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Chem 0120 Final Review GuideChapter 13: Rates of Reaction 13.1Chemical Kinetics – the study of reaction rates, how they change under varying conditions, and what molecules do during reaction.Variables Affecting Reaction Rates:1. Concentrations of reactants – generally, rate of reaction increases when concentration of reactants increases. In some reactions, rate is only affected by a reactant being present.2. Concentration of catalyst – catalyst: substance that increases the rate of reaction without being consumed. 3. Temperature – usually reaction speed increases with temperature4. Surface area of a solid reactant or catalyst – when a reaction occurs on the surface of a solid, reaction rate increases with surface areaReaction 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*sAverage rate of change – slope over a time intervalInstantaneous rate of change – slope of a tangent line13.2- To obtain the rate of reaction, you must determine the concentration of a reactant or product during the course of the reaction13.3Rate 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 + eERate = k[A]m[B]n[c]pWhere “k” is the rate constant and m and n are the experimentally determined reaction orders and “c” is the catalystRate Constant – proportionally constant in the relationship between rate and concentrations; – temperature dependentReaction Order – exponent of the concentration of a species in the rate law, usually an integer and MUST be determined experimentallyOverall Order of Reaction – sum of exponents in the rate law- Orders can be fractional, zero, and negativeIsomerization – different compounds with the same molecular formulaDetermining 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)13.4Integrated Rate Laws – mathematical relationship of concentration and timeWhere “[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: ln([A]t/[A]0) = -kt- Second order rate law: (1/[A]t) = kt + (1/[A]0)- Zero order rate law: [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: t1/2 = 0.693/k- Second order: t1/2 = 1/k[A]0 - Zero order: t1/2 = [A]0/2kThe 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 lineSummary:Order Rate Law Integrated Rate Law Half-Life Straight-Line Plot0 Rate = k [A]t = -kt + [A]0t1/2 = [A]0/2k [A] vs t1 Rate = k[A] ln([A]t/[A]0) = -kt t1/2 = 0.693/k ln[A] vs t2 Rate = k[A]2(1/[A]t) = kt + (1/[A]0) t1/2 = 1/k[A]0 1/[A] vs t13.5Collision Theory – Assumes that for a reaction to occur, reactant molecules must collide with anenergy greater than some minimum value and with proper orientationActivation Energy – “EA,” minimum energy of collision required for two molecules to reactAccording 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 Where “R” = 8.31 J/mol, “T” = temp in Kelvin, and “Mm” = molar mass- f = eEA/Rt- p does not change with temperatureTransition-State Theory – explains the reaction resulting from the collision of two molecules interms of an activated complexActivated Complex – transition state, an unstable group of atoms that can break up to form products.Potential EnergyDiagram:13.6Arrhenius Equation – expresses the dependence of the rate constant on temperatureK = 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)13.7Elementary Reaction – each step, or a single molecular event, such as a collision of molecules, resulting in a reactionReaction Mechanism – the set of elementary reactions whose overall effect is given by the net chemical equationReaction Intermediate – a species produced during a reaction that does not appear in the net equation because it reacts in a subsequent step in the mechanismMolecularity – 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 moleculeo Unimolecular: A->B+C Rate = k[A]o Bimolecular: A+B->C+D Rate = k[A][B]o Termolecular: A+B+C->D+E Rate = k[A][B][C]13.8Rate-Determining Step – slowest step in the reaction mechanism- The predicted rate law is made from the rate-determining step of the elementary reaction13.9Catalysis – the increase in the rate of a reaction that results from the addition of a catalyst by lowering the activation energyHomogeneous Catalysis – the use of acatalyst that exists in the same phase asthe reacting speciesHeterogeneous Catalysis – the use of acatalyst that exists in a different phasefrom the reacting species, usually a solid catalyst in contact with a gaseous or liquid solution of reactantsAdsorption – attraction of molecules to a surface- Physical Adsorption – provided by weak intermolecular forces- Chemiadsorption – bound by chemical bonding forcesEnzyme – 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 placeChapter 14: Chemical Equilibrium14.1Reversible reactions result in a mixture of both products and reactants when the mixture comesto equilibriumDynamic Equilibrium – consists of a forward and a reverse reaction which both occur at the same speedChemical Equilibrium – the state reached by a reaction mixture when the rates of forward and reverse reactions become equal14.2- All of the equilibrium compositions for a reaction at a given temperature are related by a quantity called the equilibrium


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Pitt CHEM 0120 - Final Review Guide

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