Forward Reaction, Reverse Reaction and Chemical & Dynamic Equilibrium

We owe our existence to equilibrium phenomenon occurring in the atmosphere. We breathe in oxygen and exhale carbon dioxide, while plants take in CO2 and release oxygen. This natural process is responsible for the presence of life on Earth. Lots of environmental systems depend for their existence on the fragile equilibrium phenomenon.

For example, the concentration of gases in lake water is managed by the principles of equilibrium. The lives of water plants and animals are indirectly related to the concentration of dissolved oxygen in the water.

For a better understanding of Chemical Equilibrium, the understanding of the following concepts is necessary. So, we start our topic with reversible and irreversible reactions.

In a chain reaction, the substances that combine are called reactants and the new substances formed are called products. For example, when H2 and O2 (reactants) combine they form water H2O (product).


Irreversible reactions

The majority of the reactions, in which the products do not recombine to form reactants, are called irreversible reactions.

They are expected to complete and are represented by putting a single arrow () in between the reactants and products.

Reversible reactions

The reactions in which the products can recombine to form reactants are called reversible reactions.

These reactions never go to completion. They are represented by a double arrow () in between reactants and products. These reactions continue in both ways, i.e., they

consist of two reactions; forward and reverse. So, a reversible reaction is one which can be made to continue in either direction depending upon the conditions.

Forward Reaction

Let us go over a reaction between hydrogen and iodine. Due to the fact that one of the reactants, iodine is purple, while the item hydrogen iodide is colourless, proceedings of the reaction are easily observable. On heating, hydrogen and iodine, vapours in a closed flask, hydrogen iodide are formed. As a result, the purple colour of iodine fades as it reacts to form colourless hydrogen iodide. This reaction is called a forward reaction.


Reverse Reaction

When hydrogen iodide is heated in a closed flask, purple colour appears because of the formation of iodine vapours. In this case, hydrogen iodide functions as a reactant and produces hydrogen and iodine vapours. This reaction is reverse of the above. Therefore, it is called a reverse reaction.


When both of these responses are written together as a reversible response, they are represented as:


Chemical Equilibrium and Dynamic Equilibrium

Equilibrium is a state of rest or balance due to the equal action of opposing forces. However, the balance might be achieved in a variety of methods.


When the rate of the forward reaction is the same as the rate of the reverse reaction, the structure of the reaction mix stays continuous, it is called a chemical equilibrium state.

Possibilities at Equilibrium
  1. When reaction stops to continue, it is called static equilibrium. This occurs mainly in the physical phenomenon. For example, a building remains standing rather than falling down because all the forces acting upon it are balanced. This is an example of static equilibrium.
  2. When a reaction does not stop, just the rates of forward and reverse reactions become equal to each other however occur in opposite directions. This is called dynamic equilibrium state. Dynamic implies reaction is still continuing. At dynamic equilibrium state:

Rate of forward reaction = Rate of reverse reaction

Dynamic Equilibrium

Dynamic equilibrium only takes place in reversible reactions, and it is when the rate of the forward reaction becomes equal to the rate of the reverse reaction. These are dynamic since the forward and reverse reactions are still happening, but the two rates are equal and changeless, so they’re likewise at balance.

Dynamic balance is an example of a system in a stable state. This implies the variables in the equation are constant in time (since the rates of reactions are equivalent). If you take a look at a reaction in dynamic equilibrium, it will look like absolutely nothing is occurring given that the concentrations of each compound stay consistent. However, reactions are in fact continuously taking place.

For example, in case of reaction between hydrogen and iodine vapours, a few of the molecules react with each other to form hydrogen iodide.




At the same time, a few of the hydrogen iodide molecules disintegrate back to hydrogen and iodine.


In the beginning, as the concentration of the reactants is higher than that of the products, the rate of the forward reaction is quicker than the reverse reaction. As the reaction proceeds, the concentration of reactants will gradually reduce while that of products will increase, as a result, the rate of the forward reaction will go on reducing and the reverse reaction will go on increasing and eventually the two rates will end up being equal to each other.

Therefore, the balance will set up and the concentration of different types (H2, I2, HI) ends up being consistent. It is represented as


Macroscopic characteristics of dynamic equilibrium

A couple of essential characteristic functions of dynamic equilibrium are provided below:

  1. An equilibrium is achievable only in a closed system (in which substances can neither leave nor enter).
  2. At equilibrium state, a reaction does not stop. Forward and reverse reactions keep occurring at the very same rate but in opposite directions.
  3. At equilibrium state, the amount (concentration) of reactants and products do not alter. Even physical properties like colour, density, etc. stay the same.
  4. An equilibrium state is obtainable from, in either case, i.e. starting from reactants or from products.
  5. Aequilibriumstate can be disturbed and again achieved under the provided conditions of concentration, pressure and temperature.