Thermochemistry 10/15/10

Part I: Thermochemistry Basics thermochemistry = the study of the transfers of energy as heat that accompany chemical reactions and physical changes. enthalpy (ΔH) a measure of the difference between the stored energy of the reactants and products in a chemical reaction. if ΔH is negative, the the reaction is exothermic if ΔH is positive, then the reaction is endothermic. thermochemical equation = a balanced chemical reaction that includes the value of  H for the reaction. Ex: 2H 2 (g) + O 2 (g)  2H 2 O (g)  H = −483.6 kJ/mol C 6 H 12 O 6 (s) + 6O 2 (g)  6CO 2 (g) + 6H 2 O (l)  H = −2808 kJ/mol  H is usually measured at a set of standard conditions of 1 atm and 298 K (25°C). standard  H can be indicated with a degree sign on the H (like this:  H°) this is not the same as STP from the gas laws!

there are a few different forms of  H you need to know:  H comb = heat of combustion = enthalpy change for the complete burning of 1 mole of a substance.  H vap = heat of vaporization = enthalpy change that occurs when 1 mole of a substance is vaporized (will always be positive)  H fus = heat of fusion = enthalpy change that occurs when 1 mole of a substance is melted (will always be positive) H 2 O (l)  H 2 O (g)  H vap = 40.7 kJ/mol H 2 O (s)  H 2 O (l)  H fus = 6.01 kJ/mol

enthalpy change measurements can be analyzed to determine whether or not a reaction will take place at all. the great majority of chemical reactions in nature are exothermic. as these reactions proceed, energy is liberated and the products have less energy than the original reactants. 4Fe (s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = kJ/mol the products are also more resistant to change, more stable, than the original reactants. the tendency throughout nature is for a reaction to proceed in a direction that leads to a lower energy state. Part II: Entropy & Reaction Tendency spontaneous process = a physical or chemical change that, once begun, occurs with no outside intervention the majority of spontaneous reactions are exothermic (-  H)

that doesn’t mean endothermic reaction can’t be spontaneous, it only means another force is responsible for making them occur! That other force is known as entropy. entropy (  S)= a measure of the degree of randomness of particles in a system. The tendency throughout nature is for a reaction to proceed in a direction that leads to a higher entropy state. Ex: a naturally occurring endothermic process is melting. an ice cube melts spontaneously at room temperature as energy is transferred from the warm air to the ice. the well-ordered arrangement of water molecules in the ice crystal is lost, and the less-orderly liquid phase of higher energy content is formed. a system that can go from one state to another without an enthalpy change does so by becoming more disordered another good example of entropy tending to increase is how gases spread out into a recently opened container....

another good example of entropy tending to increase is how gases spread out into a recently opened container: in “a” above, the stopcock is closed. The gas molecules are not allowed to mix.  S = 0 in “b” above, the stopcock has been opened and four possible arrangements of the gas molecules are shown.  S = positive

Part III: Free Energy processes in nature are driven in two directions: toward lowest enthalpy and toward highest entropy. when these two oppose each other, the dominant factor determines the direction of change. to predict which factor will dominate for a given system, a function has been defined to relate the enthalpy and entropy factors at a given temperature. free energy (  G)= the relationship between  H,  S and temp. ΔG° = ΔH° − TΔS° natural processes proceed in the direction that lowers the free energy of a system.