Chemistry

Chemistry

1. C3

   • 4
     • 4.6 - Chemical equilibrium
       • reversible reactions
         • Means products can react to make the reactants again
       • closed systems
         • no reactants or products can escape
         • equilibrium is reached when rate of forward reaction = rate of reverse reaction
           • at equilibrium both reactions happen, but amount of reactants and products remain the same
       • changing concentration of reactant or product
         • amounts of reactants or products
           • Changed by changing the reaction conditions
             • EG
               • increasing concentration of reactant causes more products to be formed as system tries to reach equilibrium
               • if product removed, more reactants will react to form more products to reach an equilibrium
               • ICL + CL2 (RR symbol) ICL3
                 • If chlorine added, concentration of chlorine increases so more ICL3 is produced
                 • if chlorine removed, concentration of chlorine decreases so more ICL produced
     • 4.7 - altering conditions
       • changing pressure
         • if different number of molecules of gas on one side of reaction than other
           • changing pressure affects position of equilibrium
             • EG
               • increasing pressure
                 • Causes position of equilibrium to shift to try to reduce pressure
                   • favouring reaction that produces fewer molecules
         • if forward reaction produces more molecules of gas
           • an increase in pressure decreases amount of products formed
           • a decrease in pressure increases the amount of products formed
         • if forward reaction produces less molecules of gas
           • an increase in pressure increases the amount of products formed
           • a decrease in pressure decreases amount of products formed
       • causes position of equilibrium to shift to try to cancel out the change
       • changing temperature
         • increasing temperature favours endothermic reaction
         • decreasing temperature favours exothermic reaction
         • if forward reaction is...
           • exothermic
             • an increase in temperature decreases amount of product formed
             • a decrease in temperature increases the amount of products formed
           • endothermic
             • an increase in temperature increases that amount of products formed
             • a decrease in temperature decreases the amount of produts formed
     • 4.8 +4.9 - the Haber process
       • Making ammonia
         • Haber process
           • Used to manufacture ammonia
             • Uses of ammonia
               • Making fertilisers and other chemicals
         • Ingredients
           • Nitrogen
           • Hydrogen
         • Conditions to make it
           • Gasses passed over iron catalyst
           • 450 degrees C
           • 200 atmospheres of pressure
           • Are used to give fast rate and reasonable yield
         • Reversible reaction
           • N2(g) +3H2(g) <----->2NH3(g)
           • Means some ammonia produced breaks back down into hydrogen and nitrogen
             • So yield = about 15%
         • Gases that come out of reactor are cooled so ammonia condenses
           • Liquid ammonia separated from unreacted gases
             • unreacted gases recycled to reduce waste
       • Economics of the haber process
         • optimum pressure
           • Why?
             • products have fewer molecules of gas than the reactants
               • So the higher the pressure, the greater the yield
                 • However high pressure has problems
                   • More NRG needed to compress the gas
                   • Stronger reaction vessels needed
                     • These are very costly
           • 200 atmospheres
           • Is a comprimise between the costs and the yield
         • Optimum temperature
           • Why?
             • Forward reaction = exothermic
               • So the lower the temperature, the greater the yield
                 • However...
                   • Reaction rate decreases with temperature
                   • At low temps, iron catalyst becomes ineffective
                     • Meaning it would take a long time to produce any ammonia
           • 450 C is used - it gived reasonable yield in a short time