Chem chemical reactions can go forward as well as in the reverse direction. concentrations of reactants and products become constant over time

Transcription

1 Chemical equilibrium Reading: Chapter 15 (omit 15.7) As you read ask yourself What is meant by chemical equilibrium? How does the equilibrium constant expression depend on the nature of the species (liquids, solids, gases) involved in the equilibrium? How does the magnitude of the equilibrium constant provide information about the relative amounts of reactants and products at equilibrium? How can the direction of a reaction (towards products or toward reactants) be predicted based on the starting amounts? How can the value of the equilibrium constant be obtained from amounts of reactants and products present in equilibrium? How can the equilibrium concentrations of reactants and products be obtained from the values of the equilibrium constant? Chem 10 1 Chemical equilibrium we have studied physical equilibria chemical reactions can go forward as well as in the reverse direction con ncentration (mol L -1 ) concentrations of reactants and products become constant over time H + I HI HI H + I HI HI time H, I H, I time Chem 10 1

2 dynamic equilibrium CO(g) + H (g) CH 3 OH(g) Chem 10 3 how can we describe the equilibrium? a specific ratio of concentrations is constant law of mass action: a A + b B d D + e E CO(g) + H (g) CH 3 OH(g) based on the balanced chemical equation Chem 10 4

3 Expt. 1 Expt. Expt. 3 CH 3 OH CO H Chem 10 5 K depends on the equilibrium conditions and the nature of the reaction concentrations can be expressed in molarity (moles/l) or in partial pressures, P CO SO (g) + O (g) SO 3 (g) recall: PV = nrt, so P = (n/v) RT when substituted into K P expression: Chem

4 The equilibrium constant does not have units K is related to rates of reactions and to thermodynamics in thermodynamics the equilibrium constant is defined in terms of activities activity is defined as a ratio of actual concentration or pressure to concentration or pressure of a reference state reference states: 1 M solutions, 1 atm for partial pressures activity = 1 for pure solids and pure liquids a CO = [CO]/c o where a is activity and c 0 is the reference value 1 M units cancel in ratios we will ignore slight numerical differences between activities and concentrations or pressure and use no units for the equilibrium constant values Chem 10 7 magnitude of equilibrium constants value of K C (or K P ) indicates how far the reaction has proceeded towards products the equilibrium i constant t says nothing about very small very large Chem

5 K values for related reactions consider the reaction multiples SO (g) + O ( g) SO 3 (g) K C = [ SO3 ] [ SO ] [ O ] If you multiply the coefficients of a chemical equation by a factor Chem 10 9 reverse reactions consider the reaction SO (g) + O ( g) SO 3 (g) [ SO3 ] K C = [ SO ] [ O ] SO O K C if you reverse the reaction, Chem

6 new reactions Given: NO(g) + Br (g) NOBr(g) K C1 =.0 NO(g) N (g) + O (g) K C =.1 x What is K c for N (g) + O (g) + Br (g) NOBr(g)? if you add reactions to obtain an overall chemical equation, Chem Heterogeneous equilibria (section 15.4) CaCO 3 (s) CaO(s) + CO (g) small amount of CaCO 3 large amount of CaO large amount of CaCO 3 small amount of CaO Chem

7 heterogeneous equilibria pure solids and pure liquids concentration (mol/l ) of solids and liquids CaCO 3 (s) CaO(s) + CO (g) Ni(s) + 4 CO(g) Ni(CO) 4 (g) Ag O(s) + HNO 3 (aq) AgNO 3 (aq) + H O(l) solvents are omitted when concentrations are low, because they are effectively a pure liquid Chem Calculating equilibrium constants from concentrations Does the data provide equilibrium concentrations? Use initial amounts and mol relationships from balanced chemical equation to calc. equilibrium concentrations put equilibrium concentrations into the equilibrium constant expression Chem

8 Example: If the equilibrium concentration of O in air is 8 x 10-3 M and that of O 3 is 9 x 10-3 M, what is the equilibrium constant for the conversion of O (g) to O 3 (g)? Chem Example: N O 4 (g) NO (g) In an experiment.00 mol of N O 4 (g) are placed in an empty 5.00 L flask and heated to 407 K. A dark red-brown gas appears indicating the formation of NO. The intensity of the colour indicates that the concentration of NO at equilibrium is 0.55 M. What is the equilibrium constant? Chem

9 Applications of equilibrium constants Predict the direction of a reaction SO 3 (g) SO (g) + O (g) Q Use value of Q defined same way as equilibrium constant expression except the concentrations are not equilibrium values Compare relative magnitude of Q and K Chem Example: A mixture of hydrogen, iodine and hydrogen iodide, each at a concentration of M, was introduced into a container heated to 783 K. At this temperature, K P = 46 for the reaction H (g) + I (g) HI(g) predict whether or not more HI has a tendency to form. Chem

10 Calculating equilibrium concentrations: write the balanced chemical equation use the equation to make a table of initial concentrations calculate Q, compare to K and predict direction of reaction represent the change in one concentration as x and use the mol relationships to define the changes in all other species in terms of x sum the initial concentration and the change represented by values of x to get expressions for the equilibrium concentrations substitute the equilibrium concentration expressions into the equation for K and solve for x use the value of x to calculate the equilibrium concentrations Chem Example In an experiment H and CO are placed in a flask so that their concentrations are both M. The mixture is heated to 40 C and equilibrium is achieved. What are the concentrations of water and carbon monoxide (the products) at equilibrium? H (g) + CO (g) H O (g) + CO(g) K c = 010( 0.10 (at t40 C) Chem

11 Example: At 5 C, the reaction I (g) + Cl ((g) ICl(g) has an equilibrium constant K P = Initially a reaction mixture at this temperature contains PI = atm, PCl = atm and PICl = atm. Calculate the equilibrium partial pressures of I, Cl, and ICl. Chem 10 1 Example: What is the equilibrium concentration of the pollutant NO produced in automobile engines at high temperature if the reactants N (g) and O (g) are present in their normal ratio in air:.4 M N and 0.56 M O? N (g) + O (g) NO(g) K C = 1.7 x 10-3 at 300 K Chem 10 11