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Heat of Combustion of any substance is defined as the energy released when it undergoes combustion. Heat of Combustion can also be termed as calorific value which is described as the amount of energy released when 1 kg of any fuel undergoes combustion in the presence of sufficient oxygen under standardised temperature and pressure conditions.
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Key Terms: Heat, Combustion, Energy, calorific value, Heat of Combustion, oxygen
Combustion
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Combustion is a chemical process in which a substance reacts with oxygen to give off heat. In some cases, the heat is also accompanied by light. For combustion the presence of air is necessary.
The substance that undergoes combustion is said to be combustible. This substance is also called fuel.
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What is Heat of Combustion?
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When combustion occurs the substance that undergoes combustion releases energy. This energy can be measured as the heat of combustion of the substance. Heat of combustion of any substance is defined as the amount of energy that 1kg of the substance releases when it undergoes combustion. The heat of combustion of any substance can be calculated from its chemical formula. Heat of combustion can also be described as the calorific value of the substance.
Units of Heat of Combustion
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Heat of combustion of any substance can be measured in different forms. As energy is released when any substance is burnt, the heat of combustion of a substance is measured in units of energy. The most common unit used to express heat of combustion is Kilojoules/kg. Other units like Joule and Kilojoule are also used to express heat of combustion.
The common way to express heat of combustion is described as follows:
- Energy (Kilojoules/mole) released when one mole of any substance is burned.
- Energy (Kilojoules/Kg) released when one Kg of any substance is burned.
- Energy (Kilojoules/Litre) released when one litre of any substance is burned.
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Sign of Heat of Combustion
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Heat of Combustion is the energy released by a substance when it undergoes combustion. The energy of a body either increases or decreases when it undergoes any transformation in any process. Similarly, when a body undergoes combustion, the heat is released out of the body. This is marked by a decrease in the total energy of the body. This is the reason the sign of the heat of combustion is negative as the total energy of the body undergoing combustion decreases.
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Determination of Heat of Combustion
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Heat of combustion can be determined in two ways. One way is the gross calorific value and the other way is the net calorific value. The two ways can also be defined as follows:
- Higher calorific value: Higher heating value or gross calorific value
- Lower calorific value: Lower heating value or net calorific value
Determination of Higher Calorific Value
The Higher Calorific Value (HCV) of a substance can be determined by bringing together all the combustion products back to the initial temperature. The initial temperature here is the temperature of the environment in which the reaction occurred prior to the combustion.
This process also involves the condensation of any vapour produced during the combustion reaction. These measurements are performed under standard conditions, in which the temperature of the reaction environment is controlled at 25°C.
The determination of Gross Calorific Value is similar to the thermodynamic combustion heat because the reaction enthalpy transition assumes a particular compound temperature both before and after the combustion phases. Such as in the case where the combustion water is condensed into a liquid.
The gross calorific value can be considered as the latent heat of water vaporization in the final combustion product. The heating values for fuels are also measured, where it is possible for the condensation of the reaction products.
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Determination of Lower Calorific Value
The lower calorific value is also known as lower heating value. It is the net calorific value of the fuel. Its value is determined by subtracting the latent heat value of water vaporization formed in the reaction from the gross calorific value.
This method involves the assumption that the water formed during the combustion reaction will be in the vapour form. Thus, the energy used during the reaction for the conversion of water into water vapour is considered during the calculation.
The lower heating value (LHV) calculations assume that the combustion process's water portion is in the vapor phase at the end of the combustion reaction, while HHV assumes that it is in liquid form at the end of the combustion reaction. The lower calorific value presumes that there exists no latent heat recovery from water vaporization both in the fuel and the reaction materials. This helps in the comparison of fuels where the combustion product condensation is impossible or where the heat cannot be used at temperatures below 150 degrees Celsius.
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Determination of Measuring Heating Values
The higher heating value is determined experimentally in a bomb calorimeter. A stoichiometric mixture of oxidizer and fuel in a steel container at 25°C undergoes combustion which is initiated by an ignition device and the reactions are allowed to complete. Water vapor is produced in this process of reaction. After this both the vessel and its contents are cooled to the original temperature of 25 °C. The higher heating value is calculated as the difference between initial temperature and final temperature.
Lower heating value (LHV) is measured when cooling stops at 150 °C, and the reaction heat is recovered only partially.
Things to Remember
- Combustion is the chemical process in which a substance produces heat after reacting with oxygen.
- The substance that undergoes combustion is combustible and is known as fuel.
- Heat of combustion is the energy produced when 1 kg of any fuel undergoes combustion.
- The common unit used for measuring heat of combustion is Kilojoules/kg. Other units used are Kilojoule/Litre and Kilojoule/mole.
- There are two ways of determining heat of combustion: high calorific value and low calorific value.
- In the High calorific value determination process, the combustion products are brought back to the initial temperature.
- The lower calorific value is determined by subtracting the latent heat value of water vaporization from the high calorific value.
Sample Questions
Ques. What is heat of combustion? Name the unit in which it is expressed. (2 Marks)
Ans. Heat of combustion is defined as the energy released by any substance when it undergoes combustion, under standard conditions. It is expressed in Kilojoules/kg. Other units of expression are Kilojoules/mole and Kilojoule/litre.
Ques. How do we determine the High Calorific Value of heat of combustion? (2 Marks)
Ans. The high calorific value of heat of combustion can be determined by bringing back all the combustion products to the initial temperature. The initial temperature here is the temperature of the environment in which the experiment is conducted.
Ques. How do we determine the low calorific value of heat of combustion? (2 Marks)
Ans. The lower calorific value of heat of combustion is determined by subtracting the latent heat of water vaporization formed in the reaction from the high calorific value.
Ques. What is the major difference between the calculation process of high calorific value and low calorific value of heat of combustion? (2 Marks)
Ans. The major difference between the calculation process of high calorific value and low calorific value is that in low calorific value calculation, the water at the end of the combustion process is considered to be in vapour form, while it is considered to be in liquid form in high calorific value.
Ques. How do we measure the high calorific value of heat of combustion? (2 Marks)
Ans. The high calorific value of heat of combustion is measured in a bomb calorimeter. A stoichiometric mixture of oxidizer and fuel in a steel container at 25°C undergoes combustion in the bomb calorimeter. After this both the vessel and its contents are cooled to the original temperature of 25 °C. The higher heating value is calculated as the difference between initial temperature and final temperature.
Ques. How do we measure the lower calorific value of heat of combustion? (2 Marks)
Ans. The lower calorific value of heat of combustion is also measured in a bomb calorimeter. A stoichiometric mixture of oxidizer and fuel in a steel container at 25°C undergoes combustion in the bomb calorimeter. After this both the vessel and its contents are cooled to the original temperature of 25 °C. Lower heating value (LHV) is measured when cooling stops at 150 °C, and the reaction heat is recovered only partially.
Ques. State the differences between the high calorific value and low calorific value of heat of combustion. (4 Marks)
Ans. The differences between the high calorific value and low calorific value of heat of combustion are as follows:
| High calorific value | Low calorific value |
|---|---|
| HCV is the amount of heat evolved when a unit weight or volume of the fuel is completely burnt and the products of combustion are cooled to normal conditions. | LCV is the amount of heat evolved when a unit weight or volume of the fuel is completely burnt and water vapor leaves with the combustion products without being condensed. |
| HCV is the total energy released when the products have cooled down and condensed. | LCV is the total energy released when the products are hot. |
| The by-products are allowed to condense in HCV. | The by-products are allowed to escape in LCV. |
| Heat or energy of the by-products can be recovered. | Heat or energy of by-products are allowed to escape. |
| The value of heat produced is high. | The value of heat produced is low. |
Ques. Why is the heat of combustion always negative? (2 Marks)
Ans. When a body undergoes combustion, the energy of the body decreases. The substance undergoing combustion releases energy into the atmosphere. This signifies that the total energy of the body has decreased. This decrease in energy is shown by negative energy. This is why the heat of combustion is always negative.
Ques. What is combustion? (2 Marks)
Ans. Combustion is a scientific term used to explain burning. Combustion is a chemical process, which includes a reaction between two components:
Fuel (source of Hydrocarbon) and
Oxygen Source (Air, or an oxidant).
Whenever fuel and oxygen react together, they release heat and light energy. Heat and light energy then result in the formation of flame. Thus, combustion can be explained easily using the formula stated below, i.e., combustion is:
Hydrocarbon + Oxygen → Carbon Dioxide + Water + Heat Energy
Ques. What is the ignition temperature? (2 Marks)
Ans. Combustion requires a minimum temperature to reach before it can be carried out, i.e., the ignition temperature. The lowest temperature at which a substance catches fire is called its ignition temperature.
Substances having a very low ignition temperature, can easily catch fire with a flame are thus called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG), etc.
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