Dissolution of NH4NO3 What does it mean?

Control of Chemical Reactions

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

The Laws of Thermodynamics 1st Law: Energy is Conserved 2nd 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

Which has the greatest entropy? Cl2(g) Cl(g) 2 Cl(g) H2O(s)

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

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 oC?

Calculating S All other reactions C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) -377 J/Kmol

Return to: What is entropy, anyway? Consider two atoms in a box: forced onto left side, viewed as quadrants

Consider two atoms in a box: forced onto left side, viewed as quadrants

Calculate entropy change for formation of rain: H2O(g)  H2O(l)

What types of reactions lead to increased entropy?

1 Inc Dec Can’t tell

2 Inc Dec Can’t tell

3 Inc Dec Can’t tell

4 Inc Dec Can’t tell

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

Question: How can rain form? H2O(g)  H2O(l) S = -118.89 J/K

Calculate Suniverse for H2O(g)  H2O(l) at: 25oC 130 oC

Entropy vs. Enthalpy Control of Reactions Which is more important at higher temperatures? Suniverse = Ssystem + Ssurroundings

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 Fe2O3(s) + 3 C(s)  4 Fe(s) + 3 CO2(g) H = +468 kJ S = +561 J/K G = H - TS What is G at 25 oC and at 1000 oC?

Breaking bonds releases energy: True False

Bond energies and enthalpy change review:

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

Temperature Domains and Reaction Favorability H + - + S -

A: High temperature Low temperature All temperatures No temperatures

A: High temperature Low temperature All temperatures No temperatures

A: High temperature Low temperature All temperatures No temperatures

A: High temperature Low temperature All temperatures No temperatures

Temperature free energy plots:

Temperature free energy plots:

Temperature free energy plots:

Temperature free energy plots:

2 Fe2O3(s) + 3 C(s)  4 Fe(s) + 3 CO2(g) H = +468 kJ S = +561 J/K In what temperature range will this reaction be favored? High or low? What temperature?

Free Energy of Formation: Only used at 25 oC

Free Energy of Formation: Only used at 25 oC