Topic 15 Year 2 Energetics HL 0 is

Topic 15 – Year 2 Energetics HL

0 ) is the enthalpy of The standard enthalpy of reaction (DHrxn a reaction carried out at 1 atm. a. A + b. B c. C + d. D DH 0 rxn = [ c. DH 0 f (C) + d. DH 0 f (D) ] - [ a. DH 0 f (A) + b. DH 0 f (B) ] DH 0 rxn = S n. DH 0 f (products) - S m. DHf 0 (reactants) Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. (Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end. ) 6. 6

C (graphite) + 1/2 O 2 (g) CO (g) + 1/2 O 2 (g) C (graphite) + O 2 (g) CO 2 (g) 6. 6

Calculate the standard enthalpy of formation of CS 2 (l) given that: 0 = -393. 5 k. J C(graphite) + O 2 (g) CO 2 (g) DHrxn S(rhombic) + O 2 (g) CS 2(l) + 3 O 2 (g) SO 2 (g) 0 = -296. 1 k. J DHrxn CO 2 (g) + 2 SO 2 (g) 0 = -1072 k. J DHrxn 1. Write the enthalpy of formation reaction for CS 2 C(graphite) + 2 S(rhombic) CS 2 (l) 2. Add the given rxns so that the result is the desired rxn. C(graphite) + O 2 (g) 2 S(rhombic) + 2 O 2 (g) + CO 2(g) + 2 SO 2 (g) 0 = -393. 5 k. J CO 2 (g) DHrxn 0 = -296. 1 x 2 k. J 2 SO 2 (g) DHrxn CS 2 (l) + 3 O 2 (g) 0 = +1072 k. J DHrxn C(graphite) + 2 S(rhombic) CS 2 (l) 0 = -393. 5 + (2 x-296. 1) + 1072 = 86. 3 k. J DH rxn 6. 6

Benzene (C 6 H 6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49. 04 k. J/mol. 2 C 6 H 6 (l) + 15 O 2 (g) 12 CO 2 (g) + 6 H 2 O (l) DH 0 rxn = S n. DH 0 f (products) - S m. DHf 0 (reactants) DH 0 rxn = [ 12 DH 0 f (CO 2) + 6 DH 0 f (H 2 O)] - [ 2 DH 0 f (C 6 H 6)] DH 0 rxn = [ 12 x– 393. 5 + 6 x– 285. 8 ] – [ 2 x 49. 04 ] = -6534. 9 k. J -6534. 9 k. J = - 3267. 4 k. J/mol C 6 H 6 2 mol 6. 6

Electrostatic (Lattice) Energy Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. Q+QE=k r Q+ is the charge on the cation Q- is the charge on the anion r is the distance between the ions Lattice energy (E) increases as Q increases and/or as r decreases. cmpd Mg. F 2 Mg. O Li. F Li. Cl lattice energy 2957 Q= +2, -1 3938 Q= +2, -2 1036 r F- < r Cl 853 9. 3

Born-Haber Cycle for Determining Lattice Energy o DHoverall = DHo 1 + DHo 2 + DHo 3 + DHo 4 + DHo 5 9. 3

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Spontaneous Physical and Chemical Processes • A waterfall runs downhill • A lump of sugar dissolves in a cup of coffee • At 1 atm, water freezes below 0 0 C and ice melts above 0 0 C • Heat flows from a hotter object to a colder object • A gas expands in an evacuated bulb • Iron exposed to oxygen and water forms rust spontaneous nonspontaneous 18. 2

Introduction to Entropy Spontaneous physical changes are easy to predict. A dropped glass will fall to the ground once it is released. The reverse of a spontaneous event, like water flowing up a waterfall, will not occur except perhaps in the world of special effects for movies. The probability of finding a highly ordered situation (e. g. , a deck of cards in order) is much lower than a highly disordered one (in which the cards are random). Chemical reactions or events are driven toward spontaneity by the energetics of the process. Water will boil if heated to 100 °C at 1 atm. Gas particles in one chamber will flow into an empty one (see figure: (a) is spontaneous while (b) is not).

Does a decrease in enthalpy mean a reaction proceeds spontaneously? Spontaneous reactions CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) DH 0 = -890. 4 k. J H+ (aq) + OH- (aq) H 2 O (l) DH 0 = -56. 2 k. J H 2 O (s) NH 4 NO 3 (s) H 2 O (l) DH 0 = 6. 01 k. J H 2 O NH 4+(aq) + NO 3 - (aq) DH 0 = 25 k. J 18. 2

Entropy (S) is a measure of the randomness or disorder of a system. order disorder S S DS = Sf - Si If the change from initial to final results in an increase in randomness Sf > Si DS > 0 For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state Ssolid < Sliquid << Sgas H 2 O (s) H 2 O (l) DS > 0 18. 3

Processes that lead to an increase in entropy (DS > 0) 18. 2

How does the entropy of a system change for each of the following processes? (a) Condensing water vapor Randomness decreases Entropy decreases (DS < 0) (b) Forming sucrose crystals from a supersaturated solution Randomness decreases Entropy decreases (DS < 0) (c) Heating hydrogen gas from 600 C to 800 C Randomness increases Entropy increases (DS > 0) (d) Subliming dry ice Randomness increases Entropy increases (DS > 0) 18. 3

Entropy State functions are properties that are determined by the state of the system, regardless of how that condition was achieved. energy, enthalpy, pressure, volume, temperature, entropy Potential energy of hiker 1 and hiker 2 is the same even though they took different paths. 18. 3

First Law of Thermodynamics Energy can be converted from one form to another but energy cannot be created or destroyed. Second Law of Thermodynamics The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. Spontaneous process: DSuniv = DSsys + DSsurr > 0 Equilibrium process: DSuniv = DSsys + DSsurr = 0 18. 4

Entropy Changes in the System (DSsys) The standard entropy of reaction (DS 0 rxn) is the entropy change for a reaction carried out at 1 atm and 250 C. a. A + b. B DS 0 rxn = c. C + d. D [ c. S 0(C) + d. S 0(D) ] - [ a. S 0(A) + b. S 0(B) ] DS 0 rxn = S n. S 0(products) - S m. S 0(reactants) What is the standard entropy change for the following reaction at 250 C? 2 CO (g) + O 2 (g) 2 CO 2 (g) S 0(CO) = 197. 9 J/K • mol S 0(O 2) = 205. 0 J/K • mol S 0(CO 2) = 213. 6 J/K • mol DS 0 rxn = 2 x S 0(CO 2) – [2 x S 0(CO) + S 0 (O 2)] DS 0 rxn = 427. 2 – [395. 8 + 205. 0] = -173. 6 J/K • mol 18. 4

Entropy Changes in the System (DSsys) When gases are produced (or consumed) • If a reaction produces more gas molecules than it consumes, DS 0 > 0. • If the total number of gas molecules diminishes, DS 0 < 0. • If there is no net change in the total number of gas molecules, then DS 0 may be positive or negative BUT DS 0 will be a small number. What is the sign of the entropy change for the following reaction? 2 Zn (s) + O 2 (g) 2 Zn. O (s) The total number of gas molecules goes down, DS is negative. 18. 4

Gibbs Free Energy Spontaneous process: DSuniv = DSsys + DSsurr > 0 Equilibrium process: DSuniv = DSsys + DSsurr = 0 For a constant-temperature process: Gibbs free energy (G) DG = DHsys -TDSsys DG < 0 The reaction is spontaneous in the forward direction. DG > 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction. DG = 0 The reaction is at equilibrium. 18. 5

The standard free-energy of reaction (DG 0 rxn) is the freeenergy change for a reaction when it occurs under standardstate conditions. a. A + b. B c. C + d. D 0 DGrxn = [ c. DG 0 f (C) + d. DG 0 f (D) ] - [ a. DG 0 f (A) + b. DG 0 f (B) ] 0 DGrxn = S n. DG 0 f (products) - S m. DG 0 f (reactants) Standard free energy of formation (DG 0 f ) is the free-energy change that occurs when 1 mole of the compound is formed from its elements in their standard states. DG 0 f of any element in its stable form is zero. 18. 5

What is the standard free-energy change for the following reaction at 25 0 C? 2 C 6 H 6 (l) + 15 O 2 (g) 12 CO 2 (g) + 6 H 2 O (l) 0 DGrxn = S n. DG 0 f (products) - S m. DG 0 f (reactants) 0 DGrxn = [12 DG 0 f (CO 2) + 6 DG 0 f (H 2 O)] - [ 2 DG 0 f (C 6 H 6)] 0 DGrxn = [ 12 x– 394. 4 + 6 x– 237. 2 ] – [ 2 x 124. 5 ] = -6405 k. J Is the reaction spontaneous at 25 0 C? DG 0 = -6405 k. J < 0 spontaneous 18. 5

DG = DH - TDS 18. 5