Introduction to the Heat of Combustion
Combustion reactions are known as one of the most important and most frequent chemical reactions taking place in our surroundings. The reason why these reactions are so important lies in the fact that a tremendous amount of energy (in the form of heat) is released as a byproduct.
We used this heat of combustion as fuel to run automobiles and other gas equipment. Yes, the fuel over which you run your cars and other vehicles burn in the combustion chemical reaction and we guess, this point is enough to predict the importance of combustion.
Most combustion reactions occur when hydrocarbons react with oxygen to form carbon monoxide and water. Since we study all the hydrocarbons in organic chemistry, the heat evolved by their combustion chemical reaction is called heat of combustion of organic compounds.
To understand it further, let's have a look at the combustion reaction examples, Let's start from methanol
In the given reaction, we can see that one mole of methanol is reacting with one mole of oxygen to produce one mole of CO2 and two moles of water. Since the heat of combustion is released only by the one mole of reacting substances, we will call it the molar heat of combustion.
How Energy is Released in Combustion Chemical Reaction?
A simple or complex chemical reaction is none other than breaking of older bonds and formation of newer ones. To break the existing bonds of reacting substances, energy is required which is called activation energy.
For example, when we ignite the spark to turn on the car, we are providing the activation energy to the fuel so the combustion could initiate.
Once the process of combustion chemical reaction begins, the heat is evolved as a by-product because the energy required to break older bonds (C-H) is lower than that evolved by the formation of newer bonds (H-O, C-O). Thus, in this way, the excess energy is released in the form of heat that is called heat of combustion.
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Calculating heat of Combustion
The tool we usually use to find the heat of combustion of any organic compound is called a bomb calorimeter. For this purpose, all you have to do is to burn your sampling compound in the calorimeter and wait.
However, while using a calorimeter, it is important to keep the oxygen concentration higher. Otherwise, the process of incomplete combustion gets started.
Similarly, you can also calculate the heat of combustion by observing the temperature change in the system. Lets calculating it using combustion reaction examples.
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Measuring Heat of Combustion by Temperature Change
Suppose you have been asked to calculate the heat of combustion evolved from the burning of 1.75g ethanol along with 200g of water. During this reaction, the temperature of the system has been observed to increase up to 55 degrees centigrade. To calculate the molar heat of combustion of 1.75g ethanol, follow this scheme
1. Write down the known values
The values that are cleared from the problem are
Mass of sample = 1.75 g
Mass of water = 200 g
Ethanol's Molar mass = 46.1 g mol-1
Temperature change = 55 oC
Cp of water = 4.18 J (goC)-1
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2. Convert the Units
First of all, convert the Ethanol's mass into the number of moles such as
1.75g/4.61 g mol-1 = 0.0336 moles
Now calculate the total energy generated such as
4.18 J (goC)-1 × 200 g × 55oC = 46024 J
Now, convert the Joule into kilojoules such as
46024J/1000 = 46.024 kJ
3. Calculate molar heat
To calculate the molar heat of combustion, solve the calculated values such as
46.024kJ / 0.0336 moles = 1369 kJ mol-1
Thus, the heat of combustion of 1.78g of ethanol is 1396 kJ mol-1
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The term combustion efficiency refers to the efficiency of equipment to burn a specific fuel. Thus, if an engine possesses 100% combustion efficiency, it means it is efficient enough to pull out all the energy from the fuel. In the same way, the amount of energy that can be pulled out from fuel by using equipment of 100% combustion efficiency, is called specific heat of combustion.
However, no equipment has been invented up till now which can achieve 100% combustion efficiency. Commonly, these efficiencies of engines range between 10%-95%.
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The factors upon which combustion efficiency depends are
- Chemical nature of fuel
- Relative percentages of CO2 and oxygen by volume
- Net temperature
The relation of combustion energy to these factors is directly proportional. In other words, we can increase the heat of combustion simply by using linear high octane fuel, increasing the temperature, surface area, the collision of reactants with each other, and vapor pressure.
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