Download Chapter 18: Chemical Thermodynamics

Chapter 18: Chemical Thermodynamics
Sections 18.8 & 18.9 will be covered
after we cover Ch 15 on Equilibrium.
1. Introduction
In the next few chapters, we will be studying the following concepts:
Thermodynamics: _________________________ (spontaneously)?
Kinetics: How _________ will the reaction occur?
Equilibrium: To what ________ will the reaction happen?

At equilibrium (not yet covered), opposing reaction rates (e.g. forward and reverse
reaction rates) are equal in magnitude
o The system is balanced between reactants and product
-may be mostly products, reactants, or a mixture
Definition: Thermodynamics is the study of the relationship between:
_____________________________________________________________
2. Thermochemistry Review
Introductory topics in thermodynamics were covered in first semester general chemistry.
You may want to re-read that chapter.
We will do a quick review the most essential facts.
Energy
Energy is the capacity to do work or to transfer heat.
Kinetic energy: the energy of _____________
KE=1/2 mv2
Potential energy: the energy of ____________
Heat energy is the kinetic energy of the particles.
First Law of Thermodynamics
(aka ________________________________________)
 Formal definition: The combined amount of matter and energy in the universe is
constant.
 More typical definition: Energy can neither be created nor destroyed in a chemical
reaction. (It can only be transferred.)
Thermodynamics - 1
Enthalpy and Enthalpy Change
 H = q at constant ______________
 It is a ___________ function (independent of the path)
 The standard enthalpy change for a reaction is:
ΔH o  ΣnΔH fo (products)  ΣmΔHfo (reactants )
 Hf is the heat of _________________.
 Formation reactions have
- _____________ product
- produce a __________ mole of that product
- use only ____________ as reactants in their standard states.
 Sign of H
(__) Rxn is exothermic, gives off heat, heat is a product.
(__) Rxn is endothermic, absorbs heat, heat is a reactant.
Standard Molar Enthalpies of Formation (aka Heat of Formation) Hof
By convention, Hf = ____
for elements at standard state.
Writing formation reactions may require using fractional coefficients.
Problem: Write the formation reaction for HBr.
Thermodynamics - 2
 Problem: for Hrxn = ΣnHproducts –ΣmHreactants

4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g)
Hf (kJ/mol)=
-45.9
0.0
91.3
-241.8
Heat Transfer
Heat lost by the system = -(heat gained by the __________________)
They are equal in magnitude, but opposite in sign.
What does the first law of thermodynamics really tell us?
 qsystem + qsurr = __
 qsystem = -qsurr
where q =_______________
Problem: Which of the following is endothermic?
E
E
Reaction I
E
Reaction II
a) I
b) II
c) III
d) I and II
e) I and III
Thermodynamics - 3
Reaction III
Enthalpy is an Extensive Property
This means that it depends on the amount of material (or how you balance the equ.)
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) + 890kJ
If
Then 2CH4(g) + 4O2(g)  2CO2(g) + 4H2O(l) + 1780kJ
H= -890kJ
H= ________
Hess’s Law: The Law of Heat Summations
Since Enthalpy is a state function, enthalpy change is the same whether a reaction occurs in
one step or many.
This means that if we add reaction (steps) we can add their enthalpies.
Note: If a reaction is doubled, the value of H is ____________
If a reaction is tripled, the value of H is ____________ .
If a reaction is reversed, the sign of H is ____________
Problem; Now It’s Your Turn! (using Hess’s Law)
Determine H for:
C2H4(g) + H2O(l) --> C2H5OH(l)
(equation c)
Given:
C2H5OH(l) + 3O2(g) --> CO2(g) + 3H2O(l) H = -1367kJ
(equation a)
H = -1411kJ
(equation b)
C2H4(g) + 3O2(g) --> CO2(g) + 2H2O(l)
Thermodynamics - 4
Calorimetry
Calorimeter is a device used to measure energy changes associated with chemical or physical
changes.
__________ calorimeter is a completely sealed device.
Measures the energy change at constant volume _____
If gases are produced or consumed, pressure will change.
_________________ style calorimeter is open to the atmosphere.
Measures the energy change at constant pressure _____ = ______
If gases are produced or consumed, the volume may change.
Determining a Calorimeter Constant.
-heat lost by the system = heat gained by the surroundings (water + calorimeter)
-qsys
=
where: m = _______ of the solution in the calorimeter
c = specific heat of the water (4.184 J/g C)
Bomb Calorimeter
T = ________
Coffee Cup Calorimeter
Thermodynamics - 5
Problem: Determining a Calorimeter Constant.
Determine the calorimeter constant if 80.g of water at 58.5C are mixed with 85g of water
in a coffee cup calorimeter at 22.8C. The final temperature after mixing was 39.7C.
Approach: heat lost by hot water = heat gained by cold water + calorimeter
-qsys
=
(m)(c)( T)
Thermodynamics - 6
+
(C’)(T)
3. Thermodynamics
Thermodynamics helps us determine the relative stabilities of reactants vs products.
 If products are more stable
Rxn is said to be ___________________-favored and spontaneous.
 If reactants are more stable
Rxn is said to be ___________________-favored a non-spontaneous.
Spontaneous Processes
Def: One that, once initiated, moves forward without any
_________________________________.
Spontaneous processes are ones in which products are favored over reactants
at the specified conditions.

Enthalpy (H) can determine a ___________________
that a reaction will be spontaneous:

o
Reactions with _____________ enthalpies (H < 0)
-products are lower in energy than the reactants
-the reaction will usually proceed spontaneously
o
Reactions with _____________ enthalpies (H >0)
-products are higher in energy than the reactants
the reaction will not usually proceed spontaneously
Enthalpy alone ________________________ to predict spontaneity!
o
Some reactions or processes are spontaneous at some temperatures but not at
others.
(i.e. ice melts spontaneously above 0ºC but not below it.)
Thermodynamics - 7
4. Entropy (S)

Entropy is a thermodynamic state function that describes the number of
_____________________ (position and/or energy levels) that are available
to a system in a given state.
o
Closely associated with probability.
The more ways a state can be achieved, the greater the likelihood (probability)
o
of finding that state.
Entropy is typically associated with:
o
- _____________________________.
- ___________________of matter and energy.
o
The SI unit of entropy is ________________________
Order vs Disorder

Highly ordered systems become more ____________________
o
Eggs will break, but not spontaneously reassemble.
o
A pile of leaves will eventually distribute themselves over my entire lawn, but
not spontaneously assemble into a nice neat pile
o
A toddler’s room will spontaneously become disordered.
These spontaneous processes also have an intrinsic direction, even though there is no
apparent net energy change
ALL SPONTANEOUS PROCESSES INVOLVE AN INCREASE IN
THE ENTROPY OF THE __________________ .
Why does a gas expand into a vacuum?
Thermodynamics - 8
Consider the following:
1. Behavior of gas in a container

I have two glass containers connect by a valve.
The left side has only gas A and the right has gas B.:

I then open the valve connecting the two containers.
What happens?

____________________________________________

Let’s analyze what’s happening.
o There was no heat transfer from the system to the
surroundings _________
o
The spontaneous reaction results in
_____________________________ .
o
Dispersal increases number of
_____________________________
(1 macro-state if no dispersal;
3 macro-states if dispersed.
See Right)
Thermodynamics - 9
There are multiple combinations and permutations within each macro-state.
For example, look at State C.
2. The behavior of objects of different temperature

I have two cubes of metal.
One cube is red hot. The other is really cold.

Then I put the two blocks next to each other:

Over time what will happen to the temperature
of the two blocks?

____________________________________________

There is a spontaneous process that involves the flow of heat energy from the hot block
to the cold block (the net heat energy is unchanged, however)
o
The spontaneous reaction results in _____________________
o
The process appears to be spontaneous and irreversible (in the absence of some
outside influence) (The heat will not re-separate.)
Thermodynamics - 10