cubes (system) absorb heat from the surrounding lemonade, melting to become part
of the liquid mixture.
(b) Work must be done on the gas being forced into the balloon (system) to cause the expansion of the balloon.
(c) Work must be done by the surroundings to move a piston to compress a gas (system) in a cylinder.
(d) Heat from the combustion of a fuel/air mixture generates an increased pressure by an increased number of moles gaseous products derived from the chemical reaction. The increased force becomes work pushing the piston from the cylinder.
(e) The electric current generates heat and possibly a coherent magnetic field capable of moving the coil. The latter will potentially be the work of a motor or a buzzer.
5-21
The Kelvin temperature of an ideal gas is postulated to be directly proportional to the internal
energy of the gas. For more complex systems, detection of changes in internal energy require that
both temperature and work done by/on the system be monitored.
5-22
The first law of thermodynamics states that energy can be transferred between a system and
its surroundings in the form of either heat or work, 
Esystem = q + w. This relationship limits the amount of
heat or work that can be extracted from a system.
5-23
When a system does work on its surroundings the internal energy of the system decreases
and Esystem is negative. When a system loses
heat to its surroundings the internal energy of the system also decreases and Esystem is negative.
5-24
The expansion of a gas pushing a piston from a cylinder is an example of a system doing work
on its surroundings. The temperature of the system will decrease. The freezing of an ice cube in a
glass of lemonade is an example of a system losing heat to its surroundings. In this case the
temperature of the system does not change. If the lemonade is considered to be the system losing
heat to the surrounding ice cubes, the temperature of the system will decrease until becoming
equal to that of the surrounding ice cubes. What we view as the system can significantly alter the
effect a transfer of energy between the system and its surroundings will have on the temperature
of the system.
5-25
If we focus our attention on a gas enclosed in a cylinder where the surroundings do work on the
system by forcing a piston into the cylinder compressing the gas, the internal energy of the
system will increase. The temperature of the system will also increase. If the same system gains
heat from the surroundings (the piston being fixed to maintain a constant volume), the internal
energy of the system will increase. The temperature of the system will also increase.
5-26
The first law of thermodynamics tells us that if we try to do nothing else but extract either heat or
work from a system, the internal energy of the system will decrease to a point at which the
system can no longer provide either heat or work.
5-27
State functions of a system are often associated with intensive properties of the system. Thus
temperature, pressure, volume, number of mols, and internal energy of an ideal gas are state
functions.
5-28
State functions of a trip:
| g) location of the car, | h) elevation, | i) latitude, | j) longitude |
Path dependent characteristics of a trip:
| a) work done | b) energy expended | c) cost |
| d) distance traveled | e) tire wear | f) gasoline consumed. |
5-29
By definition a property of a system is a state function if it depends only on the state of the
system, not on the path used to get to that state.
The state functions are:
| a) temperature | b) internal energy | c) enthalpy | d) pressure |
| e) volume |
The properties that are not state functions and are path dependent are:
| f) heat | g) heat |
5-30
By definition, a state function of a system is independent of the path used to take the system from
the initial state, Xi to the final state, Xf. Therefore only the difference,
Xf - Xi need be of concern.
5-31
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5-33
5-34
Heat energy can be exchanged by a system under conditions of constant pressure,
qp, or constant volume, qv. The steel container operates under
conditions of constant volume, the styrofoam cup operates under conditions of constant pressure.
5-35
5-36
This is the heat released in the combustion of 0.100 g of B5H9.
5-37
The change in enthalpy associated with a chemical reaction will be equal to the change in
internal energy of the system when no "PV work" is done by/on the system.
5-38
For reactions having no net change in the number of mols gaseous substances, the enthalpy
change is roughly equivalent to the change in internal energy.
(a) 3 mol gas --> 2 mol gas; PV work.
(b) All reactants and products are solids. No PV work.
(c) All reactants and products in aqueous solution. No PV work.
(d) 1 mol gaseous reactant, no gaseous products. PV work.
Reactions (b) and (c) would show roughly equivalent changes in enthalpy and internal
energy.
5-39
The change in enthalpy of a system measures the energy change not only of the internal energy
but also that associated with pressure-volume changes. Under the special condition of constant
pressure, the change in enthalpy of the system directly measures the change in internal energy.
5-40
Endothermic reactions require an input of heat energy.
(a) Breaking chemical bonds requires input of energy. Endothermic.
(b) Combustion reactions are generally exothermic.
(c) Condensation changes in phase are generally exothermic.
(d) Exothermic.
5-41
Endothermic reactions proceed with the absorption of heat energy.
(a) The reaction of sodium with water is a violent, exothermic process.
(b) Magnesium burns vigorously in oxygen. The reaction is highly exothermic.
(c) The separation of NaCl (table salt) into its component elements requires input of significant
energy. The reaction is endothermic.
(d) The combination of sodium ion and an electron is an exothermic process.
5-42
The water of the sweat or covering the dog's tongue vaporizes in an endothermic process. The
water vapor carries away the equivalent of 40.7 kilo Joules of energy per mol H2O
evaporated. This removal of heat energy serves to cool the surrounding area of the body where
the blood is passing. Heat exchange with the internal body fluids is then affected as the blood is
circulated within.
5-43
The value of H for a reaction depends on the condition under
which the reaction is run. A standard state for thermochemical measurements has therefore been
defined in which the pressure of any gas is 1 atm and the concentration of any aqueous solution
is 1 M. Measurements made under standard-state conditions are indicated by adding a superscript
o to the symbol for the quantity being measured. Thus Ho refers to measurements in which the reactants and
products are in their standard state.
5-44
The effect of pressure and concentration on thermodynamic data is controlled by defining a set of
standard conditions for thermodynamic experiments.
5-45
No heat is given off in this reaction. The reaction is endothermic and Ho is positive.
5-46
5-47
5-48
The difference between the initial and final values of the enthalpy of a system does not depend
on the path used to go from one state to the other. Therefore the enthalpy change for a reaction is
the same regardless of whether the reaction occurs in one step or in several steps.
5-49
Take the difference between the two reaction equations and get:
C (graphite) ---> C (diamond)
5-50
(1) 
(2) 
Multiply reaction (2) by 2 and reverse it to get
(3) 
Add reaction (1) and (3) to get
check: Horxn = 
Hof(products)
- Hof(reactants)
Horxn = (2)(- 285.8) + (1)(0) - (2)(-
187.8) = - 196 kJ
5-51
(1) 
(2) 
Subtract reaction (2) from (1) and get
gathering terms we get,
check: Horxn = Hof(products)
- Hof(reactants)
Horxn = (2)(+ 90.37 kJ) - 0 = 180.7
kJ
5-52
The desired equation is: 
(1) 
(2) 
(3) 
The desired reaction is obtained by adding the third reaction to twice the second and twice the first reaction.
5-53
The desired reaction is obtained as the reverse of the first plus three times the second equation plus two times the third equation.
5-54
The desired equation is attained by subtracting the first equation from the second equation.
5-55
The standard enthalpy of formation is defined to be zero for elements
in their standard states. These substances included are: F2(g)
and P4(s)
5-56
The reaction is endothermic.
5-57
The reaction is endothermic.
5-58
5-59
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5-62
5-63
5-64
For iron(III) oxide:
For Chromium(III) oxide:
The reaction with iron(III) oxide evolves more heat per mol aluminum consumed.
5-65
The bond dissociation enthalpy for one mol of Cl2 molecules
is 240 kJ. The enthalpy to form
one mol of chlorine atoms will be
half this value or 120 kJ.
5-66
Enthalpies for reactions are estimated using bond dissociation values by considering the
difference in enthalpies associated with bonds broken as compared to that for bonds formed.
Bond dissociation:
Bond Formation:
5-67
5-68
Bond dissociation
Bond Formation
5-69
H2(g) + Cl2(g)--> HCl(g)
Bond dissociation:
Bond Formation:
5-70
CS2(g)+3 O2(g)--> CO2(g)+2 SO2(g)
Bond Dissociation:
Bond Formation:
5-71
2 CH3OH(g)+3 O2(g)--> 2 CO2(g)+4 H2O(g)
Bond dissociation:
Bond Formation:
5-72
1 mol H2O(s)-->1 mol H2O(l)=6.03 kJ
1 mol H2O(l, 0°C)-->1 mol H2O(l, 100°C)=7.53 kJ
1 mol H2O(l)-->1 mol H2O(g)=40.67 kJ
Energy needed= 54.23 kJ
5-73
5-74
2 H2(g) + O2--> 2 H2O(g)
rewrite equation as
435 kJ+249 kJ-2 X=-241.83 kJ
-2 X=-925.83 kJ
X= 462.9 kJ
5-75
5-76
CH3CH2OH(l) + 3 O2(g) --> 2 CO2(g) + 3 H2O(g)
5-77
