Changing concentration shifts the equilibrium position because a system at equilibrium responds to disturbances by adjusting the rates of its forward and reverse reactions to restore balance. When the concentration of a reactant or product changes, the system is no longer at equilibrium — the forward and reverse reaction rates become unequal. To re-establish equilibrium, the system shifts in the direction that counteracts the change, a behavior described by Le Chatelier’s principle.
If the concentration of a reactant increases, particles collide more frequently in the forward direction, making the forward reaction temporarily faster than the reverse. To relieve this increase, the system consumes the added reactants and forms more products, shifting equilibrium to the right.
If a product is added, the reverse reaction suddenly becomes faster because more product particles can now collide to form reactants. The system responds by converting the extra product back into reactants, shifting equilibrium to the left.
Conversely, removing reactants or products creates the same type of shift in the direction that restores what was removed. Removing reactants pulls the equilibrium left, while removing products pulls it right.
These shifts occur because equilibrium depends on the balance between the forward and reverse reaction rates — and concentration directly affects how often particles collide. When concentrations change, collision frequencies change immediately, breaking the balance. The system then adjusts its composition until the rates of the forward and reverse reactions become equal again.
Importantly, changing concentration does not alter the equilibrium constant, K. Instead, the system shifts concentrations until the reaction quotient, Q, once again equals K. The shift stops only when equilibrium is restored.
This principle explains many real-world phenomena, such as how hemoglobin loads and unloads oxygen, why adding excess reactants increases product yield in industrial reactions and how buffer solutions resist pH changes.
Ultimately, changing concentration shifts equilibrium because a system naturally adjusts its composition to restore the balance of forward and reverse reaction rates.
Frequently Asked Questions
Does changing concentration change the value of K?
No. K depends only on temperature, not concentration.
What happens if both reactants and products increase in concentration?
The direction of shift depends on how Q compares to K after the changes.
Can equilibrium be restored instantly?
No. It takes time for the reaction rates to rebalance, though the shift begins immediately.
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