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KU CHEM 135 - Exam 2 Study Guide
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CHEM 135 1st EditionExam # 2 Study Guide Lectures: 10-17Lecture 10 (February 16)Rate vs. reaction concentration When placed in a graph format:- Zero order reaction has a straight horizontal line- First order reaction has a straight increasing slope- Second order reaction has an arched increasing slopeComplex rate law and reaction orderaA + bB  cC + dDRate=k[A]m[B]nk= rate constantm & n= reaction order with respect to A & BOverall reaction order = (m + n)Ex. Determining the order and rate constant of a reaction-Consider the reaction between nitrogen dioxide and carbon monoxide.NO2(g) + CO(g)  NO(g) + CO2(g)[NO2] (M) [CO] (M) Initial Rate (M/s)0.10 0.10 0.00210.20 0.10 0.00820.20 0.20 0.00830.40 0.10 0.033The initial rate of the reaction is measured at several different concentrations of the reactants with accompanied results. From the data, determine:a)The rate law for the reactionb)The rate constant (k) for the reaction(Answers to this example are on p. 607)Lecture 11 (February 18) Integrated Rate LawZero order graph- [A] on y-axis and time on x-axis, slope= -kFirst order graph- ln[A] on y-axis and time on x-axis, slope= -kSecond order graph- 1/[A] on y-axis and time on x-axis, slope= kEquations:Zero order- [A]= -kt + [A]initialFirst order- ln[A]= -kt + ln[A]initialSecond order- 1/[A]= kt + 1/[A]initialHalf LifeTime it takes for the concentration of the reactant to fall to one-half its original valueZero order reaction- t1/2=[Ainitial]/2kFirst order reaction- t1/2=0.693/kSecond order reaction- t1/2= 1/k[A0]Ex. Determining the concentration of a reactant at a given timeThe decomposition of SO2Cl2 is first order and has a rate constant of 2.90 x 10-4s-1. If the reaction is carried out at the same temperature, and the initial concentration of SO2Cl2 is 0.0225M, what will the SO2Cl2 concentration be after 865s?(Answer on p. 610)Ex. Half-lifeMolecular iodine dissociates at 625K with a first-order rate constant of 0.271 s-1. What is the half-life of this reaction?(Answer on p. 611)Ex. This reaction was monitored as a function of time:A B+CA plot of ln[A] versus time yields a straight line with a slope of -0.0045/s. a) What is the value of the rate constant (k) for this reaction at this temperature?b) Write the rate law for the reaction.c) What is the half-life?d) If the initial concentration of A is 0.250M, what is the concentration after 225s?(Answer on p. A-16)Ex. The decomposition of SO2Cl2 is first order in SO2Cl2 and has a rate constant of 1.42 x 10-4s-1 at a certain temperature.a) What is the half-life for this reaction?b) How long will it take for the concentration of SO2Cl2 to decrease to 25% of its initial concentration?c) If the initial concentration of SO2Cl2 is 1.00M, how long will it take for the concentration to decrease to 0.78M?d) If the initial concentration of SO2Cl2 is 0.150M, what is the concentration of SO2Cl2 after 2.00 x 102s?(Answer on p. A-16)Lecture 12 (February 20)Factors affecting the rate of a reaction1. Concentration- If there is an increase in the concentration, there is an increase in the frequency of collisions2. Temperature- If there is an increase in temperature, the frequency of collisions also increase3. Surface areaAmong the colliding molecules, only effective collisions, i.e. collisions in right orientation and with right energy can lead to product formation.Frequent collisions raise the rate of the reaction Collisions need sufficient energy to break reactant bondsLecture 13 (February 23)Activation energy in a chemical reaction (Ea)- Lower the activation energy, faster the rate of reaction- REMEMBER: frequent collisions + energetic collision + correct orientation= productArrhenius equation- Takes into account all factors that affect the rate of a reactionk =Ae− EaRTk= rate constantAe= Arrhenius factor: collision frequency and orientation factorR= gas constantT= temperature in kelvinEa= energy barrier to be surmounted in going from reactants to products- The Arrhenius equation has two derivatives:ln(k)=−EaR(1T)+ln ⁡( A)lnk2k1=EaR(1T1−1T2)Reaction Mechanism and Reaction IntermediateUnimolecular: Aproduct rate= k[A]1Bimolecular: 2A product rate= k[A]2Termolecular: A+B product rate= k[A]![B]13A product rate= k[A]3A+B+C product rate= k[A]![B]1[C]1Reaction intermediates appear in the elementary steps but not in the final balanced overall reaction.Rate Determining Step- Rate determining step (slow step) determines the rate law of the overall reaction- Rate law of slow step= rate law of overall reaction Ex. Using the two-point form of the Arrhenius equationConsider the reaction between nitrogen dioxide and carbon monoxide.NO2(g) + CO(g)  NO(g) + CO2(g)The rate constant at 701K is measured as 2.57M-1 s-1 and that at 895K is measured as 567M-1 s-1. Find the activation energy for the reaction in kJ/mol.(Answer on p. 620)Ex.The activation energy of a reaction is 56.8 kJ/mol and the frequency factor is 1.5 x 1011/s. Calculate the rate constant of the reaction at 25˚C. (Answer on p. A-16)Ex. The rate constant (k) for a reaction was measured as a function of temperature. A plot of ln k versus 1/T (in K) is linear and has a slope of -7445K. Calculate the activation energy for the reaction.(Answer on p. A-16)Lecture 14 (February 25)REVIEW DAY FOR CHAPTER 13 For extra practice on Chapter 13:P. 638 #25, 29, 31, 35-35 odds, 45, 49, 51, 57, 59, 61, 65, 69, 73Lecture 15 (February 27)Chemical EquilibriumN2 + 3H2 ⇌ 2NH3 This equation can go both ways and is reversible. The rates of the reaction are the same, meaning they are at equilibrium, no significant change inconcentrationDoes this mean the reaction stops?No, there is constant movement, even in equilibrium, in order to keep the reaction balanced.Reversible Reactions and Equilibrium ConstantsaA + bB ⇌ cC + dD- Rates of the forward and reverse reactions are equal at equilibrium.- Equilibrium does NOT mean the products are equalKeq=[ products ][reactants]Keq=[C]c[ D]d[ A ]a[B ]b- The constant within this equation is unitless.- If Keq >> 1, it consists of more product than reactant (greater numerator)- If Keq << 1, it consist of more reactant than product (greater denominator)Heterogeneous Equilibrium- Reactants and products are in different phaseso 2CO2(g) ⇌ CO2(g) + C(s)o Equilibrium constant: COC O2¿2¿[¿¿ 2]¿KC=¿- Do not include: pure solids and pure liquids in an equilibrium constant- Changing the amount of solid does not change the concentration of gas Ex.


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KU CHEM 135 - Exam 2 Study Guide

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