Thermochemistry Study of energy transformations and transfers that

Thermochemistry • Study of energy transformations and transfers that accompany chemical and physical changes. • Terminology 4 System 4 Surroundings 4 Heat (q) transfer of thermal energy 4 Chemical energy - E stored in structural unit

Energy [capacity to do work] • POTENTIAL [stored energy] • KINETIC [energy of matter] 4 K. E. = 1/2 mu 2 4 Units JOULES (J) = Kg m 2/ s 2 First Law of Thermodynamics ( Law of Conservation of Energy ) “The Total Energy of the Universe is Constant” Universe = ESystem + ESurroundings = 0

Enthalpy • • • Property of matter Heat content, symbol H Endothermic or Exothermic Fixed at given temperature Directly proportional to mass Quantitative 4 H 0 reaction = a H 0 products - b H 0 reactants 4 H 0 = q reaction and q reaction = - q water 4 q = (mass)(specific heat)( temp)

Change in Enthalpy = H Enthalpy is defined as the system’s internal energy plus the product of its pressure and volume. H = E + PV For Exothermic and Endothermic Reactions: H = H final - H initial = H products - H reactants Exothermic : H final H initial H 0 Endothermic : H final H initial H 0 Draw enthalpy diagrams

Gases Sublimation H 0 sub Deposition Condensation - H 0 vap - H 0 sub Vaporization H 0 vap Liquids Freezing - H 0 fus Melting H 0 fus Sublimation Solids Deposition

Special H’s of Reactions When one mole of a substance combines with oxygen in a combustion reaction, the heat of reaction is the heat of combustion( Hcomb): C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(g) H= Hcomb When one mole of a substance is produced from it’s elements, the heat of reaction is the heat of formation ( Hf ) : Ca(s) + Cl 2 (g) H= Ca. Cl 2 (s) Hf When one mole of a substance melts, the enthalpy change is the heat of fusion ( Hfus) : H 2 O(s) H 2 O(L) H= Hfus When one mole of a substance vaporizes, the enthalpy change is the heat of vaporization ( Hvap) : H 2 O(L) H 2 O(g) H = Hvap

Fig. 6. 14

Bond Energies • Energy of a reaction is the result of breaking the bonds of the reactants and forming bonds of the products. • H 0 reaction = bonds broken + bonds formed 4 breaking bonds requires energy – endothermic(+) 4 forming bonds releases energy – exothermic (-)

Fig. 6. 10

Calorimetry • Laboratory Measurements • Calorimeter is device used to measure temperature change. • q = (mass)(specific heat)( temp) 4 Heat capacity = amount of heat to raise temperature 1 o. C. 4 Specific heat = amount of heat to raise temperature of 1 g of substance 1 o. C. 4 J/g- o. C or molar heat J/ mol- o. C 4 heat lost =heat gained

Calorimeters Lab Coffee-Cup Bomb

Specific Heat Capacity and Molar Heat Capacity and Specific Heat heat capacity = q q = Quanity of Heat q = constant x T T J Specific heat capacity = g. K q = c x mass x T Molar Heat Capacity (C) = “C” has units of: = c J mol. K q moles x T

Stoichiometry • Thermochemical Equation CH 4 + 2 O 2 CO 2 + 2 H 2 O + 890 k. J 4 H [-] exothermic, heat product 4 H [+] endothermic, heat reactant 4 heat can be calculated using balanced chemical reaction including enthalpy information. • Example: Calculate the amount of heat released when 67 grams of oxygen is used.

Hess’s Law of Heat Summation The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps. Need overall final reaction and individual reactions with enthalpy change. Example: Calculate the enthalpy for the reaction N 2 + 2 H 2 N 2 H 4 Given: N 2 + 3 H 2 2 N H 3 H = - 92. 4 k. J N 2 H 4 + H 2 2 N H 3 H = - 183. 9 k. J H reaction = H 1 + H 2 + H 3 + …. H = ? ? ?

Entropy Summary • • Examples and activities Disorder favored for spontaneous reactions Symbol S Units: J / Kelvin or J / K • mol So standard conditions [25 o. C and 1 atm] S>0 [+] more disorder - favored Tables [elements] S 0 reaction = a S 0 products - b S 0 reactants Examples - Practice Problems

Spontaneity • Need to consider both H and S • Examples: 4 Combustion of C 4 Ice melting H __(-)__ __(+)__ S __+___ • Second Law of Thermodynamics 4 In any spontaneous process there is always an increase in the entropy of the universe 4 Suniverse = Ssystem + Ssurrounding 4 Entropy of the universe is increasing.

• Third Law of Thermodynamics 4 Entropy of a perfect crystal at 0 Kelvin is 0 4 Based on this statement can use So values from the tables and calculate Srxn 4 S 0 reaction = a S 0 products - b S 0 reactants • Outcome: Determine S 0 Rxn both qualitatively and quantitatively • Conclusion: G 0 = H 0 - T S 0 SPONTANEITY DEPENDS ON H, S & T

Free Energy Gibbs free energy–This is a function that combines the systems enthalpy and entropy: • New Thermo Quantity • When a reaction occurs some energy known as Free Energy of the system becomes available to do work. • Symbol G 4 Reactions spontaneous nonspontaneous equilibrium G — + 0 [release free energy] [absorb free energy]

Free Energy Quantitative • For a given reaction at constant T and P G = H – T S 4 H and S are given or calculated from tables 4 Remember T is absolute [Kelvin scale] 4 watch units on H and S, they need to match • Can also use Free Energy Tables G 0 reaction = a G 0 products - b G 0 reactants

Reaction Spontaneity and the Signs of Ho So Ho , So, and -T So Go Go Description - + - - Spontaneous at all T + - + + Nonspontaneous at all T + + - + or - Spontaneous at higher T; Nonspontaneous at lower T - - + + or - Spontaneous at lower T; Nonspontaneous at higher T Table 20. 1 (p. 879)

Qualitative

Temperature & Spontaniety Quantitative G = H – T S Use to calculate G at different T

Free Energy and its relationship with Equilibria and Reaction Direction o G = -RT ln K

The Relationship Between Go (k. J) 200 100 50 10 1 0 -1 -10 -50 -100 -200 K 9 x 10 -36 3 x 10 -18 2 x 10 -9 2 x 10 -2 7 x 10 -1 1 1. 5 5 x 101 6 x 108 3 x 1017 1 x 1035 Table 20. 2 (p. 883) Go and K at 25 o. C Significance Essentially no forward reaction; reverse reaction goes to completion. Forward and reverse reactions proceed to same extent. Forward reaction goes to completion; essentially no reverse reaction.

Free Energy and Equilibrium Constant Qualitative Summary G < 0 spontaneous and Kc determines extent of reaction (K>1 or large favors products) G 0 0 K = 1 at equilibrium <0 (-) >1 spontaneous forward reaction >0 (+) <1 nonspontaneous forward reaction

Free Energy and Equilibrium Constant Quantitative • G = G 0 + RT ln. Q 4 G at any conditions and G 0 standard conditions 4 at equilibrium G = 0 and Q = K therefore: 0 = G 0 + RT ln. Q and G 0 = – RT ln. K 4 R = 8. 314 J/mol • K and T in Kelvin Outcome: Be able to calculate G and K and interpret results.

Thermochemistry Summary • Study of energy transformations and transfers that accompany chemical and physical changes. • First Law of Thermodynamics: 4 Energy of the Universe is constant • Second Law of Thermodynamics: 4 Entropy of the universe increasing • Third Law of Thermodynamics: 4 Entropy of a perfect crystal at 0 Kelvin is zero.

Spontaneity Occurs without outside intervention • Enthalpy 4 H 0 reaction = a H 0 products - b H 0 reactants 4 H 0 = q reaction and q reaction = - q water 4 q = (mass)(specific heat)( temp) • Entropy 4 S 0 reaction = a S 0 products - b S 0 reactants • Free Energy 4 G 0 reaction = a G 0 products - b G 0 reactants 4 G 0 reaction = H - T S

Nonstandard Conditions • G = G 0 + RT ln Q for nonstandard conditions 4 when at equilibrium Q = K and G = 0 4 G 0 = -RT ln K 4 R = 8. 314 J/mole Kelvin • G and K both are extent of reaction indicators. 4 G < 0 K >1 spontaneous product favored 4 G > 0 K<1 non spontaneous reactant favored 4 G = 0 K =1 equilibrium