Entropy and Gibbs Energy

Remember from yesterday: From Hess’s Law, we got: H reaction = H products - H reactants Another Example: 2H2 (g) + 2CO2 (g)  2H2O (g) + 2CO (g) H reaction = [2(-241.8) + 2(-110.5)] – [2(0) + 2(-393.5)] H reaction = 82.4 kJ

Now on to Entropy

What is Entropy? Entropy is a measure of randomness or disorder of a system. It is symbolized by S. So, if there is a increase in disorder, S is positive Some Examples of an increase of entropy Diffusion – the process of dispersion Reduction of pressure of a gas Production of more gas in a chemical reaction Total number of moles of products is greater than the number of moles of reactants [Example: 2NI3 (s)  N2 (g) + 3I2 (g)]

Hess’s Law and Entropy Hess’s law can also be applied to entropy in the same way as it is applied to enthalpy. Therefore, S reaction = S products - S reactants Some entropy changes are located on page 359 and 360, but more are located in Appendix A on page 833. Example: Calculate the change in entropy for the following reaction using values from page 360 and that CH3OH has an S0 of 126.8 J/mol.K: CO (g) + 2H2 (g)  CH3OH (l)

How can we tell if a reaction will be spontaneous? Spontaneous means that a reaction will occur without continuous outside assistance. Well, if there is a decrease in enthalpy, H is negative. This helps it to be spontaneous. If there is an increase in entropy, S is positive, that helps too. A higher temperature also helps. Josiah Willard Gibbs came up with an answer by putting these facts together. He came up with what is now known as Gibbs energy, or free energy, or Gibbs free energy

Gibbs Energy Equation The equation is G = H - TS, where G is the change in Gibbs energy, H is the change in enthalpy, T is the temperature, and S is the change in entropy If G is a negative number, the reaction will be spontaneous. If G is a positive number, the reaction will be nonspontaneous. If G is a zero, the reaction is at equilibrium.

Relating Enthalpy and Entropy Changes to Spontaneity Spontaneous? Negative Positive Yes, at all temperatures Either Only if T < H/ S Only if T > H/ S Never

Example Using the following values, compute the G value for each reaction and predict whether they will occur spontaneously. Reaction H (kJ) Temperature S (J/K) 1 +95 298 K +45 2 -96.1 157 K +119 3 -266 400 C +54

Just Remember H reaction = H products - H reactants S reaction =  S products -  S reactants G reaction = G products - G reactants Just remember the coefficients in front of the compounds in the equation. Be careful of the units, H and G in the book are in kJ/mol, while S is in J/mol.K

Homework Page 371: 30, 31, 32, 33, 34, 41, 42