1. Thermochemistry Chapter 6
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2. Energy is the capacity to do work.
Radiant energy comes from the sun and is earth’s primary energy source
Thermal energy is the energy associated with the random motion of atoms and molecules
Chemical energy is the energy stored within the bonds of chemical substances
Nuclear energy is the energy stored within the collection of neutrons and protons in the atom
Potential energy is the energy available by virtue of an object’s position

3. Temperature v. Heat
Heat is the transfer of thermal energy between two bodies that are at different temperatures.
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
In other words…
Temperature is a measure of the KE of the system, related to the random motions of particles.
Heat involves a transfer of energy between two objects due to a difference in temperature.

4. Thermochemistry is the study of heat change in chemical reactions.
Dropping Lead Shot
Measure the temperature of the lead shots in your bag
Drop the bag from about 2m high to the ground
Repeat for approximately 2 minutes
Check the temperature of the lead shots again.

5. Thermochemistry is the study of heat change in chemical reactions.
The system is the specific part of the universe that is of interest in the study.
open
closed
isolated
Exchange:
mass & energy
energy
nothing

6. 2H2 (g) + O2 (g) 2H2O (l) + energy
Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.
2H2 (g) + O2 (g) H2O (l) + energy
H2O (g) H2O (l) + energy
Endothermic process is any process in which heat has to be supplied to the system from the surroundings.
energy + 2HgO (s) Hg (l) + O2 (g)
energy + H2O (s) H2O (l)

7. Schematic of Exothermic and Endothermic Processes

8. Thermodynamics is the scientific study of the interconversion of heat and other kinds of energy.
State functions are properties that are determined by the state of the system, regardless of how that condition was achieved.
energy
, pressure, volume, temperature
Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.

9. STATE FUNCTION

10. Suppose we have a gas at 2 atm, 300K, and 1L
Suppose we have a gas at 2 atm, 300K, and 1L. At constant temperature, we decrease the pressure to 1 atm. What happens to the volume?
DV = Vfinal – Vinitial
DV = 2L – 1L
DV = 1L
What if we first doubled the pressure first and then decreased the pressure to ¼ its value. What is the volume now?
DE = Efinal - Einitial
DP = Pfinal - Pinitial
DV = Vfinal - Vinitial
DT = Tfinal - Tinitial
DV = 1L

11. Law of Conservation of Energy!!
First law of thermodynamics – energy can be converted from one form to another, but cannot be created or destroyed.
Law of Conservation of Energy!!
C3H8 + 5O CO2 + 4H2O
Exothermic chemical reaction!
Chemical energy lost by combustion = Energy gained by the surroundings
system
surroundings

12. We cannot calculate the total energy of a system at any given point
We cannot calculate the total energy of a system at any given point. We can only calculate the change in energy.
DEsystem + DEsurroundings = 0 or
DEsystem = -DEsurroundings
C3H8 + 5O CO2 + 4H2O
Exothermic chemical reaction!

13. Another form of the first law for DEsystem
DE = q + w
DE is the change in internal energy of a system
q is the heat exchange between the system and the surroundings
w is the work done on (or by) the system

14. The internal energy of the balloon hasn’t changed
The internal energy of the balloon hasn’t changed. When it expands, the system does 10J of work. How much heat is added or taken away?

15. Work is not a state function.
Work Done On the System
w = F x d
w = -PDV when a gas expands against a constant external pressure
P x V = x d3 = F x d = w F d2
DV > 0
-PDV < 0
wsys < 0
initial
final
Work is not a state function.
Dw = wfinal - winitial

16. Why is work not a state function?
A sample of nitrogen gas expands in volume from 1.6 L to 5.4 L at constant temperature. What is the work done in joules if the gas expands (a) against a vacuum and (b) against a constant pressure of 3.7 atm?
w = -P DV
(a)
DV = 5.4 L – 1.6 L = 3.8 L
P = 0 atm
W = -0 atm x 3.8 L = 0 L•atm = 0 joules
(b)
DV = 5.4 L – 1.6 L = 3.8 L
P = 3.7 atm
w = -3.7 atm x 3.8 L = L•atm
w = L•atm x
101.3 J
1L•atm
= J
Why is work not a state function?

17. Problem 6.16
A gas expands in volume from 26.7 mL to 89.3 mL at constant temperature. Calculate the work done (in joules) if the gas expands (a) against a vacuum (b) against a constant pressure of 1.5 atm (c) against a constant pressure of 2.8 atm.

18. Work done when a gas is compressed in a cylinder like the one below is 462 J. During this process, there is a heat transfer of 128 J from the gas to the surroundings. Calculate the energy change for this process.
DE = q + w
DE= -128 J J
DE= 334 J

19. Problem 6.18
The work done to compress a gas is 74 J. As a result, 26 J of heat is given off to the surroundings. Calculate the change in energy of the gas.

20. Enthalpy and the First Law of Thermodynamics
DE = q + w
At constant pressure:
q = DH and w = -PDV
DE = DH - PDV
DH = DE + PDV

21. Is the following reaction endothermic or exothermic?
CO2(g) + 2H2O(l)  CH4(g) + 2O2(g) ΔH = kJ/mol 2SO2(g) + O2(g)  2SO3(g) ΔH = kJ/mol CH4(g)+ 2O2(g)  CO2(g) + 2H2O(g) H2O(l)  H2O(s)

22. DH = H (products) – H (reactants)
Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.
DH = H (products) – H (reactants)
DH = heat given off or absorbed during a reaction at constant pressure
Hproducts > Hreactants
Hproducts < Hreactants
DH > 0
DH < 0

23. Thermochemical Equations
Is DH negative or positive?
System absorbs heat
Endothermic
DH > 0
6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm.
H2O (s) H2O (l)
DH = 6.01 kJ

24. Thermochemical Equations
Is DH negative or positive?
System gives off heat
Exothermic
DH < 0
890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm.
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l)
DH = kJ

25. Thermochemical Equations
The stoichiometric coefficients always refer to the number of moles of a substance
H2O (s) H2O (l)
DH = 6.01 kJ
If you reverse a reaction, the sign of DH changes
H2O (l) H2O (s)
DH = kJ
If you multiply both sides of the equation by a factor n, then DH must change by the same factor n.
2H2O (s) H2O (l)
DH = 2 x 6.01 = 12.0 kJ

26. Thermochemical Equations
The physical states of all reactants and products must be specified in thermochemical equations.
H2O (s) H2O (l)
DH = 6.01 kJ
H2O (l) H2O (g)
DH = 44.0 kJ
How much heat is evolved when 266 g of white phosphorus (P4) burn in air?
P4 (s) + 5O2 (g) P4O10 (s) DH = kJ
1 mol P4
123.9 g P4 x
3013 kJ
1 mol P4 x
266 g P4
= 6470 kJ

27. Problem 6.26
Determine the amount of heat (in KJ) given off when 1.26x104 g of NO2 are produced according to the equation 2NO(g) + O2(g) 2NO2(g) ∆H = kJ

28. A Comparison of DH and DE
2Na (s) + 2H2O (l) NaOH (aq) + H2 (g) DH = kJ
At 25 oC and 1 atm
DE = DH - PDV
PDV = 1 atm x 24.5 L = 2.5 kJ
DE = kJ – 2.5 kJ = kJ

29. Example 6.4
Calculate the change in internal energy when 2 mol of CO are converted to 2 moles of CO2 at 1 atm and 25°C 2CO(g) + O2(g)  2CO2(g) ΔH = kJ

30. Problem 6.28
Consider the reaction H2(g) + Cl2(g) 2HCl(g) ∆H = kJ If 3 moles of H2 react with 3 moles of Cl2 to form HCl, calculate the work done (in joules) against a pressure of 1.0 atm at 25ºC. What is ∆E for this reaction? Assume the reaction goes to completion.