Chemistry - Equilibrium - Quick Guides

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Study GuideChemistry–Equilibrium1. Quiz: Two Reaction Directions1.QuestionEquilibrium is the stable situation resulting fromAnswer Choices•the forward reaction only•the reverse reaction only•two offsetting reactionsCorrect Answertwo offsetting reactionsWhy This Is CorrectChemical equilibrium occurs whentwo opposing reactions—the forward reaction and the reversereaction—are happening at the same time.These reactions offset each other so that there isno overall changein the amounts of reactants andproducts, even though reactions are still occurring.2.QuestionWhen writing equilibrium reactions, a __________ is used.Answer Choices•single arrow pointing right•single arrow pointing left•double arrow

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Study GuideCorrect Answerdouble arrowWhy This Is CorrectAdouble arrow (⇌)shows that the reaction can proceed inboth directions—forward and reverse.This symbol indicates that the system can reach equilibrium, unlike a single arrow which shows aone-direction (irreversible) reaction.3.QuestionAt 1.0 atmosphere of pressure and 0°C, water can exist asAnswer Choices•only a solid•only a liquid•both a liquid and a solidCorrect Answerboth a liquid and a solidWhy This Is CorrectAt0°C and 1 atmpressure, water is at itsmelting/freezing point.At this temperature, ice and liquid water can exist together in equilibrium, meaning melting andfreezing occur at the same rate.4.QuestionEquilibrium positions will change depending on the

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Study GuideAnswer Choices•temperature•time of the day•use of a catalystCorrect AnswertemperatureWhy This Is CorrectChanging thetemperatureaffects the energy of particles and shifts the equilibrium position accordingto Le Châtelier’s principle.A catalyst only speeds up how fast equilibrium is reached and doesnotchange the equilibriumposition, and time of day has no effect.5.QuestionAt equilibrium and at a fixed temperature, the forward reaction will occur atAnswer Choices•the same rate as the reverse reaction•a rate twice that of the reverse reaction•a rate that is half that of the reverse reactionCorrect Answerthe same rate as the reverse reactionWhy This Is CorrectAt equilibrium, theratesof the forward and reverse reactions are equal.This balance keeps the concentrations of reactants and products constant, even though reactionscontinue to occur.

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Study Guide2. Equilibrium Concentrations2.1What Does Chemical Equilibrium Mean?When a chemical reaction reachesequilibrium, it doesnotstop. Instead, theforward and reversereactions happen at the same rate. Because of this balance, the concentrations (or pressures) ofreactants and products stayconstant over time.At equilibrium, these concentrations are related by a special number called theequilibrium constant,written asK.2.2Writing an Equilibrium ExpressionLet’s start with an example:The equilibrium expression for this reaction is:Important Rules to Remember:•Substances on theright side (products)go on thetopof the expression.•Substances on theleft side (reactants)go on thebottom.•Thecoefficientsin the balanced equation becomeexponentsin the expression.Because NO₂has a coefficient of 2, its concentration is squared.

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Study Guide2.3The General Form of an Equilibrium ExpressionFor a general reaction:The equilibrium expression is:Here:•Brackets [ ] meanconcentration•Lowercase letters are thestoichiometric coefficientsfrom the balanced equationExample: Formation of AmmoniaConsider this important industrial reaction:The equilibrium expression is:This tells us how ammonia, nitrogen, and hydrogen are related at equilibrium.Example: Dissociation of Water VaporAnother example is the dissociation of water vapor:The equilibrium expression is:Each reaction has itsown value of K, depending on the reaction and the temperature.

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Study Guide2.4Equilibrium Constants and UnitsThe value ofKdepends on:•Thereaction itself•Thetemperature•How concentration is measuredFor:•Solutions→ concentration is usually inmolarity (Kc)•Gases→ concentration can be expressed aspressure (Kp)2.5Calculating K from Experimental Data (Ammonia Example)For the reaction:The equilibrium constant is:Substituting the values:This large value of K means the reaction strongly favorsammonia formation.

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Study Guide2.6Using K to Predict Reaction DirectionYou can compare current pressures or concentrations with the value of K:•If the calculated ratio isgreater than K, the reaction shiftsleft(ammonia decomposes).•If the ratio isless than K, the reaction shiftsright(more ammonia forms).The system will always adjust until equilibrium is reached.2.7Pure Solids and Liquids in Equilibrium ExpressionsPuresolidsandliquidsdonotappear in equilibrium expressions because their concentrations stayconstant.Example:The equilibrium expression is:Water (H₂O) is a liquid, so it isnot included.Another Example with SolidsThe equilibrium expression is:The solids (LiBr and LiH) are required for the reaction but arenot part of the K expression.

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Study GuidePractice ApplicationsHalogen Reaction at 100°CGiven initial amounts in a 1.0 L container, equilibrium calculations can predict which direction thereaction will move.Sulfur Trioxide Formation at 700°CGiven:•Pₛₒ₂= 0.2 atm•Pₒ₂= 0.5 atmYou can calculate the pressure of SO₃at equilibrium.Summary•Equilibrium meansno net change, not no reaction.•The equilibrium constantKrelates reactants and products.•Coefficients becomeexponents.•Pure solids and liquids arenot included.•The value of K helps predictreaction direction.

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Study Guide3. Quiz: Equilibrium Concentrations1.QuestionThe equilibrium constant is always written as a ratio ofAnswer Choices•products over reactants•reactants over products•products times reactantsCorrect Answerproducts over reactantsWhy This Is CorrectBy definition, an equilibrium constant (K) is written as the concentration of products divided by theconcentration of reactants, each raised to the power of their coefficients from the balanced equation.2.QuestionWhen writing an equilibrium expression, the coefficients from the balanced equation become theAnswer Choices•factors of the concentration terms•exponents of the concentration terms•divisor of the concentration termsCorrect Answerexponents of the concentration termsWhy This Is CorrectThe coefficients in a balanced chemical equation indicate how many moles participate and thereforebecome the exponents in the equilibrium expression.

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Study Guide3.QuestionIn a (Kc) expression, the concentrations of the species are expressed in terms ofAnswer Choices•molality•mole fraction•molarityCorrect AnswermolarityWhy This Is Correct(Kc) uses molar concentrations (mol/L) of reactants and products at equilibrium.4.QuestionIn a (Kp) expression, the concentrations of the gaseous species are expressed in terms ofAnswer Choices•pressure•molality•molarityCorrect AnswerpressureWhy This Is Correct(Kp) is written using the partial pressures of gaseous reactants and products.

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