When is a chemical equilibrium reached?

A chemical equilibrium in chemical reaction is reached at the point when the rate of forward reaction equals the rate of backward reaction.

Recall that when the rate of forward reaction equals the rate of backward reaction, the properties of the system (concentrations of reactants and products) remains constant.

At this point in a chemical reaction, the rates are equal and the concentrations constant. At chemical equilibrium, the arraignment is not really automatic, but it is achieved with time because the rates of reactions equal each other with respect to time provided the reversible chemical reaction occurs in a closed system and every other factors of rates of reactions remains constant.

So, equilibrium is always reached with time and do not just exist immediately when a reaction starts off in a chemical system.

In fact, at the initial stage of a chemical reaction, rates of reaction is not always equal but equilibrium reached with time.

It is important to mention that rates of reaction may be unequal at the initial stage but the system however reaches equilibrium and thus maintains it.

How is the Chemical equilibrium attained in a reversible chemical reaction?

When a chemical reaction starts, the rates are unequal at that point but as the reversible reaction proceeds with time …the system fights to ensure the rate of forward reaction equals the rate of backward reaction.

And once the state is attained, rate of forward reaction equals rate of backward reaction and concentration of reactants and product remains constant.

What factors can alter chemical equilibrium reached in a chemical reaction?

According to Le Chatelier Principle, if an external constraint like change in temperature, pressure or concentration is imposed on a system in equilibrium, equilibrium position shifts in order to annul the constraint.

So, the factors that can affect chemical equilibrium are temperature, concentration and Pressure.

When these factors are imposed on a reversible chemical reaction in equilibrium, the equilibrium position will shift in order to counterbalance the effect and maintain equilibrium state.

Effect of these factors on equilibrium	How they affect the system
Effect of Concentration on equilibrium reached.	Concentration when increased or decreased alter equilibrium position. Example on equilibrium position by concentration CaCO3==CaO + CO2 When concentration is increased (by adding more CaCO3), the equilibrium position reached is altered. How the equilibrium reached is maintained? The chemical equilibrium position will shift to the right favoring forward reaction.
Effect of Temperature on equilibrium reached	When temperature is increased or decreased, it will alter the equilibrium position reached. Example on equilibrium position reached. 2SO2 + O2 == 2SO3 Enthalpy change =-ve When Temperature is increased, the equilibrium position is altered. How equilibrium position reached maintained? The equilibrium position will shift to the left top neutralize the effect.
Effect of Pressure on equilibrium position	When temperature is increased or decreased, it will alter the equilibrium position reached. Example on equilibrium position reached. 2H2 + O2 == 2H2O When Pressure is increased, the equilibrium position is altered. How equilibrium position reached maintained? The equilibrium position will shift to the right to neutralize the effect.

I just quickly reviewed how these factors change equilibrium position though I did not go into full details on factors that can alter equilibrium position and how they do it.

However, it is very important to remember that equilibrium in chemical reaction is reached or attained with time in a reversible chemical system.