Unit 4 Thermodynamics
By: Elliot Prizant and Zack 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)]
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
 Endothermic reactions take energy from the surrounding system to start the reaction
 Exothermic reactions release energy after forming products
Energy and Changes of State:
 Heat of Fusion(C): quantity of heat required to melt a substance
 Heat of Vaporization(C): quantity of heat required to vaporize a substance
 Takes more energy to vaporize than to melt (need to break more bonds to get gas)
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 (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)
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.
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)
Spontaneity:
 If a reaction is spontaneous, then the reaction is product favored.
Is the reaction spontaneous? T =∆H / ∆S
∆S < 0
∆S > 0
∆H > 0
Never Spontaneous
Maybe (at high temps)
∆H < 0
Maybe (at low temps)
Always Spontaneous
Gibbs Free Energy (G):
∆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
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