Chapter 19: Thermodynamics The Essentials

Join me to learn about: SpONTANEITY, Entropy, and Free Energy! Learn how to… Learn how to… –Predict whether a reaction will happen –How much work a reaction can do

Spontaneous reactions occur everywhere…all the time! From a cracked egg, From a cracked egg, To an erupting volcano, To an erupting volcano, To melting ice cream… To melting ice cream… Every Spontaneous Reaction is said to be IRREVERSIBLE! Every Spontaneous Reaction is said to be IRREVERSIBLE!

Ice melts to water.. Ice melts to water.. …and water freezes back into ice. …and water freezes back into ice. How could this be called “irreversible”? How could this be called “irreversible”?

We’re looking at 2 processes: Ice to water: Ice to water: –Start with ice at -10 o C –Add heat –End with water at +10 o C –This is a spontaneous, irreversible process.

Water to ice: Water to ice: –Start with water at +10 o C –Remove heat –End with ice at -10 o C …also a spontaneous, irreversible process. So, how do we set up a reversible process with water and ice? So, how do we set up a reversible process with water and ice?

A reversible process happens at the same TEMPERATURE! Put the water and ice together Put the water and ice together –Water turns into ice at 0 o C –Ice turns into water at 0 o C TO SUM IT UP SO FAR: Reversible processes can’t change in temperature. Irreversible processes do.

Something you should know… Spontaneous reactions spread things out. Spontaneous reactions spread things out. –Atoms may be spread out. –Energy (like heat) may be spread out. This spreading out is called “ entropy ” This spreading out is called “ entropy ” –Entropy is defined by AP as “disorder”. –The sign for entropy is “ S ” or “ ∆S ”

How can we relate “S” to reversible and irreversible processes? For reversible reactions: For reversible reactions: –There is no change in disorder. –There is no change in entropy. –∆S = 0 For irreversible reactions: For irreversible reactions: –There is an increase in disorder. –There is an increase in entropy. –∆S is positive or greater than zero (∆S > 0).

How do we find entropy? Change in Entropy = Heat / Temperature Change in Entropy = Heat / Temperature –∆S = q (in Joules) / T (in Kelvins)

Yes, a gas is more disordered than a liquid, which is more disordered than a solid! Think of how the atoms are arranged: Think of how the atoms are arranged: –A gas has its atoms bouncing off the walls. –A liquid has its atoms sliding around. –A solid has its atoms vibrating in place. Therefore: S gas > S liquid > S solid Therefore: S gas > S liquid > S solid

The 3 rd Law of Thermodynamics The entropy of a pure crystal at 0 Kelvins is... The entropy of a pure crystal at 0 Kelvins is... ZERO!!! ZERO!!!

The 2nd Law of Thermodynamics: The entropy of the universe is always increasing. The entropy of the universe is always increasing. –Everything in the universe is spreading out, –Which leads to more disorder, more entropy –If something appears to have become more ordered, then a lot of energy/heat has been spread out, which leads to positive S.

Entropy values can be added up Just like enthalpy values (ΔH) and Hess’ Law, you can subtract the values of a reaction to get the overall entropy change. Just like enthalpy values (ΔH) and Hess’ Law, you can subtract the values of a reaction to get the overall entropy change. ΔS products - ΔS reactants = ΔS reaction ΔS products - ΔS reactants = ΔS reaction Do you notice a pattern? Do you notice a pattern?

How does this relate to chemistry? It helps to predict whether a reaction will happen, whether it is spontaneous. It helps to predict whether a reaction will happen, whether it is spontaneous. How can we make such a prediction? How can we make such a prediction? –We use an extremely important equation.

ΔG = ΔH - TΔS What the heck is ΔG? What the heck is ΔG? –This is called free-energy, after a man named Dr. Gibbs. It tells us how much work a reaction can do. –Something you ABSOLUTELY need to know: If ΔG is negative, the reaction is spontaneous. It will happen. If ΔG is negative, the reaction is spontaneous. It will happen. If ΔG is positive, the reaction is nonspontaneous. It will NOT happen. If ΔG is positive, the reaction is nonspontaneous. It will NOT happen.

Hey, there is something else that you can do to calculate ΔG: It is similar to Hess’ law…can you guess? It is similar to Hess’ law…can you guess? ΔG products - ΔG reactants = ΔG reaction ΔG products - ΔG reactants = ΔG reaction Just remember: many calculations in chemistry is either products – reactants or products / reactants!

TO SUM IT UP… A negative delta G is spontaneous. A negative delta G is spontaneous. A positive delta G is nonspontaneous. A positive delta G is nonspontaneous. What happens when delta G is ZERO? What happens when delta G is ZERO? –No way it can do any work, right? –Do you remember a situation like this? If ΔG = 0, the reaction is at EQUILIBRIUM! If ΔG = 0, the reaction is at EQUILIBRIUM!

Which brings us to… ΔG = ΔG o – RT ln Q ΔG = ΔG o – RT ln Q If we’re at equilibrium, we can change 2 things…which two do you think they are? If we’re at equilibrium, we can change 2 things…which two do you think they are? 0 = ΔG o – RT ln K 0 = ΔG o – RT ln K This is supposed to show how the equilibrium constant is related to temperature. This is supposed to show how the equilibrium constant is related to temperature.

The End!