1. Chapter 5 Thermochemistry
Lecture Presentation
Chapter 5 Thermochemistry
John D. Bookstaver
St. Charles Community College
Cottleville, MO
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Energy
Energy is the ability to do work or transfer heat.
Energy used to cause an object that has mass to move is called work.
Energy used to cause the temperature of an object to rise is called heat.
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Kinetic Energy
Kinetic energy is energy an object possesses by virtue of its motion: 1 2
Ek =  mv2
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Potential Energy
Potential energy is energy an object possesses by virtue of its position or chemical composition.
The most important form of potential energy in molecules is electrostatic potential energy, Eel:
KQ1Q2 d
Eel =
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Units of Energy
The SI unit of energy is the joule (J):
An older, non-SI unit is still in widespread use: the calorie (cal):
1 cal = J
1 J = 1 
kg m2 s2
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6. Definitions: System and Surroundings
The system includes the molecules we want to study (here, the hydrogen and oxygen molecules).
The surroundings are everything else (here, the cylinder and piston).
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Definitions: Work
Energy used to move an object over some distance is work:
w = F  d
where w is work, F is the force, and d is the distance over which the force is exerted.
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Heat
Energy can also be transferred as heat.
Heat flows from warmer objects to cooler objects.
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Conversion of Energy
Energy can be converted from one type to another.
For example, the cyclist in Figure 5.2 has potential energy as she sits on top of the hill.
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Conversion of Energy
As she coasts down the hill, her potential energy is converted to kinetic energy.
At the bottom, all the potential energy she had at the top of the hill is now kinetic energy.
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11. First Law of Thermodynamics!!!!!!!
Energy is neither created nor destroyed.
In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, and vice versa.
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Internal Energy
The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E.
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Internal Energy
By definition, the change in internal energy, E, is the final energy of the system minus the initial energy of the system:
E = Efinal − Einitial
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14. Changes in Internal Energy
If E > 0, Efinal > Einitial
Therefore, the system absorbed energy from the surroundings.
This energy change is called endergonic.
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15. Changes in Internal Energy
If E < 0, Efinal < Einitial
Therefore, the system released energy to the surroundings.
This energy change is called exergonic.
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16. Changes in Internal Energy
When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w).
That is, E = q + w.
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E, q, w, and Their Signs
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18. Exchange of Heat between System and Surroundings
When heat is absorbed by the system from the surroundings, the process is endothermic.
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19. Exchange of Heat between System and Surroundings
When heat is absorbed by the system from the surroundings, the process is endothermic.
When heat is released by the system into the surroundings, the process is exothermic.
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State Functions
Usually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem.
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State Functions
However, we do know that the internal energy of a system is independent of the path by which the system achieved that state.
In the system depicted in Figure 5.9, the water could have reached room temperature from either direction.
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Enthalpy
Enthalpy is a measure of the heat flow in and out of a system
Enthalpy can be measured if a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure–volume work
Enthalpy is the internal energy plus the product of pressure and volume:
H = E + PV
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Enthalpy
When the system changes at constant pressure, the change in enthalpy, H, is
H = (E + PV)
This can be written
H = E + PV
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Enthalpy
Since E = q + w and w = −PV, we can substitute these into the enthalpy expression:
H = E + PV
H = (q + w) − w
H = q
So, at constant pressure, the change in enthalpy is the heat gained or lost.
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25. Endothermicity and Exothermicity
A process is endothermic when H is positive.
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26. Endothermicity and Exothermicity
A process is endothermic when H is positive.
A process is exothermic when H is negative.
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PRACTICE: Label each of the following as endothermic (EN) or exothermic (EX), then show whether H would be positive or negative.
Water boils on a stove
Rain freezing into snow
A chemical reaction in a hand warmer makes it feel warm
A candle burning
Salt is added to ice; the temperature drops
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28. Endothermic, because heat is released to the surroundings.
Exothermic, because heat is released to the surroundings.
Endothermic, because heat is absorbed from the surroundings.
Exothermic, because heat is absorbed from the surroundings.
Answer: C

29. The Truth about Enthalpy
Enthalpy is an extensive property.
H for a reaction in the forward direction is equal in size, but opposite in sign, to H for the reverse reaction.
H for a reaction depends on the state of the products and the state of the reactants.
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Calorimetry
Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow.
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31. Heat Capacity and Specific Heat
The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity.
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32. Heat Capacity and Specific Heat
We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K (or 1 C).
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33. Heat Capacity and Specific Heat
Specific heat, then, is
Specific heat =
heat transferred
mass  temperature change
s = q
m  T
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34. When a piece of iron at 356 Kelvins is placed in water at 298 Kelvins, what happens?
Energy flows from iron to water.
Energy flows from water to iron.
Energy does not flow.
d. Energy is not conserved.
Answer: a

35. Hg(l) Fe(s) Al(s) H2O(l) Answer: A
Editor: The question refers to Table 5.2 and thus this table needs to be provided with the question.

36. Specific heat practice!
A sample of iron absorbs 67.5 J of heat, upon which the temperature of the sample increases from 21.5 °C to 28.5 °C. If the specific heat of iron is J/g-K, what is the mass (in grams) of the sample?
A) 4.3
B) 11
C) 21
D) 1100
E) 1.1 x 103
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37. Specific heat practice #2
The temperature of a 35.2 g sample of iron increases from 23.7 °C to 29.5 °C. If the specific heat of iron is J/g-K, how many joules of heat are absorbed?
A) 4.3
B) 92
C) 0.450
D) 1100
E) 1.1 x 103
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AP Chem bellringer – 1/30/14
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ENTHALPY!!!
INFO FOR PACKET:
ΔHrxn: heat of reaction (enthalpy change for a chemical rxn)
ΔHf: heat of formation (enthalpy change when 1 mole of a substance is formed from constituent elements)
ΔHcomb: heat of combustion (enthalpy change for combustion reaction)  
ΔHsol: heat of solution (enthalpy change when a solute dissolves)
ΔHvap: Heat of vaporization (enthalpy change when a liquid vaporizes)
ΔHfus: Heat of fusion (enthalpy change when a solid melts)
ΔHBDE: Bond dissociation energy (heat required to break a chemical bond)
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Today’s focus:
ΔHrxn: heat of reaction ΔHf: heat of formation
IMPORTANT NOTES:
If a problem just says “ΔH”, it’s usually ΔHrxn.
If the degree sign is included (ex: ΔH°) it means the reaction is occurring at standard temperature and pressure, or STP
Standard temperature: 25 °C
Standard pressure: 1 atm
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41. Why do we need to know enthalpy of reaction so well?
Next week’s lab: designing the best hand warmer
Goal of lab: compare the enthalpy of reaction for different hand warmer solutions in order to pick the best one
It is an INQUIRY lab, meaning you write your own procedure (we will go over that on Monday)
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42. Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter.
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43. Constant Pressure Calorimetry
Because the specific heat for water is well known (4.184 J/g-K), we can measure H for the reaction with this equation:
q = m  s  T
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Bomb Calorimetry
Reactions can be carried out in a sealed “bomb” such as this one.
The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction.
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Bomb Calorimetry
Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy, E, not H.
For most reactions, the difference is very small.
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Enthalpy of Reaction
The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants:
H = Hproducts − Hreactants
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Enthalpy of Reaction
This quantity, H, is called the enthalpy of reaction, or the heat of reaction.
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Enthalpy of reaction
H is an extensive property, so its value changes depending on mass.
If you know mass AND H, you can figure out the heat change for a reaction.
Example: The value of ΔH° for the reaction below is -72 kJ. Calculate how many kJ of heat are released when 35 grams of HBr is formed in this reaction.  
H2 (g) + Br2 (g) → 2HBr (g)
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49. Solving enthalpy of reaction problems
BACK TO STOICHIOMETRY!
Known: 35 g HBr, ΔH° for -72 kJ/mol
H2 (g) + Br2 (g) → 2HBr (g)
Set up a railroad to find heat:
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Hess’s Law
H is well known for many reactions, and it is inconvenient to measure H for every reaction in which we are interested.
However, we can estimate H using published H values and the properties of enthalpy.
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Hess’s Law
Hess’s law states that “[i]f a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”
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Hess’s Law
Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.
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Hess’s Law: Example
Given the following reactions, what is the enthalpy of the reaction of nitrogen with oxygen to produce nitrogen dioxide? (1) 2NO → N2 + O2 ΔH = -180 kJ (2) 2NO + O2 → 2NO2 ΔH = -112 kJ N2 + 2O2 → 2NO2
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Hess’s Law: Example
Given the following reactions: 2S (s) + 3O2 (g) → 2SO3 (g) ΔH = -790 kJ S (s) + O2 (g) → SO2 (g) ΔH = -297 kJ the enthalpy of the reaction in which sulfur dioxide is oxidized to sulfur trioxide is _____________ kJ. 2SO2 (g) + O2 (g) → 2SO3 (g)
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55. Enthalpies of Formation
An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.
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56. Standard Enthalpies of Formation
Standard enthalpies of formation, Hf°, are measured under standard conditions (25 °C and 1.00 atm pressure).
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Calculation of H
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
Imagine this as occurring
in three steps:
C3H8(g)  3C(graphite) + 4H2(g)
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Calculation of H
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
Imagine this as occurring
in three steps:
C3H8(g)  3C(graphite) + 4H2(g)
3C(graphite) + 3O2(g) 3CO2(g)
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Calculation of H
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
Imagine this as occurring
in three steps:
C3H8(g)  3C(graphite) + 4H2(g)
3C(graphite) + 3O2(g) 3CO2(g)
4H2(g) + 2O2(g)  4H2O(l)
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Calculation of H
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
The sum of these equations is
C3H8(g)  3C(graphite) + 4H2(g)
3C(graphite) + 3O2(g) 3CO2(g)
4H2(g) + 2O2(g)  4H2O(l)
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
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Calculation of H
We can use Hess’s law in this way:
H = nHf,products – mHf°,reactants
where n and m are the stoichiometric coefficients.
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Calculation of H
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
H = [3(−393.5 kJ) + 4(−285.8 kJ)] – [1(− kJ) + 5(0 kJ)]
= [(− kJ) + (− kJ)] – [(− kJ) + (0 kJ)]
= (− kJ) – (− kJ) = − kJ
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63. Extra Credit Opportunity
Knowing how enthalpy changes as a substance undergoes a phase change is very important. Section 11.4 (pgs ) in your textbook describes how to solve problems relating to those changes. Using the information in practice problem 11.3 (pg 441), calculate the TOTAL enthalpy change that occurs when 50 g ice at -10°C is heated to water at 50°C.
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Energy in Foods
Most of the fuel in the food we eat comes from carbohydrates and fats.
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Energy in Fuels
The vast majority of the energy consumed in this country comes from fossil fuels.
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