Methane bond enthalpies

What is the average enthalpy of atomization of the four C—H bonds in methane Compare this value with the accepted value of the C—H bond enthalpy. 

Calculate the bond enthalpy of the C—C bond in ethane using only the enthalpies of atomization of methane and ethane. Compare this result with the accepted result. 

Write the balanced chemical equation for the complete fluorination of methane to tetrafluoromethane. Using bond enthalpies, estimate the enthalpy of this reaction. The corresponding reaction using chlorine is much less exothermic. To what can this difference be attributed ... 

To calculate the average bond enthalpy of the (C-H) bond in methane, E (C-H), the average of the four bond dissociation enthalpies is calculated ... 

Worked Example 3.13 Use the bond enthalpies in Table 3.3 to calculate the enthalpy of burning methane (Equation (3.28)). Assume all processes occur in the gas phase. 

This value of the C-H bond enthalpy does not correspond to the enthalpy of dissociation of the carbon-hydrogen bond in methane, as represented in Equation (4.35). 

For example, the usual structural description of a methane molecule (CH4) in terms of four C—H single bonds suggests that A//rxn for the total dissociation to gaseous atoms is simply four times the bond enthalpy DH°[C—H] of a typical C—H bond ... 

One such important measurable quantity is the enthalpy change when a bond is broken in the gas phase, called the bond enthalpy. This is invariably positive because heat must be added to a collection of stable molecules to break their bonds. For example, the bond enthalpy of a C—H bond in methane is 438 kj mol , measured as the standard enthalpy change for the reaction... 

Bond enthalpy terms, on the other hand, are quantities assigned to each bond in a molecule such that the sum over all bonds is equal to the enthalpy change associated with the conversion of the molecule into separate atoms. Bond enthalpy terms are assumed to be constant, and therefore transferable from molecule to molecule. So for methane ... 

It is relatively simple to assign bond enthalpies to the bond in a diatomic molecule because in these cases the bond enthalpy is just the energy required to break the molecule into its atoms. However, many important bonds, such as the C—H bond, exist only in polyatomic molecules. For these bonds, we usually use average bond enthalpies. For example, the enthalpy change for the following process in which a methane molecule is decomposed into its five atoms (a process called atomization) can be used to define an average bond enthalpy for the C—H bond, H(C—H) ... 

Because there are four equivalent C—H bonds in methane, the enthalpy of atomization is equal to the sum of the bond enthalpies of the four C—H bonds. Therefore, the avereige C—H bond enthalpy for CH4 is D(C—H) = (1660/4)kJ/mol = 415kJ/mol. 

Figure 8.15 Using bond enthalpies to calculate AH, . Average bond enthalpies are used to estimate AH for the reaction of methane with chlorine to make methyl chloride and hydrogen chloride. 

Use bond enthalpies (Table 8.4) to estimate AH for these two reactions. Why is the conversion of methane to ethane more favorable when oxygen is used ... 

Experiments show that it takes 1656 kJ/mol to break all the bonds in methane (CH4) and 4006 kJ/mol to break all the bonds in propane (CsHg). Based on these data, calculate the average bond enthalpy of the C—C bond. 

Many bond enthalpies are average bond enthalpies. For example, the C-H bond enthalpy is based upon the average bond energies in methane, alkanes and other hydrocarbons. 

Bond enthalpies can be used to determine the enthalpy change for a particular reaction involving molecules in the gaseous state, for example the combustion of methane ... 

Figure 15.1 Hess s law cycle to calculate the average bond enthalpy in methane... 

The Hess s law cycle in Figure 15.1 can be used to calculate the average bond enthalpy in methane using the enthalpy of formation of methane and the enthalpies of atomization of carbon and hydrogen ... 

Using the following information and the fact that the average C -H bond enthalpy is 414 kj/mol, estimate the standard enthalpy of formation of methane fCHfr. 

Write and run an MM3 input file for methane from scratch, that is, open an empty file and put in all the necessary infomiation to do the MM3 calculation on CH4. What is the enthalpy of formation of CH4 What are the C—H bond lengths and angles ... 

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