Dear PF Forum,(adsbygoogle = window.adsbygoogle || []).push({});

I have read this link in Wiki.

But I don't understand what it means. Can someone help me?

https://en.wikipedia.org/wiki/Heat_of_combustion

A: Heat of combustion of CH_{4}is 50.09 MJ/kg

https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table)

B: Enthalpy of water: -285.88 KJ/mol

C: Enthalpy of CO_{2}: -393.509 KJ/mol

Then, I'll try to do a simple calculation.

A: CH_{4}heat of combustion is 50.09 MJ/kg.

What does it mean?

Does it mean that if we burn 1 kg of CH_{4}completely

CH_{4}+ 2O_{2}-> CO_{2}+ 2H_{2}O

It will produce 50.09 mega joules?

B: Enthalpy of water is 285.88 kj/mol

What does it mean?

If we combine 2 moles of Hydrogen and 1 moles of Oxygen, it will explode and gives 285.88 kilo joules?

C: Then I did some calculation...

The heat of combustion of

1 moles CH_{4}+ 2 moles O_{2}->1 molesCO_{2}+2 molesH_{2}O

16 gr CH_{4}+ 32 gr O_{2}-> 44 gr CO_{2}+ 36 gr H_{2}O will gives 50.09 MJ/kg * 16 gr =801.44 KJ

The enthalpy of

1 moles H_{2}O: -285.88 KJ

1 moles CO^{2}: -393.519 KJ

Combining those two:

2 molesH_{2}O: -571.76 KJ/mol

1 molesCO^{2}: -393.519 KJ/mol

=965.269 KJ

801.44 KJ ≠ 965.269 KJ

Where did I go wrong?

Or my understanding of the concept of heat combustion and enthalpy is wrong.

Thanks for any answer.

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# Heat of Combustion, Enthalpy

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