Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10General Chemistry Topic 7:Chemical EquilibriaObjectives: By the end of this topic, you should:1. understand the concept of “chemical equilibrium,” and why it is dynamic.2. be able to write the expression for the equilibrium constant for a reaction, and know what a big vs small Keq means.3. understand the difference between exothermic and endothermic chemical reactions, and know how the heat of reaction is involved. 4. understand the effect of concentration, temperature, and catalysts on reaction rates. 5. understand the lack of influence of concentration and catalysts on equilibrium constants. 6. be able to use LeChâtelier’s principle to predict changes in equilibrium positionSuggested reading: DTC, Chapter 7, pp. 253-265 (245-257)Homework problems: 7.71 (7.65), 7.72 (7.66), 7.85 (7.77), 7.91 (7.87), 7.99 (7.97)Wait awhileWait awhile longerLooks the same(still at equilibrium)The condition of equilibrium can be illustrated by imagining two fish tanks connected by a narrow channel. One tank initially contains a dozen goldfish. Equilibrium: “stable” but dynamicNo net change“At equilibrium”Average of 6 fishin each tankDynamic Equilibrium – “dynamic” signifies continued activity - “equilibrium” signifies no net changeThe 2 tanks are“out of equilibrium”but fish haven’t stopped swimming!NOT always the same 6 fish on each sideInitially 12 Initially 12 No net change:6 and6 in each tankSystem is “at equilibrium”Equilibrium is dynamic – fish are constantly moving between the two tanks, but there is no net change. Opposing rates of flow are equal.Rate of flow  [ ] Rate of flow  [ ]Systems out of equilibrium tend to move towards equilibriumEventuallyInitial rate of flowto other tank is hi;initially all in theirown tankSo far, we’ve considered chemical rx as changes in reactants to products,but many rx are reversible – products can change back into reactants.The study of equilibrium involves understanding whether a chemical rxwill occur on its own, given enough time.Remember - rate of a rx is proportional to concentration of reactantsRate of forward rx (production of C + D)  [A] [B]Rate of reverse rx (production of A + B)  [C] [D]Eventually, rates of forward and reverse rx will be equalEquilibrium has been reachedRate of production of A+B = rate of production of C+D - concentrations no longer change - individual rx haven’t stopped – occurring at equal ratesChemical EquilibriumConsider the reaction: A + B C + DThis can be considered as two separate rx: Forward rx: A + B C + D Reverse rx: A + B C + DChemical EquilibriumSome reversible rx have high proportions of products at equilibriumSome reversible rx have mostly reactants at equilibriumWe need to have a quantitative measure of the relative amountsof reactants and products present at equilibrium - to what extent will the reaction occur as written?We use the equilibrium constant (Keq) = [products] [reactants]If Keq is large: [products][reactants]Almost all reactant convertedto product at equilibriumIf Keq is small: [products][reactants]Almost all reactant remains atequilibrium; little product formedIf Keq is 1: [products][reactants]Amounts of reactants and productsare equal at equilibriumA little bit more about Keq (equilibrium constant)Dynamic equilibrium is the state in which the rate of the forward rx equals the rate of the reverse rx. The equilibrium constant (Keq) tells us the relative amounts of reactants and products present at equilibrium (how likely the forward rx is to occur).Consider the reaction: mA + nB pC + qD (m, n, p, & q are molar ratios)Keq = k1k-1[products][reactants]= [C]p [D]q[A]m [B]n= [CO2][C] [O2]Keq = [SO3]2[SO2]2 [O2]Keq = k1k-1C + O2 CO22 SO2 + O2 2 SO3We can change the rate of the forward or reverse reaction by changing the relative amounts of reactants and products.When we alter a reaction that is at equilibrium, we say there is a stress on the equilibrium. Effect of concentration on equilibriumLe Châtelier’s Principle: Relief from stress!!Le Châtelier’s Principle – when the equilibrium is stressed, the rates of the forward and reverse rx will change to relieve the stress.Consider the reaction below:N2 + O2 2 NOAt equilibrium, forward and reverse rx rates are equal;[N2], [O2], & [NO2] constantStress: Add O2ShiftStress: Add NOShiftAdding or removing substances in a rxhas the effect of pushing or pullingthe rx in the direction that relieves“the stress on the equilibrium”We can “drive” a rx in one direction or the otherby adding or removing reactants or products.Effect of temperature on equilibriumTemperature changes the equilibrium of a reaction (and Keq)- increasing temperature (adding heat energy) shifts equilibrium in the direction that absorbs energy (heat of rx).- decreasing temperature (removing heat energy) shifts equilibrium in the direction that releases energy (heat of rx).To understand this, think of heat as either a product (exothermic rx) or a reactant (endothermic rx).Temperature changes the equilibrium constant - different values at different temperatures - standard values are at 25C and 1 atm (for gases)heat addedheat addedExothermic Rx(heat energyreleased)Endothermic Rx(heat energyabsorbed)Increasing the concentration of reactants will accelerate the reaction (rate of formation of product). However, once it reaches equilibrium, the RELATIVE amounts of reactants and products are constant (Keq) - RELATIVE amounts same as before adding extra reactantsConcentration of reactants or products has no effect on KeqConcentration and catalysts don’t affect KeqCatalysts (including enzymes) only accelerate reaction rates – they do NOT alter the equilibrium.They lower the activation energy (speed up Rx)but don’t change energy levels ofreactants and products.don’t change relative amounts ofreactants and products.Vocabulary – do you know the meaning of these terms?Reversible chemical RxChemical equilibrium (dynamic)Equilibrium constant (Keq)Exothermic vs endothermic Rx (and how heat of rx is involved)CatalystLe Châtelier’s PrincipleStress (on an equilibrium) – and how to relieve itDo you know the effect of concentration, temperature, and catalysts on rates of chemical Rx and on Keq?Can you write the expression for Keq given