CHAPTER 6 CHEMICAL EQUILIBRIUM

Transcription

1 For updated version, please click on BSK1133 PHYSICAL CHEMISTRY CHAPTER 6 CHEMICAL EQUILIBRIUM PREPARED BY: DR. YUEN MEI LIAN AND DR. SITI NOOR HIDAYAH MUSTAPHA Faculty of Industrial Sciences & Technology yuenm@ump.edu.my and snhidayah@ump.edu.my

2 Description Aims To discuss the equilibrium state of chemical and physical equilibrium. To express and calculate equilibrium constants for homogeneous equilibria, heterogeneous equilibria and multiple equilibria. To discuss the factors that may affect the position of equilibrium. To learn the Le Châtelier s principle in the prediction of changes.

3 Description Expected Outcomes Able to describe the equilibrium state of chemical and physical equilibrium. Able to express and calculate the equilibrium constants. Able to study the factors that may affect the position of an equilibrium. Able to apply the Le Châtelier s principle. References Atkins, P & Julio, D. P. (2006).Physical Chemistry (8th ed.). New York: Oxford. Chang, R. (2005).Chemistry (8th ed.). New York: McGraw Hill. Atkins, P & Julio, D. P. (2012). Elements of Physical Chemistry (sixth ed.). Freeman, Oxford. Silbey, R. J., Alberty, A. A., & Bawendi, M. G. (2005). Physical Chemistry. New York: John Wiley & Sons Mortimer R. G. (2008) Physical Chemistry, Third Edition, Elsevier Academic press, USA.

4 Contents 6.1 The Concept of Equilibrium 6.2 Relate the Kinetics in Chemical Equilibrium 6.3 Factors Conclusion

5 6.1 The Concept of Equilibrium

6 I. Introduction The Concept of Equilibrium and the Equilibrium Constant o o Chemical equilibrium can be obtained when the rates of the forward and reverse reactions must be equal. In addition, the concentrations of the reactants and products are unchanged. Example: 2NO 2 (g) N 2 O 4 (g) Physical equilibrium is referring equilibrium of two phases with the same substance. Example: H 2 O (l) H 2 O (g)

7 II. Equilibrium Constant a) Homogeneous Equilibria same phase of reacting species. N 2 O 4 (g) 2NO 2 (g) K c = [NO 2] 2 [N 2 O 4 ] K c shows the reacting species (gas) (units = molarity, moles per liter and partial pressure). Pressure of a gas is to concentration in mol/l of gas (at constant temperature) P = ( n )RT K V p = P NO 2 P N2 O 4 P NO2 and P N2 O 4 are equilibrium partial pressure (atm). K p shows equilibrium concentration expressed in terms of pressure. K c K p cause their units of expression are different. Example: H 2 (g) + Br 2 (g) 2HBr (g) Since Δ n = 2 2 = 0, therefore, K p = (0.0821T) Δn K c K p = (0.0821T) 0 K c K p = K c (Special Case for this reaction where K p = K c ) 2

8 Explanation: Assume ideal gas behaviour aa (g) bb (g) (a,b = stoichiometric coefficients) K c = [B]b [A] a P A = n ART V Substitute in K p K p = (n BRT V )b ( n A RT V )a = ( b K p = P B P B = n BRT V n B V )b ( n A n A V P A a K c K p and n B V can be replaced by [A] and [B]. (RT)b-a = [B]b V )a [A] a (RT)Δn = K c (RT) Δn K p = (0.0821T) Δn K c P A = partial pressure of A P B = partial pressure of B V = volume of the container in liters have units of mol/l. Thus, Where Δ n = b a (moles of gaseous products moles of gaseous reactants) Pressures are usually expressed in atm, hence, gas constant R is atm L/(K mol).

9 b) Heterogeneous Equilibria o Refer to a reversible reaction where different phases of reactants and products that are in. CaCO 3 (s) CaO (s) + CO 2 (g) K c = CaO [CO 2] [CaCO 3 ] K c is used as symbols to differentiate it from final equilibrium constant at the end. o SPECIAL: Concentration (means density) of a solid does not depend on how much of the substance is present. Example: [CaCO 3 ] and [CaO] are constant and combine with K c to form final equilibrium constant. [CaCO 3 ] [CaO] K c = Kc = [CO 2 ] (new equilibrium constant) In thermodynamic, concentration replace with activity of pure solid or liquid = 1. Thus, K c = [CO 2 ] or K p = P CO2

10 c) Multiple Equilibria Example: These equilibria can be split into two steps: K c = A2 H + 2 [H 2 A] K c is equal to the product of the equilibrium expressions for the two reaction steps. Thus, K c = K 1 K 2

11 d) The Form of K and the Equilibrium Equation Condition I N 2 O 4 (g) 2NO 2 (g) 2NO 2 (g) N 2 O 4 (g) K c = [NO 2] 2 = 4.63 x [N 2 O 4 ] 10-3 K c = N 2O 4 = 1 = [NO 2 ] 2 K c o K c K c = 1. (Must specify the equilibrium equation) x 10 Condition II 1 N 2 2O 4 (g) NO 2 (g) N 2 O 4 (g) 2NO 2 (g) K c = [NO 2] 1 K c = [NO 2] 2 [N 2 O 4 ] [N 2 O 4 ] 2 o K c = K c, since K c = 4.63 x 10-3, Hence, K c = = 216 SUMMARY K p = (0.0821T) Δn K c Equilibrium constant (K p or K c ) is unitless.

12 6.2 Relate the Kinetics in Chemical Equilibrium

13 Relate Kinetics and Chemical Equilibrium a) A single elementary step k f rate constant A + 2B AB k 2 Forward: r rate f = k f [A][B] 2 Reverse: rate r = k r [AB] 2 At equilibrium (no net changes): rate f = rate r K c = k f = [AB 2] k r [A][B] 2 b) More than one elementary step k f Step 1: 2B B 2 Step 2: A + B AB 2 Overall reaction: A + 2B AB 2 (Example of multiple equilibria) K = k f K = k f k r = [B 2] [B] 2 k r k f k r k = [AB 2] r A [B 2 ] Overall reaction: K c = K K = AB 2 A [B] 2

14 6.2 Relate the Kinetics in Chemical Equilibrium

15 Le Chatelier s Principle It states that if external stress is introduced to an equilibrium system, the system will adjust to offset it and achieve a new equilibrium position. It helps in prediction of the position of an equilibrium reaction when a change of conditions (concentration, pressure, volume and temperature) occurs. When a new equilibrium reaches, the concentrations of reactants and products remain constant.

16 Forward Reaction A (g) + 3B (g) C (g) + D (g) ΔH = kj/mol Factors Position of Equilibrium shift to right shift to left Concentration Increase (substance A) Decrease (substance A) Pressure Increase Decrease Temperature Decrease Increase Catalyst Unchange

17 Conclusion Chemical equilibrium is referring different substances as reactants and products in an equilibrium, whereas, physical equilibrium is equilibrium between two same phases of same composition of reactants and products. Equilibrium constants is used as an indicator to show relative amount between reactants and products. The direction where an equilibrium reaction will shift is assessed by Le Châtelier s Principle when an occurrence of varying conditions.

18 AUTHOR INFORMATION DR. YUEN MEI LIAN (SENIOR LECTURER) INDUSTRIAL CHEMISTRY PROGRAMME FACULTY OF INDUSTRIAL SCIENCES & TECHNOLOGY UNIVERSITI MALAYSIA PAHANG Tel. No. (Office): DR. SITI NOOR HIDAYAH MUSTAPHA (SENIOR LECTURER) INDUSTRIAL CHEMISTRY PROGRAMME FACULTY OF INDUSTRIAL SCIENCES & TECHNOLOGY UNIVERSITI MALAYSIA PAHANG Tel. No. (Office):