Thermochemistry
Chapter 6
Chemistry E-1a
Energy: What is it? How is it transformed?
•  What is energy? What are the SI units for energy?
•  What types of energy are we going to be interested in studying?
Thermal Energy: The Flow of Heat
•  What is heat? Heat is a process by which thermal energy can be transferred from one object to another or one substance to another.
We define the heat, q, for an object as the amount of heat gained or lost by an object during the process of transferring thermal energy.
Whenever two objects of different temperatures come in contact with each other, energy flows from the hotter object to the colder one.
How is heat flowing in the following process?
http://www208.pair.com/sumphan/Mini%20Food%2006-4.jpg
Law of Conservation of Energy:
Determining the Amount of Heat Gained or Lost
• The equation used to determine the amount of head lost or gaining by something is below,
let’s look at it carefully:
!
q = m s ΔT!
!
Why does your finger feel cold in room temperature water but not cold in room temperature air?!
Air
What Can Heat Do? Change the Temperature
•  You take an insulated beaker containing 100 g of water at 20°C. You add 50 grams of copper metal that has been heated to 100°C.
What will be the final temperature of the system when it reaches thermal equilibrium (everything at the same temperature)?
• What are Exothermic and Endothermic processes?
Work: A Different Process Where Energy Can be Transferred
•  We have seen that heat (q) is the transfer of thermal energy, which is the random motion of atoms and molecules.
•  What is work?
• We can calculate the work done when a gas expands or contracts against a constant external pressure:
Clicker Question:
Determine work (in Joules) for a sample of gas that expands from an
initial volume of 1.00 liters to a final volume of 3.50 liters against a constant
external pressure of 1.20 atm.
1) w = –3.0 J
2) w = –4.2 J
3) w = –303.9 J
4) w = –425.5 J
Work, Heat, and Internal Energy
• Joule’s experiment that demonstrated that heat and work are both ways to transfer energy into and out of a system.
• What relationship can we establish between heat and work?
• Clicker Questions:
An enclosed sample of gas expands against a constant external pressure within a
perfectly insulated container so that there is no transfer of heat into or out of the gas.
A. What can you conclude about q for this gas?
1) q < 0
2) q = 0
3) q > 0
B. What can you conclude about w for this gas?
1) w < 0
2) w = 0
3) w > 0
C. What can you conclude about ∆U for this gas?
1) ∆U < 0
2) ∆U = 0
3) ∆U > 0
http://lectureonline.cl.msu.edu/~mmp/kap11/picts/mechequi.gif
State Functions
•  What are state functions?
• What are some examples of state functions?
• Are heat and work state functions?
Defining a New State Function: Enthalpy
• Let us define a new state function called Enthalpy, H. This function will allow us to determine the heat released or consumed
during a process that is open to the atmosphere:
• What is the sign of ∆H for an exothermic reaction?
• What is the sign of ∆H for an endothermic reaction?
Energy Changes in Chemical Reactions
•  Ultimately, we want to understand Chemical Energy. What is Chemical Energy?
Consider the following reaction:
NH4Cl(s) + H2O(l) ! NH4+(aq) + Cl–(aq)
Clicker Question: When this reaction occurs, is energy released or absorbed?
1) Released
2) Absorbed
How do you know this? Where does the energy go?
What is the sign of ΔU, the change in ( internal ) energy for this process? Why?
What is the sign of ΔH for this reaction? Why?
Enthalpies of Reaction (Part 1)
•  Consider the following reaction, the combustion of methane:
CH4 (g) + 2 O2 (g) ! CO2 (g) + 2 H2O (l)
ΔH = –890 kJ/mol
The “per mole” in the units of ∆H for the reaction above means per mole of the reaction as it is written with the
Stoichiometric coefficients above
How can we define the number of moles of a reaction?
nrxn = Number of moles of reaction
=
If reaction goes to completion:
nrxn = Number of moles of reaction =
What is the expression for the heat absorbed or released in a given reaction?
Enthalpies of Reaction (Part 2)
•  Consider (again!) the following reaction, the combustion of methane:
CH4 (g) + 2 O2 (g) ! CO2 (g) + 2 H2O (l)
ΔH = –890 kJ/mol
How much heat will be released if 17.5g of CH4 reacts with an excess of oxygen gas?
How much heat will be released if this reaction produces 12.2 g of H2O?
Enthalpies of Reaction (Part 3)
•  Consider (yet again!) the following reaction, the combustion of methane:
CH4 (g) + 2 O2 (g) ! CO2 (g) + 2 H2O (l)
ΔH = –890 kJ/mol
Clicker Question:
How much heat will be released if 3 moles of CH4 (g) are mixed with 5 moles of O2 (g) and this reaction proceeds to completion?
1) 1780 kJ of heat released
2) 2225 kJ of heat released
3) 2670 kJ of heat released
4) 4450 kJ of heat released
5) 7120 kJ of heat released
Determine ΔH for the following reactions:
2 CH4 (g) + 4 O2 (g) ! 2 CO2 (g) + 4 H2O (l)
ΔH =
CO2 (g) + 2 H2O (l) ! CH4 (g) + 2 O2 (g)
ΔH=
Measuring the Enthalpies of Reaction: Constant Pressure Calorimetry
•  We can measure the enthalpy of reaction ( heat released per mole of reaction ) by using a technique called calorimetry.
This is a closed container specifically designed for this purpose. Here is an example of a high tech, expensive version:
Let’s see how calorimetry can be used to find ΔH:
100 ml solutions of 1.50 M NaCl and 1.00 M AgNO3 are both at a temperature
of 22.4°C. These solutions are mixed together in a coffee cup calorimeter, a
precipitate of solid silver chloride forms, and the resulting mixture reaches a
temperature of 30.2°C. Assume the density of the original solutions was 1.0 g/mL
and the specific heat of the entire mixture is 4.18 J/g°C.
Determine ΔH for this reaction.
Measuring the Enthalpies of Reaction: Constant Volume (Bomb) Calorimetry
• When measuring the enthalpies of reactions for solids and gasses we use a constant volume calorimeter.
This is called a bomb calorimeter.
A particular chemical reaction takes place in a bomb calorimeter, and the temperature of
the calorimeter decreases. What can you conclude about ∆H for this chemical reaction?
1) ∆H < 0
2) ∆H = 0
3) ∆H > 0
4) Impossible to tell with the information given
Enthalpies of Reaction From Calculation: Hess s Law
•  Hess’s law: The overall enthalpy of a process is the sum of the enthalpies from any series of reactions that, taken together, transform
the desired reactants into the desired products [Munowitz, 2000]
Suppose we are given the ΔH for the following reactions:
CH4 (g) + 2 O2 (g) ! CO2 (g) + 2 H2O (l)
H2O (g) ! H2O (l)
ΔH = – 890 kJ/mol
ΔH = – 44 kJ/mol
Can we determine the ΔH for the following reaction?
CH4 (g) + 2 O2 (g) ! CO2 (g) + 2 H2O (g)
ΔH = ???
Enthalpies of Reaction: Formation reactions
•  Enthalpies of every kind of reaction or process can be tabulated. For example
Enthalpy of neutralization
Enthalpy of combustion
Enthalpy of vaporization (boiling)
Enthalpy of fusion (melting)
Enthalpy of formation
When 1 mole of a substance is formed from its elements in their
standard states, then the enthalpy change for that reaction is called
the standard enthalpy of formation, ΔHof
The standard state is the form of an element (solid, liquid, or gas,
diatomic or monatomic etc.) found at 1 atm of pressure.
Any temperature may be used but most data is collected at 25°C.
Formation Reactions
Clicker Questions:
For each of the following reactions, identify if the reaction is a formation reaction:
A.
½ N2 (g) + 3/2 H2 (l) ! NH3 (g)
B.
½ N2 (g) + 3/2 H2 (g) ! NH3 (l)
C.
N2 (g) + 3 H2 (g) ! 2 NH3 (g)
D.
½ N2 (g) + ½ O2 (g) ! NO (g)
E.
N (g) + ½ O2 (g) ! NO (g)
F.
O2 (g) ! O2 (g)
What is the ∆H of the last reaction (reaction F) above?
What is the heat of formation of an element in its standard state?
Using Enthalpies of Formation
Use the tabulated data at right, plus the additional
information that the ΔHof for CH4 (g) is –74.8 kJ/mol,
to determine ∆H for the following reaction:
CH4 (g) + 2 O2 (g) ! CO2 (g) + 2 H2O (l)
Enthalpies of Reaction: Bombardier Beetle
•  Lets consider this very interesting bug…
Putting it All Together: Thermochemistry
If 247 grams of ice at -15°C are added to 300 grams of hot tea at 90°C, what will be
the final temperature of this mixture? (Tea has the same specific heat as water.)
ΔHfus= 6.01 kJ/mol
Specific heat of liquid = 4.184 J/g°C!
Problem: Gas Laws, Work, Heat, and Energy
(Chang, Essentials, 6.77)
A 1.00-mole sample of ammonia at 14.0 atm and 25°C in a cylinder fitted with a movable piston expands against a constant external!
pressure of 1.00 atm. At equilibrium, the pressure and volume of the gas are 1.00 atm and 23.5 L, respectively. Calculate the !
q, w and ΔU for this process. The specific heat of ammonia is 0.0258 J/g°C. !