1. Topic 15 – Year 2
Energetics HL

2. The standard enthalpy of reaction (DH0 ) is the enthalpy of a reaction carried out at 1 atm.
rxn
aA + bB cC + dD
DH0
rxn
dDH0 (D) f
cDH0 (C) = [ + ] -
bDH0 (B)
aDH0 (A)
DH0
rxn
nDH0 (products) f = S
mDH0 (reactants) -
Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.
(Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end.)
6.6

3. C (graphite) + 1/2O2 (g) CO (g)
CO (g) + 1/2O2 (g) CO2 (g)
C (graphite) + O2 (g) CO2 (g)
6.6

4. Calculate the standard enthalpy of formation of CS2 (l) given that:
C(graphite) + O2 (g) CO2 (g) DH0 = kJ
rxn
S(rhombic) + O2 (g) SO2 (g) DH0 = kJ
rxn
CS2(l) + 3O2 (g) CO2 (g) + 2SO2 (g) DH0 = kJ
rxn
1. Write the enthalpy of formation reaction for CS2
C(graphite) + 2S(rhombic) CS2 (l)
2. Add the given rxns so that the result is the desired rxn.
rxn
C(graphite) + O2 (g) CO2 (g) DH0 = kJ
2S(rhombic) + 2O2 (g) SO2 (g) DH0 = x2 kJ
rxn +
CO2(g) + 2SO2 (g) CS2 (l) + 3O2 (g) DH0 = kJ
rxn
C(graphite) + 2S(rhombic) CS2 (l)
DH0 = (2x-296.1) = 86.3 kJ
rxn
6.6

5. 2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)
Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is kJ/mol.
2C6H6 (l) + 15O2 (g) CO2 (g) + 6H2O (l)
DH0
rxn
nDH0 (products) f = S
mDH0 (reactants) -
DH0
rxn
6DH0 (H2O) f
12DH0 (CO2) = [ + ] -
2DH0 (C6H6)
DH0
rxn =
[ 12x– x–285.8 ] – [ 2x49.04 ] = kJ kJ
2 mol
= kJ/mol C6H6
6.6

6. Electrostatic (Lattice) Energy
Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.
Q+ is the charge on the cation
E = k
Q+Q- r
Q- is the charge on the anion
r is the distance between the ions
cmpd
lattice energy
MgF2
MgO
2957
3938
Q= +2,-1
Q= +2,-2
Lattice energy (E) increases as Q increases and/or
as r decreases.
LiF
LiCl
1036
853
r F- < r Cl-
9.3

7. Born-Haber Cycle for Determining Lattice Energy
DHoverall = DH1 + DH2 + DH3 + DH4 + DH5 o
9.3

8. 9.3

9. Spontaneous Physical and Chemical Processes
A waterfall runs downhill
A lump of sugar dissolves in a cup of coffee
At 1 atm, water freezes below 0 0C and ice melts above 0 0C
Heat flows from a hotter object to a colder object
A gas expands in an evacuated bulb
Iron exposed to oxygen and water forms rust
spontaneous
nonspontaneous
18.2

10. Introduction to Entropy
Spontaneous physical changes are easy to predict. A dropped glass will fall to the ground once it is released. The reverse of a spontaneous event, like water flowing up a waterfall, will not occur except perhaps in the world of special effects for movies. The probability of finding a highly ordered situation (e.g., a deck of cards in order) is much lower than a highly disordered one (in which the cards are random). Chemical reactions or events are driven toward spontaneity by the energetics of the process. Water will boil if heated to 100 °C at 1 atm. Gas particles in one chamber will flow into an empty one (see figure: (a) is spontaneous while (b) is not).

11. Does a decrease in enthalpy mean a reaction proceeds spontaneously?
Spontaneous reactions
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH0 = kJ
H+ (aq) + OH- (aq) H2O (l) DH0 = kJ
H2O (s) H2O (l) DH0 = 6.01 kJ
NH4NO3 (s) NH4+(aq) + NO3- (aq) DH0 = 25 kJ
H2O
18.2

12. Entropy (S) is a measure of the randomness or disorder of a system.
DS = Sf - Si
If the change from initial to final results in an increase in randomness
Sf > Si
DS > 0
For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state
Ssolid < Sliquid << Sgas
H2O (s) H2O (l)
DS > 0
18.3

13. Processes that lead to an increase in entropy (DS > 0)
18.2

14. (a) Condensing water vapor
How does the entropy of a system change for each of the following processes?
(a) Condensing water vapor
Randomness decreases
Entropy decreases (DS < 0)
(b) Forming sucrose crystals from a supersaturated solution
Randomness decreases
Entropy decreases (DS < 0)
(c) Heating hydrogen gas from 600C to 800C
Randomness increases
Entropy increases (DS > 0)
(d) Subliming dry ice
Randomness increases
Entropy increases (DS > 0)
18.3

15. energy, enthalpy, pressure, volume, temperature , entropy
State functions are properties that are determined by the state of the system, regardless of how that condition was achieved.
energy, enthalpy, pressure, volume, temperature
, entropy
Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.
18.3

16. First Law of Thermodynamics
Energy can be converted from one form to another but energy cannot be created or destroyed.
Second Law of Thermodynamics
The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.
Spontaneous process:
DSuniv = DSsys + DSsurr > 0
Equilibrium process:
DSuniv = DSsys + DSsurr = 0
18.4

17. Entropy Changes in the System (DSsys)
The standard entropy of reaction (DS0 ) is the entropy change for a reaction carried out at 1 atm and 250C.
rxn
aA + bB cC + dD
DS0
rxn
dS0(D)
cS0(C) = [ + ] -
bS0(B)
aS0(A)
DS0
rxn
nS0(products) = S
mS0(reactants) -
What is the standard entropy change for the following reaction at 250C? 2CO (g) + O2 (g) CO2 (g)
S0(CO) = J/K•mol
S0(CO2) = J/K•mol
S0(O2) = J/K•mol
DS0
rxn
= 2 x S0(CO2) – [2 x S0(CO) + S0 (O2)]
DS0
rxn
= – [ ] = J/K•mol
18.4

18. Entropy Changes in the System (DSsys)
When gases are produced (or consumed)
If a reaction produces more gas molecules than it consumes, DS0 > 0.
If the total number of gas molecules diminishes, DS0 < 0.
If there is no net change in the total number of gas molecules, then DS0 may be positive or negative BUT DS0 will be a small number.
What is the sign of the entropy change for the following reaction? 2Zn (s) + O2 (g) ZnO (s)
The total number of gas molecules goes down, DS is negative.
18.4

19. For a constant-temperature process:
Gibbs Free Energy
Spontaneous process:
DSuniv = DSsys + DSsurr > 0
Equilibrium process:
DSuniv = DSsys + DSsurr = 0
For a constant-temperature process:
Gibbs free energy (G)
DG = DHsys -TDSsys
DG < The reaction is spontaneous in the forward direction.
DG > The reaction is nonspontaneous as written. The
reaction is spontaneous in the reverse direction.
DG = The reaction is at equilibrium.
18.5

20. DG0 of any element in its stable form is zero.
The standard free-energy of reaction (DG0 ) is the free-energy change for a reaction when it occurs under standard-state conditions.
rxn
aA + bB cC + dD
DG0
rxn
dDG0 (D) f
cDG0 (C) = [ + ] -
bDG0 (B)
aDG0 (A)
DG0
rxn
nDG0 (products) f = S
mDG0 (reactants) -
Standard free energy of formation (DG0) is the free-energy change that occurs when 1 mole of the compound is formed from its elements in their standard states. f
DG0 of any element in its stable form is zero. f
18.5

21. 2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)
What is the standard free-energy change for the following reaction at 25 0C?
2C6H6 (l) + 15O2 (g) CO2 (g) + 6H2O (l)
DG0
rxn
nDG0 (products) f = S
mDG0 (reactants) -
DG0
rxn
6DG0 (H2O) f
12DG0 (CO2) = [ + ] -
2DG0 (C6H6)
DG0
rxn =
[ 12x– x–237.2 ] – [ 2x124.5 ] = kJ
Is the reaction spontaneous at 25 0C?
DG0 = kJ
< 0
spontaneous
18.5

22. DG = DH - TDS
18.5