Monochloroethane (C2 H5 Cl) can be produced by the direct reaction of ethane gas (C2 H6) with ch

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Monochloroethane (C2 H5 Cl) can be produced by the direct...

Monochloroethane $\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\right)$ can be produced by the direct reaction of ethane gas $\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$ with chlorine gas or by the reaction of ethylene gas $\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)$ with hydrogen chloride gas. The second reaction gives almost a $100 \%$ yield of pure $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}$ at a rapid rate without catalysis. The first method requires light as an energy source or the reaction would not occur. Yet $\Delta G^{\circ}$ for the first reaction is considerably more negative than $\Delta G^{\circ}$ for the second reaction. Explain how this can be so.

Key Concepts

Thermodynamic Favorability

This concept refers to the overall driving force of a reaction, often expressed as the Gibbs free energy change (?G). A negative ?G indicates that a reaction is energetically favorable and could, in principle, proceed spontaneously. However, thermodynamic favorability does not guarantee a fast or efficient reaction, as it only pertains to the relative energies of reactants and products, not the pathway between them.

Reaction Kinetics

Reaction kinetics deals with the rate at which a chemical reaction proceeds and is governed by the energy barrier between reactants and products, known as the activation energy. Even a thermodynamically favored reaction can have a slow rate if this energy barrier is high, meaning that molecules must acquire enough energy (e.g., via heat or light) to undergo the transformation.

Activation Energy and Energy Input

Activation energy is the minimum energy required for reactants to transform into products by overcoming the transition state barrier. In cases where the activation energy is significant, external energy (like light in photochemical reactions) is necessary to initiate the reaction. Without this energy input, the reaction rate remains slow even if the overall reaction is thermodynamically favorable.

Photochemical Initiation

Photochemical initiation involves using light energy to overcome the activation barrier, particularly in reactions that proceed via radical mechanisms. Light can excite molecules to form radicals, which then propagate a chain reaction process. This is crucial in reactions such as the halogenation of ethane, where a radical chain mechanism is operative and requires light to start the process.

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