Lecture Presentation Chapter 5 Thermochemistry 2012 Pearson Education Inc John D Bookstaver St Charles Community College Cottleville MO Energy Energy is the ability to do work or transfer heat Energy used to cause an object that has mass to move is called work Energy used to cause the temperature of an object to rise is called heat 2012 Pearson Education Inc Thermochemistry Kinetic Energy Kinetic energy is energy an object possesses by virtue of its motion Ek mv2 1 2 2012 Pearson Education Inc Thermochemistry Potential Energy Potential energy is energy an object possesses by virtue of its position or chemical composition The most important form of potential energy in molecules is electrostatic potential energy Eel Eel KQ1Q2 d 2012 Pearson Education Inc Thermochemistry Units of Energy The SI unit of energy is the joule J kg m2 1 J 1 s2 An older non SI unit is still in widespread use the calorie cal 1 cal 4 184 J 2012 Pearson Education Inc Thermochemistry Definitions System and Surroundings The system includes the molecules we want to study here the hydrogen and oxygen molecules The surroundings are everything else here the cylinder and piston 2012 Pearson Education Inc Thermochemistry Definitions Work Energy used to move an object over some distance is work w F d where w is work F is the force and d is the distance over which the force is exerted 2012 Pearson Education Inc Thermochemistry Heat Energy can also be transferred as heat Heat flows from warmer objects to cooler objects 2012 Pearson Education Inc Thermochemistry Conversion of Energy Energy can be converted from one type to another For example the cyclist in Figure 5 2 has potential energy as she sits on top of the hill 2012 Pearson Education Inc Thermochemistry Conversion of Energy As she coasts down the hill her potential energy is converted to kinetic energy At the bottom all the potential energy she had at the top of the hill is now kinetic energy 2012 Pearson Education Inc Thermochemistry First Law of Thermodynamics Energy is neither created nor destroyed In other words the total energy of the universe is a constant if the system loses energy it must be gained by the surroundings and vice versa 2012 Pearson Education Inc Thermochemistry Internal Energy The internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E 2012 Pearson Education Inc Thermochemistry Internal Energy By definition the change in internal energy E is the final energy of the system minus the initial energy of the system E Efinal Einitial 2012 Pearson Education Inc Thermochemistry Changes in Internal Energy If E 0 Efinal Einitial Therefore the system absorbed energy from the surroundings This energy change is called endergonic 2012 Pearson Education Inc Thermochemistry Changes in Internal Energy If E 0 Efinal Einitial Therefore the system released energy to the surroundings This energy change is called exergonic 2012 Pearson Education Inc Thermochemistry Changes in Internal Energy When energy is exchanged between the system and the surroundings it is exchanged as either heat q or work w That is E q w 2012 Pearson Education Inc Thermochemistry E q w and Their Signs 2012 Pearson Education Inc Thermochemistry Exchange of Heat between System and Surroundings When heat is absorbed by the system from the surroundings the process is endothermic 2012 Pearson Education Inc Thermochemistry Exchange of Heat between System and Surroundings When heat is absorbed by the system from the surroundings the process is endothermic When heat is released by the system into the surroundings the process is exothermic 2012 Pearson Education Inc Thermochemistry State Functions Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem 2012 Pearson Education Inc Thermochemistry State Functions However we do know that the internal energy of a system is independent of the path by which the system achieved that state In the system depicted in Figure 5 9 the water could have reached room temperature from either direction 2012 Pearson Education Inc Thermochemistry State Functions Therefore internal energy is a state function It depends only on the present state of the system not on the path by which the system arrived at that state And so E depends only on Einitial and Efinal 2012 Pearson Education Inc Thermochemistry State Functions However q and w are not state functions Whether the battery is shorted out or is discharged by running the fan its E is the same But q and w are different in the two cases 2012 Pearson Education Inc Thermochemistry Work Usually in an open container the only work done is by a gas pushing on the surroundings or by the surroundings pushing on the gas 2012 Pearson Education Inc Thermochemistry We can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston Work w P V 2012 Pearson Education Inc Thermochemistry Enthalpy If a process takes place at constant pressure as the majority of processes we study do and the only work done is this pressure volume work we can account for heat flow during the process by measuring the enthalpy of the system Enthalpy is the internal energy plus the product of pressure and volume H E PV 2012 Pearson Education Inc Thermochemistry Enthalpy When the system changes at constant pressure the change in enthalpy H is This can be written H E PV H E P V 2012 Pearson Education Inc Thermochemistry 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 So at constant pressure the change in enthalpy is the heat gained or lost 2012 Pearson Education Inc Thermochemistry Endothermicity and Exothermicity A process is endothermic when H is positive 2012 Pearson Education Inc Thermochemistry Endothermicity and Exothermicity A process is endothermic when H is positive A process is exothermic when H is negative 2012 Pearson Education Inc Thermochemistry Enthalpy of Reaction The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants H Hproducts Hreactants 2012 Pearson Education Inc Thermochemistry Enthalpy of Reaction This quantity H is called the enthalpy of reaction or the heat of reaction 2012 Pearson Education Inc Thermochemistry The Truth about Enthalpy 1 Enthalpy is an extensive property 2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse