Thermodynamics

Every physical or chemical change is accompanied by energy change Thermodynamics: branch of chemistry that studies energy changes –specifically: changes in heat energy

Thermodynamics Tells us if a reaction will occur 2 considerations: –enthalpy (heat energy) –entropy (chaos/randomness)

Enthalpy, H enthalpy: heat content of system at constant pressure – symbol = H

Changes in Enthalpy are measurable cannot measure enthalpy content of system directly changes can measure changes in enthalpy! symbol =  H  H = H final – H initial = H products - H reactants

Net gain in energy Endothermic ProcessEndothermic Process: energy absorbed H final > H initial so H final – H initial results in positive value  H is positive

Net loss in energy Exothermic ProcessExothermic Process: energy released H final  H initial so H final – H initial results in negative value  H is negative [ see footnote to table I]

Energy of universe is conserved Universe energy can move between system and the environment Environment System A B Which arrow represents an endothermic change? ? exothermic change A B

Change in Energy choose how measure energy change –depends on how set up experiment monitor the system monitor the environment* * usually easier

Energy lost = Energy gained how do you know energy has moved? can measure energy gained or lost by environment –equals energy lost or gained by system change in temperature!

source reaction is carried out in water in styrofoam cup temperature of water is monitored water is the environment! cup is the universe!

Q = mC  T Q = energy change m = mass of water c = specific heat of water  T = temperature change = T f – T i

Different kinds of  H’s  H of dissolving: heat of solution  H of phase change: heat of fusion/heat of vaporization  H of reaction: heat of reaction –categorized by rxn type

Table I: Heats of Reaction rxns #1-6: combustion rxnsrxns #1-6: combustion rxns  H: heat of combustion rxns #7-18: formation reactionsrxns #7-18: formation reactions –substance is formed from its elements  H: heat of formation rxns #19-24: dissolving equationsrxns #19-24: dissolving equations  H: heat of solution

Energy depends on amount remember – it takes more energy to heat up water in bathtub than to make a cup of tea

CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O (l)  H = kJ 1 mole of methane + 2 moles of oxygen → 1 mole of carbon dioxide gas & 2 moles of liquid water reaction is exothermic (negative sign for ΔH) kJ energy released per mole of CH 4 (g) burned

Energy depends on amount burn 2 moles of CH 4 (g) with 4 moles of O 2 (g), get 2 times as much energy out Stoichiometry! (2)(890.4 kJ) = kJ is released

Phase Change: Energy depends on direction PE Solid Liquid Gas Up is endothermic Down is exothermic melting/fusion boiling/ vaporization sublimation condensation freezing deposition

Reactions: Energy depends on direction too! N 2 (g) + 3H 2 (g)  2NH 3 (g)  H = kJ 2NH 3 (g)  N 2 (g) + 3H 2 (g)  H = 91.8 kJ If look at reverse reaction, then need to reverse sign of  H

Thermochemical Equations balanced chemical equations show physical state of all reactants & products energy change can be given in 2 ways –energy term written as reactant or product OR –  H is given right after equation

Exothermic Rxn: energy = product 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = kJOR 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) kJ Exothermic

Endothermic Rxn: energy = reactant NH 4 NO 3 (s)  NH 4 + (aq) + NO 3 - (aq)  H = 27 kJ OR NH 4 NO 3 (s) + 27 kJ  NH 4 + (aq) + NO 3 - (aq)

Changes of State H 2 O(s)  H 2 O(l)  H fusion = J/g at 0 o C H 2 O(l)  H 2 O(s)  H = J/g at 0 o C energy is absorbed when water melts & energy is released when water freezes! H 2 O(l)  H 2 O(g)  H vapor = 2260 J/g at 100 o C H 2 O(g)  H 2 O(l)  H = J/g at 100 o C energy is absorbed when water evaporates & energy is released when water condenses!