HONORS CHEMISTRY Unit G: ENERGY, KINETICS, and EQUILIBRIUM CHAPTER SIXTEEN: REACTION ENERGY

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1 HONORS CHEMISTRY Unit G: ENERGY, KINETICS, and EQUILIBRIUM CHAPTER SIXTEEN: REACTION ENERGY

2 THERMOCHEMISTRY uchemical REACTIONS are accompanied by an ENERGY CHANGE uchemical bonds BREAK & REFORM absorbing and releasing energy in a reaction uphase CHANGES also absorb or release energy uthermochemistry is the study of energy transfer as HEAT in chemical reactions or phase changes

3 HEAT & TEMPERATURE utemperature (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!

4 HEAT & TEMPERATURE

5 HEAT & TEMPERATURE

6 SPECIFIC HEAT uspecific 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 ºC uspecific HEAT (c) & HEAT (q) ARE TWO DIFFERENT THINGS!

7 SPECIFIC HEAT uspecific HEAT is dependent upon the identity of the substance uspecific HEAT quantitatively describes heat transfer ability of a substance

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9 CALORIMETER ucalorimeter is the instrument used to measure HEAT involved or transferred in a reaction ucan be used to measure ENTHALPY CHANGE uenthalpy CHANGE is the amount of energy (heat) gained or lost during a reaction or phase change

10 CALORIMETER

11 ENTHALPY of REACTION uenthalpy OF REACTION is the quantity of energy transferred as HEAT during a chemical reaction usometimes called Heat of Reaction uenthalpy of reaction = H rxn uthe difference between the energy stored in the bonds of the reactants and products

12 ENTHALPY of REACTION uexothermic REACTIONS release energy as a PRODUCT (HEAT OUT HOT) uendothermic REACTIONS absorb energy as a REACTANT (HEAT IN COLD) uthermochemical EQUATIONS include the H rxn in the equation

13 ENTHALPY of REACTION

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18 THERMOCHEMICAL EQUATIONS uthe equations represent molar amounts and the H rxn is proportional to the molar amounts uphysical states must be included uthe sign reverses if the equation reverses u Temperature is not important

19 ENTHALPY OF FORMATION uenthalpy OF FORMATION is the enthalpy change that occurs when one mole of a compound forms from its standard conditions (product) u H 0 f = ENTHALPY OF FORMATION u H 0 f = 0 (for elements in standard state) u H 0 f is NEGATIVE for STABLE COMPOUNDS u H 0 f is POSITIVE FOR UNSTABLE COMPOUNDS uusually obtained from a reference table uuseful for HESS S LAW CALCULATIONS

20 HESS S LAW uhess S LAW uses thermochemical equations for reactions uthe parts of a reaction SUM to the whole uhess S LAW is that the overall enthalpy change in a reaction is equal to the sum of the individual steps of the process

21 HESS S LAW uthe component equations are always provided uif the reaction needs to be reversed, then reverse the H 0 f sign (+ to -) or (- to +) usometimes multiply the equation to give desired stoichiometric values, then also multiply the H 0 f value uit S A LITTLE TRICKY, BUT HANG IN THERE!

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23 SEE THE SAMPLE PROBLEMS IN THE TEXTBOOK FOR MORE!

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26 ENTHALPY? ~ENERGY!... H EXOTHERMIC (- H) ENDOTHERMIC (+ H) HEAT & q

27 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

28 REACTIONS: DRIVING FORCES

29 REACTIONS: DRIVING FORCES umost reactions are exothermic (favorable H ) uendothermic reactions rare (unfavorable H) uhow ARE ENDOTHERMIC REACTIONS POSSIBLE? uentropy (disorder) make endothermic reactions possible u S is the change in ENTROPY (disorder)

30 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 HN 4 NO 3 (s) à 2 N 2 (g) + 4 H 2 O (l) + O 2 (g)

31 ENTROPY ( S) unaturally occurring reactions tend to favor greater disorder ( HIGHER ENTROPY) umelting, decomposition, solutions u S units: (kj/mol K) uhowever it is possible to for a system to become MORE ORDERED (LOWER ENTROPY)!

32 ENTHALPY & ENTROPY unatural process are driven in 2 directions: ulower ENTHALPY (energy) - H uhigher ENTROPY (disorder) + S uthe two forces can work together or oppose each other ugibbs FREE ENERGY ( G) is a function of ENTHALPY and ENTROPY and is TEMPERATURE dependent upredicts SPONTANEITY OF REACTION ( G)

33 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

34 GIBBS FREE ENERGY

35 GIBBS FREE ENERGY

36 GIBBS FREE ENERGY u- G = SPONTANEOUS (will happen) u+ G = NOT SPONTANEOUS (won t happen)