© 2009, Prentice-Hall, Inc. Thermochemistry (Ch. 5 & Sections 8.8 & 11.4 ) Lecture 1 - Intro

© 2009, Prentice-Hall, Inc. Energy – Energy is the capacity to do work or produce heat – Energy used to cause an object to move is called work. – Energy used to cause the temperature of an object to rise is called heat.

Energy Definitions Potential Energy: Energy due to position or composition Kinetic Energy: Energy due to the motion of the object Internal Energy: The sum of all Potential Energy and Kinetic Energy  E = E final - E initial

Units of Energy The SI unit of energy is the joule (J). An older, non-SI unit is still in widespread use: the calorie (cal). 1 cal = J C Food calories, C alories, are really kilocalories. 1 J = 1  kg m 2 s2s2

© 2009, Prentice-Hall, Inc. Definitions: System and Surroundings The system includes the molecules we want to study The surroundings are everything else

© 2009, Prentice-Hall, Inc. First Law of Thermodynamics Energy is neither created nor destroyed. If the system loses energy, it must be gained by the surroundings, and vice versa.

© 2009, Prentice-Hall, Inc. Changes in Internal Energy Energy exchanged between the system and the surroundings, is either heat (q) or work (w). That is,  E = q + w.

© 2009, Prentice-Hall, Inc. Changes in Internal Energy Energy GAINED by the system is positive (+) Energy LOST by the system is negative (-)

© 2009, Prentice-Hall, Inc. Exchange of Heat between System and Surroundings When heat is absorbed by the system, the process is endothermic. When heat is released by the system, the process is exothermic.

© 2009, Prentice-Hall, Inc. Enthalpy ENTHALPY, H – from the Greek word, enthalpein, meaning “to warm” Accounts for heat flow in processes occuring at CONSTANT PRESSURE with only PV work. Enthalpy is the internal energy plus the product of pressure and volume:  H =  E + P  V

© 2009, Prentice-Hall, Inc. Enthalpy Since  E = q + w and w = -P  V, we can substitute these into the enthalpy expression:  H =  E + P  V  H = (q+w) − w  H = q At constant pressure, the change in enthalpy is the heat gained or lost.

© 2009, Prentice-Hall, Inc. Endothermic and Exothermic Positive  H is endothermic.

© 2009, Prentice-Hall, Inc. Endothermic and Exothermic Positive  H is endothermic. Negative  H is exothermic.

© 2009, Prentice-Hall, Inc. Enthalpy of Phase Changes The heat of fusion,  H fus (kJ/mol), is the energy required to change between a solid and liquid. The heat of vaporization,  H vap (kJ/mol), is the energy required to change between a liquid and gas.  H fus >  H vap More energy is required to break IMFs (boiling) than to weaken IMFs (melting). melting/freezing boilng/condensing

Calculating Heat of Vaporization Calculate the heat required to boil 13 grams of water.  H vap is kJ/mol q =  H = n  H vap = kJ/mol x 13 g x 1 mol/18 g = 29 kJ

Calculating Heat of Fusion Calculate the heat required to melt 13 grams of water.  H fus is 6.03 kJ/mol q =  H = n  H fus = 6.03 kJ/mol x 13 g x 1 mol/18 g = 4.4 kJ