Entropy Chemical Reactions 2 nd Law of Thermodynamics

Entropy & Chemical Reactions

2 nd Law of Thermodynamics • A process will be spontaneous is the • entropy of the universe increases Now we will look at entropy regarding to chemical reactions

What is the sign for S? • N 2(g) + 3 H 2(g) 2 NH 3(g) • First look at the ______ • If all state are the same…then look at # ______

What is the sign for S? • 4 NH 3 + 5 O 2 4 NO + 6 H 2 O

Calculating S • Calculating S is just like calculating H • S° of any element or diatomic molecule is NOT zero. • You must look these up!

Example • Calculate S for the following reaction: • 2 Ni. S(s) + 3 O 2(g) 2 SO 2(g) + 2 Ni. O (s)

Example • Calculate S for the following reaction: • Al 2 O 3(s) + 3 H 2(g) 2 Al(s) + 3 H 2 O(g)

Gibbs Free Energy & Chemical Reactions • 1. 2. 3. You can calculate G in 3 ways… Like Hess’s Law Like H° With the equation G = H - T S

Example • Calculate H, S, & G at 25°C using the • following data… 2 SO 2 + O 2 2 SO 3 Substance H (KJ/mol) S (J/K mol) SO 2 -297 248 SO 3 -396 257 O 2 0 205

Calculate G • Using the following data at 25°C • • Cdiamond + O 2(g) CO 2 (g) G = -397 KJ Cgraphite + O 2(g) CO 2 (g) G = -394 KJ Calculate G for the reaction: C diamond C graphite

Calculating G • Methanol is a high octane fuel used in high performance racing engines. Calculate G for the • • following reaction 2 CH 3 OH(g) + 3 O 2(g) 2 CO 2(g) + 4 H 2 O(g) Given the following free energies of formation: Substance G°(KJ/mol) CH 3 OH (g) -163 O 2 (g) 0 CO 2 (g) -394 H 2 O (g) -229