Control of Chemical Reactions Thermodynamic Control of Reactions

Control of Chemical Reactions

Thermodynamic Control of Reactions Enthalpy Bond Energies Entropy Randomness – Forming stronger bonds favors reactions. – Molecules with strong bonds are more stable. – Reactions that increase randomness are favored. – Forming gases favors reactions.

Remember Enthalpy? Enthalpy of formation for elements in their natural states = 0.

H 2 O(g) Gaseous water -242

The Laws of Thermodynamics 1 st Law: Energy is Conserved 2 nd Law: Any “spontaneous” process leads to an increase in entropy of the universe.

Entropy A measure of randomness. Units of J/K.

Trends in entropy

Entropy Change • First define: System Surroundings

Entropy Change For the System If the system gets more random, S is positive. (Favors the reaction) If the system gets more ordered, S is negative. (Disfavors the reaction)

Calculating S Chemical Reactions C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O(l)

Calculating S Special Case: Phase Changes Heat of fusion (melting) of ice is 6000 J/mol. What is the entropy change for melting ice at 0 o. C?

Calculate entropy change formation of rain: H 2 O(g) H 2 O(l)

What types of reactions lead to increased entropy?

Entropy vs. Enthalpy Control of Reactions Second law of thermodynamics: Suniverse = Ssystem + Ssurroundings

Question: How can rain form? H 2 O(g) H 2 O(l)

Calculate Suniverse for H 2 O(l) H 2 O(g) at: 90 o. C 110 o. C

Putting S, H and Temperature Together Gibb’s Free Energy: G = H - T S When G is negative, reaction is favored. When G is positive, reaction is disfavored.

2 Fe 2 O 3(s) + 3 C(s) 4 Fe(s) + 3 CO 2(g) H = +468 k. J S = +561 J/K G = H - T S What is G at 25 o. C and at 1000 o. C?

Enthalpy vs. Entropy Control of Reactions G = H - T S At high temperatures: At low temperatures:

Temperature Domains and Reaction Favorability H + S + - -

2 Fe 2 O 3(s) + 3 C(s) 4 Fe(s) + 3 CO 2(g) H = +468 k. J S = +561 J/K In what temperature range will this reaction be favored? High or low? What temperature?

Free Energy vs. Temperature Curves

Catalytic Converters Nitrogen oxides cause smog. N 2(g) + O 2(g) 2 NO(g) H = +180 k. J S = +25 J/K

Free Energy of Formation: Only used at 25 o. C 2 Ba. O(s) + C(s) 2 Ba(s) + CO 2(g)