Chapter 19 – Reaction Rates and Equilibrium

Chapter 19 – Reaction Rates and Equilibrium Rates of Reaction Reversible Reactions and Equilibrium Determining Whether a Reaction Will Occur Calculating Entropy and Free Energy The Progress of Chemical Reactions

19.1 Rates of Reaction • How long does a chemical reaction take? • Collision Theory • IF colliding particles have enough energy (KE) and collide at the right orientation, they react to form a new product:

19.1 Rates of Reaction (cont.) The minimum amount of energy that particles must have in order to react is called the activation energy. Activation energy acts like a barrier that the reactants must cross to be converted into products. Activated complex, transition state…

19.1 Rates of Reaction (cont.) • Factors Affecting Reaction Rates • Here they are again… • Temperature • Concentration • Particle size • Catalysts

19.2 Reversible Reactions and Equilibrium • Reversible Reactions • Reactions that can occur in the other direction.. • 2SO2 + O2 2SO3

19.2 Reversible Reactions and Equilibrium (cont.) • Chemical Equilibrium • A state in which the forward and reversible reactions take place at the same rate. • Factors Affecting Equilibrium: Le Chatelier’s Principle • Obviously, when discussing equilibrium there is “a fine line” of balance in the reaction – changes of almost any kind can disrupt this balance. • Le Chatelier’s Principle: If a stress (temp, conc., pressure) is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

19.2 Reversible Reactions and Equilibrium (cont.) • Equilibrium Constants • A ratio of product concentrations… • aA + bB cC + dD • Keq = [C]c x [D]d / [A]a x [B]b where [X] is expressed in mol/L and x is the number of moles from the balanced chemical equation. • A Keq value > 1 favors products at eq. Keq value < 1 favors reactants at eq. Examples…

Ch 19 Assignments • Ch 19 CPQs #1 pg. 572 • #38,39,40,41,43,44,46,47,48,49,50

19.3 Determining Whether a Reaction Will Occur • Free Energy and Spontaneous Reaction • To do work, you need available energy. • Free energy: energy that is available to do work. (Not always efficient. • Reactions may OR may not occur spontaneously… • Entropy • Reactions tend to occur to attain the lowest possible energy for a system: the law of disorder. • The disorder in a system is measured as entropy, (S).

19.3 Determining Whether a Reaction Will Occur (cont.) • Entropy explained and visualized…

19.3 Determining Whether a Reaction Will Occur (cont.) • Heat, Entropy, and Free Energy • The size and direction of heat (enthalpy) changes combined with entropy determine whether or not a reaction is spontaneous, i.e., whether it favors products and releases free energy. • Table 19.1, pg. 554

19.3 Determining Whether a Reaction Will Occur (cont.)

19.4 Calculating Entropy and Free Energy • Entropy Calculations • Standard entropy change (entropy change at standard conditions: • DSo = So (products) – So (reactants) • Use Table 19.2 • Examples… • Free Energy Calculations • The maximum amount of energy that can be combined with another process to do work: Gibbs Free-Energy (G). • DG = DH – TDS where H = enthalpy, T = Temperature • DGo = DGof (products) – DGof (reactants) • Tables 19.3 and 19.4 • Examples…

19.5 The Progress of Chemical Reactions • Rate Laws • Fortunately, we don’t have to settle for explanations that are qualitative in nature- in other words, “the reaction will likely favor the products…” • Mathematically calculating rates of reaction… • Rate Law: an expression relating the rate of a reaction to the concentration of reactants: Rate = DA / Dt = k x [A] • For the reaction aA + bB cC + dD, the rate equation takes the form: Rate = k[A]a[B]b

19.5 The Progress of Chemical Reactions (cont.) • Rate Laws (cont.) • The specific rate constant,k, determines the relationship between the concentrations and the rate of reaction. • If k is large, the reaction will form products quickly. • If k is small, the reaction will form products slowly. • The order of a reaction is the power to which the concentration must be raised to give the experimentally observed relationship between concentration and rate: 1st order reaction – proportional to one reactant. • Reaction mechanisms… • Showing the reactions involved in a complex reaction • Figure 19.3, decomposition of N2O…

Ch 19 Assignments • Ch 19 CPQs #1 pg. 572 • #38,39,40,41,43,44,46,47,48,49,50 • Ch 19 CPQs #2 pg. 572 • #51,53,54,55,56,59,61,62

Chapter 19 – Reaction Rates and Equilibrium