# How can I calculate heat of combustion and enthalpy using Hess's Law?

• Stephanus
In summary, the conversation discusses the concepts of heat of combustion and enthalpy, specifically in relation to the combustion of methane (CH4) and the formation of water (H2O) and carbon dioxide (CO2). The participants also mention the importance of considering the phase of water and the use of Hess's Law equation in calculating the heat of reaction. They also suggest looking up tables for enthalpies of formation to aid in calculations.
Stephanus
Dear PF Forum,
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 CH4 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 CO2: -393.509 KJ/mol

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

A: CH4 heat of combustion is 50.09 MJ/kg.
What does it mean?
Does it mean that if we burn 1 kg of CH4 completely
CH4 + 2O2 -> CO2 + 2H2O
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 CH4 + 2 moles O2 -> 1 moles CO2 + 2 moles H2O
16 gr CH4 + 32 gr O2 -> 44 gr CO2 + 36 gr H2O will gives 50.09 MJ/kg * 16 gr = 801.44 KJ

The enthalpy of
1 moles H2O: -285.88 KJ
1 moles CO2: -393.519 KJ

Combining those two:
2 moles H2O: -571.76 KJ/mol
1 moles CO2: -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.

Check the "phase" of water (liq/vap).

Stephanus
Bystander said:
Check the "phase" of water (liq/vap).
Of course. Thanks. The numbers are close now.

Stephanus said:
Of course. Thanks. The numbers are close now.
Liquid water vs water vapor accounts for only about half the difference. You also forgot to subtract the heat of formation of methane, which is -75 kJ/mole.

Chet

Stephanus
You might also read about Hess's Law Equation. That is, if you haven't already done so. Heat of Rxn (or, Heat of Combustion in this case) = (Sum of Enthalpies of Formation of Products) - (Sum of Enthalpies of formation of Reactants) Google 'Enthalpy of Formation Tables' You'll get many 'energy of formation' values to apply.

Stephanus

## 1. What is the heat of combustion?

The heat of combustion, also known as the enthalpy of combustion, is the amount of energy released when a substance undergoes complete combustion with oxygen. It is typically measured in units of joules per mole (J/mol) or kilojoules per gram (kJ/g).

## 2. How is the heat of combustion calculated?

The heat of combustion is calculated by measuring the amount of heat released during the combustion reaction and dividing it by the amount of substance that is burned. This can be done experimentally in a calorimeter or calculated using the enthalpy changes of formation of the reactants and products.

## 3. What factors affect the heat of combustion?

The heat of combustion can be affected by several factors including the type of fuel being burned, the amount of oxygen available for combustion, and the temperature and pressure of the reaction environment. The composition of the fuel and the presence of impurities can also impact the heat of combustion.

## 4. Why is the heat of combustion important?

The heat of combustion is an important measurement in the study of thermodynamics and energy production. It can be used to determine the efficiency of different fuels and processes, as well as the energy potential of a substance. It is also essential in understanding and predicting the environmental impact of combustion reactions.

## 5. What are some real-world applications of the heat of combustion?

The heat of combustion has many real-world applications, including in the design and optimization of engines and power plants, as well as in the development of new and more efficient fuels. It is also used in the production of energy from biomass and other renewable resources. Additionally, the heat of combustion is important in the fields of environmental science and air quality control, as it can help in assessing the emissions and pollutants produced by combustion processes.

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