Equilibria

Question 1

What is the equilibrium constant, K_c?

Answer 1

• The equilibrium constant calculated from equilibrium concentrations for a system at constant temperature.
• The equilibrium constant always has the same value (provided you don’t change the temperature), irrespective of the amounts of A, B, C and D you started with. It is also unaffected by a change in pressure or whether or not you are using a catalyst.
• Substances on the right-hand side of the equation are written at the top of the K_c expression.
• Those on the left-hand side at the bottom.
• The indices (the powers that you have to raise the concentrations to - for example, squared or cubed or whatever) are just the numbers that appear in the equation; the moles, the coefficients.

Question 2

Construct an expression for K_c for an homogeneous system in equilibrium; The Contact Process.

‘Be able to perform calculations involving such an expression’

Answer 2

• K_c expression will include some visible powers in this instance due to the different moles.
• [concentrations] measured in mol dm^-3
  > cancel out to find units of K_c when putting the units into expression instead of numbers
• Units vary; must work them out for each reaction by cancelling out the units for each term.
• Rearrange expression to find concentration etc.

Question 3

Be able to predict the effects of changes of temperature on
the value of the equilibrium constant

Answer 3

• Equilibrium constants are changed if you change the temperature of the system. K_c is constant at constant temperature, but varies the temperature changes.
• If K_c increases in value, the equilibrium moves to the right; the forward direction, more product.
• If K_c decreases in value, the equilibrium moves to the left; the reverse direction (less product). [products]/[reactants]
• For an exothermic reaction (ΔH = negative), increasing the temperature decreases the equilbrium constant; equilibrium moves to counter the change, in the reverse (left) direction, forming more reactant thus it being a bigger divisor.
• For an endothermic reaction, increasing the temperature increases the equilibrium constant.

Question 4

Is the value of the equilibrium constant affected by changes either in concentration or the addition
of a catalyst?

Answer 4

No, it is not; K_c is fixed at a given temperature, thus if the concentration of one thing changes then the concentrations of the others must change to keep the equilibrium constant the same. Position of equilibrium changes.

Catalysts just mean equilibrium is reaced faster; no effect on yield or K_c or position of equilibrium.

Question 5

What does the size of Kc tell you about the composition of the equilibrium mixture?

Answer 5

• If Kc is much greater than 1, products predominate over reactants, thus we usually say that the equilibrium is over to the right.
• If Kc is much less than 1, reactants predominate, and the equilibrium position is over to the left.

This is because [products]/[reactants].

• Reactions where K_c is greater than 10¹⁰ are usually regarded as going to completion.
• Reactions where K_c is less than 10^-10 are regarded as not taking place at all.