HONORS CHEMISTRYUnit G: ENERGY, KINETICS, and EQUILIBRIUM

CHAPTER SIXTEEN: REACTION ENERGY

THERMOCHEMISTRYuCHEMICAL REACTIONS are

accompanied by an ENERGY CHANGEuChemical bonds BREAK & REFORM – absorbing

and releasing energy in a reactionuPHASE CHANGES also absorb or release energyuTHERMOCHEMISTRY is the study of energy

transfer as HEAT in chemical reactions or phase changes

HEAT & TEMPERATUREuTEMPERATURE (T) is a measure of

the average KE of particles in a sample of matter (ºC or K)

uHEAT (q) is the energy transferred between matter at different temps (Joule (J) or (kJ))

uTHESE TWO THINGS ARE DIFFERENT!

HEAT & TEMPERATURE

HEAT & TEMPERATURE

SPECIFIC HEATuSPECIFIC HEAT is the amount of energy

required to raise the temperature of 1 g of a substance by 1 K or 1ºC

uUnit: (J/g・ºC)uSPECIFIC HEAT à (c)uSpecific Heat of Water = 4.18 J/g・ºCuSPECIFIC HEAT (c) & HEAT (q) ARE TWO

DIFFERENT THINGS!

SPECIFIC HEATuSPECIFIC HEAT is dependent upon the

identity of the substanceuSPECIFIC HEAT quantitatively describes

heat transfer ability of a substance

CALORIMETERuCALORIMETER is the instrument used to measure

HEAT involved or transferred in a reactionuCan be used to measure ENTHALPY CHANGEuENTHALPY CHANGE is the amount of energy (heat)

gained or lost during a reaction or phase change

CALORIMETER

ENTHALPY of REACTIONuENTHALPY OF REACTION is the quantity of energy

transferred as HEAT during a chemical reactionuSometimes called “Heat of Reaction”uEnthalpy of reaction = △Hrxn

uThe difference between the energy stored in the bonds of the reactants and products

ENTHALPY of REACTIONuEXOTHERMIC REACTIONS release energy as a

“PRODUCT” (HEAT OUT – HOT)uENDOTHERMIC REACTIONS absorb energy as a

“REACTANT” (HEAT IN – COLD)uTHERMOCHEMICAL EQUATIONS include the △Hrxn

in the equation

ENTHALPY of REACTION

THERMOCHEMICAL EQUATIONS

uThe equations represent molar amounts and the △Hrxn is proportional to the molar amounts

uPhysical states must be includeduThe sign reverses if the equation reversesu Temperature is not

important

ENTHALPY OF FORMATIONuENTHALPY OF FORMATION is the enthalpy change

that occurs when one mole of a compound forms from its elements @ standard conditions (product)

u△H0f = ENTHALPY OF FORMATION

u△H0f = 0 (for elements in standard state)

u△H0f is NEGATIVE for STABLE COMPOUNDS

u△H0f is POSITIVE FOR UNSTABLE COMPOUNDS

uUsually obtained from a reference tableuUseful for HESS’S LAW CALCULATIONS

HESS’S LAWuHESS’S LAW uses thermochemical equations for

reactionsuThe parts of a reaction SUM to the wholeuHESS’S LAW is that the overall enthalpy change in

a reaction is equal to the sum of the individual steps of the process

HESS’S LAWuThe component equations are always provideduIf the reaction needs to be reversed, then reverse

the △H0f sign (+ to -) or (- to +)

uSometimes multiply the equation to give desired stoichiometric values, then also multiply the △H0

fvalue

uIT’S A LITTLE TRICKY, BUT HANG IN THERE!

SEE THE SAMPLE PROBLEMS IN THE

TEXTBOOK FOR MORE!

ENTHALPY?~ENERGY!

...△H…EXOTHERMIC (-△H)

…ENDOTHERMIC (+△H) …HEAT & q

REACTIONS: DRIVING FORCES

uReactions can be SPONTANEOUS (favorable) or NON-SPONTANEOUS (unfavorable)

uTwo factors determine SPONTANIETY:uENTHALPY (energy) (H)uENTROPY (disorder) (S)

uENTROPY is the degree of randomness of particles

REACTIONS: DRIVING FORCES

REACTIONS: DRIVING FORCES

uMost reactions are exothermic (favorable △H )uEndothermic reactions rare (unfavorable △H)uHOW ARE ENDOTHERMIC REACTIONS POSSIBLE?uENTROPY (disorder) make endothermic reactions

possibleu△S is the change in ENTROPY (disorder)

ENTROPY (△S)

u+△S = increased disorder (favorable)u-△S = decreased disorder (unfavorable)uThe production of “more moles” of products or

products in a higher energy physical state have +△S (greater entropy)

u(s) < (l) < (aq) < (g)u 2 HN4NO3 (s) à 2 N2 (g) + 4 H2O (l) + O2 (g)

ENTROPY (△S)

uNaturally occurring reactions tend to favor greater disorder ( HIGHER ENTROPY)

uMelting, decomposition, solutionsu△S units: (kJ/mol・K)uHOWEVER… it is possible to for a system to

become MORE ORDERED (LOWER ENTROPY)!

ENTHALPY & ENTROPY uNatural process are driven in 2 directions:uLOWER ENTHALPY (energy) -△HuHIGHER ENTROPY (disorder) +△SuThe two forces can work together or oppose each

otheruGIBBS FREE ENERGY (△G) is a function of

ENTHALPY and ENTROPY and is TEMPERATURE dependent

uPREDICTS SPONTANEITY OF REACTION (△G)

GIBBS, ENTHALPY & ENTROPY: △G, △H, △S

uREACTIONS BOTH EXOTHERMIC (-△H) AND INCREASE DISORDER (+△S) à SPONTANEOUS

uReactions can be endothermic and still spontaneous

uReactions can be more ordered and still be spontaneous

uREACTIONS BOTH ENDOTHERMIC (+△H) AND DECREASE DISORDER (-△S) à NOT SPONTANEOUS

GIBBS FREE ENERGY

GIBBS FREE ENERGY

GIBBS FREE ENERGY

u-△G = SPONTANEOUS (will happen)

u+△G = NOT SPONTANEOUS (won’t happen)