following 2008 chem spec

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  • Created by: danni :)
  • Created on: 05-06-13 11:11

what is dynamic equilibrium?

  • rate of forward reaction= rate of reverse reaction
  • therefore, no further change in the concentration of reactants or products
  • important equilibrium processes in industry
  • -haber process for ammonia
  • N2(g) + 3H2(g) Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) 2NH3(g)
  • -contact process for sulfuric acid
  • H2S2O7(l) + H2O(l) → 2 H2SO4(l)
  • -esterification
  • (http://www.chemguide.co.uk/organicprops/alcohols/estergeneq.gif)
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hydrogen and iodine

H2(g) + I2(g) <===> 2HI(g)

  • when the reactants are first placed in a closed system, only the forward reaction occurs because there is no product yet
  • once some product molecules have been formed, the rate of the reverse reaction increases, as the rate of the forward reaction decreases, until the two are happening at the same rate
  • an increase in the concentration of one side will drive equilibrium towards the other side
  • equilibrium can also be established between physical changes:

e.g., I2(s) <===> I2 (g) 

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equilibrium constants

H2(g) + I2(g) <===> 2HI(g)


[H2(g)] [I2(g)]  = Kc the square brackets represent concentrations in moldm-3. the concentrations are those at equilibrium. individuals are raised to powers that correspond to the number of moles in the balnced equation, therefore, Kc varies with the equation and the mole ratios.

In a heterogenous system, the concentration of solids are taken to always be constant, therefore, they are omitted from the equilibrium expression

the value of the equilibrium depends on Stotal :   Stotal= (8.31)lnK          

Stotal = ∆Ssystem + ∆SsurroundingsSsystem –∆H / T: so K depends on temperature and nature of the reaction

Kc< 1: products favoured, Kc>1 reactants favoured, very large Kc: reaction goes to completion

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gaseous equilibria and Kp

  • partial pressure is the pressure the gas would exert if it was alone in a filled container
    • total pressure= sum of partial pressures
      • mole fraction= moles of A/ moles of A+ B
        • partial presssure of A= mole fraction of A x total pressure of the system
  • all rules for writing the expression for Kp are the same as Kc
  • only temperature affects Kp pr Kc: although other things, such as pressure, can alter the POSITION of equilibrium
  • Kp and Kc can tell us how far a reaction has gone but they cant tell us reaction rate
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worked example: calculating Kc

at 313k, a mixture of 0.5 mol ethanol and 1.0 mol ethanoic acid are reacted until equilibrium is established. at equilibrium, the amount of ester was 0.4 mol. find Kc

vol of system = Vdm3

                     CH3COOH (l) + C2H5OH(l) <===> CH3COOC2H5(l) + H2O(l)      

initial mol                1.0               0.5                               0                      0             

equilib. mol             0.6               0.1                              0.4                    0.4           

Kc=    [CH3COO2H5] [H2O]

           [CH3COOH] [C2H5OH] =                      (0.4/V)^2   = 2.7


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equilibrium using immiscible solvents

  • when a substance is soluble in two immiscible solvents, the equilibrium constant is the partition coefficient
  • partition coefficient= [substance in solvent A] / [substance in solvent B] 
  • partition coefficients >1 mean the substance is more soluble in solvent A
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entropy and the equilibrium constant

for reversible systems:

  • ΔStotal= ΔSsys + ΔSsurr = 0 : entropy CHANGE = 0
  • balance between amounts of products and reactants determined by K

total entropy of a system is max at equilibrium

  • ΔStotal= RlnK  : R is constant so and increase in entropy means an increase in K, therefore, entropy change indicates the direction of a spontaneous reaction and, how far to completion the reaction goes, the larger K, the further equilibrium shifts towards the products
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using entropy to explain the effect of temperature

 ΔS surroundings= - ΔH/ T  

 ΔH in an exothermic reaction is negative which makes ΔS surroundings positive, exothermic reactions increase the entropy of the surroundings

ΔS total= RlnK or lnK = (1/R) x (ΔS surroundings + ΔS system)

so lnK=  (1/R) x (ΔS surroundings +  ΔH/ T )


  • for an exothermic reaction, when T increases, K decreases
  • for an endothermic reaction, when T increases, K increases
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rates and equilibria in industry

  • catalysts speed up the forward and reverse reactions
  • they lower activation energy
  • reduce the time taken to establish equilibrium
  • have no effect on te value of equilibrium constants
  • if a reaction is  ΔH positive, K rises with temperature increase giving more products.
  • When  ΔH is negative, K falls with temperature increase, giving more reactants.
  • Increased temperature increases te rate constant k, so rate increases wit increasing temperature
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industry 2

  • adding more reactants to an equilibrium mixture accelerates the forward reaction
  • as the amounts of products increase, the reverse reaction reaction starts to speed up. a new position of equilibrium is established but K remains unchanged
  • the pressure of the gas is proportional to its concentration. Changes in pressure dont affect the value K
  • a change in pressure affects the position of equilibrium according to le chateliers principle
  • increasing pressure means more molecules in a given volume so collisions occur more frequently so the rate of reaction increases

Atom economy

  • atom economy (%) = mass of atoms in desired product     x 100
  • total mass of atoms in reactants
  • atom economy can be improved by:
  • developing more effiecient synthesis, e.g., new catalyst/ replacing elimination reactions with addition reactions
  • finding new uses for any by-products
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