Reaction Rates Chapter 17 Expressing reaction rates ∆ quantity Average rate = - ________________ ∆ time This equation defines the average rate at which reactants produce products. Chemists look at changes in molar concentration (mol/L or M) of reactants or products per unit time. Average rate is expressed as mol/L· s Reaction rates are determined experimentally by measuring concentrations in a reaction. Rates CANNOT be calculated from balanced equations. Average reaction rates • ALWAYS POSITIVE • Negative sign (-) in front of equation is used to calculate rate of consumption of reactants. The Collision Theory • Atoms, ions, and molecules must collide in order to react. • Explains why reactions occur and how rates of chemical reactions can be modified. The Collision Theory 1. Reacting substances (atoms, ions, or molecules) must collide. 2. Reacting substances must collide with the correct orientation. 3. Reacting substances must collide with sufficient energy to form the activated complex. 2H2 + O2 → 2H2O • According to the collision theory, H2 and O2 must collide in order to produce water (H2O). CO + NO2 → CO2 + NO • In this reaction, CO and NO2 must also collide to produce CO2 and NO. • However, only a small fraction of collisions produce reactions. Orientation • In the previous example, few collisions result in reactions because the C in CO must collide with the O in NO2 for a temporary bond to form. • Any other contact results in rebound (or bouncing off) of reactants. Activated Complex • When orientation is correct at collision, and intermediate substance (activated complex) is formed. • Often acts as a transition state (or first step) leading to the products. Activation energy • Collisions between reactants must also have enough activation energy in order to produce the activated complex. • Correct orientation (at collision) will not produce the activated complex if there is insufficient energy. Exothermic reaction Endothermic reaction Influence of Spontaneity • Spontaneity only indicates the natural tendency for a reaction to occur. • Spontaneity will NOT affect the reaction rate. 17.2 Factors affecting reaction rates 1. Nature of reactants Reactive nature of the reactants Some react more readily than others Factors cont 2. Concentration Higher concentrations of reacting particles increases the rate of reaction. Higher concentrations increases number of particles present and increases number of collisions. Factors cont 3. Surface Area Increasing surface area increases reaction rates. Increased surface area gives more room for collisions in the same amount of time. Factors cont 4. Temperature Increasing temperature increases the reaction rates. Higher temperatures raises kinetic energy of reacting particles and raises frequency of collisions and collision energy. Factors cont 5. Catalysts Increase reaction rates without being consumed in the reaction (metabolic reactions). Catalysts (enzymes) lower the activation energy needed for a reaction to occur which increases the reaction rate. Enzyme catalyzed reaction Factors cont 6. Inhibitors Catalysts that slow or stop reactions. Food industry: preservatives to prevent spoilage or weed herbacides that stop weed growth. Catalysts Heterogenous: exist in a physical state different than that of the reaction it catalyzes. Homogenous: exist in the same physical state as the reaction it catalyzes. 17.3 Reaction Rate Laws • Rate law - equation that expresses the mathematical relationship between the rate of reaction and the concentration of reactants • A → B Rate = k [A] • k=specific rate constant [A]=concentration of A Specific rate constant k • unique for every reaction • Concentration per time; usually in mol/s • Reaction rate is directly proportional to molar concentration • k does NOT change w/concentration but will change with temperature Instantaneous reaction rates • Rate of decomposition at a specific time • Use rate law: Rate=k [A] • Constant at specific temperature and concentration of reactant will be given Reaction Mechanisms • Most chemical reactions consist of a series of simple reactions (elementary reactions) • A complex reaction consists of 2 or more elementary reactions • Reaction mechanism is made of the complete sequence of elementary reactions that make the complex reaction Intermediates • A substance produced in one elementary step that is consumed in the next elementary step Rate-determining step • Reactions cannot occur any faster than the slowest elementary step. • The slowest elementary step is the rate-determining step since it controls the rate of the reaction.