Adding more C or more D to the system has an analogous but opposite effect.
Think about it this way. When you add more A to the system, you’re increasing
the amount of reactant available to react and form products. Similarly, if you add
more C or D to the system, you’re increasing the amount of product available to
react and form reactants. The system will adjust by moving to the left to
reestablish equilibrium.
When we increase the concentration of one species on the left side of an
equation, what happens to the concentration of the other species on the left side
of the equation? Say we add more A to the system: There will be more collisions
between A particles and B particles. Since we did not add any B to the system,
the increased collisions among A and B particles, along with the increased
production of C and D, will tend to reduce the concentration of B at equilibrium.
After equilibrium has shifted, there will be more A particles than there were
before we added any more A. There will be fewer B particles than there were