Question

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.

Short answer

While reaction 1 (ethane gas with chlorine gas) is more thermodynamically favorable, having a larger negative ΔGº, it is kinetically less favorable due to a higher activation energy, resulting in a slower rate and energy source requirement. Conversely, reaction 2 (ethylene gas with hydrogen chloride gas) is kinetically favorable due to a lower activation energy, allowing it to proceed rapidly without catalysis or external energy, despite being thermodynamically less favorable. Thermodynamics and kinetics are separate aspects of chemical reactions, meaning a reaction being thermodynamically favorable does not guarantee it would also be kinetically favorable.

Step-by-step solution

Writing out the reactions

: Let's represent each reaction with their respective chemical formulas: Reaction 1: Ethane gas reacting with chlorine gas \[C_2H_6(g) + Cl_2(g) \rightarrow C_2H_5Cl(g) + HCl(g)\] Reaction 2: Ethylene gas reacting with hydrogen chloride gas \[C_2H_4(g) + HCl(g) \rightarrow C_2H_5Cl(g)\] Note that both reactions produce monochloroethane as the main product.

Understanding thermodynamics and kinetics

: Thermodynamics refers to the study of energy and its transformations. It tells us the direction of a process as well as if a reaction is spontaneous or not. A negative ΔGº value indicates a spontaneous reaction, meaning the reaction favors the formation of products. Kinetics, on the other hand, deals with the rate of a reaction. It is concerned with how fast a reaction proceeds, rather than the direction it takes. Faster reactions have higher rates, which means that they proceed at a faster pace and complete in a shorter span of time. Now the task is to explain the relationship between the two factors in the given reactions.

Discussing the thermodynamic aspects of the reactions

: The direct reaction of ethane gas with chlorine gas (reaction 1) has a more negative ΔGº than the reaction of ethylene gas with hydrogen chloride gas (reaction 2). A more negative ΔGº means that the energy transformation during the reaction (from reactants to products) is more favorable in reaction 1 than in reaction 2 from a thermodynamic standpoint. However, this favorable transformation in energy does not necessarily imply that the reaction will occur readily or quickly. The energy barrier for a reaction, known as the activation energy, plays a crucial role in the rate of reaction.

Addressing the kinetic aspects of the reactions

: Though reaction 1 is more thermodynamically favorable, it is not as kinetically favorable as reaction 2. Reaction 1 requires an energy source (light) to overcome its activation energy and proceed. This shows that reaction 1 likely has a higher activation energy compared to reaction 2. On the contrary, reaction 2 exhibits a much faster rate without requiring an energy source or catalyst. This implies that its activation energy is lower, which allows it to occur with little energy input. Even though reaction 2 is less thermodynamically favorable (with a less negative ΔGº), it occurs faster due to its lower energy barrier.

Concluding the explanation

: Thermodynamics and kinetics are two separate aspects of chemical reactions. A reaction being thermodynamically favorable does not guarantee that it would be kinetically favorable as well. In this example, the reaction between ethane gas and chlorine gas (reaction 1) has a larger negative ΔGº, making it more thermodynamically favorable than the reaction between ethylene gas and hydrogen chloride gas (reaction 2). However, the larger activation energy in reaction 1 results in a slower rate and the requirement of an energy source, making it kinetically less favorable. In contrast, reaction 2, despite being thermodynamically less favorable, is kinetically favorable due to its lower activation energy, allowing it to proceed rapidly without the need for catalysis or external energy.