Heterogeneous and also Multiple Equilibria

Heterogeneous equilibria show off reactions through compounds in different phases; many equilibria show off reactions v two or much more steps.

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Learning Objectives

Calculate the equilibrium constant of a multiple-step reaction, offered the equilibrium constant for every step

Key Takeaways

Key PointsIn heterogeneous equilibria, compounds in different phases react. However, the concentration the a pure hard or fluid per unit volume is always the same. As such, the activity (ideal concentration) of a hard or fluid is 1, and these phases have no impact on the equilibrium expression.In multiple equilibria, the equilibrium have the right to be break-up into 2 or an ext steps. Both measures must be contained in the equilibrium continuous equation.The product the the equilibrium constants for each action in a reaction device is equal to the equilibrium expression the the as whole reaction.Key Termsheterogeneous: Having an ext than one phase (solid, liquid, gas) present in a device or process.

Heterogeneous Equilibria

In heterogeneous equilibria, compound in different phases react. For example, equilibrium might exist in between solid and gaseous species, in between liquid and aqueous species, etc.


The complying with equilibrium system requires both gas and also solid phases:

extC( exts)+ extCO _ 2 ( extg) ightleftharpoons 2 extCO( extg)

Therefore, the equilibrium expression for this reaction will certainly be created as:

extK _ exteq =frac < extCO>^2< extC>< extCO_2>

C(s) is omitted from the expression since it exist in the hard phase. The factor for this is due to the fact that the concentration that a pure hard or a pure fluid is constantly the same; that “concentration” is really its density, which is uniform regardless of sample size. Together a result, the activity, or appropriate concentration, of a fluid or a hard is defined as 1. Since their task is unity, and anything multiplied by 1 remains itself, solids and liquids have no result whatsoever top top the equilibrium expression. The over expression reduce to:

extK _ exteq =frac < extCO>^2(1)< extCO_2> =frac < extCO>^2< extCO_2>

Multiple Equilibria

In lot of equilibria, the equilibrium can be split into two or more steps. Both steps must be consisted of in the equilibrium continuous equation.


Consider the case of a diprotic acid, such as sulfuric acid. Diprotic acids have the right to be composed as H2A. When liquified in water, the mixture will certainly contain H2A, HA–, and A2–. This equilibria can be separation into 2 steps:

extH_2 extA leftrightharpoons extHA^- + extH^+quadquad extK_1 = frac< extHA^->< extH^+>< extH_2 extA>


Sulfuric acid: Sulfuric acid, the molecule pictured here, is an example of a diprotic acid.

extHA^- leftrightharpoons extA^2- + extH^+quadquad extK_2 = frac< extA^2->< extH^+>< extHA^->

K1 and also K2 are instances the equilibrium constants because that each step. Next, we deserve to write out the as whole reaction equation, i beg your pardon is a amount of these two steps:

extH_2 extA ightleftharpoons extA^2-+2; extH^+quadquad extK_ exteq=frac< extA^2->< extH^+>^2< extH_2 extA>

Notice the the equilibrium expression because that the as whole reaction, Keq, is equal to the product the the equilibrium expressions for the 2 reaction steps. Thus, for a reaction including two primary school steps:

extK_ exteq= extK_1 extK_2

Specialized Equilibrium Constants

Common reactions, such together the self-ionization the water, have specially named equilibrium constants.

Learning Objectives

List the assorted special types of equilibrium constants

Key Takeaways

Key PointsThe self-ionization that water is the dissociation of water into a proton and a hydroxide ion. The expression Kw is defined as the product that the concentration that hydrogen ions and the concentration the hydroxide ions.The acid dissociation constant, Ka, steps the loved one strength of one acid.The base dissociation constant, Kb, measures the loved one strength of a base.Key Termsself-ionization: The procedure by i m sorry a water molecule donates a proton come a surrounding water molecule, yielding hydronium and hydroxide ions.dissociation: The procedure of break molecules apart right into ions in solution.

Many reactions are so typical or advantageous that they have actually their very own special equilibrium constants.

Self-Ionization that Water

The self-ionization, or autodissociation, that water is a reaction that occurs to a very small extent in neutral water. In this process, one molecule of water donates a proton to a surrounding water molecule, which yields hydronium and hydroxide ions.


Autodissociation of water: A water molecule protonates a neighboring water molecule, yielding hydronium and also hydroxide ions.

The equilibrium expression for this reaction is composed as follows:

extK_ extW=< extH^+>< extOH^->=1.0 imes 10^-14

Note that due to the fact that water is a liquid, that is omitted indigenous this equilibrium expression. The worth of the dissociation constant of water, KW, is 1.0 imes 10^-14. This will certainly come into use later, in a future conversation on acids and bases.

Acid Dissociation Constant, Ka

An mountain dissociation constant, Ka, is the equilibrium consistent for the dissociation the an acid in aqueous solution. The general kind of the well balanced equation is:

extHA( extaq) ightleftharpoons extH^+( extaq)+ extA^-( extaq)

HA is a generic mountain that dissociates by separating into A−, well-known as the conjugate basic of the acid, and also a hydrogen ion, or proton, H+. As explained previously, hydrogen ion actually exist together solvated hydronium ions in aqueous solutions. The equilibrium expression is given as:

extK_ exta=frac< extH^+>< extA^->< extHA>

Recall that solid acids dissociate totally or almost fully into your ions. As such, strong acids will have huge values that Ka that are higher than one, which suggests that the front reaction the dissociation is strongly favored. Weak acids, on the other hand, will have small values the Ka that are less than one, indicating the the reverse reaction is strong favored; weak acids dissociate only to a small extent. Together such, Ka acts a relative indicator of acid strength.

Acetic mountain dissociation: A ball-and-stick design of the dissociation of acetic acid to acetate. A water molecule is protonated to type a hydronium ion in the process. The acidic proton that is transferred from acetic acid to water is displayed in green.

Base Dissociation Constant, Kb

The base dissociation constant, Kb, is analogous come the mountain dissociation constant. For a generalized reaction of a base, B, in water, we have:

extB( extaq)+ extH_2 extO( extl) ightleftharpoons extBH^+( extaq)+ extOH^-( extaq)

The equilibrium expression, Kb, is offered by:

extK_ extb=frac< extBH^+>< extOH^->< extB>

As through the acid dissociation constant, large values the Kb are indicative that a stronger base, while tiny values of Kb are indicative the a weaker base.

Reaction Quotients

The reaction quotient is a measure of the relative amounts of reactants and also products during a chemical reaction at a given allude in time.

Learning Objectives

Calculate the reaction quotient, Q, and also use it to predict even if it is a reaction will continue in the forward or turning back direction

Key Takeaways

Key PointsJust favor the equilibrium constant, Keq, the reaction quotient is a function of activities and/or concentrations of reactants and products.The reaction quotient have the right to be offered to recognize in i m sorry direction a reaction will certainly proceed. If Q = Keq, the reaction is at equilibrium. If Q eq, the reaction will relocate toward the products to with equilibrium. If Q > Keq, the reaction will move toward the reaction in order to with equilibrium.As the reaction proceeds, the species’ concentrations, and hence the reaction quotient, change. At some point the concentrations come to be constant; at this point, the reaction is in ~ equilibrium.Key Termsreaction quotient: A measure of the activities or concentration of the chemical varieties involved in a chemical reaction in ~ a given allude in time.equilibrium constant: A number value derived from the ratio of the concentration of products and also reactants at equilibrium.equilibrium: The state the a reaction in i beg your pardon the rates of the forward and reverse reactions are the same.

The reaction quotient, Q, is a measure of the relative amounts of reactants and also products throughout a chemistry reaction at a given allude in time. By to compare the worth of Q come the equilibrium constant, Keq, for the reaction, we have the right to determine whether the front reaction or turning back reaction will certainly be favored. Take it the complying with generic equation:

extaA + extbB ightleftharpoons extcC + extdD

The reaction quotient, Q, take away the complying with form:

extQ = frac< extC>^ extc< extD>^ extd< extA>^ exta< extB>^ extb

Note that the reaction quotient takes the precise same kind as the equilibrium constant, and also is a role of concentration and/or activities of the reactants and products. The distinction is that Q uses when the reaction is at non-equilibrium conditions, and also therefore that is value deserve to vary. Just as for the equilibrium constant, the reaction quotient can be a function of tasks or concentrations.

The reaction quotient can be used to determine whether a reaction under specified conditions will continue spontaneously in the front direction or in the turning back direction. 3 properties have the right to be derived from this definition of the reaction quotient:

If Q = Keq, the reaction is in ~ equilibrium.If Q eq, the reaction will relocate to the appropriate (in the front direction) in stimulate to reach equilibrium.If Q > Keq, the reaction will relocate to the left (in the turning back direction) in stimulate to with equilibrium.

Moving towards equilibrium: The sphere in the early stage state is indicative a reaction in i m sorry Q eq, deserve to be expressed as follows:

extK_ exteq=lim_ extt oinfty extQ( extt)

This expression reflects that Q will certainly eventually become equal come Keq, given an boundless amount that time. However, most reactions will generally reach equilibrium in a finite period of time.

Expressing the Equilibrium consistent of a Gas in terms of Pressure

For gas-phase reactions, the equilibrium consistent can be expressed in regards to partial pressures, and is provided the designation KP.

Key Takeaways

Key PointsAccording come the best gas equation, press is directly proportional come concentration, presume volume and also temperature are constant.Since press is directly proportional come concentration, we have the right to write our equilibrium expression because that a gas-phase reaction in terms of the partial pressures of every gas. This special equilibrium continuous is well-known as KP.KP bring away the exact same type as KC. To stop confusion between the two, execute not use brackets (< >) as soon as expressing partial pressures.Key Termspartial pressure: The push that one component of a mixture of gases contributes come the full pressure.equilibrium: The state the a reaction in i beg your pardon the rates of the forward and reverse reactions space the same.

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Liquefied gas: inside this tank, propane is compressed right into a liquid, which is in equilibrium through its gaseous headspace. The interior pressure of the gas propane is a function of temperature.