THERMOCHEMISTRY
We will return to this topic in chapter 16 (Spontaneity, Entropy, and Free Energy)
ENERGY AND ITS FORMS
1) energy
a) the capacity to do work or transfer heat
2) work
a) force x distance
3) potential energy
a) energy due to position
b) can be associated with attraction and repulsion of charges
4) kinetic energy
a) energy of motion
b) KE = ½ mv2
c) due to motion
d) temperature is associated with K.E.
5) principle of the conservation of energy
a) energy cannot be created or destroyed, only change form
6) heat energy
a) form of energy associated with changing temperature
b) will establish thermal equilibrium
i) objects in contact will eventually have the same temperature
7) thermal energy
a) same as heat
b) also used when discussing the motions of molecules in solids,
liquids, or gases
8) radiant energy
a) heat energy passing through space
b) example: heat energy from the sun
9) electrical energy
a) can be generated mechanically
b) may be produced chemically in a battery (voltaic or galvanic cell)
10) thermochemistry
a) study of thermal energy
b) application to chemistry
i) heat energy produced or absorbed in chemical reactions
STATE FUNCTIONS
1) State Function
a) a quantity whose value is determined only by the state of the
system.
b) independent of the path that you use
c) enthalpy is a state function since the ΔH does not depend on the
path you choose to follow.
2) Examples:
a) pressure
b) volume
c) temperature
3) ONLY the change in enthalpy can be determined experimentally.
Chemical Energy
1) Terms
a) System
i) object under investigation
ii) material being studied
b) Surroundings
i) anything that is not the system
c) Universe
i) system + surroundings
ii) EVERYTHING is included in the universe
2) Types of reactions
a) endothermic
i) heat flows from surroundings to the system
ii) heat flows in
iii) ΔHrxn = positive (+)
b) exothermic
i) heat flows from the system to the surroundings
ii) heat flows out
iii) ΔHrxn = negative (-)
3) ΔE
a) change in internal energy of the system
b) must be equal to the sum of the heat + work transferred to or from
the system
c) ΔE = q + w; first law of thermodynamics; law of conservation of
energy
4) ΔHrxn = HfProducts - HfReactants
UNITS OF ENERGY
1) Chemistry
a) calorie
i) amount of heat needed to raise the temperature of exactly 1.00
gram of water 1.00 OC
ii) Temperature is really measured from 14.4OC to 15.4OC.
b) Calorie (with a capital C)
i) The amount of heat energy associated with food.
ii) Equal to kilocalories
c) kilocalorie
2) Physics
a) joule (J)
b) SI unit of energy;
3) 1 cal = 4.184 J (remember this conversion factor).
HEAT CAPACITY & SPECIFIC HEAT
1) Specific heat capacity (c)
a) Amount of heat energy needed to raise 1.0 g of a substance 1.0 OC
2) Molar specific heat (Cp)
a) Same as above except that instead of 1.0 gram it is 1.0 mole
3) Water’s specific heat
a) cal/(g x K) = 4.18 J/(g x K)
b) May substitute OC for K
Both scales are
measured in
degrees, but for
absolute
temperature
scales, there is no
degree symbol, it
is OC and K.
4) q = mΔTc
a) q = quantity of heat
b) c = specific heat
c) m = mass in grams
d) ΔT = Tf - Ti
Alphabet: a b c d e F g h I
“Tf” before “Ti”
Since T is the change of temperature, and the change in
degree Celsius and degree Kelvin are the same, they may cancel each other
out…but ONLY IN THIS TYPE OF PROBLEM.
NOTE:
5) Water has a very, very, very (get the idea) high specific heat.
6) Many problems are carried out in a calorimeter.
a) device to measure the heat energy produced in a reaction
b) Bomb calorimeter
i) a calorimeter with a steel inside chamber to explode samples
and measure their heat energy
ii) qreaction = -(qbomb + qwater)
c) qabsorbed = -qlost
7) ΔT is negative
a) Therefore q is negative
b) Reaction is endothermic
c) Heat flows from the system to the surroundings
8) ΔT is positive
a) Therefore q is positive
b) Reaction is exothermic
c) Heat flows from the surroundings to the system
9) Heat associated with phase changes
a) latent heat
i) heat associated with a change in phase of a substance
(1) solid  liquid or liquid  solid
(2) liquid  gas or gas  liquid
ii) temperature remains constant
iii) Two types
(1) latent heat of fusion
(a) heat energy required to go from solid to liquid
(b) melt ice and turn into water
(i) Ice at 0.0OC
(ii) Water will be at 0.0OC
(iii)Value for water is 333 J/g
(iv) Since no temperature change, no temperature unit
for 333 J/g
(2) latent heat of vaporization
(a) heat required to vaporize a liquid at the boiling point
and turn into gas.
(b) Water at 100.0OC to steam at 100.0OC
(i) Value for water is 2260 J/g
(ii) No temperature unit (no temperature change)
HESS’S LAW
Hess’s Law = if a reaction is the sum of two or more other reactions, then
ΔHrxn for the overall process must be the sum of the ΔHrxn of each of the
reactions that make up the overall equation.
STANDARD ENTHALPIES OF
FORMATION
1) standard conditions
a) 1 atm pressure (standard pressure)
b) 25OC (298 K) (room temperature)
c) 1 M solutions
2) ground state
a) physical state in which it exists at standard conditions
3) superscript zero o automatically means standard conditions, i.e. Hfo
4) Types of Enthalpies
a) Enthalpy of a reaction (ΔHrxn)
i) Energy associated with any reaction.
ii) Most general of the types encountered.
iii) If reactions meet specific definitions, then the (ΔHrxn) becomes
something else…a special type of enthalpy reaction…see next
page for the types
b) Special types of (ΔHrxn)
i) molar enthalpy of formation (ΔHf)
(1) Energy associated with making one mole of a compound
from its elements in their ground states.
(2) ΔHf for the elements in their standard states = 0.
(a) It doesn’t take any energy to make one mole of
Al from Al
ii) Molar Enthalpy of Combustion (ΔHcomb)
(1) Energy associated with combusting one mole of a substance
iii) Molar Enthalpy of Neutralization (ΔHneut)
(1) Energy associated with neutralizing one mole of a
substance
iv) Molar Enthalpy of Vaporization (ΔHvap)
(1) Energy associated with vaporizing one mole of a substance
REMEMBER:
ΔHrxn = Σ ΔHfproducts - Σ ΔHfreactants
 Remember your alphabet.
Laws of Thermodynamics
First Law:
The total energy of the universe is a constant.
Law of Conservation of Energy
Second Law: The total entropy (disorder) of the universe always
increases in a spontaneous process.
Third Law:
The entropy of every pure, crystalline substance at absolute
zero is zero.
AP Question 1988 B
Substance
C(s)
H2(g)
C2H5OH(l)
H2O(l)
Enthalpy of Combustion, Hocomb
(kiloJoules/mol)
-393.5
-285.8
-1366.7
----------
Absolute Entropy, So
(Joules/mol–K)
5.740
130.6
160.7
69.91
(a)
Write a separate, balanced chemical equation for the combustion of each of the following:
C(s), H2(g), and C2H5OH(l). Consider the only products to be CO2 (g), and/or H2O (l).
(b)
In principle, ethanol can be prepared by the following reaction:
2 C(s) + 2 H2(g) + H2O(l)  C2H5OH(l)
Calculate the standard enthalpy change, Horxn, for the preparation of ethanol, as shown in
the reaction above.
(c)
Calculate the standard entropy change,So, for the reaction given in part (b).
Note:
There are two ways to do this problem – using Hess’s Law and algebraically – we are going
to do it both ways. This problem is set up to “guide” you to one method, namely Hess’s Law, but for
the algebra method, you have to understand the key concept of ___________________.
AP Question 1979 B
Hf°
S°
Compound (kilocalories/mole)(calories/mole K)
H2O(l)
-68.3
16.7
CO2(g)
-94.1
51.1
O2(g)
0.0
49.0
C3H8
?
64.5
When 1.000 gram of propane gas, C3H8, is burned at 25°C and 1.00 atmosphere, H2O(l) and
CO2(g) are formed with the evolution of 12.03 kilocalories.
(a)
Write a balanced equation for the combustion reaction.
(b)
Calculate the molar enthalpy of combustion, H°comb, of propane.
(c)
Calculate the standard molar enthalpy of formation, Hf°, of propane gas.
(d)
Calculate the entropy change, S°comb, for the reaction and account for the sign S°comb.
1984 B
Substance
C(s)
CO2(g)
H2(g)
H2O(l)
O2(g)
C3H7COOH(l)
Standard Heat of Formation, Hf°
in kJ mol-1
0.00
-393.5
0.00
-285.85
0.00
?????
Absolute Entropy, S°
in J mol-1 K-1
5.69
213.6
130.6
69.91
205.0
226.3
The enthalpy change for the combustion of butyric acid at 25°C, H°comb, is -2,183.5 kilojoules
per mole. The combustion reaction is
C3H7COOH(l) + 5 O2(g)  4 CO2(g) + 4 H2O(l)
a) From the above data, calculate the standard heat of formation, Hf°, for butyric acid.
b) Write a correctly balanced equation for the formation of butyric acid from its elements.
c) Calculate the standard entropy change, Sf°, for the formation of butyric acid at 25°C. The
entropy change, S°, for the combustion reaction above is -117.1 J K-1 at 25°C.
d) Calculate the standard free energy of formation, G°f, for butyric acid at 25°C.
Note:
There are two ways to do part b – using Hess’s Law and algebraically – we are going to do it
both ways. If you understand both ways, you can do any problem that they give you on the AP test!
The algebra method contains both types of problems that you can encounter on the AP exam.
1)
A hot dog provides 160 Calories. What is this energy in joules?
\
2)
Mr. A’s tea is too hot, so he wants to cool it down, so he goes to
the freezer and gets a block of aluminum (which he keeps in the
freezer just for this purpose). The aluminum block has a mass of
250. g and you want to cool 250. g of coffee (with a specific heat
of 4.20 J/g x K) from 60 degrees Celsius to 45 degrees Celsius. To
accomplish this, what must the initial temperature of the aluminum
block be?
3)
Assume you have 1 cup of ice (237 mL) at 0.0 degrees Celsius.
How much heat is required to melt the ice and then warm the
resulting water to 25 degrees Celsius?
4)
Calculate the enthalpy change, ∆H, for the formation of 1 mole of
strontium carbonate (the material that gives the red color in
fireworks) from its elements.
Sr(s) + C(graphite) + 3/2 O2(g)  SrCO3(s)
The information available to you is:
Sr(s) + ½ O2(g)  SrO(s)
SrO(s) + CO2(g)  SrCO3(s)
C(graphite) + O2(g)  CO2(g)
∆H = -592 kJ
∆H = -234 kJ
∆H = -394 kJ
5)
The first step in the production of nitric acid from ammonia
involves the oxidation of NH3. Find the enthalpy change for this
reaction. Is the reaction exothermic or endothermic and why?
4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g)
6)
Calculate the standard molar enthalpy of formation of SiCl4(g)
given the equation below.
SiO2(s) + 4 HCl(g) + 139.5 kJ  SiCl4(g) + 2 H2O(g)