Chem 0120 Final Review Guide Chapter 13 Rates of Reaction 13 1 what molecules do during reaction Variables Affecting Reaction Rates Chemical Kinetics the study of reaction rates how they change under varying conditions and 1 Concentrations of reactants generally rate of reaction increases when concentration of reactants increases In some reactions rate is only affected by a reactant 2 Concentration of catalyst catalyst substance that increases the rate of reaction being present without being consumed 3 Temperature usually reaction speed increases with temperature 4 Surface area of a solid reactant or catalyst when a reaction occurs on the surface of a solid reaction rate increases with surface area Reaction Rate the increase in molar concentration of product of a reaction per unit time OR the decrease in molar concentration of reactants per unit time Units mole L s Average rate of change slope over a time interval Instantaneous rate of change slope of a tangent line 13 2 To obtain the rate of reaction you must determine the concentration of a reactant or product during the course of the reaction 13 3 Rate Law an equation that relates the rate of a reaction to the concentration of reactants and catalyst raised to various powers For the reaction aA bB c dD eE Rate k A m B n c p Where k is the rate constant and m and n are the experimentally determined reaction orders and c is the catalyst Rate Constant proportionally constant in the relationship between rate and concentrations temperature dependent Reaction Order exponent of the concentration of a species in the rate law usually an integer and MUST be determined experimentally Overall Order of Reaction sum of exponents in the rate law Orders can be fractional zero and negative Isomerization different compounds with the same molecular formula Determining the rate law perform a series of reactions where the concentration of 1 reactant varies for each see Ebbing 10th ed Pg 544 for an example Integrated Rate Laws mathematical relationship of concentration and time Where A 0 is the initial concentration of reactant A A t is the concentration of A at time t k is the rate constant and t is the time First order rate law Second order rate law Zero order rate law ln A t A 0 kt 1 A t kt 1 A 0 A t kt A 0 see Ebbing 10th ed Pg 548 for an example using rate law Half Life t1 2 the time it takes for the reactant concentration to decrease one half of its initial value First order Second order Zero order t1 2 0 693 k t1 2 1 k A 0 t1 2 A 0 2k The order of reaction can also be found by graphing y mx b First order plotting ln A t y t x and k m gives a straight line Second order 1 A t y t x k m gives a straight line Zero order A y and t x gives a straight line 13 4 Summary Order 0 1 2 Rate Law Rate k Rate k A Rate k A 2 Integrated Rate Law A t kt A 0 ln A t A 0 kt 1 A t kt 1 A 0 Straight Line Plot Half Life A vs t t1 2 A 0 2k t1 2 0 693 k ln A vs t t1 2 1 k A 0 1 A vs t 13 5 Collision Theory Assumes that for a reaction to occur reactant molecules must collide with an energy greater than some minimum value and with proper orientation Activation Energy EA minimum energy of collision required for two molecules to react According to Collision Theory rate constant k depends on 1 Collision frequency z 2 Fraction of collisions with energy EA f 3 Fraction of collisions with proper orientation p So k zfp and z 3RT Mm for gases f eEA Rt p does not change with temperature Where R 8 31 J mol T temp in Kelvin and Mm molar mass Transition State Theory explains the reaction resulting from the collision of two molecules in terms of an activated complex Activated Complex transition state an unstable group of atoms that can break up to form products Potential Energy Diagram 13 6 Arrhenius Equation expresses the dependence of the rate constant on temperature K Ae EA RT Where R 8 31 J mol T temp in Kelvin and A Frequency factor const ln k2 k1 EA R 1 T1 1 T2 13 7 Elementary Reaction each step or a single molecular event such as a collision of molecules Reaction Mechanism the set of elementary reactions whose overall effect is given by the net resulting in a reaction chemical equation Reaction Intermediate a species produced during a reaction that does not appear in the net equation because it reacts in a subsequent step in the mechanism Molecularity the number of molecules on the reactant side of an equation For an elementary reaction the rate is proportional to the product of the concentration of each reactant molecule o Unimolecular o Bimolecular o Termolecular Rate k A Rate k A B Rate k A B C A B C A B C D A B C D E 13 8 Rate Determining Step slowest step in the reaction mechanism The predicted rate law is made from the rate determining step of the elementary reaction 13 9 Catalysis the increase in the rate of a reaction that results from the addition of a catalyst by lowering the activation energy Homogeneous Catalysis the use of a catalyst that exists in the same phase as the reacting species Heterogeneous Catalysis the use of a catalyst that exists in a different phase from the reacting solution of reactants species usually a solid catalyst in contact with a gaseous or liquid Adsorption attraction of molecules to a surface Physical Adsorption provided by weak intermolecular forces Chemiadsorption bound by chemical bonding forces Enzyme biological catalyst uses the reaction E S ES E P Substrate substance the enzyme catalyzes Active Site where the substrate bonds to the enzyme and catalysis takes place Chapter 14 Chemical Equilibrium 14 1 14 2 Reversible reactions result in a mixture of both products and reactants when the mixture comes to equilibrium Dynamic Equilibrium consists of a forward and a reverse reaction which both occur at the Chemical Equilibrium the state reached by a reaction mixture when the rates of forward and same speed reverse reactions become equal All of the equilibrium compositions for a reaction at a given temperature are related by a quantity called the equilibrium constant K Equilibrium Constant Expression an expression obtained by multiplying the concentrations of products dividing by the concentrations of reactants and raising each concentration term to the power equal to the coefficient in the chemical equation Equilibrium Constant Kc the value obtained for the equilibrium constant expression when equilibrium concentrations are substituted For the reaction aA bB cC dD c D Kc C A a B b d or Coefficient Products Reactants Coefficient Law of