Kc

• Kc represent the equilibrium constant, it is calculated from the concentrations of reactant and products (in $mol\ dm^{-3}$) for reactions in solution or homogeneous gaseous reactions = reversible reactions.

  If any other factor is varied, such as pressure, concentration of reactants or the addition of a catalyst, the value of the equilibrium constant remains constant.
• $K_c$ example: $aA+$$bB\leftrightarrow cC+$$dD$

  $K_c=$$\frac{\left[C\right]^c\left[D\right]^d}{\left[A\right]^a\left[B\right]^b}$

  [A],[B],[C] and [D] represent the concentration in $mol\ dm^{-3}$.

  a,b,c and d are the balancing numbers in the equation for the reaction (the big number at the front of the molecule).
• Units of $K_c=$$\frac{\left(mol\ dm^{-3}\right)^{\left(c+d\right)}}{\left(mol\ dm^{-3}\right)^{\left(a+b\right)}}$

  This is a general formula for the units but the overall power depends on the balancing number in the equation for the reaction.
• Examples of calculating Kc:

  $PCl_5\leftrightarrow PCl_3+$$Cl_2$

  $K_c=$$\frac{\left[PCl_3\right]\left[Cl_2\right]}{\left[PCl_5\right]}$
• Examples of calculating Kc:

  $CH_3COOH+$$CH_3CH_2OH\leftrightarrow CH_3COOCH_2CH_3+$$H_2O$

  $K_c=$$\frac{\left[CH_3COOCH_2CH_3\right]\left[H_2O\right]}{\left[CH_3COOH\right]\left[CH_3CH_2OH\right]}$
• The equilibrium constant for the reverse reaction is the reciprocal of the value for the forward reaction. Use $\frac{1}{K_c}$, the units for the equilibrium constant for the forward reaction will be filled, so for example $\left(mol^{-1}dm^3\right)^{-1}=$$mol\ dm^{-3}$.
• What is the only factor that affects Kc?

  Temperature is the only factor that affects the value of Kc for an equilibrium reaction. When temperature affects the position of equilibrium and the concentration of reactants and products, Kc will change.
• When the forward reaction is exothermic, an increase in temperature shifts the position of equilibrium to the left as it absorbs heat. This lowers the concentration of the products and increases the concentration of the reactants, so Kc decreases.