When is equilibrium reached in a chemical reaction?
Chemical equilibrium is reached when the rate of forward reaction equals the rate of the backward reaction in such a way that there is no difference in the concentrations of reactants and products.
It’s been said that chemical equilibrium in Chemistry refers to chemical reactions that can attain equilibrium when the rates of reactions are equal, and this leads to equal concentrations of reactants and products.
So, equilibrium is a state that is reached with time in a way that there is not net difference in the properties of the system with respect to time.
When is Chemical equilibrium reached?
Equilibrium is reached when rates of reactions become equal.
At the initial stage of a reversible chemical reaction, it is possible for any of the reactions to proceed faster either forward or backward however none is permitted to continue thus with respect to time.
Equilibrium sets in 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 other factors of rates of reactions constant.
So, equilibrium is always attained and do not just exist immediately when a reaction starts off in a chemical system.
How is Chemical equilibrium attained?
Equilibrium is attained on the premise that a time comes when the rate of forward reaction equals the rate of backward reaction
When this happens, one of the hallmarks is that the properties of the system in chemical equilibrium remains constant.
When equilibrium is reached, the system tries to maintain the equilibrium with respect to time.
Event though, an external constraint like change in temperature, concentration or pressure is imposed on the system,- the system will fight to neutralize the effect and maintain the equilibrium state or position.
How is chemical equilibrium maintained when reached?
Another important aspect of chemical equilibrium is understanding how chemical equilibrium is maintained after being reached.
So I’m going to explain with few examples on how to maintain chemical equilibrium in reversible reactions.
However the main reason behind the mechanism on how equilibrium is maintained in reversible system is by the shift on equilibrium position.
Reversible chemical reaction tends to maintain equilibrium state by a shift on equilibrium position when the system is upset by external factors.
When I say shift, I mean a shift to either right hand side or left hand side of the equation.
The shift in equilibrium is boldly defended by Le Chatelier when he stated the Le Chatelier’s Principle.
Le Chatelier’s Principle that when an external constraint (such as change in Temperature, Pressure and concentration) is imposed on a system in equilibrium, the equilibrium position will shift so as to annul or neutralize the effect.
Le Chatelier
3 factors that can upset Chemical equilibrium when reached?
There are three factors that can affect the state of chemical equilibrium when reached and these factors according to the French Scientist Le Chatelier are Temperature, Pressure and concentration.
| Factors that upset chemical equilibrium | How the factors shift equilibrium position. |
| Effect of Temperature | I am using this sign == for reversibility sign Recall Increase in temperature will favour endothermic reaction. Decrease in temperature will favour exothermic reaction. Consider the reaction below. 2SO2 + O2 ===2SO3 Change in enthalpy= -ve What is the effect of change in Temperature on equilibrium reached? Answer Effect of increase in temperature will favour backward reaction shifting the equilibrium position to the left since backward reaction is endothermic. Effect of decrease in temperature will favour forward reaction shifting the equilibrium position to the right since forward reaction is exothermic. |
| Effect of Pressure | Recall Increase in pressure will favour the reaction side with less volume (a smaller number of moles) Decrease in pressure will favour the reaction side with larger volume (higher number of moles) Consider the reaction below 2H2 + O2 == 2H2O Answer Effect of increase in pressure Increase in pressure will favour forward reaction shifting the equilibrium position to the right since it tit the side with a smaller number of moles Effect of decrease in pressure Decrease in pressure will favour backward reaction shifting the equilibrium position to the left since it the side with higher number of moles |
| Effect of concentration | Hint Increasee in concentration one side of the equation favours the other side of the equation Decrease in concentration one side favours the same side of the equation Consider the equation Ca + ZnCl2 ==CaCl2 + Zn What is the effect of changes in concentration on equilibrium reached? Answer Effect of increase in amount of zinc. When more calcium particles are added, the equilibrium position will shift to the right favouring forward reaction producing more of CaCl2 and Zn. Effect of decrease in amount of zinc When zinc particles are removed, the equilibrium position shifts to the left favouring backward reaction producing more of zinc and ZnCl2. |
So, in conclusion, when equilibrium is reached in a chemical reaction, it is maintained by the system even when an external constraint is imposed on the system, equilibrium position can only shift either to the left or right in order to neutralize the effect of the constraint imposed.
