Fundamentals of Chemistry: Theory and Practice: DH2K 34 Topic 7 - Equilibrium

Chemical Reactions When we study chemical reactions we often assume that reactions go to completion. Reactants Products Most chemical systems are not that straight forward. Almost always reactions do not go to completion Reactants Products We have what is called a chemical equilibrium

Reversible reactions naturally reach a point where:- Equilibrium Reversible reactions naturally reach a point where:- Rate of forward reaction = Rate of reverse reaction In chemistry we deal with dynamic equilibrium the concentration of reactants and products remains constant but the forward and back reactions continue.

Dynamic Equilibrium Consider the reversible reaction A + B C + D Initially there are no products C + D and only the forward reaction occurs. The rate of the forward reaction decreases as the reactants are used up. As the products are formed the rate of the reverse reaction increases A dynamic equilibrium is reached when rate of forward reaction = rate of reverse reaction.

Rate of forward reaction = rate of reverse reaction

It is rare in an equilibrium that the mixture will contain 50% reactants and 50% products

More often the concentration of the reactants is greater than the concentration of the products The equilibrium lies to the left

Or the concentration of the products is greater than the concentration of the reactants. The equilibrium lies to the right

The equilibrium in this case lies heavily to the left. The Water Equilibrium H2O H+ + OH- The equilibrium in this case lies heavily to the left. Most water exists as molecules not as ions H2O H+ + OH-

Summarise A chemical reaction is at equilibrium when the conc of reactants and products remain constant. Equilibrium is reached when rate of forward reaction = rate of backwards reaction At equilibrium the conc of reactants and conc of products need not be equal. The same equilibrium position is reached whether we start with reactants and form products or start with products and form reactants.

The Equilibrium Constant: Kc Consider the following generalised reaction: aA + bB cC + dD The law of mass action states that at equilibrium the following expression can be applied: Equilibrium constant Kc = [C]c[D]d [A]a[B]b Concentration of products Indices from balanced equation Concentration of reactants In terms of concentration

Equilibrium Constant :Kc Write an equilibrium expression for the following reaction: Fe3+(aq) + 6CN-(aq) Fe(CN)63-(aq) Kc= [Fe(CN)63-] [Fe3+] [CN-]6 Try The following: 2Fe3+(aq) + 3I- 2Fe2+(aq) + I3-(aq) H3PO4(aq) 2H+(aq) + HPO42-(aq)

Equilibrium Constant: Kc Equilibrium constant is the ratio of the concentration of products divided by the concentration of reactants. The actual value gives guidance to the extent of a reaction once it has reached equilibrium. The greater the value of Kcthe greater the concentration of the products compared to the reactants. ie. the further the reaction has gone to completion

Equilibrium lies to the left Generally in time all reactions can be considered to have reached equilibrium and the following general assumption may be made Value of Kc Extent of Reaction <10 -2 Equilibrium lies to the left 10 -2 – 10 2 Significant quantities of reactants and products at equilibrium >10 2 Equilibrium lies to the right

Equilibrium constant involving gases (Kp) For reactions involving gaseous reactants and products the equilibrium constant may be expressed in terms of partial pressures. Gases inside a closed container exert a pressure proportional to the number of moles of the particular gas present. For example if 2 gases are present in equimolar amounts and the total pressure is 1 atmosphere then the partial pressure of each gas is 0.5 atmosphere.

Equilibrium constant involving gases (Kp) For the general equation: aA + bB cC + dD Equilibrium constant Kp= PCc PDd PAa PBb Pressure of products Pressure of reactants In terms of pressure

Equilibrium constant involving gases (Kp) The equation for the Haber process used in the manufacture of ammonia is: N2(g) + 3H2(g) 2NH3(g) Kp= PNH32 PN2PH23 Try the following 2NOCl(g) 2NO(g) + Cl2(g) 2SO2(g) + O2(g) 2SO3(g)

The Solubility Product Constant When an ionic compound is dissolved in water it usually goes into solution as ions. When an excess of a slightly soluble ionic compound is mixed with water an equilibrium occurs between the solid compound and the ions in the saturated solution.

The Solubility Product Constant CaC2O4(s) Ca2+(aq) + C2O42-(aq) An equilibrium constant exists for this solubility process called the solubility product constant. For CaC2O4this is written Ksp= [Ca2+][C2O42-] In general the solubility product constant (Ksp) represents the solubility of a salt in a saturated solution.

Solubility Product cont. The solubility product equals the product of the equilibrium concentrations of the ions in the compound, each concentration raised to a power equal to the number of such ions in the formula of the compound. PbI2(s) Pb2+(aq) + 2I- Ksp= [Pb2+] [I-]2

Examples to Try Write the solubility product expressions for the following salts. AgCl BaCO3 Pb(OH)2 Mg3(PO4)2 Pb3(AsO4)2 Use page 7 in databook at also look at what group an element is in to determine its charge – remember they must balance to form a neutral molecule.