Chapter 20 Energy and Disorder

Objectives Understand the concepts of enthalpy, entropy, and free energy and the relationship among them Use these concepts to solve problems dealing with any of them Understand energy as it is related to chemical systems

Why Reactions Occur Exothermic reactions generally take place spontaneously Endothermic reactions generally do not take place spontaneously Natural processes tend to go from an orderly state to a disorderly one High energy  low energy Order  disorder

Isothermal Processes – reactions taking place at constant temperature Isobaric Processes – reactions taking place at a constant pressure Thermodynamics – the study of the flow of energy

State Functions A state function is one whose value depends only on the current state of the system T = T2 – T1 V = V2 – V1 P = P2 – P1

Internal Energy Every system has some internal energy Internal energy, U, is a state function Ways to transfer energy By heating the system/surroundings By doing work

U = q + w q = heat gained by the system w = the amount of work done on the system Neither q nor w are state functions q has a positive sign if heat is flowing in, negative if heat is flowing out w is negative if work is done on the surroundings, and positive if done on the system

Assignment 1-5 page 394 Read through the rest of chapter 20 Due: Tomorrow

Enthalpy Enthalpy (H) = U + PV Enthalpy is a state function H = H2 – H1 Exothermic Rxn: H < 0 Endothermic Rxn: H > 0

Enthalpy Change

Which is Which?

What about this one?

Standard States Standard state refers to the enthalpy substances have at 298.15 K and 100.000 kPa Not the same as with the gas laws In measuring enthalpy, set the enthalpy of any free elements to be equal to zero A free element is one that is not in a compound

Enthalpy of Formation Enthalpy of formation is the change in enthalpy when one mole of a compound is produced from free elements in their standard states Units: KJ/mole Symbol: Hfº º means at standard state Thermodynamic stability depends on the amount of energy that would be required to decompose the compound See table A-6 in appendix for values Thermodynamically stable compounds have large negative enthalpies of formation

Calculation of Enthalpy of Reaction Hfº (products) = Hfº (reactants) + Hrº  means summation Hrº means change in enthalpy If the enthalpy of formation of each product is known, you can calculate the amount of energy produced or absorbed, which then tells you if the reaction will be endothermic or exothermic Assignment: Due at end of class 6-7 page 397

Hess’s Law Hess’s Law – The enthalpy change for a reaction is the sum of the enthalpy changes for a series of reactions that add up to the overall reaction

Consider reaction A  C Break into two parts (1)AB and (2)B  C Hr(1) = Hf°B - Hf°A Hr(2) = Hf°C - Hf°B So, the enthalpy change for the overall change of A to C is H° = Hr(1)° + Hr(2)°

Entropy Entropy, S, is derived from the second law of thermodynamics. This law places limits on the conversion of heat into work and prohibits perpetual motion Entropy (S) is a measure of disorder in a system Entropy is a state function

Entropy cont. Examples of S > 0 = Increase in disorder - S > 0 = Increase in disorder S < 0 = Decrease in disorder Assignment: Due at end of class 8-10 pg 400 10 points

Gibbs Free Energy Gibbs free energy determines whether a reaction will occur or not G = H - T S If G < 0 the reaction is exergonic (spontaneous) If G > 0 the reaction is endergonic The reaction can only occur is T S is very large If G = 0 the system is at equilibrium

Gibbs Free Energy Calculations Appendix A-6 Gr° =  Gf°(products) -  Gf°(reactants) Assignment Problems 11-14 pg 403-404

Chapter Review Complete questions