Vertigo3729
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What is the bond enthalpy of the reaction between fluorine and methane?
CH4 + 4F2 --> CF4 + 4HF
[Data: C-H = 412, F-F = 158, C-F = -484, H-F = -562 (all values are in kJ mol^-1)]
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ppabla
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(Original post by Vertigo3729)
What is the bond enthalpy of the reaction between fluorine and methane?
CH4 + 4F2 --> CF4 + 4HF
[Data: C-H = 412, F-F = 158, C-F = -484, H-F = -562 (all values are in kJ mol^-1)]
△H = Bond in reactants (broken bonds) + Bonds in products (made bonds)
= [(412 x 4) + (158 x 4)] + [(-484 x 4) + (-562 x 4)]
= 2280 + (-4184)
= -1904
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Vertigo3729
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(Original post by ppabla)
△H = Bond in reactants (broken bonds) + Bonds in products (made bonds)
= [(412 x 4) + (158 x 4)] + [(-484 x 4) + (-562 x 4)]
= 2280 + (-4184)
= -1904
I have a question, why did you add them?
Because our teacher gave us this formula: △H = sum of bonds broken on left - sum of bonds made on right. So using this formula you'd get 6464 kJ mol^-1.
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charco
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(Original post by Vertigo3729)
I have a question, why did you add them?
Because our teacher gave us this formula: △H = sum of bonds broken on left - sum of bonds made on right. So using this formula you'd get 6464 kJ mol^-1.
Energy is NEEDED to break bonds
Energy is RELEASED when bonds are made.
You break the reactants bonds and make the products bonds.
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Vertigo3729
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(Original post by charco)
Energy is NEEDED to break bonds
Energy is RELEASED when bonds are made.
You break the reactants bonds and make the products bonds.
I get what you're saying, but I don't understand why we add the values. Is there a specific rule for this?
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Vertigo3729
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(Original post by charco)
Energy is NEEDED to break bonds
Energy is RELEASED when bonds are made.
You break the reactants bonds and make the products bonds.
So, I found this formula, but I don't understand when we use either of them:
ΔH =potential energy of product bonds−potential energy of reactant bonds
=energy added to break reactant bonds+energy released when making product bonds
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qwert7890
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(Original post by Vertigo3729)
I have a question, why did you add them?
Because our teacher gave us this formula: △H = sum of bonds broken on left - sum of bonds made on right. So using this formula you'd get 6464 kJ mol^-1.
The two approaches are the same, but when you use your interpretation of the formula, “sum of bonds on the right” is not going to be in negative.

In the formula provided by ppabla the negative sign is added on each bond on the right. In your formula the negative term is collectively taken on the entire right side. Both are the same, you are interpreting it incorrectly.

According to your formula:

Sum of bonds on left - sum of bonds on right
= + 2280 - (4184)
= -1904
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Vertigo3729
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(Original post by qwert7890)
The two approaches are the same, but when you use your interpretation of the formula, “sum of bonds on the right” is not going to be in negative.

In the formula provided by ppabla the negative sign is added on each bond on the right. In your formula the negative term is collectively taken on the entire right side. Both are the same, you are interpreting it incorrectly.

According to your formula:

Sum of bonds on left - sum of bonds on right
= + 2280 - (4184)
= -1904
Alright, so correct me if I'm wrong.

If we use the formula: potential energy of product bonds−potential energy of reactant bonds.
We are subtracting the energies of all the bonds (sum of product bonds - the sum of reactant bonds) and we exclude the negative sign in the values because we are merely trying to find out the difference in potential energy of the bonds. Which, in the case of the reactants, would be the minimum energy needed to break the bonds (activation energy) so 2280 is the activation energy.

If we use the formula: =energy added to break reactant bonds+energy released when making product bonds.
We are trying to find out how much energy was used as a whole. So how much energy was taken in and then how much was released. And how much of this released energy wasn't used to break the bonds in the reactants.
So if we say 2280 kJ mol^-1 was taken in to make the bonds and then once the bonds were formed, -4184 kJ mol^-1 was given out. Therefore, 1904 kJ mol^-1 energy was released extra.
Because: the energy released would be -2280 - x = -4184. So the extra energy released is 1904 kJ mol^-1 and it's the enthalpy change.
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