Final answer:
The position of equilibrium is said to shift due to a disturbance (like a change in concentration or pressure) that alters the concentrations of reactants and products, all while maintaining the same Keq, except in cases of temperature change which can affect Keq.
Step-by-step explanation:
We say the position of equilibrium is shifted when a reaction at equilibrium is disturbed because even though the equilibrium constant (Keq) remains the same at a given temperature, the system compensates for the disturbance by changing the concentrations of reactants and products.
Disturbing the equilibrium by adding or removing reactants or products will shift the equilibrium position to re-establish the ratio defined by the Keq expression.
This concept is explained using Le Chatelier's principle, which states that if an equilibrium system is stressed, the reaction shifts to reduce the stress and restore equilibrium without changing the value of Keq, except when there are changes in temperature, as Keq is temperature-dependent.