Equilibria

• What is a reversible reaction?


  Where reactions can go forwards and backwards.
• Describe a forwards reaction?


  Initially reactants are used up quickly but slow as their conc drops.
• Describe a backwards reaction?


  Initially reactants are reformed slowly but speed up as conc of products increases.
• When is dynamic equilibrium reached?


  When forwards reaction equals same rate as backwards and conc of each substance remains constant.

  Only occurs in a CLOSED SYSTEM.
• Le Chatelier's Principle?


  If a reaction at equilibrium changes in pressure, temp or conc the position of equilibrium shifts to counteract the change.
• Increasing conc affect on equilibrium?


  If we increase the conc of a product or reactant, equilibrium will shift to reduce the conc.

  eg 3H2 + N2 > 2NH3.

  If we increase the conc of H2, equilibrium shifts right to use it up and reduce the conc. More NH3 will produce to counteract change.
• Increasing pressure affect on equilibrium?


  If we increase pressure, equilibrium shifts to side with lower no. gas particles to reduce pressure.

  eg 3H2 + N2 > 2NH3. More NH3 will be produced and equilibrium shifts right.
• Increasing temp affect on equilibrium?


  If we increase the temp, equilibrium shifts in the endothermic direction to decrease temp.

  eg 2H2 + N2 > 2NH3. Enthalpy= -46.2.

  More H2 and N2 will be produced as equilibrium shifts left.
• Catalysts affect on equilibrium?


  Catalysts don't affect the position of equilibrium.

  They speed up the rate of reaction equally.

  It has NO EFFECT on yield.
• Making ethanol?


  C2H4 + H2O > C2H5OH. Enthalpy change= -46KJmol-1.

  High pressure shifts equilibrium right to produce more ethanol and increase reaction rate. High pressure is expensive as thicker pipes needed.

  Decreasing temp shifts equilibrium right, producing more ethanol. But lower temp means lower rate of reaction.
• Conditions of making ethanol?


  Pressure= 60atm.

  Temp= 300 degrees.

  Catalyst= Phosphoric acid.
• How can Kc be calculated?


  From the molar concentrations in a reaction.

  Kc= [products] divided by [reactants]
• How does temp affect kc value?


  Kc is only valid for 1 temp.

  Changing temp, changes equilibrium conc, changing kc.

  If temp change shifts equilibrium right, kc will increase.

  If temp change shifts equilibrium left, kc will decrease.
• How does conc affect kc?


  Kc value is unaffected by changes in conc.
• How does a catalyst affect kc value?


  No effect on kc value but speeds up rate of reaction.

  Helps reach equilibrium at a faster rate.
• Why is chlorine used in swimming pools even though its toxic?


  Only used in small amounts.

  Health benefits outweigh risks.
• Define dynamic equilibrium?


  The forward and backward reaction occurs at the same rate so the conc of reactants/products is constant in a CLOSED SYSTEM.
• Le Chateliers Principle?


  If conc, pressure, temp changes then the position of equilibrium shifts to oppose the change.
• Increasing temp effect on equilibrium?


  Shifts temp in endothermic direction to decrease temp.
• Increasing pressure effect on equilibrium?


  Shifts to side with less moles to reduce it
• Catalyst effect on equilibrium?


  No effect on equilibrium position but reaches equilibrium faster.
• Effects on equilibrium?


  Low temp produces a higher yield but at a slower rate.

  High pressure produces a higher yield but is more expensive as requires thicker pipes.
• Kc expression?


  Products conc/ reactants conc
• Units of kc?


  More moles of products: mol mol mol/ mol mol= moldm-3

  mol mol mol mol mol/ mol mol mol= mol2dm-6

  More moles of reactants: mol/ mol mol= mol-1dm3

  mol mol/ mol mol mol mol= mol-2dm6
• Calculating equilibrium moles?


  Requires knowing starting reactant/product moles and equilibrium mole of 1 chemical.

  eg PCl5 >< PCl3 + Cl2 Volume=2dm3

  Start moles= 6, 0, 0

  Equilibrium moles= 4 (-2), 2 (+2), 2 (+2)

  Conc= moles/ volume

  4/2= 2. 2/2= 1. 2/2=1. [1][1]/[2]= 0.5moldm-3
• Calculating equilibrium moles?


  eg N2 + 3H2 >< 2NH3 Volume= 3dm3

  Starting moles= 12, 15, 0

  Equilibrium moles= 9 (-3), 6 (-3x2), 6(+3x2)

  9/3= 3. 6/2= 2. 6/3=2. [2]^2/[3][2]^3= 0.167mol-2dm6
• Factor affecting kc?


  Only temp affects kc. If it shifts equilibrium right, kc increases.

  If temp shifts equilibrium left, kc decreases.

  Catalyst speeds up rate of reaction.

  Pressure and conc don't as the change cancels out.
• Density=


  mass/volume
• Forwards reaction?


  Initially reactants are used up quickly but slow as their conc drops.
• backwards reaction?


  Initially reactants are reformed slowly but speed up as their conc of products increases.
• Making ethanol?

  C2H4(g) + H2O(g) >< C2H5OH(g). Enthalpy change= -46Kjmol-1

  Temp= 300 degrees

  Pressure= 60atm

  Catalyst= Phosphoric acid H3PO4
• Using kc to workout equilibrium conc?


  eg CH3COOH + C2H5OH >< CH3COOC2H5 + H2O Kc= 3.5

  2.8moldm-3 3.2moldm-3 3.5= [CH3COOC2H5][H2O]/ [1.8][3.2]

  Rearranged= 3.5 x 1.8 x 3.2= 20.16

  [CH3COOC2H5 ]=[H2O] so 20.16 square rooted= 4.49moldm-3
• If the forwards reaction is endothermic what temp should be used to increase the yield of products?

  Higher temp should be used.

  The forwards reaction is endothermic so equilibrium shifts right to oppose increase in temp.
• Why is operating at a high pressure used in industry?

  Increases the rate of reaction. High pressure leads to particles being closer, so an increased collision frequency.
• Explain why the catalyst is more effective when coated onto an unreactive honeycomb?

  Greater surface area so more successful collisions.
• Explain the effect of a change of temperature on both the position of equilibrium and the rate of reaction, and justify why a compromise temperature is used industrially?

  Exothermic reaction shifts equilibrium in endothermic direction at high temps. This will decrease the yield if the forwards reaction is endothermic.

  At higher temps, molecules have more energy. Therefore an increased collision frequency, so rate of reaction increases.

  Industrial conditions achieve a balance of a suitable yield at a reasonable rate.
• Why would a data bond enthalpy be different from a calculated?

  Mean bond enthalpies aren't the same as actual bond enthalpies in CO2, CH4 and H2O
• Why might catalysts be less effective if there are impurities?

  Impurities/ sulfur compounds block the active sites.
• State how the forward and reverse reaction are related to eachother at equilibrium?


  They occur at the same time.
• Why would a data book bond enthalpy be different from a calculated?

  Mean bond enthalpies aren't the same as actual bond enthalpies in CO2.