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Which of the following reactions has zero activation energy?
A) $C{H_4} + Cl \cdot \to \cdot C{H_3} + HCl$
B) $C{l_2} \to 2Cl \cdot $
C) \[ \cdot C{H_3} + \cdot C{H_3} \to C{H_3} - C{H_3}\]
D) \[ \cdot C{H_3} + Cl - Cl \to C{H_3} + Cl + Cl \cdot \]

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Last updated date: 13th Jun 2024
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Answer
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Hint: The activation energy is defined as the energy that must be provided to compounds so that they result in a chemical reaction. Activation energy can be thought of as the magnitude of the potential barrier (sometimes called the energy barrier) separating minima of the potential energy surface pertaining to the initial and final thermodynamic state.

Complete step by step answer:
The reactions that favor forming products on their own are said to have zero activation energy.
Among the given four reactions only one reaction will have zero activation energy. The reaction is:
\[ \cdot C{H_3} + \cdot C{H_3} \to C{H_3} - C{H_3}\]
This is so because the reactants in the reaction consist of highly reactive free methyl radicals which are so energetic that they react very rapidly as soon as another free radical is introduced to it.
All the other reactions mentioned above are following a free radical mechanism which requires energy in the form U.V radiation which has very high activation energy.
In the above reaction, the free radicals (which are unstable due to the presence of high energy) are bonded together and thus, lower their energy by increasing the stability.

Thus, the correct option is C.

Note:
The activation energy is defined as the energy required converting the inactive reactant into their excited states such that their activity increases and they form the corresponding product. When this energy is zero, it means that either the reaction is too feasible that it takes place without any external push or energy; or it also means that the solution in which the reactant takes place consists of highly reactive reaction intermediates.