Presentation on theme: "Chapter 14 Chemical Equilibrium"— Presentation transcript:
1Chapter 14 Chemical Equilibrium Watch animation…chemical reactionreaching equilibriumequilibrium being dynamicDynamic equilibriumPhysical processes:solid-liquidvapor-liquid….Chemical reactions:reactantsproductsDynamic equilibria are responsiveto changes in the conditions.This chemical engineer is testing a process for the formation of new liquid fuels from coal and petroleum. Such methods may transform the generation of energy worldwide.
3No chemical reaction is ‘complete’, Figure In the synthesis of ammonia, the molar concentrations of N2, H2, and NH3 change with time until there is no further net change and the concentrations settle into values corresponding to a mixture in which all three substances are present.N2(g)+3H2(g)2NH3(g)N2(g)+3H2(g)2NH3(g)No chemical reaction is ‘complete’,No chemical reaction is ‘impossible’.
4Figure When we plot the rates of the forward and reverse reactions for the formation of ammonia on one graph, we can see that as the forward rate decreases, the reverse rate increases, until they are equal. At this point, the reaction is at equilibrium and the rates remain constant.ReactantsProducts
5N2(g)+2HD (g)+D2(g)2NHD2(g) Figure 14.3 In an experiment showing that equilibrium is dynamic, a reaction mixture in which N2 (pairs of blue spheres), D2 (pairs of yellow spheres), and ND3 have reached equilibrium is mixed with one with the same concentrations of N2, H2 (pairs of gray spheres), and NH3. After some time, the concentrations of nitrogen, hydrogen, and ammonia are found to be the same, but the D atoms are distributed among the hydrogen and ammonia molecules.N2(g)+3D2(g)2ND3(g)N2(g)+2HD (g)+D2(g)2NHD2(g)N2(g)+H2(g)+2HD(g)2NH2Dg)N2(g)+3H2(g)2NH3(g)Chemical reaction is dynamic, forwardand reverse reactions taking place all the time.
6Try Yourself to Define the Equilibrium Constant? aA+bB+cC+…pP+qQ+rR+…Kc=[P]+[Q]+[R]+…-[A]-[B]-[C]-…?Kc=p[P]+q[Q]+r[R]+…-a[A]-b[B]-c[C]-…?Kc=[P][Q][R]…/[A][B][C]…?Kc=pqr…[P][Q][R]…/abc…[A][B][C]…?
7Equilibrium Constant aA+bB+cC+…pP+qQ+rR+… Cato Guldberg & Peter Waage (1864)
8‘Justification’ of the Definition of Equilibrium Constant aA+bB+cC+…pP+qQ+rR+…A+A+…B+B+…+C+C+…P+P+..+Q+Q+…+R+R+…
9Example Is the same for all experiment no matter what the initial compositions are.
12Classroom ExerciseWrite the equilibrium constant for the following reactions:
13Multiply a reaction: Reverse a reaction: Using smallest possible stoichiometric coefficients to write equilibrium constantsMultiply a reaction:Reverse a reaction:
14Classroom ExerciseWrite down the equilibrium constant of the following reaction:H2+D22HDSuppose the equilibrium constant of above reaction at 500 Kis 3.6, what is the equilibrium constant of its reverse reaction?Kc=[HD]2/[H2][D2]Reverse reaction: Kc-1=0.28
18Figure The equilibrium constant for a reaction is equal to the ratio of the rate constants for the forward and reverse elementary reactions that continue in a state of dynamic equilibrium. (a) A relatively large forward rate constant means that the forward rate can match the reverse rate even though only a small amount of reactants is present. (b) Conversely, if the reverse rate constant is relatively large, then the forward and reverse rates are equal when only small amounts of products are present.
20Homogeneous Equilibria: All products and reactants are of the same phase. Heterogeneous Equilibria: Reacting systems with more than one phase.
21Ignored!Molar concentration of pure solid and liquid is a constant, independent of the amount present . It is ignored in the calculation of equilibrium constant.Another way of understanding: the concentration of pure solid/liquidIs always 100%1. Since 1a =1, it does not affect Kc.
27Classroom Exercise2S(s)+3O2(g)2SO3(g) T=300K, equilibrium concentrations: [O2]=0.25 mol/L, [SO3]=0.3 mol/L. calculate the equilibrium constant Kc and Kp.
28Gaseous EquilibriaEquilibrium constants for gaseous reactions can be written by using either molar concentrations or partial pressures.
29Summary of Equilibrium Constants The equilibrium constant is the ratio of the concentrations or partial pressures of the products to those of the reactants, each concentration raised to a power equal to its stoichiometric coefficient in the balanced equation.
30Using Equilibrium Constants The Extent of ReactionThe Direction of ReactionEquilibrium Tables
31The Extent of Reaction K>1000, product dominant; 0.001<K<1000, neither reactants nor products dominate equilibrium;K<0.001,reactants dominant.
32Figure The size of the equilibrium constant indicates whether the reactants (blue squares) or the products (yellow squares) are favored. Note that reactants are favored when Kc is small (left), products are favored when Kc is large (right), and reactants and products are in almost equal abundance when Kc is close to 1 (middle). Here, for simplicity, we compare reactions with Kc 102 and Kc 102.
34Classroom Exercise H2(g)+Cl2(g)2HCl(g) Suppose that the equilibrium molar concentrations of H2 and Cl2 at 300 K are both 1.0×10-16 mol/L. What is the equilibrium molar concentration of HCl, given Kc= 4.0×1031?H2(g)+Cl2(g)2HCl(g)
38Figure The relative sizes of the reaction quotient Q and the equilibrium constant K indicate the direction in which a reaction mixture tends to change. The arrows point from reactants to products when Q, K (left) or from products to reactants when Q K (right). There is no tendency to change once the reaction quotient has become equal to the equilibrium constant.
39Quiz At certain stage, it is measured that Will the reaction be moving to formation of more product or not?The reaction will be moving to formation of more product.
40Equilibrium TablesA table that shows the initial concentrations, the changes needed to reach equilibrium, and the final equilibrium compositions.
45Use of Le Chatelier Principle (I) Adding a reactant or removing a product reaction tends to form products.Adding a product or removing a reactant more reactant tends to form.
46Use of Le Chatelier Principle (II) Compression of a reaction mixture the reaction that reduces the number of gas-phase molecules.Increasing by introducing an inert gas no effect on the equilibrium!
49Introducing inert gas (yellow) has no effect on the equilibrium composition. More generally, introducing anything that does not react with any productand reactant of a reaction would not change the equilibrium compositionof the reaction. (A catalyst can increase reaction rate but not equilibriumconstant.)
52Use of Le Chatelier Principle (III) Raising the temperature of an exothermic reaction reaction tends to form more reactants.Raising the temperature of an endothermic reaction reaction tends to form more products.