HONORS CHEMISTRY Unit G: ENERGY, KINETICS, and...
Transcript of HONORS CHEMISTRY Unit G: ENERGY, KINETICS, and...
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)