Chemical Equilibrium, Study Guides, Projects, Research of Physical Chemistry

Download Chemical Equilibrium and more Study Guides, Projects, Research Physical Chemistry in PDF only on Docsity! C H E M I C A L E Q U I L I B R I U M P H Y S I C A L C H E M I S T R Y 2 P R E P A R E D B Y : E N G R . L . A . A G U S T I N C H E M I C A L E Q U I L B R I U M - the dynamic state of a reversible reaction when the two opposing reactions occur at the same rate and the concentrations of reactants and products do not change with time A + B ⇌ C + D A + B à C + D (Forward Reaction) C + D à A + B (Reverse Reaction) L A W O F M A S S A C T I O N à“the rate of a chemical reaction is proportional to the active masses of the reactants” *active mass – molar concentration (mols/L) Consider the general reaction: A + B ⇌ C + D According to Law of Mass Action: Rate of Forward reaction ∝ [A][B] = k1[A][B] Rate of Reverse reaction ∝ [C][D] = k2 [C][D] E Q U I L I B R I U M C O N S T A N T : E Q U I L I B R I U M L A W From the given reaction: A + B ⇌ C + D k1 k2 = C [D] A [B] At any specific temperature, k1/k2 is constant, and this ratio is called: Equilibrium constant, Kc Kc = ! [#] % [&] à This equation is known as the Equilibrium constant expression or Equilibrium Law E Q U I L I B R I U M C O N S T A N T E X P R E S S I O N F O R A R E A C T I O N I N G E N E R A L T E R M S Consider the general reaction: aA + bB ⇌ cC + dD Where a, b, c and d are numerical quotient of the substance, A, B, C, and D respectively. The equilibrium constant expression is Kc = " ! # " $ # % $ Note: The equilibrium constant has no units. E X A M P L E : Predict the extent of reactions of the following: 1. H2(g) + Cl2(g) ⇌ 2HCl at 300K has Kc = 4.0x1031 2. H2(g) + I2(g) ⇌ 2HI(g) has Kc=57.0 A P P L I C AT I O N S O F E Q U I L I B R I U M C O N S TA N T 2. Predicting the Direction of the Reaction *Qc à reaction quotient at any time a.) If Qc > Kc, the reaction will proceed in the direction of the reactants (the reverse reaction will take place) b.) If Qc < Kc, the reaction will proceed in the direction of the products (forward reaction will take place) c.) If Qc = Kc, the reaction is already in equilibrium E X A M P L E : The value of Kc for the reaction 2A ⇌ B + C is 2x10-3. At a given time, the concentration of the reaction mixture is [A]=[B]=[C] = 3x10-4M. In which direction will the reaction proceed? E X A M P L E : At certain temperature, 0.100 mole of H2 and 0.100 mole of I2 were placed in a one-liter flask. The purple color of iodine vapor was used to monitor the reaction. After a time, equilibrium H2 + I2 ⇌ 2HI was established and it was found that the concentration of I2 decreased to 0.020 mol/L. Calculate the value of Kc for the reaction at the given temperature. E X A M P L E The reaction 2NO (g) + Cl2(g) ⇌ 2NOCl was studied at 25oC. The partial pressure at equilibrium were found to be PNOcl =1.2 atm PNO = 5.0x10-2 atm PCl2 = 3.0x10-1 atm Calculate for the Kp for the reaction at 25oC. A P P L I C AT I O N S O F E Q U I L I B R I U M C O N S TA N T 4. Calculation of the Degree of Dissociation EXAMPLE: 25 mL of H2 and 18 mL of I2 vapor were heated in a sealed glass bulb at 465oC when at equilibrium 30.8 ml of HI was formed. Calculate the degree of dissociation of pure HI at 465oC. E X A M P L E : At 500oC, the reaction between N2 and H2 to form ammonia has Kc = 6.0x10-2. What is the numerical value of Kp for the reaction? E X A M P L E The value of Kp at 25oC for the reaction 2NO(g) + Cl2(g) ⇌ 2NOCl(g) is 1.9x103 atm-1. Calculate the value of Kc at the same temperature. L E ’ C H AT E L I E R ’ S P R I N C I P L E “If an equilibrium is subjected to stress, the equilibrium shifts in such a way as to reduce the stress” PRESSUREINCREASED Pa rei \Pa : . The eaitbrimpostionshits | Dg Te squmprumeesvenss ta | SIDE OF REACTION WITH FEWER SIDE OF REACTION WITH MORE . GAS MOLECULES FAVOURED . GAS MOLECULES FAVOURED i: inshewamplebstow here Hn inthe example above the new more C and D, and less A and B. more A and B, and less C and D. E X A M P L E : Consider the reaction below: 2SO3 (g) ⇌ 2SO2 (g) + O2 (g) ∆H = +198kJ What change would shift the equilibrium to the left? For a reaction in equilibrium, the following relation of the standard Gibb’s free energy and K is given: ∆Go= -RT (ln K ) ∆Go= -2.303 RT (log K ) Where ΔGo = standard Gibb’s Free Energy, K = equilibrium constant The sign of ΔGo indicates whether the forward or reverse reaction is spontaneous. • If ΔGo is negative, the reaction proceeds spontaneously in the forward reaction • If ΔGo is positive, the reverse reaction is spontaneous. • If ΔGo =0, the reaction is at equilibrium. VAN ’ T HO F F I S OTH E RM E X A M P L E The partial pressure of CO2 in the reaction CaCo3 (s) ⇌ CaO (s) + CO2(g) is 0.773 mm at 500oC. Calculate the Kp at 600oC for the above reaction. Δ H of the reaction is 43.2 kcal per mole and does not change in the given range of temperatures. E X A M P L E The equilibrium constant of a reaction doubles on raising the temperature from 25oC to 35oC. Calculate the ΔHo for the reaction. T H A N K YO U F O R L I S T E N I N G !