topic 2 - equilibrium constant Kc

• what is a homogenous equilibrium reaction?


  one in which all of the reactants and products are in the same physical state,
• what happens once a state of equilibrium is reached in a reversible reaction?


  the reaction mixture is analysed to determine the concentration/number of moles of a reactant/product present.
• how is the equilibrium constant Kc used?

  as a measure of the ratio of the concentration of the reactants to the concentration of the products.
• what is the general expression for Kc?

  Kc = conc products / conc reactants
• what is concentration measured in?

  moldm^-3
• how do we work out units for Kc?

  1. write the expression for Kc.
  2. substitute the units for conc into the expression.
  3. cancel out any units which are the same on the top and bottom lines of the expression.
• does a change in concentration have an effect on Kc?

  nope
• why doesn't a change in concentration affect Kc?

  if the concentration of 1 reactant/product changes, the equilibrium shifts to oppose the change by changing the concentration of all of the other species. The changes to the concentrations cancel each other out and the value of Kc doesn't alter.
• does using a catalyst affect Kc?

  nope
• why doesn't using a catalyst affect Kc?

  it doesn't affect the position of equilibrium and so has no affect on the value of Kc.
• does a change in temperature affect Kc?

  yes.
• what will an increase in the temperature of the surroundings cause?

  a system at equilibrium to shift in the endothermic direction to decrease the temperature again.
• what will a decrease in the temperature of the surroundings cause?


  a system at equilibrium to shift in the exothermic direction to increase the temperature again.
• how does the shifting of equilibrium affect Kc value?

  increase or decrease it depending on the direction of the change.