EQUILIBRIA

AQA AS CHEM2 EQUILIBRIA 

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  • Created by: melissa
  • Created on: 04-04-12 10:37

Types of Equilibria

dynamic equilibria

forward rate = backwards rate 

concentrations if each species remains constant

phase

any uniform part of a system which is a different from the other part of the system and seperated by a distinct boundary

eg oil and water

homogeneous & hetrigeneous equilibria

homogeneous...one phase (All reactants in the same phase) eg ethanol and water

hetrogeneous...more than one phase eg iron and water

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Le Charelier's Principle

changing the concentration of a reactant/product

if concentration of reactants is increased equilibria moves right and it increases the rate. 

changing the total pressure (gaseous)

increasing the pressure favours the side with the least moles of gas and it increases the rate

changing the temperature

increasing the temperature favors endothermic route and it increases the rate 

adding a catalyst

adding a catalyst has no effect on the position but it increases the rate 

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The Haber Process

N2(g) + 3H2(g) Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) 2NH3(g)

exothermic reaction

decrease in number of moles 

highest and quickest yeild will be achieved by a low temperature and a high pressure

BUT...

low temperature is slow so a comprimise is used (650-720K)

iron catalyst is used to speed up

pressure used is 2.0 x10-

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The Contact Process

SO3(g) + H2O(l) =>H2SO4(aq) /\H = -130kJ mol-

 oxidising sulfur... 2SO2 + O2 → 2SO

exothermic reaction

decrease in number of moles 

highest and quickest yeild will be achieved by a low temperature and a high pressure

BUT...

at high pressure, SOliquifies and at low temperature it is to slow

PRESSURE=  2-3 atm

TEMPERATURE= 450°c

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Ethanol production

C2H4(g)      +      H2O(l)         arrow (http://www.gcsescience.com/arrow.gif)          C2H5OH(l)

exothermic reaction

Temperature...eqm goes left

high yield favoured by a low temperature but it is slow so a compromise of 300°c

Pressure...2 moles to 1 mole

eqm goes right 

high yield favoured by high pressure but it is expensive so a compromise  of 6.5m Pa

 Phosphoric acid, H3PO4 is added to increase the rate of both reactions equally so eqm is unaffected but reaction is much quicker

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Methanol production

CO + 2 H2 → CH3OH

exothermic reaction

Temperature...eqm goes left

high yield favoured by a low temperature but it is slow so a compromise of 400°c

Pressure...3 moles to 1 mole

eqm goes right 

high yield favoured by high pressure but it is expensive so a compromise of 5mPa

zinc oxide and chromium (III) oxide Cr2O3  added to increase the rate of both reactions equally so eqm is unaffected but reaction is much quicker

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