Reaction Rates and Equilibrium

1. Reaction Rates andEquilibrium Chemistry Chapter 18

2. Rates of Reaction • The rate of a chemical reaction is usually expressed as the change in the amount of a reactant per unit of time. • Collision theory – particles collide & react if enough energy • Activation energy – minimum energy required to react • What factors affect reaction rates? • The rate of a chemical reaction depends upon temp, concentration, particle size, and the use of a catalyst. • A catalyst lowers the activation energy without taking part in the reaction. • An inhibitor is a substance that interferes with a catalyst

4. Reversible Reactions • A reversible reaction is one in which the conversion of reactants to products and products to reactants occur simultaneously. • When the forward & reverse reaction are equal the reaction has reached a state of balance; chemical equilibrium. • At a chemical equilibrium, no net change occurs in the amounts of components of the system.

5. Factors Effecting Equilibrium • Equilibrium position shifts as a result of changing conditions. • LeChaterler’s Principle– if stress is applied to a system in dynamic equilibrium, the system changes in a way that releases the stress. These stresses include: • Changes in the concentration of reactants or products • Changes in the temp • Changes in the pressure

6. LeChaterler’s Principle • Suppose you have an equilibrium established between four substances A, B, C and D.

7. Using Le Chatelier's Principle with a change of concentration • What would happen if you changed the conditions by increasing the concentration of A?

8. Using Le Chatelier's Principle with a change of pressure • This only applies to reactions involving gases: • What would happen if you changed the conditions by increasing the pressure?

9. Using Le Chatelier's Principle with a change of temperature • For this, you need to know whether heat is given out or absorbed during the reaction. • Assume that our forward reaction is exothermic (heat is released):

10. Using Le Chatelier's Principle with a change of temperature (cont) • What would happen if you changed the conditions by increasing the temperature? What would happen if you changed the conditions by increasing the temperature?

11. Equilibrium Constant • In a general reaction, a mol of reactant A and b mol of reactant B react to give c mol of product C and d mol of product D at equilibrium. • The equilibrium constant (Keq)is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation. A value of Keq greater than 1 means that products are favored over reactants; a value of Keq less than 1 means that reactants are favored over products.

12. Solubility Product Constant • The solubility product constant(Ksp), equals the product of the concentrations of the ions each raised to a power equal to the coefficient of the ion in the dissociation equation. • The lower the numerical value, the lower the solubility of the compound.

13. The Common Ion Effect • If the product of the concentrations of two ions in the mixture is greater than the Ksp of the compound formed from the ions, a precipitate will form. A precipitate of barium sulfate forms as barium nitrate (Ba(NO3)2 and sodium sulfate (Na2SO4) solutions are mixed. Applying Concepts - What is the product of the concentrations of barium ion and sulfate ion after precipitation is complete?

14. Free Energy & Spontaneous Reactions • Excess energy released in a chemical reaction is referred as free energy as is thus available to do work. • A spontaneous reaction is one that occurs naturally and favors the formation of substantial amounts of products at the specified conditions; also releases free energy. • A nonspontaneous reaction is one that does not favor the formation of products at the specified conditions.

15. Entropy • Entropyis a measure of disorder in a system. • The law of disorder states that the natural tendency of a system is to move in the direction of maximum disorder or randomness. An increase in entropy favors the spontaneous chemical reaction; decrease favors the non-spontaneous reaction. Applying Concepts - Which has the greater entropy, an assembled jigsaw puzzle, or the pieces in the box?

16. Enthalpy, Entropy, & Free Energy • The size & direction of enthalpy changes & entropy changes together determine whether a reaction is spontaneous; that is, whether it favors products and releases free energy.

17. Gibbs Free-Energy • All spontaneous reactions release some energy (“Gibbs” Free Energy) that becomes avalible to do work. • Gibbs Free Energy change is the max. amount of energy that can be used to do work.. A change in Gibbs free energy is related to the change in entropy (ΔS) and the change in enthalpy (ΔH) of the system by the free-energy equation. ΔG = ΔH − TΔS (The temperature (T) is in kelvins.) The numerical value of ΔG is negative in spontaneous processes because the system loses free energy.

18. Rate Laws A → B • The rate at which A forms B can be expressed as the change in A (ΔA) with time, where concentration A1 is the initial concentration of A at time t1 and concentration A2 is the concentration of A at a later time, t2.

19. Rate Laws (cont) • A rate law is an expression for the rate of a reaction in terms of the concentration of reactants. • The specific rate constant (k) for a reaction is a proportionality constant relating the concentration of reactants to the rate of the reaction. • The value of the specific rate constant, k, is large if the products from quickly; the value is small if the products form slowly.

20. Reaction Mechanisms • Graphs (a reaction progress curve) of the progress of a reaction show peaks that correspond to the energies of activated complexes and valleys that correspond to the energies of intermediates & products.