For a reaction to proceed, colliding molecules must have:

To understand why a reaction occurs, we use **Collision Theory**. According to this theory, for a chemical reaction to take place, reactant particles must collide with each other. However, not every collision results in a reaction. For a collision to be 'effective' (leading to product formation), two criteria must be met: 1. **Activation Energy ($E_a$):** The colliding molecules must possess a minimum amount of kinetic energy, known as the activation energy. This energy is required to break the existing chemical bonds in the reactants. If the energy is less than $E_a$, the molecules simply bounce off each other. 2. **Correct Orientation:** Even if molecules have enough energy, they must strike each other at the correct angle or spatial orientation so that the specific atoms involved in the reaction can interact and form new bonds. Therefore, the correct requirement is **Energy $\ge$ activation energy + correct orientation**.