CHM104 1st Edition Exam 1 Study Guide Lectures 2 10 Lecture 2 September 5 Textbook section 13 1 13 2 Reaction Rates What is chemical kinetics o The study of change with time to determine how a reaction occurs how to characterize the speed rate at which chemical change occurs What is rate o The speed at which a reaction takes place Can be quantified by either measuring how fast the reactants are being consumed or by measuring how fast the products are being made Most reactions slow down over time as the amount of reactant decreases R ate concentration concentration t concentration t time t 2 t 1 2 1 Consider thereaction H 2 I 2 2 HI g g g Write equations 3 that show how the rate of this reactioncan be expressed terms of rates of H 2 I 2 HI change 1 H 2 1 I 2 1 HI t t 2 t General equation for relating the rate of a reaction to changes in concentration of chemical species 1 A 1 B 1 C 1 D a t b t c t d t If I 2 isbeing used up at 0 00225 M at some time during thereaction how fast is H 2 being used s H 2 being used 0 0025 M s How fast is HI being produced HI be ing produced 0 005 M s What is the rate of the reaction 3 Rate 5 0 E M s 1 Consider thereaction H 2 O 2 H 2 O g 2 g g If therate of the reactionis 0 0010 M at some point time how fast is O 2 beingused s O2 1 0 E 3 5 0 E 4 M s t 2 Factors that influence rates of reaction o Reactant Concentration o Temperature o Catalyst Rate of reaction ALWAYS POSITIVE Rate being consumed NEGATIVE Rate being formed POSITIVE Lecture 3 September 8 Textbook section 13 3 The rate law equation shows how the rate of a chemical reaction is influenced by the concentrations of the reactants aA bB cC dD Rate k A m B n o Reactant orders m n show how strongly the reaction rate is influenced by a chemical Unit less exponent Common values for n m 0 1 2 1 1 2 o Fractions 2 3 3 o The rate constant k is a proportionality constant Units can be M s 1 s 1 Ms Depends on the reaction If n 0 reactant is 0th order rate is independent of reactant 0 rate k MI 0 k Unit for k M s If n 1 reactant is 1st order directly proportional to reactant rate reactant rate k MI 10 Unit for k 1 1 s s If n 2 reactant is 2nd order rate reactant 2 2 rate k MI 1 M 1 s 1 Unit for k Ms How to determine the order of a reaction o Strategy 1 Method of initial rate Rate is measured for a short time at beginning of the reaction Perform several times with different initial concentrations of reactant 1 N 2 O5 2 NO2 O2 2 N Rate k g g g Exp N2O5 0 Initial Rate Exp 1 0 100 M 0 020 M s Exp 2 0 200 M 0 040 M s n N 2 O5 N 2 O5 n N 2 O5 nexp 2 N 2 O5 nexp 2 k k rate 2 rate 1 N 2 O5 exp 2 N 2 O5 exp 1 rate 2 log nlog rate 1 log 0 200 nlog Isolaten n 1 0 040 0 020 0 100 If n 0 Exp 1 Exp 2 N2O5 0 Initial Rate If n 2 N2O5 0 Initial Rate 0 100 M 0 020 M s Exp 1 0 100 M 0 020 M s 0 200 M 0 020 M s Exp 2 0 200 M 0 080 M s Lecture 4 September 10 Textbook Section 13 4 Integrated form of the rate law equation shows how the concentration of the reactant changes over time Different from rate law equation Mathematical equation for the integrated rate law depends on the reaction order For a reaction A B If the Reaction Order is Then the Rate Law Equation is And the Integrated Rate Law Equation is And a graph of vs will be Linear straight line 0 Rate k A 0 1 Rate k A 1 ln A t ln A 0 kt ln A vs t 2 Rate k A 2 1 A t 1 A 0 kt 1 A vs t A t A 0 kt A vs t The half life equations are used to calculate how long it takes for the reactant concentration to be HALF its original value For a reaction A B Reaction Order 0 Integrated Rate Law Equation A t A 0 kt Half life equation t1 2 A 0 2k 1 ln A t ln A 0 kt t1 2 ln 2 k 2 1 A t 1 A 0 kt t1 2 1 k A 0 General Rate Law Rate k A n n can equal 0 1 2 A n0 A k t This equation can be integrated results of integration depends on the value of n If a reaction is zero order we expect a graph of A vs time to appear linear If a reaction is first order we expect a plot of ln A vs time will appear linear If a reaction is second order we expect a plot of 1 A vs time to be linear A 0 kt A t y mx b A 0 slope k x t A t y intercept y Remember zero order slope of A vs time does not change as A decreases Half life time it takes A 0 to drop by 50 or 0 50 Depend on reaction order life depends on starting reactant and k Larger rate constant faster reaction shorter life zero order half life t 1 2 first order half life t 1 2 A 0 2k 0 693 k A 0 k 1 second order half life t 1 2 Example At a high temperature with k 0 100 s 1 Consider reaction Cl2 2Cl reaction is first order What is the half life time t 1 2 0 693 0 693 6 93 seconds k 100 s 1 How long will it take to reduce to 30 0 of its initial concentration What is the time when we have 0 30x Cl2 Cl 2 Cl2 0 0 100 s 1 t Cl2 t ln ln Cl2 0 0 30 ln t Lecture 5 September 12 Textbook Section 13 5 Rates of a reaction are highly temperature dependent A 10 increase will double or triple a reaction rate Only a change in T not a change in reactant How do we explain this reactant product Rate k reactant n0 k is ONLY constant at constant K as T The relationship between k and T was quantified by Swedish chemist Arrhenius in 1889 Arrhenius equation Ae Ea RT 8 314 J k rate constant A …