The methane molecule, CH4, has the geometry shown in
Figure 2.17. Imagine a hypothetical process in which the
methane molecule is 'expanded,' by simultaneously extending
all four C—H bonds to infinity. We then have the
process
CH41g2¡C1g2 + 4 H1g2
(a) Compare this process with the reverse of the reaction
that represents the standard enthalpy of formation of
CH41g2.
Verified step by step guidance
1
insert step 1> Identify the given process: CH4(g) -> C(g) + 4 H(g).
insert step 2> Understand the reverse process: The standard enthalpy of formation of CH4(g) is the reaction C(s) + 2 H2(g) -> CH4(g).
insert step 3> Reverse the standard enthalpy of formation reaction: CH4(g) -> C(s) + 2 H2(g).
insert step 4> Compare the given process with the reversed formation reaction: The given process involves breaking CH4 into gaseous atoms, while the reversed formation reaction involves breaking CH4 into solid carbon and gaseous hydrogen molecules.
insert step 5> Conclude that the given process is different from the reverse of the standard enthalpy of formation, as it involves gaseous atoms rather than solid carbon and molecular hydrogen.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy of Formation
The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. For methane (CH4), this involves the reaction of carbon (C) and hydrogen (H2) to produce CH4. Understanding this concept is crucial for comparing the enthalpy changes in the given process and its reverse.
Bond energy is the amount of energy required to break a bond between two atoms. In the context of the methane molecule, extending the C—H bonds to infinity implies breaking these bonds, which requires energy input. This concept is essential for analyzing the energy changes associated with the hypothetical process described in the question.
Thermodynamic processes refer to the changes in energy and matter that occur during chemical reactions. The comparison of the hypothetical expansion of methane with the reverse reaction of its formation involves understanding how energy is absorbed or released in these processes. This concept helps in evaluating the overall energy balance and directionality of the reactions.
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