Outline. 9. The Second Law of Thermodynamics: Entropy. 10.Entropy and the Third law of thermodynamics 11.Spontaneous change: Free energy

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1 hermochemistry opic 6. hermochemistry hermochemistry

2 Outline. Getting Started: Some terminology. State functions 3. Pressure-Volume Work 4. he First Law of hermodynamics: Heat, work and enthalpy 5. Heat capacity and specific heat 6. Measurement of heat of reaction 7. Variation of the heat of reactions ( H & E) with temperature 8. Enthalpies of formation a) Enthalpy and chemical reactions b) he Indirect Determination of H: Hess s Law 9. he Second Law of hermodynamics: Entropy..Entropy and the hird law of thermodynamics.spontaneous change: Free energy opic 6. hermochemistry

3 6.. GEING SARED: SOME ERMINOLOGY hermodynamics is a branch of physics which deals with the energy and work of a system. hermochemistry: hermochemistry is a branch of thermodynamics focused on the study of heat change in chemical reactions. - Heat (H) Heat flows from hotter to colder bodies. - Work (W) Work includes all energies but heat. Universe = System + Surroundings SYSEM: the specific part of the universe that is of interest to us. SURROUNDINGS: the rest of the universe outside the system. opic 6. hermochemistry 3

4 System Open system Closed system Isolated system EXOHERMIC PROCESS Heat is released (Q f Q i < ) he reactants have more energy than the products. ENDOHERMIC PROCESS Heat is absorbed (Q f Q i > ) he products have more energy than the reactants. opic 6. hermochemistry 4

5 6.. SAE FUNCIONS UNIVERSIDAD CARLOS III DE MADRID Any property that has a unique value for a specified state of a system is said to be a State Function. X = X final X initial State function (P, V, ) State equation hermodynamic State Functions: hermodynamic properties that are dependent on the state of the system only. (Example: E and H) Other variables will be dependent on pathway (Example: Heat (q) and Work (w)). hese are NO state functions. In these cases, the pathway from one state to the other must be defined. opic 6. hermochemistry 5

6 4.3. PRESSURE-VOLUME WORK UNIVERSIDAD CARLOS III DE MADRID Gas expansion w = F r F = Force r = Dis tan ce Work is done by the system. w = p V w = p V HEA Energy is transferred due to the different temperature of the bodies. ( cal <> 4,8 J). opic 6. hermochemistry 6

7 4.4. HE FIRS LAW OF HERMODYNAMICS Law of Conservation of Energy: he energy of an isolated system is constant. Internal Energy, U (E) - otal energy (potential and kinetic) in a system. U universe = U universe = U system + U surroundings = U system = - U surroundings U = q + w W > W < SYSEM Q > Q < opic 6. hermochemistry 7

8 V = constant UNIVERSIDAD CARLOS III DE MADRID 4.4. HE FIRS LAW OF HERMODYNAMICS V = W = U U = q + w U U = Q v P = constant P = U U = q + w Q p = U w = U (-p p V) H H = U U + (PV) H = State function : H r = H(products) H(reactants) Solids, Liquids: Gases ( = constant): H E H = E + R n opic 6. hermochemistry 8

9 6.5. SPECIFIC HEA AND HEA CAPACIY Heat Capacity P = V = C C dq p p = = d dq d v v = = dh d de d Q Q p v = n = n Cpd = ncp d = Cvd = ncv d = d nc nc p v H = nc p E = ncv C v and C p ( mole) H = E + pv dh de d( pv = + ) d d d Solids, Liquids: Cp = Cv + d( pv ) d Gases: d(pv)/d small C p C v PV = R C p = C v + R opic 6. hermochemistry 9

10 Calorimeter UNIVERSIDAD CARLOS III DE MADRID 6.6. MEASUREMEN OF HEA OF CHEMICAL REACIONS Images from: opic 6. hermochemistry

11 6.7. VARIAION OF HE HEA OF REACIONS ( H & E) WIH EMPERAURE Enthalpy ( )? aa + bb cc + dd H =?? H (aa + bb) a (cc + dd) a P = constant H H H is known H? H (aa + bb) a (cc + dd) a H = H + H + H C p (reactants) = ac p (A) + bc p (B) C p (products) = cc p (C) + dc p (D) H' H" = = C C p p (reactants)d (products) d H" = C p (products)d opic 6. hermochemistry

12 6.7. VARIAION OF HE HEA OF REACIONS ( H & E) WIH EMPERAURE Enthalpy + Cp(products)d H = H Cp(reactants)d P = constant + H = H Cpd Usually Cp is small H f() Internal Energy V = constant + E = E Cvd Molar heat capacity at constant volumen, C v opic 6. hermochemistry

13 6.8. ENHALPIES OF FORMAION A) Enthalpy and Chemical Reactions Standard Enthalpies of Formation he enthalpy change that occurs in the formation of one mole of a substance in the standard state from the reference forms of the elements in their standard states. he standard enthalpy of formation of a pure element in its reference state is. Standard Enthalpies of Reaction For the reaction: H H rxn = rxn aa + bb cc+ dd = n H f ( products) n H f ( rea) [ c H ( C) + d H ( D) ] a H ( A) + b H ( B) f f [ ] f f opic 6. hermochemistry 3

14 6.8. ENHALPIES OF FORMAION B) he Indirect Determination of H: Hess s Law he direct Method C (graphite) + O (g) CO (g) H r = -393,5 kj H r [( mol) H ( C, graphite) + (mol ) H ( O, g) ] = ( mol ) H f ( CO, g) f f [ + ] H r = ( mol ) H f ( CO, g) H f ( CO, g) = he Indirect Method: Hess s Law Hess's Law says that enthalpy changes are additive. opic 6. hermochemistry 4 For a given reaction, H is the same whether it occurs in one step or in a series of steps. For example: If Reaction () = Reaction () + Reaction (3) H = H + H 3

15 6.8. ENHALPIES OF FORMAION B) he Indirect Determination of H: Hess s Law Example: enthalpy of formation of acethylene C (graphite) + H (g) C H (g) (non possible rxn) C (graphite) + O (g) CO (g) H (g) + ½ O (g) H O (l) H r = -393,5 kj H r = -85,8 kj C H (g) + 5 O (g) 4 CO (g) + H O (l) H r = -598,8kJ 4C (graphite) + 4O (g) 4CO (g) H r = -574, kj H (g) + /O (g) H O (l) H r = -57,6 kj 4 CO (g) + H O (l) C H (g) + 5 O (g) H r = 598,8 kj 4C (graphite) + H (g) C H (g) H r = 453, kj C (graphite) + H (g) C H (g) H r = 6,6 kj H f (C H ) = H r (C H )/mol = 6,6 kj/mol opic 6. hermochemistry 5

16 6.9. HE SECOND LAW OF HERMODYNAMICS: ENROPY. It is a measure of the randomness of molecules in a system. Entropy is a state function: S = S final S initial a) Solid Liquid Examples: a) Melting of a solid b) Vaporization of a liquid c) Solution formation d) Increase of the temperature of a body b) c) Liquid Liquid Vapour In every case S = S final S initial > d) Solid Solution System at System at ( > ) opic 6. hermochemistry 6

17 6.9. HE SECOND LAW OF HERMODYNAMICS: ENROPY. he Second Law of hermodynamics S total = S universe = S system + S surroundings S universe = S system + S surroundings > Spontaneous Process S universe = S system + S surroundings = Equilibrium All spontaneous processes produce an increase in the entropy of the universe. Evaluating entropy changes Given the reaction: aa + bb cc + dd Like H, the entropy is a state function and so the standard entropy of a chemical reaction can be calculated as follows: system reaction [ cs + ds ] [ as bs ] S = S = + S reaction C = nsproducts ms D reactants opic 6. hermochemistry 7 A B

18 Evaluating entropy changes UNIVERSIDAD CARLOS III DE MADRID P = constant S surroundings H system Exothermic Process < H > system S surroundin gs Endothermic Process H > < system S surroundin gs S surroundings H = system Spontaneous processes. Reversibility and Spontaneity. Reversible Processes quasistatic processes Reversible Expanssion and compression: (P int = P ex ± dp) Reversible change of temperature: ( ex = int ± d) Irreversible Processes the changes in the system are finite opic 6. hermochemistry 8

19 Reversible and irreversible processes W reversible < W irreversible Reversible Compression (P int = P ex dp): W rev = - P ex dv = - (P int + dp) dv - P int dv Irreversible Compression (Pex > P int ): w V V irrev = Pex dv > V V P int dv = w rev Reversible Compression of an Ideal Gas: W rev = - P int dv; (P int = nr/v) w rev = V V nr dv = nr V V V V dv V w irrev = Pex dv = P (V V ) > V = nrln V V opic 6. hermochemistry 9

20 Reversible and irreversible processes Q rev > Q irrev Q rev = E - w rev Q irrev = E w irrev Q rev Q irrev = w irrev - w rev si w reversible < w irreversible Q rev > Q irrev Calculating Entropy SS δqq rev S = δq rev = δqrev = Q For an Ideal Gas, according to the kinetic theory: E c = 3/R If = constant E = constant E = w + Q = Q = - w rev Q rev = - w rev = nr ln V /V S = Q rev / = nr ln V /V For an expanssion: V > V S > Irreversible isothermal Expanssion: S gas = nr ln V /V (S state function) opic 6. hermochemistry

21 6.. ENROPY AND HE HIRD LAW OF HERMODYNAMICS he hird Law of hermodynamics he entropy of a pure perfect crystal at K is zero. S i = Initial Entropy S f = Final Entropy S = S f - S i S i = entropy at K S = S f he entropy of a certain substance increases with temperature For one mole: S = p d S S = C C p d Since S = S C = p d S 98 for a substance that melts at f < 98 K S 98 f Cp Hf = d + + f 98 f Cp' d opic 6. hermochemistry

22 6.. SPONANEOUS CHANGE: FREE ENERGY According to the second law of thermodynamics, for an spontaneous process: S = Reversible Process S > Irreversible or Spontaneous Process S surroundings is difficult to calculate, therefore: S univ = S syst + S surround > ; S univ = S syst - H syst / > ; S univ = S syst - H syst > ; Gibbs Free Energy G = H S G = H S Since G is a state function G reaction = G products G reactants G < G = Spontaneous Process Equilibrium is established Criteria for spontaneous changes G > Non-Spontaneous Process opic 6. hermochemistry

23 Bibliography R. Chang. Chemistry. McGrawHill, Ninth edition. Chapters 6, 8. R. H. Petrucci, W. S. Harwood, F. G. Herring, J. Madura. General Chemistry. Principles and Modern Applications. Prentice Hall, Ninth edition. Chapters 7 and 9. J. McMurry, R.C. Fay. Chemistry. Pearson, Fourth edition. Chapters 8. opic 6. hermochemistry 3

CHEM Thermodynamics. Entropy, S

CHEM Thermodynamics. Entropy, S hermodynamics Change in Change in Entropy, S Entropy, S Entropy is the measure of dispersal. he natural spontaneous direction of any process is toward greater dispersal of matter and of energy. Dispersal