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 or energy –Entropy or chaos

Enthalpy, H usually do experiments at constant pressure (1 atm) Enthalpy = heat content of a 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 is positive  H is positive

Net loss in energy Exothermic ProcessExothermic Process: Energy released H final  H initial so H final – H initial is negative  H is negative –In case you forget the sign of  H for an exothermic process, look at the footnote to table I

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

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

Energy lost = Energy gained What tells you that energy has moved? can measure energy gained or lost by environment it equals energy lost or gained by system Changes in temperature!

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

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 on dissolving = heat of solution  H on phase change = heat of fusion or heat of vaporization  H on reaction = heat of reaction –Categorized by rxn type

Look at 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) + O 2 (g)  CO 2 (g) + 2H 2 O (l)  H = kJ 1 mole of methane + 1 mole 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 2 moles of O 2 (g), get 2 times as much energy out Stoichiometry! 2 x kJ = kJ will be released

Phase Change: Energy depends on direction P.E. 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 reverse equation, reverse sign of  H

Hess’s Law Can add 2 or more equations by adding the  H’s Enables you to calculate  H for # of rxns Say you’re interested in 2S(s) + 3O 2 (g)  2SO 3 (g)

Have  H’s for the following: a) S(s) + O 2 (g)  SO 2 (g)  H = -297 kJ b) 2SO 3 (g)  2SO 2 (g) + O 2 (g)  H = 198 kJ Multiply (a) by 2 and reverse (b)

Adding … 2S(s) + 2O 2 (g)  2SO 2 (g)  H = -594 kJ 2SO 2 (g) + O 2 (g)  2SO 3 (g)  H = -198 kJ 2S(s) + 3O 2 (g)  2SO 3 (g)  H = -792 kJ

Thermochemical Equations balanced chemical equation shows physical state of all reactants & products gives energy change (2 ways) –energy term can be 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 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 released when water condenses!