CHEMICAL EQUILIBRIUM

Collision Model Molecules must collide in order for a reaction to occur. Rate depends on concentrations of reactants and temperature. Copyright © Cengage Learning. All rights reserved

Temperature – increases rate. Why? Concentration – increases rate because more molecules lead to more collisions. Temperature – increases rate. Why? Copyright © Cengage Learning. All rights reserved

Activation Energy Minimum energy required for a reaction to occur. Copyright © Cengage Learning. All rights reserved

Catalyst A substance that speeds up a reaction without being consumed. Enzyme – catalyst in a biological system Copyright © Cengage Learning. All rights reserved

HC2H3O2 + H2O  H3O+ + C2H3O2- Equilibrium occurs when two opposing processes occur at the same rate. The rates are equal in both directions. Once equilibrium is reached, the concentrations don’t change, but the reaction constantly takes place. Almost all systems come to equilibrium. Composition of the equilibrium mix does not depend on which side of equation we begin.

Graph of Equilibrium

The mass action expression equals [Products]x/[Reactants]y . This manner of expression is a convention. [ NaCl] indicates concentration of NaCl. The equilibrium constant (Keq) is the value of the mass action expression when equilibrium concentrations are used. It is only valid at a given temperature.

Keq or Kc Consider the general chemical equation The exponents in the mass action expression are the same as the stoichiometric coefficients

MEANING OF Keq VALUES K is very large -- reaction proceeds far to completion; position of equilibrium is far toward product side. K = 1 -- Concentration of products & reactants about same at equilibrium; equilibrium is midway between products & reactants. K is very small -- little product formed; position of equilibrium is far toward reactants.

Kc and Kp

HETEROGENEOUS EQUILIBRIUM When more than one phase exists in a reaction, it is called a heterogeneous reaction. The concentration of a pure solid (sodium carbonate) or a pure liquid (water) is a constant. Equilibrium law expressions are written without concentration of pure solids and liquids.

Heterogeneous Equilibria The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present Write the equilibrium expression for the reaction: PCl5(s)  PCl3(l) + Cl2(g) Pure solid Pure liquid

ICE CHART Concentration must be in moles/L Concentration must be at equilibrium In the change row products and reactants must have opposite signs. In the change row products and reactants are in the same ratio as the coefficients for the reaction’s equation. A + B ↔ C + D Reaction A B C D Initial concentration   Change Equilibrium

Reaction Quotient (Q) The reaction quotient (Q) is the value of the mass action expression when nonequilibrium concentrations are used. If Q < Keq, products will form If Q> Keq, reactants will form If Q = Keq, reaction is at equilibrium

If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change. Copyright © Cengage Learning. All rights reserved

Effect of a Change in Concentration When a reactant or product is added the system shifts away from that added component. If a reactant or product is removed, the system shifts toward the removed component. Copyright © Cengage Learning. All rights reserved

Effect of a Change in Volume The system is initially at equilibrium. The piston is pushed in, decreasing the volume and increasing the pressure. The system shifts in the direction that consumes CO2 molecules, lowering the pressure again. Copyright © Cengage Learning. All rights reserved

Effect of a Change in Volume Decreasing the volume The system shifts in the direction that gives the fewest number of gas molecules. Increasing the volume The system shifts in the direction that increases its pressure. Copyright © Cengage Learning. All rights reserved

Effect of a Change in Temperature The value of K changes with temperature. We can use this to predict the direction of this change. Exothermic reaction – produces heat (heat is a product) Adding energy shifts the equilibrium to the left (away from the heat term). Endothermic reaction – absorbs energy (heat is a reactant) Adding energy shifts the equilibrium to the right (away from the heat term). Copyright © Cengage Learning. All rights reserved

Ksp = [M+][X] = solubility product constant The equilibrium conditions also applies to a saturated solution containing excess solid, MX(s). Ksp = [M+][X] = solubility product constant The value of the Ksp can be calculated from the measured solubility of MX(s). Copyright © Cengage Learning. All rights reserved

Solubility Equilibria Solubility product (Ksp) – equilibrium constant; has only one value for a given solid at a given temperature. Solubility – an equilibrium position. Bi2S3(s) 2Bi3+(aq) + 3S2–(aq) Copyright © Cengage Learning. All rights reserved