Chemical Equilibrium · 8 min read
Reversible reactions and dynamic equilibrium
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Introduction to chemical balance
In some chemical reactions, once the reactants change into products, they can never go back. Imagine burning a piece of paper; you cannot turn the ash back into paper. These are called irreversible reactions. However, in many other reactions, the products can react together to form the original reactants again. This is common in a closed room or a sealed container.
Understanding reversible reactions
A reversible reaction is like a two-way street or a commercial bus going from CMS to Ajah and then coming back. We represent these reactions using a double arrow sign pointing both ways. This shows that the forward and backward reactions are happening at the same time.
Simply put, it is a reaction that can go forward and backward.
The concept of dynamic equilibrium
Imagine a busy market square where people are entering and leaving at the exact same speed. Even though people are moving, the total number of people inside the market stays the same. This state of balance is what we call equilibrium. In chemistry, it is 'dynamic' because the molecules are still moving and reacting, even though we cannot see a change in the amount of substances.
It is a state where the forward and backward reactions happen at the same speed so that everything looks stable.
Conditions for reaching equilibrium
1. The reaction must be reversible.
2. The system must be closed (like a pot with a tight lid), so no gas or liquid escapes.
3. The temperature and pressure must remain constant.
Difference between static and dynamic
If you stack blocks and they don't move, that is static equilibrium. But chemical equilibrium is like a man running on a treadmill at 5 km/h while the treadmill moves backward at 5 km/h. He stays in the same spot, but he is definitely moving! This is why we call it dynamic.
Worked example: Nitrogen and Hydrogen
In the production of ammonia, N2(g) + 3H2(g) <=> 2NH3(g). If we start with 2 moles of N2 and 6 moles of H2 in a sealed container, the reaction starts fast. As ammonia (NH3) forms, it starts breaking back down into nitrogen and hydrogen. When the speed of forming ammonia equals the speed of breaking it down, the system has reached dynamic equilibrium.
Calculation of reaction rates
If at equilibrium, 0.5 moles of N2 are reacting per second to form NH3, then exactly 0.5 moles of N2 must be produced every second by the backward reaction for the system to stay in equilibrium.
Key points
- •Reversible reactions use a double arrow symbol.
- •Equilibrium only occurs in a closed system.
- •At equilibrium, the rate of forward reaction equals the rate of backward reaction.
- •Concentrations of reactants and products remain constant at equilibrium.
- •Dynamic equilibrium means the reaction is still happening at a molecular level.