Unit 4
Thermodynamics
By: Elliot Prizant and Zak Abecassis
Thermodynamics
 The study of the laws that govern the
conversion of energy from one form to
another, the direction in which heat will
flow, and the availability of energy to do
work.
Specific Heat Capacity (C)
 This is the heat required to produce a given temperature
change per gram of material.
 Specific heat capacity (C) = Quantity of heat supplied
(mass of object) (temperature change)
 C(J/g(K) = ___q(J)__
[m(g)] x [ΔT(K)]
The average specific heat capacity of a human
body is approximately 3500 J.kg-1.K-1. A
certain Japanese sumo wrestler has a body
mass of 200 kg. What is the wrestler's heat
capacity?
The heat capacity of a body is obtained by
multiplying its specific heat capacity by its
mass in kilograms. In this case, the heat
capacity of the wrestler is:
3500 (J.kg-1.K-1) x 200 (kg) = 700000
(7.0x105) J.K-1
Heat Transferred (q)
 q = mCΔT Units: mass in grams, C is the
specific heat capacity in J/g(K), ΔT is the
temperature final minus initial in Kelvins.
 +q means heat is transferred to a substance.
 -q means heat is transferred from the system =
q transferred into the surroundings
Bond Energies
 q = Σ Reactants Bond Energy – Σ
Products Bond Energy
 Potential energy of the elemental state is
ZERO
 Breaking bonds requires energy to be
added to the system
 Forming bonds requires energy to be
released from the system
Bond Energies
DE= +(reactant bonds)-(product bonds)
EX: 2N02 + H20 →HNO3 + HNO2
DE = (4 N-O + 2 H-O) – (5 N-O + 2 N-H)
DE =(4(201 KJ/mol) + 2(463 KJ/mol)) – (5(201 KJ/mol) +
2(391 KJ/mol))
DE = -57 KJ/mol
(note: some N-O bonds are probably double)
Energy and Changes of State
 Heat of Fusion (Hf): quantity of heat required to
melt a substance
 Heat of Vaporization (Hv): quantity of heat
required to vaporize a substance
 Takes more energy to vaporize than to melt
(need to break more bonds to get gas)
 : q=mHf or q=mHv
Hf=enthalpy of fusion
Hv=enthalpy of vaporization units: J/g, J/mol
Energy and Changes of
State
Calorimetry




mmetalcΔT = mwatercΔT
mmetalc(Tf-Ti) = mwaterc(Tf-Ti)
c for water is 4.18 J/g*C
q reaction = -(q water + q bomb)
Enthalpy
 Enthalpy (H): Heat transferred into or out of a
system at a constant pressure.
 ΔH = Σ Product Potential Energy - Σ Reactant
Potential Energy Units: Joules (J) or
Kilojoules (KJ).
 Negative ΔH is Exothermic/Positive ΔH is
Endothermic
 Enthalpy change for a reaction = ΔHºrxn = Σ
ΔHºf products - Σ ΔHºf reactants
 q = n(ΔH) or q = m(ΔH)
Enthalpy
 DHrxn=S DHproducts – SDHreactants
 Ex: C3H8 + 5 O2 = 3 CO2 + 4
H2O
 DHrxn=[3(-393.5 KJ/mol)+4(-241.8 KJ/mol)] –
[(-103.8 KJ/mol)]

DHrxn=-2043.91 KJ
Hess’s Law
 Hess’s Law: If a reaction is the sum of
two or more other reactions, then ΔH for
the overall process must the sum of ΔH
values of the constituent reactions.
 Reversing an equation, causes the sign
on ∆H to change.
 Multiplying an equation, multiply ∆H by
same number
Hess’s Law





PbS (s) + 3/2 O2 → PbO (s) + SO2 (g)
ΔH = -413.7 kJ
PbO (s) + C (s) → Pb (s) + CO (g)
ΔH = +106.8 kJ
PbS (s) + C (s) + 3/2 O2 (g) → Pb (s) + CO (g) + SO2
(g) ΔH = - 325.3 kJ
Entropy
 Entropy (S): A measure of randomness in a
system
 In order of greatest to least random
Gases>Liquids>Solids.
 ∆S = Σ S (products( – Σ S (reactants) Units:
J/K(mol)
 Increasing # of moles of gas in reaction from
reactants to products makes more entropy and
vice versa
 Product-favored reactions have higher entropy
and vice versa
Entropy
Spontaneity
 If a reaction is spontaneous, then the
reaction is product favored.
 Is the reaction spontaneous? T =∆H / ∆S
 ∆H > 0
 ∆H < 0
∆S < 0
Never Spontaneous
Maybe (at low temps)
∆S > 0
Maybe (at high temps)
Always Spontaneous
Gibbs Free Energy
 ∆G = ∆H - T∆S Units: T is in Kelvin, S is in J/(mol*K)
 A reaction is SPONTANEOUS when ∆G is NEGATIVE.
 A reaction is NOT SPONTANEOUS when ∆G is
POSITIVE.
 A reaction is at Equilibrium when ∆G is zero.
 ∆G = ∆G˚ + RT lnQ
 ∆G˚ = -RT lnK (at equilibrium)
 R=8.314 J/(mol*K) T=Kelvin
 K is the Thermodynamic Equilibrium Constant
Gibbs Free Energy
 When ∆G˚ < 0 and K > 1 Products are
favored
 When ∆G˚ = 0 and K = 1 It is at
equilibrium (Rare)
 When ∆G˚ > 0 and K < 1 reactants are
favored
THE END
Sadi Carnot (1796-1832): the "father" of
thermodynamics