What Is the Heat of Combustion Definition Formula and Calculation Methods
The heat of combustion of a given substance, which is also called the energy value or calorific value, can be described as the amount of heat liberated when a given substance undergoes combustion. In general, the heat of combustion is recognized to be a synonym of calorific value, which can be described as the total amount of energy liberated when the mass of a substance that is given undergoes combustion completely in the presence of an adequate quantity of oxygen under the standard conditions for pressure and temperature. The heat of combustion of ethanol can also be calculated using its chemical formula.
Units of Heat of Combustion of a Substance
Generally, the hydrocarbons potential, which is used in fuels, is expressed in terms of calorific value, according to the combustion reaction, which they undergo with oxygen to produce carbon dioxide and water. It should note that the heat of combustion of a substance is expressed in terms of the units given below.
Energy (either in joules or kilojoules) is liberated when one mole of the fuel undergoes complete combustion with the oxygen.
Energy (either in joules or kilojoules) is liberated when one liter of the fuel undergoes complete combustion with the oxygen.
Energy (either in joules or kilojoules) is liberated when either one gram or one kilogram of the fuel undergoes complete combustion with the oxygen.
It should also note that the heat of combustion of fuels can be calculated using a bomb calorimeter.
Determination of Heat of Combustion
The heat of combustion is often categorized into two types, as listed below.
Higher Calorific Value - It is also called higher heating value and gross calorific value,
Lower Calorific Value - It is also called lower heating value and net calorific value.
Let Us Discuss Determining Each Type of Heat of Combustion
Determination of Higher Calorific Value
The higher calorific value of a substance (which is often abbreviated to HCV) is determined by bringing back the total combustion products into the initial temperature (the reaction environment's temperature prior to the combustion occurs).
It should also note that this involves condensation of any vapour produced during the combustion reaction. It should also note that these measurements are performed under some standard conditions, where the temperature of the reaction environment is placed at 25℃.
The Gross Calorific Value (GCV) can be counted similarly to the thermodynamic combustion heat because the reaction enthalpy transition assumes a particular compound temperature both before and after the combustion phases, in a case, where the combustion water is condensed into a liquid.
It should also be noted that the gross calorific value is considered as the latent heat of water vaporization in the final combustion product. It is also quite useful to measure the heating values for fuels where it is possible for the condensation of the reaction products, in the case of a space heat gas-fired boiler, as an example.
Determination of Lower Calorific Value
The lower calorific value of a substance can also be known as the lower heating value and the fuel's net calorific value. It can be determined by subtracting the latent heat value of water vaporization formed in the reaction from the gross calorific value (which is the higher heating value). This method of expressing the heat of combustion includes the assumption that the water formed (if any) during the combustion reaction will be in the vapour form. Thus, the energy expended during the reaction for the conversion of water into water vapour (and, hence, not liberated as heat during the reaction) is considered during this quantity 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, compared to the higher heating value (HHV). Where HHV assumes that all the water present in the combustion process is in a liquid state after the combustion process takes place.
The lower calorific value (LCV) presumes there exists no latent heat recovery of water vaporization both in the fuel and the reaction materials. This helps compare fuels in such cases, either where the combustion product condensation is impossible or where the heat can not be used at temperatures below 150℃.
Determination of Measuring Heating Values
The higher heating value can be determined experimentally in a bomb calorimeter. The combustion of a stoichiometric mixture of oxidizer and fuel (for example, one mole of oxygen and two moles of hydrogen) in a steel container at 25°C can be initiated by an ignition device, and the reactions, which are allowed to complete. When oxygen and hydrogen react during combustion, there produces water vapor. Then, both the vessel and its contents are cooled to the original temperature of 25 °C, and the higher heating value is obtained as the heat released between identical initial temperature and final temperatures.
When there determines a lower heating value (LHV), cooling stops at 150 °C, and the reaction heat is recovered only partially, where the limit of 150 °C is based on the dew-point of acid gas.
It should be noted that a Higher Heating Value (HHV) can be calculated with the product of water being in liquid form. In contrast, a Lower Heating Value (LHV) can be calculated using the product of water being in vapor form.
Did You Know?
Gross heating value (GHV) accounts for exhaust water, leaving as vapour, and includes liquid water in the fuel before combustion. And, this value is necessary for fuels such as coal or wood, which usually contain an amount of water prior to burning.
FAQs on Heat of Combustion in Thermochemistry
1. What is heat of combustion?
The heat of combustion is the amount of heat released when one mole of a substance completely burns in excess oxygen under standard conditions. It is an exothermic reaction, so its enthalpy change (ΔHc) is negative.
Key points:
- Measured at constant pressure as enthalpy of combustion (ΔHc).
- Usually expressed in kJ mol-1.
- Products are typically CO2(g) and H2O(l) for hydrocarbons.
2. What is the formula for calculating heat of combustion?
The heat of combustion is calculated using q = mcΔT for calorimetry experiments.
Where:
- q = heat released (J)
- m = mass of water or solution (g)
- c = specific heat capacity (4.18 J g-1 °C-1 for water)
- ΔT = temperature change (°C)
- Calculate moles of fuel burned.
- Divide heat released (in kJ) by moles of fuel.
3. Why is heat of combustion always negative?
The heat of combustion is negative because combustion reactions release heat to the surroundings, making them exothermic.
Explanation:
- Energy is released when strong bonds in CO2 and H2O form.
- The products have lower enthalpy than the reactants.
- Thus, ΔHc < 0.
4. How do you calculate the enthalpy of combustion experimentally?
The enthalpy of combustion is determined experimentally using a calorimeter to measure heat released during burning.
Steps:
- Burn a known mass of fuel in excess oxygen.
- Measure the temperature rise of a known mass of water.
- Use q = mcΔT to calculate heat absorbed by water.
- Convert to kJ and divide by moles of fuel burned.
5. What is the difference between heat of combustion and heat of formation?
The heat of combustion is the heat released when one mole of a substance burns completely, while the heat of formation (ΔHf) is the heat change when one mole of a compound forms from its elements in their standard states.
Key differences:
- Combustion involves reaction with O2.
- Formation involves elements in standard states.
- ΔHc is usually negative and large in magnitude.
6. What is the standard enthalpy of combustion?
The standard enthalpy of combustion (ΔH°c) is the heat released when one mole of a substance burns completely in oxygen under standard conditions (1 bar, 298 K).
Conditions include:
- Pressure = 1 bar
- Temperature = 298 K (25°C)
- All substances in their standard states
7. Can you give an example of a combustion reaction with its heat of combustion?
An example of a combustion reaction is the burning of ethanol with ΔH°c ≈ −1367 kJ mol-1.
Balanced equation:
- C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
8. How is heat of combustion related to bond energies?
The heat of combustion depends on the difference between bond energies of reactants and products.
Relationship:
- Energy is required to break bonds in fuel and O2.
- Energy is released when new bonds form in CO2 and H2O.
- ΔH ≈ Σ(bond energies broken) − Σ(bond energies formed).
9. Why do hydrocarbons have high heats of combustion?
Hydrocarbons have high heats of combustion because they contain many C–H and C–C bonds that release large amounts of energy when converted to CO2 and H2O.
Reasons:
- Formation of strong C=O bonds in CO2.
- Formation of strong O–H bonds in H2O.
- Longer carbon chains generally release more total energy per mole.
10. What factors affect the experimental value of heat of combustion?
The experimental heat of combustion can be affected by heat loss, incomplete combustion, and measurement errors.
Main factors:
- Heat lost to surroundings instead of water.
- Incomplete combustion forming CO or soot instead of CO2.
- Evaporation of fuel during burning.
- Inaccurate temperature or mass measurements.
