The Equilibrium Constant Kc (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the equilibrium constant K c: writing its expression, calculating its value, and knowing what changes it.

The balanced equation tells you how to write K c, equilibrium concentrations give you its value, and of everything you can change about a reaction, only temperature changes that value.

K c is the equilibrium constant. It links the concentrations of the products and the reactants at equilibrium, taking the stoichiometry of the equation into account. The equation fixes how the expression is written, the equilibrium concentrations fix its value, and once it has a value, only a change in temperature will move it.

We start with the expression itself and what goes into it. Then a value for K c, calculated from equilibrium concentrations. After that, which changes affect that value and which leave it alone. And last, the conditions used in the Haber and Contact processes.

The equilibrium constant expression links K c to the concentrations of the reactants and the products at equilibrium, taking the stoichiometry of the equation into account. For a general reaction, a A plus b B going to c C plus d D, Kc is the product concentrations on the top and the reactant concentrations on the bottom. Each concentration is raised to the power of its balancing number from the equation, so a two in front of a species becomes a squared in the expression. Solids are ignored, so they do not appear in the expression at all. And the constant is specific to a given equation.

Every square bracket in the expression is a concentration in moles per decimetre cubed. Some questions give you the number of moles of each reactant and product at equilibrium, along with the volume of the reaction mixture. Concentration is the number of moles divided by the volume, so the concentrations come straight out of that. Other questions give you the initial and equilibrium concentrations of the reactants, but not of the products. There you set out an initial, change and equilibrium table, and use the molar ratio in the balanced equation to find the equilibrium concentrations of the products. The units of K c depend on the form of the expression, and they can cancel out completely, leaving K c with no units at all. The equilibrium concentrations are what give K c its value.

Most of what you can change about a reaction does not alter K c. Changing a concentration does not change it: adding more of one substance makes the ratio of products to reactants temporarily wrong, and the system shifts until that ratio is back to the value it had before.

Changing the pressure does not change K c either, because pressure only moves the position of the equilibrium. A catalyst does not change it, because a catalyst speeds up the forward and the reverse reaction at the same rate, so the ratio of products to reactants is left exactly as it was.

Temperature is the exception. For an endothermic reaction, raising the temperature increases the product concentrations and decreases the reactant concentration, so K c increases. For an exothermic reaction, raising the temperature decreases the product concentration and increases the reactant concentrations, so K c decreases. Temperature is the only condition that changes the value of K c.

Because temperature is the only thing that moves K c, and because rate matters as well as yield, industrial conditions are a compromise. The Haber process makes ammonia from nitrogen and hydrogen, and it is exothermic. Raising the pressure shifts the equilibrium towards the side with fewer gas molecules, which is the ammonia side, so the yield rises, and the particles are closer together, so the rate rises too. Very high pressures are however expensive, so a compromise pressure of 200 atmospheres is used.

Lowering the temperature would shift the equilibrium further towards ammonia, but at a low temperature the gases do not have enough kinetic energy to collide and react, so a compromise temperature of 400 to 450 degrees Celsius is used.

The ammonia is removed by condensing it to a liquid, which shifts the position of the equilibrium further to the right, and an iron catalyst is added to speed the reaction up.

The Contact process makes sulfuric acid, by way of sulfur trioxide, and it is run at only 1 atmosphere, because K c for that reaction is already very high and the equilibrium already lies well over to the right, so higher pressures would be unnecessary and expensive. It uses a compromise temperature of 450 degrees Celsius, and vanadium(V) oxide as its catalyst.

Kc comes from the balanced equation: products over reactants, each raised to its balancing number, with solids left out. Its value comes from the equilibrium concentrations substituted into that expression. Changing a concentration, changing the pressure, or adding a catalyst leaves that value alone; only temperature changes it. And because temperature is the only lever on Kc, and because it affects the rate as well, the conditions chosen for the Haber and Contact processes are compromises rather than the conditions that would give the highest possible yield. One equilibrium, one constant: fixed by the equation, and moved only by temperature.