Comments (0)

Transcript of Chapter 6: Equilibrium Chemistry

Chapter 6: Equilibrium ChemistryReversible Reactions and ThermodynamicsActivity EffectsBuffersLadder DiagramsSolving Equilibrium ProblemsEquilibrium Constants for Chemical ReactionsReactions are thought of as proceeding in a forward and reverse direction. In other words:

Both occurWhen the rate of the forward reaction balances the reverse, it reaches a steady stateWe say that the reaction has reached equilibriumThe amount of products and reactant will not change unless the equilibrium is disturbedReversible ReactionsThe direction in which a reaction moves in order to reach equilibrium is determined by the change in Gibbs Free Energy, G. If delta G is negative, the reaction moves in the forward direction (as written).Delta G is given by:

Where reaction quotient, Q is:

At equilibrium, delta G = 0 and the reaction quotient becomes the equilibrium constant, K.Thermodynamics and Equilibrium ChemistryFor a general equation:

When an equation is reversed:

If a reaction is the sum of two or more reactions, the individual equilibrium constants can be multiplied.Manipulating Equilibrium ConstantsDefined as two of more soluble chemicals reacting to form a solid product (precipitate)e.g.

The ability of a precipitate to dissolve is called the solubility product constant, Kspe.g.Precipitation ReactionsUsing the Bronsted-Lowry definition an acid-base reaction involves the transfer of a proton (H+) from an acid to a base.Strong acids and bases are considered to react complete (no equilibrium exists)e.g.Strong Acid-Base ReactionsWeak acids and bases do not fully react with the solventThe acid dissociation constant defines the strength of the acid

The base dissociation constant defines the strength of the baseWeak Acid-Base ReactionsWater can act as a acid and base

pH is defined as the –log[H3O+]For a neutral solution:

For a conjugate acid base pair:Autoionisation of Water and pHComplexation ReactionsReactions involving the transfer of electronsOxidation involves the loss of electronsReduction involves a gain in electronsThe relative strengths of oxidation or reduction agents are found from their reduction potentials. A large reduction potential suggests a species with a large potential to gain electrons and remove an electrons from another species (therefore a good oxidizing agent)Oxidation-Reduction ReactionsThe potential of a reaction at standard conditions is given by:

The potential is related to the equilibrium constant, K, according to the equation:

At non-standard conditions, we find the potential using the Nernst equation:Reduction Potential and Equilibrium ConstantThe equilibrium position of a reaction will move in order to relieve an applied stressFor example if a reactant is added to a reaction at equilibrium, it will produce more products in order to relieve the stressThe value of the equilibrium constant does not changeLa Chatelier’s PrinciplepH3.7HFF-When pH>3.7, F- is the dominant speciesWhen pH<3.7HF dominatespNH3.313.91pKalogK2logK1Ag+Ag(NH )Ag(NH )33+3when pNH3>3.91Ag+ dominatesWhen pNH3<3.31Ag(NH3)2+ dominatesWhen pNH3 is between 3.31 and 3.91Ag(NH3)+ dominatesE (V)ICE Table ApproachWrite the balanced chemical equationWrite the equilibrium constant expressionDraw an ICE (Initial, Change, Equilibrium) TableInsert concentrations, using x for unknownsInsert equilibrium concentrations into the equilibrium constant expressionApply assumptions (if any)Solve for xCheck AssumptionsA Systematic ApproachWrite all relevant reactions and equilibrium constant expressionsCount unique species appearing in the equilibrium constant expressions. If the number of unknowns is great than the number of equilibrium expressions, add a mass balance and/or charge balance equationCombine equations and solve for one unknown making appropriate assumptions when possibleCheck any assumptionsMass BalanceCharge BalanceReactions involving the formation of coordinative covalent bond between a metal ion and a ligandA ligand is a species with one or more lone pairs of electronsThe product is called the complexComplex formation is characterized by the formation constant, Kf.EE00Fe /Fe2+3+Sn/Sn4+2++0.771V+0.154Sn2+ & Fe2+dominateSn4+ & Fe2+dominateSn4+ & Fe3+dominateA statement of conservation of matterThe products must contain the same number and type of atoms as the reactantse.g. when HF ionizes, the number of moles of F- and HF produced must equal the initial moles of HF. For a constant volume:The charge of cations must equal the charge of anionsIn other words, the solution must have an overall neutral chargeEquation must include every ion in solution.E.g. for a solution of magnesium fluoride:2+A buffer solution is one that resists a change. For example, an acid/ base buffer will resist changes in pH due to addition of acid or base to the solutionBuffers can also exists for other types of reactionsBuffer SolutionBuffer problems can be solved using the ICE table approach or by applying the Henderson-Hasselbach Equation

Be aware that this equation includes assumptions.Solving Buffer ProblemsBuffer capacity is the ability of a buffer to resist a change in pH when adding a strong acid or baseThe buffer capacity depends on the concentrations of the buffer componentsAcid-base buffers are made by combining known amounts of acid and conjugate base or by titrating an acid or base in order to produce the conjugatePreparing BuffersLadder diagrams can be used to illustrate buffersThe diagram shows a) an acid-base buffer b) a complexation buffer and c) a redox buffer with the buffer regions indicated by grey boxesIf an equilibrium occurs in a solution containing other ions (not common ions) the measured concentration will be different to the calculated valuee.g. for the reaction:

In a potassium nitrate solution, the potassium and nitrate ions form an ionic atmospheres around the silver and iodate ions causing drag and slowing down the reverse reaction, shifting the equilibrium and making the silver iodate more soluble

The “effective concentration” of silver and iodate ions is called the activity, a.ActivityEquilibrium constants are really in terms of activities not concentrations

Activity is related to concentration by the activity coefficient:

At infinite dilution the activity coefficient is one, and activity equals concentrationThe activity coefficient can be calculated using the extended Debye-Huckel equation