7 7-1 © 2003 Thomson Learning, Inc. All rights reserved Bettelheim, Brown, and March General, Organic, and Biochemistry, 7e

7 7-2 © 2003 Thomson Learning, Inc. All rights reserved Chapter 7 Reaction Rates and Chemical Equilibrium Reaction Rates and Chemical Equilibrium

7 7-3 © 2003 Thomson Learning, Inc. All rights reserved Chemical Kinetics Chemical kinetics: Chemical kinetics: the study of the rates of chemical reactions consider the reaction that takes place when chloromethane and sodium iodide are dissolved in acetone; the net ionic equation for this reaction is to determine the rate of this reaction, we measure the concentration of iodomethane at periodic time intervals, say every 10 minutes

7 7-4 © 2003 Thomson Learning, Inc. All rights reserved Chemical Kinetics the rate of reaction is the increase in concentration of iodomethane divided by the time interval for example, the concentration might increase from 0 to 0.12 mol/L over a 30 minute time period the reaction rate over this period is this unit is read mole per liter per minute

7 7-5 © 2003 Thomson Learning, Inc. All rights reserved Reaction Rates The rates of chemical reactions are affected by the following factors molecular collisions activation energy nature of the reactants concentration of the reactants temperature presence of a catalyst On the following screens, we examine these factors one at a time

7 7-6 © 2003 Thomson Learning, Inc. All rights reserved Molecular Collisions in order for two species, A and B (they may be molecules or ions), to react, they must first collide it is possible to calculate how many collisions will take place between A and B in a given period of time such calculations indicate that the rate at which A and B collide is far greater than the rate at which they react the conclusion is that most collisions do not result in a reaction effective collisiona collision that does result in a reaction is called an effective collision there are two main reasons why some collisions are effective and others are not; activation energy and the orientation of A and B at the time of collision

7 7-7 © 2003 Thomson Learning, Inc. All rights reserved Molecular Collisions Activation energy: Activation energy: the minimum energy required for a reaction to take place in most chemical reactions, one or more covalent bonds must be broken and energy is required for this to happen this energy comes from the collision between A and B if the collision energy is large, there is sufficient energy to break the necessary bonds, and reaction takes place if the collision energy is too small, no reaction occurs

7 7-8 © 2003 Thomson Learning, Inc. All rights reserved Molecular Collisions Orientation at the time of collision the colliding particles must be properly oriented for bond breaking and bond making for example, to be an effective collision between H 2 O and HCl, the oxygen of H 2 O must collide with the H of HCl so that the new O-H bond can form and the H-Cl bond can break

7 7-9 © 2003 Thomson Learning, Inc. All rights reserved Energy Diagrams Energy diagram for an exothermic reaction

© 2003 Thomson Learning, Inc. All rights reserved Energy Diagrams The reaction of H 2 and N 2 to form ammonia is exothermic in this reaction, six covalent bonds are broken and six now ones formed breaking a bond requires energy, and forming a bond releases energy in this reaction, the energy released in making the six new bonds is greater than the energy required to break the six original bonds; the reaction is exothermic

© 2003 Thomson Learning, Inc. All rights reserved Energy Diagrams Energy diagram for an endothermic reaction

© 2003 Thomson Learning, Inc. All rights reserved Energy Diagrams Transition state: Transition state: a maximum on an energy diagram the transition state for the reaction between H 2 O and HCl probably looks like this, in which the new O-H bond is partially formed and the H-Cl bond is partially broken

© 2003 Thomson Learning, Inc. All rights reserved Factors Affecting Rate Nature of reactants in general, reaction between ions in aqueous solution are very fast (activation energies are very low) in general, reaction between covalent compounds, whether in water or another solvent, are slower (their activation energies are higher) Concentration in most cases, reaction rate increases when the concentration of either or both reactants increases for many reactions, there is a direct relationship between concentration and reaction rate; when concentration doubles the rate doubles

© 2003 Thomson Learning, Inc. All rights reserved Factors Affecting Rate Temperature in virtually all reactions, rate increases as temperature increases an approximate rule for many reactions is that for a 10°C increase in temperature, the reaction rate doubles when temperature increases, molecules move faster (have more kinetic energy), which means that they collide more frequently; more frequent collisions mean higher reaction rates not only do molecules move faster at higher temperatures, but the fraction of molecules with energy equal to or greater than the activation energy also increases

© 2003 Thomson Learning, Inc. All rights reserved Factors Affecting Rate The distribution of kinetic energies (molecular velocities) at two temperatures

© 2003 Thomson Learning, Inc. All rights reserved Factors Affecting Rate Catalyst: Catalyst: a substance that increases the rate of a chemical reaction without itself being used up

© 2003 Thomson Learning, Inc. All rights reserved Factors Affecting Rate Many catalysts provide a surface on which reactants can meet the reaction of ethylene with hydrogen is an exothermic reaction if these two reagents are mixed, there is no visible reaction even over long periods of time when they are mixed and shaken with a finely divided transition metal catalyst, such as Pd, Pt, or Ni, the reaction takes place readily at room temperature

© 2003 Thomson Learning, Inc. All rights reserved Reversible Reactions Reversible reaction: Reversible reaction: one that can be made to go in either direction if we mix CO and H 2 O in the gas phase at high temperature, CO 2 and H 2 are formed we can also make the reaction take place the other way by mixing CO 2 and H 2 the reaction is reversible, and we can discuss both a forward reaction and a reverse reaction

© 2003 Thomson Learning, Inc. All rights reserved Reversible Reactions Equilibrium: Equilibrium: a dynamic state in which the rate of the forward reaction is equal to the rate of the reverse reaction at equilibrium there is no change in concentration of either reactants or products reaction, however, is still taking place; reactants are still being converted to products and products to reactants, but the rates of the two reactions are equal

© 2003 Thomson Learning, Inc. All rights reserved Equilibrium Constants Equilibrium constant, K: Equilibrium constant, K: the product of the concentration of products of a chemical equilibrium divided by the concentration of reactants, each raised to the power equal to its coefficient in the balanced chemical equation for the general reaction the equilibrium constant is

© 2003 Thomson Learning, Inc. All rights reserved Equilibrium Constants Problem: Problem: write the equilibrium constant for this reversible reaction solution:solution: for this reaction, K is note that no exponents are shown in this equilibrium constant; by convention the exponent “1” is not written

© 2003 Thomson Learning, Inc. All rights reserved Equilibrium Constants Problem: Problem: when H 2 and I 2 react at 427°C, the following equilibrium is reached the equilibrium concentrations are [I 2 ] = 0.42 mol/L, [H 2 ] = mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K Solution:Solution: this K has no units because molarities cancel

© 2003 Thomson Learning, Inc. All rights reserved Equilibrium and Rates There is no relationship between a reaction rate and the value of K reaction rate depends on the activation energy of the forward and reverse reactions; these rates determine how fast equilibrium is reached but not its position it is possible to have a large K and a slow rate at which equilibrium is reached it is also possible to have a small K and a fast rate at which equilibrium is reached it is also possible to have any combination of K and rate in between these two extremes

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle LeChatelier’s Principle: LeChatelier’s Principle: when a stress is applied to a chemical system at equilibrium, the position of the equilibrium shifts in the direction to relieve the applied stress We look at three types of stress that can be applied to a chemical equilibrium addition of a reaction component removal of a reaction component change in temperature

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle Addition of a reaction component Addition of a reaction component suppose this reaction reaches equilibrium suppose we now disturb the equilibrium by adding some acetic acid the rate of the forward reaction increases and the concentrations of ethyl acetate and water increase as this happens, the rate of the reverse reaction also increases in time, the two rates will again become equal and a new equilibrium will be established

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle at the new equilibrium, the concentrations of reactants and products again become constant, but not the same as they were before the addition of acetic acid the concentrations of ethyl acetate and water are now higher, and the concentration of ethanol is lower the concentration of acetic acid is also higher, but not as high as it was immediately after we added the extra amount the system has relieved the stress by increasing the components on the other side of the equilibrium we say that the system has shifted to minimize the stress

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle Removal of a reaction component Removal of a reaction component removal of a component shifts the position of equilibrium to the side that produces more of the component that has been removed suppose we remove ethyl acetate from this equilibrium if ethyl acetate is removed, the position of equilibrium shifts to the right to produce more ethyl acetate and restore equilibrium the effect of removing a component is the opposite of adding one

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle Problem: Problem: when acid rain attacks marble (calcium carbonate), the following equilibrium can be written how does the fact that CO 2 is a gas influence the equilibrium? Solution:Solution: CO 2 gas diffuses from the reaction site, and is removed from the equilibrium mixture; the equilibrium shifts to the right and the marble continues to erode

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle Change in temperature Change in temperature the effect of a change in temperature on an equilibrium depends on whether the forward reaction is exothermic or endothermic consider this exothermic reaction we can look on heat as a product of the reaction adding heat (increasing the temperature) pushes the equilibrium to the left removing heat (decreasing the temperature) pushes the equilibrium to the right

© 2003 Thomson Learning, Inc. All rights reserved LeChatelier’s Principle summary of the effects of change of temperature on a system in equilibrium

© 2003 Thomson Learning, Inc. All rights reserved End Chapter 7