# What is the Average C-H Bond Energy in Methane?

• Janiceleong26
In summary, the term average C-H bond energy in methane can be illustrated by a balanced chemical equation where CH4 gas is broken into gaseous carbon and hydrogen atoms. This allows for the calculation of the bond energy without including the energy of converting carbon from gas to solid state, which would result in a higher calculated energy. It is important to note that bond energy is not affected by the standard states of the elements involved and only considers the energy required to break the bond.
Janiceleong26

## Homework Statement

"By means of a balanced chemical equation, including state symbols, illustrate the term average C-H bond energy in methane."
Ans: CH4 (g) -> C(g) + 4H(g)
Average bond energy = +x/4 kj/mol

Why C (g) ? Why is the state of C gas?

To avoid including unrelated C(g) → C(s) reaction energy in the C-H bond.

Bond energy is the energy required to dissociate the bond itself, if the final product would be C(s) you would not deal with just a bond dissociation.

Borek said:
To avoid including unrelated C(g) → C(s) reaction energy in the C-H bond.

Bond energy is the energy required to dissociate the bond itself, if the final product would be C(s) you would not deal with just a bond dissociation.

Sorry I'm abit lost..what do you mean to avoid unrelated C(g)-> C(s) ?
So does it mean that, naturally, when CH4 is broken.. The carbon formed is in gas state? But I thought it is more stable for C to exists as C(graphite) ?

No, it doesn't mean carbon exists in the gas state (at least not in typical for us temperatures and pressures). But if you would use the enthalpy of the reaction CH4(g) → C(s) + 4H(g) to calculate energy of the C-H bond, you would include energy of converting carbon from gas state into the solid state (C(g) → C(s), actually just a reversed sublimation), making the calculated energy much higher than it really is.

Please remember bond energy has nothing to do with the standard states of the elements involved. It is not only a problem with carbon, hydrogen in standard state doesn't exist as H(g), but as H2(g). But when talking about the bond energy all we care about is the amount of energy required to break the bond, we don't care about what happens to products. And when all you do with gaseous CH4 is breaking all four bonds, what you get is a gaseous mixture of carbon and hydrogen atoms. Yes, they will react/condense after that, but these are separate processes that we don't care about when determining the bond energy.

Borek said:
No, it doesn't mean carbon exists in the gas state (at least not in typical for us temperatures and pressures). But if you would use the enthalpy of the reaction CH4(g) → C(s) + 4H(g) to calculate energy of the C-H bond, you would include energy of converting carbon from gas state into the solid state (C(g) → C(s), actually just a reversed sublimation), making the calculated energy much higher than it really is.

Please remember bond energy has nothing to do with the standard states of the elements involved. It is not only a problem with carbon, hydrogen in standard state doesn't exist as H(g), but as H2(g). But when talking about the bond energy all we care about is the amount of energy required to break the bond, we don't care about what happens to products. And when all you do with gaseous CH4 is breaking all four bonds, what you get is a gaseous mixture of carbon and hydrogen atoms. Yes, they will react/condense after that, but these are separate processes that we don't care about when determining the bond energy.
I see, thanks thanks.

## 1. What is the average C-H bond energy in methane?

The average C-H bond energy in methane is approximately 435 kJ/mol.

## 2. How is the average C-H bond energy in methane determined?

The average C-H bond energy in methane is determined by measuring the amount of energy required to break a large number of C-H bonds in methane and then taking the average of these values.

## 3. What factors can affect the average C-H bond energy in methane?

The average C-H bond energy in methane can be affected by factors such as temperature, pressure, and the presence of other atoms or molecules that can interact with the C-H bond.

## 4. Why is the average C-H bond energy in methane important?

The average C-H bond energy in methane is important because it provides information about the stability and reactivity of methane, which is an important molecule in many chemical reactions and processes.

## 5. How does the average C-H bond energy in methane compare to other molecules?

The average C-H bond energy in methane is relatively low compared to other molecules, as methane is a simple and stable molecule with strong C-H bonds. However, the exact value can vary depending on the specific molecule and its environment.

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