Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Chemistry FIFTH EDITION Chapter 6 Thermochemistry

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Energy The capacity to do work or to produce heat.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Law of Conservation of Energy Energy can be converted from one form to another but can neither be created nor destroyed. (E universe is constant)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 The Two Types of Energy Potential: due to position or composition - can be converted to work Kinetic: due to motion of the object KE = ½ mv 2 (m = mass, v = velocity)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Potential Energy 1. Water behind a dam has PE that can be converted to work when the water flows down through turbines  creating electricity.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Potential Energy 2. Attractive & repulsive forces also lead to PE. The energy released when gasoline is burned results from differences in attractive forces between nuclei & electrons in the reactants & products.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 See Figure 6-1 on page 230 (a) Ball A is higher than Ball B. Therefore, Ball A initially has greater PE than ball B. Release A  PE converted to KE Ball A hits Ball B and transfers some of the KE to Ball B. (b) Final position of Ball B is lower than original position of Ball A. Some of the PE lost by Ball A transferred to the hill as Frictional Heating. (hill temp. rises slightly.)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Temperature v. Heat Temperature reflects random motions of particles, therefore related to kinetic energy of the system. Heat involves a transfer of energy between 2 objects due to a temperature difference

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Heat is not a substance contained by an object.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 In Figure 6-1, Ball B gains PE because work was done by Ball A on Ball B.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 WORK: FORCE ACTING OVER A DISTANCE Work is required to raise Ball B from its initial position to its final one. Part of the original PE of Ball A transferred through work to Ball B thereby increasing the PE of Ball B.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 TWO WAYS TO TRANSFER ENERGY: THROUGH WORK THROUGH HEAT

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 ENERGY CHANGE IS INDEPENDENT OF THE PATHWAY. As Ball A goes from the top of the hill to the bottom, Ball A always loses the same amount of PE.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 WORK & HEAT ARE DEPENDENT OF THE PATHWAY. Depending on the specific conditions of the hill, how much energy is transferred through work and through heat might differ.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 State Function Depends only on the present state of the system - not how it arrived there. It is independent of pathway. Energy change is independent of pathway Energy change does not depend in any way on the system’s past (or future).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 ENERGY IS A STATE FUNCTION. WORK & HEAT ARE NOT STATE FUNCTIONS.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 CHEMICAL ENERGY : CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O (g) + energy (heat)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 System and Surroundings System: That on which we focus attention (Reactants & Products) Surroundings: Everything else in the universe (Rxn. Container, room, etc.) Universe = System + Surroundings

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 How does energy flow out of a system in an Exothermic Reaction? Energy is lost by the system because PE of products is less than PE of reactants. Some of the PE stored in the chemical bonds is converted to thermal energy (random KE) via heat.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Exothermic and Endothermic Heat exchange accompanies chemical reactions. Exothermic: Heat flows out of the system (to the surroundings). Endothermic: Heat flows into the system (from the surroundings).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Figure 6.2 The Combustion of Methane

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Exothermic (On average) Bonds in products stronger than those of the reactants. That is, more energy is released by forming the new bonds in the products than is consumed to break the bonds in the reactants.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Endothermic SITUATION IS REVERSED!! (On average) Bonds in products weaker than those of the reactants. That is, Energy flows into the system as heat.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Figure 6.3 The Energy Diagram for the Reaction of Nitrogen and Oxygen to Form Nitric Oxide

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 THERMODYNAMICS: Study of energy and its interconversions

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 First Law of Thermodynamics: The energy of the universe is constant. Law of Conservation of Energy

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Internal Energy (E) Sum of the Kinetic Energies and the Potential Energies of all the “particles” in the system.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 First Law  E = q + w  E = change in system’s internal energy q = heat w = work

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Thermodynamic quantities always consist of two parts. 1. a number 2. a sign

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 HEAT (q) HEAT FLOWS INTO A SYSTEM q = + HEAT FLOWS OUT OF A SYSTEM q = -

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 WORK (w) SYSTEM DOES WORK ON SURROUNDINGS w = - SURROUNDING DOES WORK ON THE SYSTEM w = + In this textbook, we always take the systems point of view.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Common type of work associated with Chemical processes is Work done by a Gas (through Expansion) Work done to a Gas (through Compression)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Pressure of the gas = F/A that is, Force acting on a piston of Area, A Gas in a cylindrical Cylinder with Movable piston

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35 The Change in the Volume of the Cylinder  V = A x  h Gas in a cylindrical Cylinder with Movable piston

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 36 Work work = force  distance Since pressure = force / area, Then force = P x A. Thus, work = P x A x  h work = pressure   volume w system =  P  V

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 37 w system =  P  V If  V is “+”, gas is expanding. System is doing work on the surroundings, so work must be negative. If  V is “-”, gas is compressed. Work is being done on the system and work is positive (work flows into the system.)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 38 HOMEWORK # 17 – 29 odd. Let’s do #17

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 39 Section 6.2 Enthalpy & Calorimetry E: Internal Energy of a System H: Enthalpy Enthalpy is a state function. A change in enthalpy (  H) does not depend on the pathway between 2 states. H = E + PV

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 40 H = E + PV  H =  E +  (PV)  H =  E + P  V + V  P At constant pressure  H =  E + P  V

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 41 Consider a process under constant pressure & work is pressure-volume work.  E = q p + w  E = q p - P  V Therefore, q p =  E + P  V

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 42 Since  H =  E + P  V and q p =  E + P  V Then  H = q p That is,  H = energy flow as heat only at constant Pressure

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 43 Heat of Reaction Same as change in enthalpy at constant press. For a chemical Reaction,  H = H products - H reactants If  H > 0, then the rxn. is endothermic. If  H < 0, then the rxn. is exothermic.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 44 Calorimetry Science of measuring heat based on observing the temperature change when a body absorbs or discharges energy. Used to determine the heat exchange (q) associated with a reaction.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 45 Figure 6.5 A Coffee-Cup Calorimeter Made of Two Styrofoam Cups (for constant press. experiments = Atm. Press.). Calorimeter: Device used to determine heat associated with a chemical reaction.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 46 Heat Capacity

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 47 Some Heat Exchange Terms specific heat capacity (energy required to raise the temperature of 1 gram of a substance by 1  C. heat capacity per gram = J/°C g or J/K g molar heat capacity(energy required to raise the temperature of 1 mole of a substance by 1  C. heat capacity per mole = J/°C mol or J/K mol

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 48 Heat of reaction is an extensive property, that is, it depends directly on the amount of substance present. Intensive properties are not related to the amount of substance present.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 49 Table 6.1 on page 237 Table of Specific Heat Capacities Units J /  C · g Energy absorbed or released: specific heat capacity x mass of solution x  T = s · m ·  T Don’t forget proper signs! Homework: 31 – 55odd.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 50 Some Calorimetry Experiments at Constant Volume Constant Volume: No pressure – volume work No work done.  E = q + w = q v (constant volume) Use Bomb Calorimeter

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 51 Figure 6.6 A Bomb Calorimeter

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 52 Read Sample Exercise 6.6. Let’s do: # 55