Essays /

14 Equilibrium Essay

Essay preview

14.1 THE CONCEPT OF EQUILIBRIUM AND THE EQUILIBRIUM CONSTANT Many chemical reactions do not go to completion but instead attain a state of chemical equilibrium.
Chemical equilibrium: A state in which the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant. ⇒ Equilibrium is a dynamic process – the conversions of reactants to products and products to reactants are still going on, although there is no net change in the number of reactant and product molecules.

For the reaction: N2O4(g)



Forward rate




Reverse rate


The Equilibrium Constant
For a reaction:

aA + bB

equilibrium constant:

cC + dD
Kc =

[C ]c [ D] d
[ A] a [ B]b

The equilibrium constant, Kc, is the ratio of the equilibrium concentrations of products over the equilibrium concentrations of reactants each raised to the power of their stoichiometric coefficients.

Example. Write the equilibrium constant, Kc, for N2O4(g)


Law of mass action - The value of the equilibrium constant expression, Kc, is constant for a given reaction at equilibrium and at a constant temperature. ⇒ The equilibrium concentrations of reactants and products may vary, but the value for Kc remains the same.

Other Characteristics of Kc
1) Equilibrium can be approached from either direction.
2) Kc does not depend on the initial concentrations of reactants and products. 3) Kc does depend on temperature.

Magnitude of Kc
⇒ If the Kc value is large (Kc >> 1), the equilibrium lies to the right and the reaction mixture contains mostly products.
⇒ If the Kc value is small (Kc

Calculating Equilibrium Constants, Kc
Kc values are listed without units ⇒ don't include units when calculating Kc. If equilibrium concentrations are known, simply substitute the concentrations into the equilibrium constant expression:

Example. For the reaction, CO + 3H2
CH4 + H2O, calculate Kc from the following equilibrium
concentrations: [CO] = 0.0613 M; [H2] = 0.1839...

Read more


-91.8 0 0.0387 0.0613 0.0821 0.162 0.1839 0.200 0.250 0.500 1 1.15 1.5 1.7 10 10.0 1000 14 14.1 14.2 14.3 14.4 14.5 1l 2 2.00 2.8 2a 2b 2co 2co2 2h2 2h2o 2hf 2hi 2n2 2nh3 2no 2no2 2no3 2o 2so2 2so3 3 3.0 3.000 3cu 3cu2 3h2 3o2 4 430 4h2o 4nh3 5 54.3 566 6 6.000 6h2o 8h aa action ad add adjust affect alreadi also although amount approach aq aqueous assign assum atm attain away b back bb becom c caco3 calcul cao catalyst cc certain ch4 chang chapter characterist chateli chemic choos co co2 coeffici complet concentr concept condit consid constant consum contain convers d dd decreas densiti depend direct disturb dynam e e.g ea effect either endotherm equal equat equilibria equilibrium exampl exist exotherm express extern f f2 factor find fix flask follow form formula forward found g gas gase gaseous get given go greater h h2 h2o heat heterogen hf hi homogen i2 ice ideal includ incorpor increas increasing/decreasing initi instead introduc k k1 k2 k3 kc kelvin kf kinet kj known kp kr l larg larger law le left lie liquid list lower m magnitud mani mass math may method mixtur ml modifi mol molar mole molecul most move multipl multipli must n n/v n2 n2o4 net new nh3 no2 note number o2 o4 occur offset omit one optimum order otherwis p page part partial perfect phase place plug pn pno posit power predict present pressur principl problem proceed process product pure q quadrat quotient r rais rate ratio reach reactant reaction rearrang recal reciproc reestablish relat relationship remain remov replac result revers right root rt rxn select set shift side simpli sinc singl small smaller so2 so3 solid solut solv solvent speci specifi squar state still stoichiometr stoichiometri stress substanc substitut suppos system tabl take taken tell temperatur term thus time toward unit unless us use usual v valu vari variabl vessel volum without write written x x102 x1027