Fundamentals of Chemistry: Theory and Practice: DH2K 34 Equilibrium 2

Learning Points A chemical reaction is at equilibrium when the composition of the reactants and products remains constant. This state occurs when the rate of the forward reaction is equal to the rate of the back reaction. The same equilibrium mixture is obtained whether you start with the reactants or the products.

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 Constant: Kc The explosive reaction between hydrogen and fluorine: H2 + F2 2HF Has an equilibrium constant of 1 x1047 This means at equilibrium negligible amounts of reactants remain. By contrast the dissociation of chlorine molecules to atoms : Cl2 2Cl- Has a Kc=1 x10-38 at room temp, indicating that the reaction hardly occurs under these conditions

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

1)The Kcvalue for the reaction PCl5 PCl3 + Cl2 is 0.021 at 160oC. Which compound is present in the greatest concentration at equilibrium? 2) The following equilibrium constants apply at room temperature 25oC. Zn (s)+ Cu2+(aq) Cu(s) + Zn2+(aq) Kc=2x1037 Mg(s) + Cu2+(aq) Cu(s) + Mg2+(aq)Kc =6 x1090 Fe(s) + Cu2+(aq) Cu(s) + Fe2+(aq) Kc = 3x 1026 Of the metals Zn, Mg and Fe which removes Cu(II) ions from solution most completely?

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 0.5atmosphere.

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

Another example BaCO3 Ba2+ + CO32- Ksp = [Ba2+] [CO32-]

Reaction Quotient aA + bB cC + dD Q = [C]c[D]d [A]a[B]b The reaction quotient applies at any point before equilibrium is reached. At equilibrium Q = Kc

When Q = K equilibrium has been reached E + F G + H predominates predominates When Q>K E + F G + H When Q = K equilibrium has been reached

Le Chatilier’s Principle If a system in dynamic equilibrium is subjected to a change , processes will occur to counteract this change and restore equilibrium.

Affecting an Equilibrium Equilibrium can be affected in a number of ways:- Catalyst Concentration Temperature Pressure

Adding a Catalyst Catalyst lowers the energy needed for a reaction to take place. It does not alter the mass of reactants or products Adding a catalyst does not alter the position of equilibrium Equilibrium will be reached faster

Effect of Concentration A + B C + D Increasing the concentration of the reactants speeds up the forward reaction producing more products until a new equilibrium is reached. A new equilibrium is reached further to the right. A similar result is obtained by removing the products which slows the back reaction.

Effect of Concentration Similarly the equilibrium may be moved to the left by:- Removing the reactants. Addition of product

Example 1: ICl(l) + Cl2(g) ICl3(s) brown liquid yellow crystals Adding Chlorine gas shifts the equilibrium to the right This can be seen by an increase in the mass of yellow crystals Removing Chlorine has the opposite effect,shifting the equilibrium to the left.

Example 2: Fe3+(aq) + CNS-(aq) FeCNS(aq) Colourless Red If the red colour is diluted with water to give an orange colour,the position of equilibrium may be shown by the intensity of the colour. NaCl added KCNS FeCl3 Control

Effect of Pressure A change in pressure effects the equilibrium only if gases are involved. Increasing the pressure favours whichever reaction brings about a reduction in the total number of gas molecules. N2O4(g) 2NO2(g) 1 mole 2moles Colourless brown

Effect of Pressure cont. Increasing the pressure favours the production of dinitrogen tetroxide (N2O4) Increasing pressure moves the equilibrium to the left.

Exothermic Reactions During an exothermic reaction energy stored in the reactants (potential energy) is released to the surroundings. The products of an exothermic reaction have less energy than the reactants Potential Energy Reaction Path Reactants Products

Endothermic Reactions During an endothermic reaction energy stored in the reactants (potential energy) absorb energy from the surroundings. The products of an endothermic reaction have more energy than the reactants Reaction Path Potential Energy Reactants Products

Exothermic reactions have negative enthalpy values Enthalpy Change The difference in potential energy between products and reactants is the enthalpy change Exothermic reactions have negative enthalpy values Endothermic Reactions have positive enthalpy values The units of enthalpy are kilojoules (kJ) or kilojoules per mole (kJmol-1)

A rise in temperature will affect both reactions but not equally. Effect of Temperature In a reversible reaction if the forward reaction is exothermic then the back reaction must be endothermic. A rise in temperature will affect both reactions but not equally.

Effect of temperature cont. A rise in temperature favours the endothermic reaction (the reaction that needs to have energy supplied.) Conversely a fall in temperature favours the exothermic reaction (the reaction that gives out energy)

colourless exothermic brown Example endothermic N2O4(g) 2NO2(g) colourless exothermic brown Increased temperatures favour the production of NO2 and the equilibrium shifts to the right,causing the brown colour to darken.