CHEM 1212 1nd Edition Lecture 7 Outline of Last Lecture I. Rates of Chemical ReactionsII. Reaction Conditions and RateIII. Effects of Concentration on Reaction RatesIV. Rate EquationsV. The Order of a ReactionVI. The Rate Constant kVII. Determining a Rate EquationOutline of Current Lecture I. ConcentrationII. Collision TheoryIII. The Arrhenius EquationIV. Effects of Catalysts on Reaction RatesCurrent LectureI. Concentration – Time Relationships: Integrated Rate Lawsa. Equations used to calculate the concentrations of reactions and products after anelapsed period of timeb. First-order reactionsi. Integrated rate equation: ln[R]2 = -kt [R]0ii. The R’s in this equation are the concentrations of the reactants at time t=0 and at a later timeiii. Calculations where this would be used:These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.1. If [R]2/[R]0 is measured in the laboratory after some amount of time has elapsed, then k can be calculated2. If [R]0 and k are known, then the concentration of material remaining after a given amount of time ([R]t) can be calculated3. If k is known, then the time elapsed until a specific fraction ([R]t/[R]0) remains can be calculatedc. Second-order reactionsi. Rate equation: 1 – 1 = kt[R]t [R]0d. Zero-order reactionsi. Rate equation: [R]0 – [R]t = ktii. Units of k are mol/L x se. Graphical methods for determining reaction order and the rate constanti. Rate equations, if rearranged, have the form y = mx + bf. Half-Life and First-Order Reactionsi. The half-life (t1/2) of a reaction is the time required for the concentration of a reaction to decrease to one half its initial valueii. The longer the half-life, the slower the reactioniii. Usually used when dealing with first order reactionsiv. The half life is the time when [R]t = ½ [R]0v. In order to find the half life, we substitute in the ½ as the concentration fraction and replace t with t1/2vi. For first order reaction half life: t1/2 = 0.693 KII. Collision Theory: Concentration and Reaction Ratea. Collision theory of reaction ratesi. The reacting molecules must collide with one anotherii. The reacting molecules must collide with sufficient energy to initiate the process of breaking and forming bondsiii. The molecules must collide in an orientation that can lead to rearrangement of the atoms and the formation of productsb. The number of collisions between the two reactant molecules is directly proportional to the concentration of each reactant, and the rate of the reaction shows a first-order dependence on each reactantc. Temperature and Reaction Ratei. Higher temperatures allow reactions to occur more rapidlyii. Lowering a temperature slows down a reactioniii. Molecules gain the energy to react through heat/kinetic energyd. Activation energy (Ea)i. Defined as the amount of energy a molecule must obtain in order to react; a sort of energy barrier to reactionsii. If the barrier is low, less energy is required for the reaction and vice versa iii. Transition state: state at which the molecules have obtained sufficient energye. Effect of Molecular Orientation on Reaction Ratei. Heat along is not enough to ensure that the reaction will occurii. The reaction also requires that the molecules come together in the correct orientationiii. The lower the probability of achieving the proper alignment, the smaller the value of k, and the slower the reactionIII. The Arrhenius Equationa. This is the observation that reaction rates depend on the energy and frequency of collisions between reacting molecules, on the temperature, and on whether the collisions have the correct geometryb. k=rate constant = Ae-Ea/RTi. A = frequency factor; specific to each reaction and is temperature dependentii. The superscript of the equation = the fraction of molecules having the minimum energy required for reactions (value is always less than 1)c. This equation allows you to…i. Calculate Ea from the temperature dependence of the rate constantii. Calculate the rate constant for a given temperature, if Ea and A are knownd. The activation energy can be obtained if k is known at two different temperaturesi. Ln k1 = - (Ea/RT1) + ln AND ln k2 = - (Ea/RT2) + ln A IV. Effects of Catalysts on Reaction Ratea. Catalysts are speed up the rate of reactions by lowering the activation energyb. Catalysts are not consumed in a chemical reactionc. Catalysts allow reactions to occur at lower temperatures than they normally wouldd. Catalysts do not appear in balanced equations, but appears in the reaction rate lawe. If a catalyst is present in the same phase of the reacting substance, it is called a homogeneous catalystf. Catalyzed reactions occur much quicker than the un-catalyzed reactioni. Un-catalyzed reactions are a 1 step process ii. Catalyzed reactions have multiple