Download Problems du Jour

Chapter 5: Thermochemistry
and Energy
Thermodynamics: study of energy and its transformations
Thermochemistry: relationships between chemical
reactions and energy changes
From Ch. 4: Are the 'driving forces' (formation of solids,
gases, etc.) the sole motivating factors behind metathesis
reactions?
What drives the 'driving forces'?
From Ch 13: recall the steps in solution formation –
energetics was one of the factors in solution formation
The Nature of Energy
Energy may be defined in two equivalent ways:
The capacity to do work (generally mechanical work):
W=Fxd
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The capacity to transfer heat
-what is heat?
Types of energy
Kinetic energy (KE): energy by virtue of motion
1
mv
2
Definition: KE =
2
Potential energy (PE): energy by virtue of position
PE is a result of attractions and repulsions that objects
experience in relation to other objects
Form of PE depends on the problem
E.g., PE of object of mass m at height h above earth:
PE = mgh
E.g. Coulombic potential between two charges q1, q2
at a distance r:
PE =
kq q
r
1
2
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What type of PE is of interest in chemistry?
In a chemical reaction, the chemical energy released as
reactants convert to products is due to the PE stored in
the arrangement of the atoms in the substance
How is kinetic energy manifest in chemistry?
SI energy unit is the Joule:
kg m
1J= 1
s
2
2
E.g., What is the KE in Joules of a 950-lb motorcycle
moving at 68 mph?
By what factor will the kinetic energy change of the speed
of the motorcycle is decreased to 34 mph?
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Thermodynamic definitions
We partition the universe into
(1) The part we are studying, i.e., the system
And
(2) The part of the universe which interacts with
the system, i.e., the surroundings
Why must we do this?
Any change in the system is manifest by a change in the
surroundings – we measure changes in the surroundings
to determine what happened in the system
Examples?
What types of systems exist?
Isolated: no exchange of matter or energy with
surroundings
Closed: exchange energy but not matter with
surroundings
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Open: exchange energy and matter with
surroundings
We will study closed systems – these can exchange
energy with its surroundings
Since heat and work are both forms of energy, we will
study how a closed system exchanges heat and work with
its surroundings......
E.g. suppose a bar of Al at 95oC is dropped into 1 L of
water at 25oC.
What is the system?
What constitutes the surroundings?
Is energy exchanged between the two?
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The First Law of Thermodynamics
Energy is conserved; it is neither created nor destroyed
The energy lost by a system must equal the energy gained
by the surroundings (and vice versa)
In the above example, the energy lost by the Al was gained
by the water and both ended up at the same temperature
When we speak of energy in a chemical context, what kind
of energy are we talking about?
Total energy of a system = KE + PE + internal energy (E)
What is the internal energy?
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An exact value of E cannot be determined for a chemical
system; however, changes in E, i.e.,
E = Efinal – Einitial

as a result of a chemical or physical process can be
measured
What does a (+)E mean? A (-)E?
Chemically, what do the initial and final notations refer
to?
E.g., Na(s) and Cl2(g) react to form NaCl(s), and a large
amount of heat is evolved
The system loses energy to the surroundings - the internal
energies of Na(s) and Cl2(g) are greater than that of NaCl(s)
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In the presence of Cl2(g), Na(s) will readily form NaCl(s) –
the product is lower in internal energy than the reactants
and the difference in internal energy is given off as heat,
which is measured as a temperature change in the
surroundings!
Systems (chemical or otherwise) tend towards the lowest
energy state possible!
How can we relate changes in internal energy to variables
which can be observed or measured?
Closed systems exchange energy with their surroundings
as heat or work
When a closed system undergoes any chemical or
physical change, the change in internal energy E can be
calculated from
E = q + w

q = heat; (+) for heat flowing into the system
w = work: (+) for work done on the system
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E = q + w is the mathematical statement of the first law of
thermo!
E.g., suppose a system releases 113 kJ of heat to the
surroundings and does 39 kJ of work on the
surroundings.
Calculate E
Is this process endothermic or exothermic?
A process is endothermic if the system absorbs heat
from the surroundings!
Exothermic process: system loses heat to
surroundings!
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State functions
Internal energy (E) is an extensive property

E is fixed for a given set of conditions, regardless of
how those conditions were arrived at

E is a state function: E = Efinal – Einitial
The value of a state function is independent of the
path taken to reach the initial and final states!
Is work a state function?
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Heat and enthalpy changes
We're trying to find a convenient way to measure
changes in internal energy.....
What variables can we manipulate to make our lives a
little easier?
Choices are pressure (P), temperature (T), and
volume (V)
If we hold V constant, then no P-V work can be done,
and E = q
This isn’t a very convenient (or safe) way to measure
internal energy changes......
When a transformation occurs at constant volume, most
of the energy gained or lost by the system is in the form of
heat
At constant pressure, the system and surroundings
can still exchange energy as heat and work
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If we’re dealing only with P-V work (due to expansion
of a gas), then
w = -PV
At constant P, the first law becomes
E = qp - PV
Massage this a little.......consider a transformation from
state 1 (P, V1) to state 2 (P, V2)
E2-E1 = qp – P(V2-V1)
Regroup...
E2+PV2 – (E1+PV1) = qp
We define a new function called the enthalpy, H:
H = E +PV
Then,
H2-H1 = H = qp
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Whoa! If a process is undertaken at constant P, then the
heat flow between system and surroundings = the change
in enthalpy, H!
Now: if a system undergoes a transformation at constant
P and absorbs heat from the surroundings, we say that the
process is endothermic, and H > 0
If a system undergoes a transformation at constant P and
evolves heat, we say that the process is exothermic and
H < 0
What is the easiest way to do an experiment at constant
P?
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E.g. Suppose we start with 100 mL water at 25oC. We add
10 g of NH4NO3. After the NH4NO3 dissolves, the
thermometer reads 18oC.
Define the system and surroundings
What are the initial and final states in this transformation?
What is the sign of H for this process?
Something to note and remember: H refers to the
enthalpy change occurring in the system!!!!
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Problems du Jour
At one time, a common means of forming small amounts
of oxygen gas in the lab was to heat KClO3:
2KClO3(s)  2KCl(s) + 3O2(g)
H = -89.4 kJ
For this reaction, calculate H for the formation of:
(a) 1.24 mol O2
(b) 4.89 g KCl
(c) 1.76 g KClO3 from KCl and O2
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Problem du Jour
Two solid objects, A and B, are placed in boiling water and
allowed to come to temperature there. Each is then lifted
out and placed in separate beakers containing 1000 g
water at 10.0 oC. Object A increases the temperature of
the water by 3.50oC; object B increases the temperature of
the water by 2.60 oC.
(a) Which object (A or B) has the larger heat capacity?
(b) What can you say about the specific heats of A and B?
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