Reaction Rates and Equilibrium

1. Reaction Rates and Equilibrium Chapter 19 C.Smith

2. I. Rates of ReactionA. Collision Theory • 1. Rate is defined as the measure of speed change that occurs over a period of time. • 2. In chemistry, rates of chemical change usually are expressed as the amount of reactant changing per unit time. • 3. Visible changes caused by chemical reactions are related to changes in the properties of individual atoms, ions, and molecules through a model called collision theory. • 4. Atoms, ions, and molecules can react to form products when they collide provided that the particles have enough kinetic theory.

3. I. Rates of ReactionA. Collision Theory • 5. The minimum amount of energy that particles must have in order to react is called activation energy. • 6. Activation energy is a barrier that reactants must cross to be converted into products • a. During a reaction, unstable particles that are neither products or reactants are temporarily formed. • b. The particle is called an activated complex and exist for a very short time. (approximately 10-13 sec.) • c. This period is called the transition state in where the activated complex can either reform reactants or form products.

4. I. Rates of ReactionA. Collision Theory • 7. Collision theory explains why some naturally occurring reactions are immeasurably slow at room temperature and essentially at a rate of zero. • 8. If the activation energy of the products is lower than the energy of the reactants, the reaction is exothermic. • 9. If the activation energy of the products is higher than the energy of the reactants, the reaction is endothermic.

5. I. Rates of ReactionA. Collision Theory • Addison-Wesley Chemistry p.535

6. I. Rates of ReactionB. Factors Affecting Reaction Rates • 1. Temperature a. At higher temperatures, molecules move faster and more chaotic and has an increase in kinetic energy. b. The effect increases the number of collisions between molecules and the more colliding particles are energetic enough to slip over the activation energy barrier.

7. I. Rates of ReactionB. Factors Affecting Reaction Rates • 2. Concentration a. Increasing the number of particles increase the number of collisions between particles. b. Increasing collisions between particles increases the frequency of particles that become activated complexes and possible products. c. Cramming more particles into a fixed volume increases the concentration of reactants, the collision frequency, and therefore the reaction rate.

8. I. Rates of ReactionB. Factors Affecting Reaction Rates • 3. Particle Size a. The smaller the particles size, the larger is the surface area for given mass of particles. b. An increase in surface area increases the amount of the reactant exposed for reaction, which further increases the collision frequency and the reaction rate. c. One way to increase the surface area of solid reactants is to dissolve them. d Another way to increase the surface area is to grind solids into fine powder. (explosion hazard)

9. I. Rates of ReactionB. Factors Affecting Reaction Rates • 4. Catalysts a. A catalyst is a substance that increases the rate of a reaction without being involved in or used up in the reaction. b. Catalysts lower the activation energy of a reaction which allows more particles to form products. • 5. Inhibitors a. An inhibitor interferes with the action of a catalyst. b. An inhibitor “poisons” a catalyst and can slow and even stop a reaction.

10. II. Reversible Reactions and EquilibriumA. Reversible Reactions • 1. Reactions sometimes do not go to completion. • 2. Some reactions are reversible. • 3. In reversible reactions, the formation of products occurs along with the decomposition of products and reforming of reactants. • 4. This is represented by a double arrow. • 5. When the forward and reverse reactions occur at the same rate in a reaction, a chemical equilibrium is reached.

11. II. Reversible Reactions and EquilibriumA. Reversible Reactions • 6. At a chemical equilibrium, there is no net change in the actual amounts of the components of the systems. • 7. The equilibrium position of a reaction is given by the relative concentration of the system’s components. • 8. The equilibrium position indicates whether the components on the left or right side of a reversible reaction are at a higher concentration. a. If the higher concentration is on the right side of the reaction, it is a forward reaction. b. If the higher concentration is on the left side of the reaction, it is a reverse reaction.

12. II. Reversible Reactions and EquilibriumA. Reversible Reactions • 9. Example: 2SO2 + O2D 2SO3 Forward reaction if more SO3 present Reverse reaction if more O2 or SO2

13. II. Reversible Reactions and EquilibriumB. Factors Affecting Equilibrium • 1. A delicate balance exist in a system at equilibrium. • 2. Any change in the system will cause it to shift to restore equilibrium. • 3. Henri Le Châtelier studied shifts in equilibrium and concluded the following principle: If stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

14. II. Reversible Reactions and EquilibriumB. Factors Affecting Equilibrium • 4. Stresses that upset equilibrium can be concentration of reactants or products, changes in pressure, and changes in temperature. • 5. Changes in concentration a. If products are removed, the reaction will shift to replace them and removal of reactants will cause a shift to replace them. b. If products are added, the reaction will shift to form reactants and addition of reactants will cause a shift to form products.

15. II. Reversible Reactions and EquilibriumB. Factors Affecting Equilibrium • 6. Affects of Temperature a. Increasing temperature will cause a shift in the direction that absorbs heat. b. Decreasing temperature will cause a shift in the direction that releases heat. • 7. Affects of Pressure - Changes in pressure only affect equilibrium that have an unequal number of moles of gaseous reactants and products. a. Increase in pressure will shift to side of the reaction with the least number of moles. b. Decrease in pressure will shift to side the reaction with the greatest number of moles

16. II. Reversible Reactions and EquilibriumC. Equilibrium Constants • 1. Chemist represent equilibrium as numerical value. • 2. This value relates the amount of reactants to products at equilibrium. • 3.This is called the equilibrium constant Keq. • Keq = [products] [reactants] aA + bB D cC + dD Keq = [C]c[D]d [A]a[B]b

17. II. Reversible Reactions and EquilibriumC. Equilibrium Constants • 4. If the equilibrium constant (Keq) is greater than one, the products are favored at equilibrium. • 5. A Keq less than one means the formation of reactants is favored at equilibrium. • 6. Example: In the reaction below, at the equilibrium point, dinitrogen tetroxide has a concentration of 0.0055M and nitrogen dioxide has a concentration of 0.025M. What is the equilibrium constant for the reaction? • N2O4D 2NO2

18. III. Determining Whether a Reaction Will Occur A. Free Energy and Spontaneous Reactions • 1. Some chemical and physical processes release energy that can be used to bring about other changes. • 2. This energy is called free energy. • 3. Free energy is the energy that is available to do work • 4. Spontaneous reaction is a reaction that will occur naturally and favors the formation of the product (Keq>1) • 5. All spontaneous reactions release free energy!

19. III. Determining Whether a Reaction Will Occur A. Free Energy and Spontaneous Reactions • 6. Nonspontaneous reaction is a reaction that does not favor formation of the product and does not occur naturally. • 7. Reactions can be spontaneous for one set of conditions but not for another. • 8. Sometimes nonspontaneous reactions can occur when combined with spontaneous reactions. • 9. This does NOT refer to speed of reaction, only whether or not it naturally occurs.

20. III. Determining Whether a Reaction Will Occur A. Free Energy and Spontaneous Reactions • 10. We define enthalpy change (DH) to be the measure of change in heat content for a reaction. • 11. Since all spontaneous reactions release free energy, you would think that these reactions or processes would always be exothermic (release heat = - DH). • 12. This is NOT the case with the melting of ice to water which requires energy (endothermic), but it happens spontaneously. • 13. Therefore, enthalpy change is not the only factor that determines whether a reaction will be spontaneous.

21. III. Determining Whether a Reaction Will Occur B. Entropy • 1. The law of disorder states that all processes occur in such a way that they move towards maximum disorder (randomness). • 2. Entropy is defined as the measure of the amount of disorder in a system. • 3. A reaction that absorbs energy (endothermic) can be spontaneous if entropy increases.

22. III. Determining Whether a Reaction Will Occur B. Entropy • 4. How does entropy increase? a. Changing to a less organized phase 1. Solid changing to liquid or gas 2. Liquid changing to gas b. Substance is divided into parts: for example NaCl is separated to Na+ and Cl- in solution c. The product has more molecules than the reactants : for example 2H2O a 2H2 + O2 • 5. Entropy also increases as heat is applied to a system.

23. III. Determining Whether a Reaction Will Occur B. Entropy • 6. You have to look at both the change in enthalpy (heat) and the change in entropy (disorder) to determine if a reaction is spontaneous. a. Exothermic + entropy increase = Spontaneous reaction b. Endothermic + entropy increase = Spontaneous reaction ONLY if entropy change is more than heat absorbed c. Exothermic + entropy decrease =spontaneous reaction ONLY if heat change is more than increased order d. Endothermic + entropy decrease = nonspontaneous reaction