The conversion of natural gas, which is mostly methane, into products that contain two or more carbon atoms, such as ethane (C2H6), is a very important industrial chemical process. In principle, methane can be converted into ethane and hydrogen: 2 CH4(g) → C2H6(g) + H2(g) In practice, this reaction is carried out in the presence of oxygen: 2 CH4(g) + 1/2 O2(g) → C2H6(g) + H2O(g) (c) Explain how the preceding reactions are an example of driving a nonspontaneous reaction, as discussed in the 'Chemistry and Life' box in Section 19.7.
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1. The first reaction, 2 CH4(g) → C2H6(g) + H2(g), is nonspontaneous. This means it does not occur naturally without the input of energy. The conversion of methane to ethane and hydrogen requires a significant amount of energy to break the bonds in the methane molecules and form new bonds in the ethane and hydrogen molecules.
2. In order to make this reaction occur, we need to 'drive' it by providing the necessary energy. This is often done by coupling the nonspontaneous reaction with a spontaneous one. A spontaneous reaction is one that occurs naturally without the need for an input of energy.
3. The second reaction, 2 CH4(g) + 1/2 O2(g) → C2H6(g) + H2O(g), is an example of such a spontaneous reaction. The reaction of methane with oxygen to form ethane and water is exothermic, meaning it releases energy.
4. By carrying out the nonspontaneous reaction in the presence of oxygen, the energy released from the spontaneous reaction can be used to drive the nonspontaneous reaction. This is an example of how a nonspontaneous reaction can be driven by coupling it with a spontaneous reaction.
5. This concept is discussed in the 'Chemistry and Life' box in Section 19.7, where it is explained how nonspontaneous reactions are often driven in biological systems by coupling them with spontaneous reactions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Spontaneity of Reactions
In thermodynamics, a spontaneous reaction is one that occurs without external intervention, typically characterized by a decrease in free energy. Nonspontaneous reactions, on the other hand, require an input of energy to proceed. Understanding the spontaneity of a reaction is crucial for determining whether a reaction can occur naturally or if it needs to be driven by external factors, such as heat or catalysts.
Reaction coupling involves linking a nonspontaneous reaction to a spontaneous one, allowing the overall process to proceed. In the context of the given reactions, the conversion of methane to ethane and hydrogen is nonspontaneous, but it can be driven by coupling it with the combustion of methane, which is spontaneous. This principle is often utilized in biochemical processes and industrial applications to facilitate desired reactions.
Oxygen plays a critical role in combustion reactions, acting as an oxidizing agent that facilitates the conversion of hydrocarbons into products like water and carbon dioxide. In the provided reaction, the presence of oxygen allows the nonspontaneous conversion of methane to ethane and hydrogen to occur by providing the necessary energy through combustion. This highlights the importance of reactants in driving chemical processes and altering reaction spontaneity.
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