Kinetics

• What is meant by 'activation energy'?

  The amount of energy required to cause a chemical reaction
• What does Le Chatelier's principle state?

  If a stress is applied to a system at equilibrium, equilibrium shifts to relieve that stress
• What is meant by rate of reaction?

  The amount of substance formed per unit of time
• CH4 + H2O >< CO + 3H2

  Reaction is done at temperature of 800 degrees c and low pressure of 300kPa with nickel catalyst

  Explain in terms of equilibrium yield and cost why these conditions are used

  Stage 1: Temperature

  • Reaction is endothermic (positive value) so equilibrium shifts to RHS to reduce the temperature. So higher temp increases yield, higher temps are costly
• What is Kc


  equilibrium constant
• Effect of increase in temperature on equilibrium position

  Endothermic- Moves right, so increase of Kc

  Exothermic- Moves left, so decrease of Kc
• Expression for Kc


  [products]/[reactants]
• exothermic reaction


  A reaction that releases energy in the form of heat
• What is an endothermic reaction?

  A reaction that ABSORBS energy in the form of heat
• Bond making


  exothermic (releases energy)
• Bond breaking


  endothermic (requires energy)
• Le Chatelier's Principle


  the position of equilibrium moves to oppose any change
• What condition affects the value of Kc?

  temperature
• What does Kc being greater or less than 1 suggest for the position of equilibrium?

  Greater than 1= over to the right

  Lesser than 1= over to the left
• dynamic equilibrium


  the rate of the forward reaction is equal to the rate of the reverse reaction. Concentration of reactants and products remain constant
• Why the operation of this process at a much higher pressure would be very expensive

  Energy required to provide high pressure, strong pressure vessel needed (to withstand high pressure)
• State and explain the effect of an increase in pressure and the effect of an increase in temperature, on the yield of nitrogen monoxide in the above equilibrium (+180)

  Effect of increase in pressure on the yield: no change, equal number of gaseous moles on either side therefore both sides affected equally

  Effect of increase in temperature on the yield: increases, equilibrium moves to lower the temperature or oppose the change forward reaction is endothermic
• Effect of increase in pressure on equilibrium position

  The pressure is lowered by favouring side with fewest number of gaseous moles
• State why the volume V need not be known in calculating the value of Kc for the reaction.

  Equal number of moles on each side of equation
• When does Le Chatelier's Principle only apply?

  homogeneous equilibria (products and reactants int the same state)
• Effect of catalyst on position on equilibrium


  No effect as will speed up the rate of the forward and backward reaction equally
• Difference between Kc and Kp

  Kp and Kc are theequilibrium constantof an ideal gaseous mixture. Kp is equilibrium constant used when equilibrium concentrations are expressed inatmospheric pressureand Kc is equilibrium constant used when equilibrium concentrations are expressed inmolarity
• Steps to working out Kc


  1) Use ICE table to find moles of each species

  2) Calculate concentrations at equilibrium, you do moles/volume and the units is moldm-3

  3) Put these concentrations to do

  e.g. Kc= [NO]2 [O2] / [NO2]2 , put in the concentration values you just worked out.

  4) Find units by also doing e.g. Kc= moldm-3/ moldm-3 moldm-3
• Using Kc to find equilibrium concentrations


  In notes
• Effect of changing concentration on Kc

  no effect
• Effect of adding catalyst on value of Kc

  Adding a catalyst has no effect on value of Kc as catalyst speeds up forwards and reverse reaction equally.Catalyst speeds up rate at whichequilibrium is established
• The reaction is done at a temperature of 800 °C and a low pressure of 300 kPa in the presence of a nickel catalyst. Explain, in terms of equilibrium yield and cost, why these conditions are used.

  Stage 1:Temperature1a. The reaction is endothermic (so equilibrium shifts to RHS to reduce the temperature) 1b. So, higher temperature increases the yield 1c. High temperatures are costly (so compromise temperature used)Stage 2:Pressure2a. More moles of gas on the right hand side, (so equilibrium shifts to RHS to increase the yield) 2b. So, lower pressure increases the yield 2c. A low pressure means a low costStage 3:Catalyst3a. Catalyst has no effect on yield 3b. Adding a catalyst allows a lower temperature to be used 3c. So, this lowers the cost
• Which statement is not always correct for a reaction at equilibrium? reactants ⇌ products

  The concentrations of the reactants and products are equal.