8/3/2019 Thermodynamics Overall (Brief)

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1412 Lecture notes – Chapter 15 - Thermodynamics

Thermodynamics 

• First Law of Thermodynamics = energy is neither created nor destroyed

Enthalpy: ∆H

• +∆H, endothermicfeels cold because heat is converted into chemical energy

lower energy reactants higher energy products

•  – ∆H, exothermicfeels hot because chemical energy is converted into heathigher energy reactants lower energy products

• ∆Hrxn = Σ(n×∆Hf)prod – Σ(n×∆Hf)react 

• CaCO3(s)  CaO(s) + CO2(g)  ∆Hrxn = _____________ 

• 2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(g) ∆Hrxn = _____________ 

• How much heat is produced by the decomposition of 2 moles of CaCO3?

• How much heat is produced by the combustion of 5 grams of C2H2?

Entropy: ∆S

• Change in entropy (∆S) is a measure of change in disorder for a reaction.

• Less disordered reactants more disordered products +∆S

• More disordered reactants less disordered products – ∆S

• Second Law of Thermodynamics = the universe favors disorder.

• Third Law of Thermodynamics = at zero Kelvin, entropy (S) is equal to zero.

• ∆Srxn = Σ(n×∆Sf)prod – Σ(n×∆Sf)react 

• 2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(g) ∆Srxn = _____________ 

Some values taken from appendix K

Substance ∆Hf0

(kJ/mol) ∆Sf0

(J/mol) ∆Gf0

(kJ/mol) CaCO3(s) –1207 92.9 –1129

CaO(s) –635.5 40 –604.2

CO2(g) –393.5 213.6 –394.4

C2H2(g) 226.7 200.8 209.2

O2(g) 0 205.0 0

H2O(g) –241.8 188.7 –228.6

8/3/2019 Thermodynamics Overall (Brief)

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1412 Lecture notes – Chapter 15 - Thermodynamics

Spontaneity - Will a reaction occur?

• Exothermic reactions (-∆H) are favored over endothermic reactions because the total potentialenergy is lowered.

• Increasing disorder (+∆S) is favored over decreasing disorder.

Gibbs Free Energy: ∆G 

• Gibbs free energy (∆G) tells whether a reaction (as written) is spontaneous.

• ∆Grxn = Σ(n×∆Gf)prod – Σ(n×∆Gf)react 

• ∆Grxn = ∆Hrxn – T∆Srxn temperature must be in Kelvin 

• ∆Grxn < 0 means reaction is spontaneous (product favored)

• ∆Grxn > 0 means reverse reaction is spontaneous (reactant favored)

• ∆Grxn = 0 means forward and reverse are equally favored (equilibrium)

• 2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(g) ∆Grxn = _____________ 

Hess’s Law

• C2H4 + H2O C2H5OH ∆H = ___________ 

Also given:

C2H5OH + 3O2  2CO2 + 3H2O ∆H = -1367 kJ/mol

C2H4 + 3O2  2CO2 + 2H2O ∆H = -1411 kJ/mol

Internal Energy and Work

• ∆E = q + w w = -P∆V = -∆nRT R = 8.314 J/mol·K ∆H = qp 

•  –w = work done by the system, +w = work done on the system

• H2O( ℓ  ) H2O(g)  ∆Hvap = 40.7 kJ/mol ∆E = ______________ 

Practice Problems: 11, 14, 30, 34, 36, 88c-f, 92, 94, 95, 96, 108