1. Quantitative Analysis
Energy is the capacity to do work (w) and exchange heat (q): E = q + w
A system can do work (-w) or have work done on it (+w). In chem, usually see with expansion and compression, respectively.
A system can gain heat (+q, endo) or lose heat (-q, exo)
Units of E = J or kJ; Cal or cal
1 J = 1 kgm cal = J
s Cal = 1000 cal

2. Energy as a state function
State Function: Independent of pathway, depends on initial and final state only
Path 1 and path 2 have different values of q and w, but overall E is the same

3. A (s)  A (l)
If this process involves 15.6 kJ of heat and 1.4 kJ of work done on the system, calc E.

4. Relate Energy to Enthalpy
E = q + w
Heat energy is related to enthalpy
H = qP
At constant pressure, q = qP and w = 0
E = qP + 0 = H
H is a state function (see with reaction profile) and an extensive property (see this with units, next slide)

5. Enthalpy as Extensive Property
Units: kJ/mol or q/n
2 A + 3 B  C H = 100 J
100 J needed for 2 mol A to react with 3 mol B to make 1 mol C
H wrt A = 50 J; wrt B = 33 J; wrt C = 100 J
What is H for 4 A + 6 B  2 C
C  2 A + 3 B

6. Problem #1
Write a thermochemical equation for A  B if decomposition of 1 mol A requires 10 kJ energy.
How much energy is required for 2 mol A? Would thermochem eqn change?
What about B  A? What is the thermochem eqn?

7. Problem #2
When 1 mol liquid nitroglycerine (C3H5(NO3)3) decomposes, it causes an explosion that releases 5720 kJ heat.
Is rxn endo or exo? + or – q?
Calculate the amount of heat released when 4 mol of nitroglycerine decomposes.

8. Problem #3
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (l) H = -890 kJ Is rxn endo or exo? Write the thermochem eqn another way. What does -890 kJ mean in words? How much heat is released if 4 g CH4 combusted?

9. 4 Common Types of H Hcomb = heat of combustion: 1 mol combusted
Hf = heat of formation: form 1 mol from elements
Hfusion = heat of fusion: melt 1 mol
Hvap = enthalpy of vaporization (l to g): 1 mol vaporizes
Ex of each? Sign + or -?

10. Enthalpy as State Function/Hess’s Law
Because H is a state function, the H of R  P will be the same no matter the pathway
Path Path 2
R  A same NIE R  A
A  P same H A  B
different rxn profile B  P
For a series of rxns, overall HNIE is sum of H for each step
recall: if reverse eqn, flip sign of H
if mult coeff by factor, mult H by same factor

11. Hess Problem #1
What is H for the following reaction given the info below?
Want: 4 Al (s) + 3 MnO2 (s)  2 Al2O3 (s) + 3 Mn (s)
Know: 2 Al (s) + 3/2 O2 (g)  Al2O3 (s) H = kJ
Mn (s) + O2 (g)  MnO2 (s) H = -521 kJ

12. Hess Problem #2
The bombardier beetle uses an explosive discharge as a defensive mechanism. The chemical reaction involved is the oxidation of hydroquinone by hydrogen peroxide to produce quinone and water: C6H4(OH)2 (aq) + H2O2 (aq)  C6H4O2 (aq) + 2 H2O (l) Calculate H for the reaction from the following info C6H4(OH)2 (aq)  C6H4O2 (aq) + H2 (g) H = kJ H2 (g) + O2 (g)  H2O2 (aq) H = kJ H2 (g) + ½ O2 (g)  H2O (g) H = kJ H2O (g)  H2O (l) H = kJ

13. 2nd Way to Calculate H (1st way = Hess’s Law)
Note: 3rd way is calorimetry (after midterm)
For reactions that are too slow to determine via calorimetry, use Hf values (can do this for S and G too) to determine Hrxn
Use standard heats of formation, Hfo (1 atm and 25oC)
Elements have Hfo = 0 kJ/mol; Cmpds = appendix
Hrxno = nHfo Products -  nHfo Reactants

14. Hrxn = nHfo P -  nHfo R
What is Hrxn for combustion of methane?
Need balanced equation
What are units? Does sign make sense?
Can do the same for Srxn or Grxn
Srxn = nSo P -  nSo R
Grxn = nGfo P -  nGfo R

15. Calculate S Can use equation on previous slide
Can use dimensional analysis (units = J/K, where temp is absolute not change)
S = q = n H
T T
Calc S when 68.3 g ethanol (g) condenses at n-bp.
n-bp C2H5OH = 78.3oC
Hvapo = kJ/mol

16. Calculate Go Can use sum equation and standard values
Go = Ho - T So
When we do equilibrium, we learn other ways using non standard G and equilibrium constant, K
Calculate Go for a rxn whose Ho = 25.6 kJ and So = 132 J/K at 25.0oC
Is reaction spont/favorable always, never, sometimes? If sometimes, what conditions are favorable?