What Is Heat of Formation Definition Formula and How to Calculate It
In the field of Physics as well as Chemistry, the influence of compounds and mole is hugely significant. The reason behind such significance is that through the study and analysis of one mole of any and every compound, a broader, more generalized idea about the compound, or even similar compounds can be deduced and inferred. Therefore, the answer to “What is enthalpy of formation” is- it is the heat amount that is absorbed or even evolved during the process of formation of one mole of any compound from the constituent elements of the compound.
Standard Enthalpy of Formation
As has been mentioned before, the process of heat absorption or heat evolution during the formation of a compound’s mole is called the enthalpy of formation. The term “enthalpy” is used to refer to the internal energy of a thermodynamic mechanism or system in its totality, along with the combined product of the system’s volume as well as its pressure.
Therefore, as it can be understood, the standard heat of the formation of a mole of a compound is the combined heat of the sum of its internal energy and the product of volume and pressure. Since assumptions and deductions are necessary for the formation of any and every mole, it has been observed that generally, a compound initiates its formation at 77- degree Fahrenheit temperature, where the necessary pressure is of one atmosphere. Therefore, this is considered the standardised amount of heat necessary for the formation of a mole of a compound.
Define Standard Enthalpy of Formation
Since the law of energy conservation states that the heat that is transferred to the system is equal to the change of the system’s internal energy, the amount of heat that is provided externally to material for it to change its own phase is known as enthalpy, where the heat necessary for converting a liquid material to its gaseous form is called the enthalpy of vaporization. Since the differences in the phases of materials are also concerning the standard enthalpy of formation, it is necessary to keep in mind the various phases that materials consist in.
As is known to almost all of us, there are three primary phases for a material. The three fundamental states of matter that are the most prevalent in our day to day lives are- Solid, Liquid and Gas or Vapour. The reason why these three are the most prevalent forms of thermodynamic phases is that they are the three phases that we are the most accustomed with. We have seen how water exists in its liquid form in lakes and oceans, whereas it exists in its solid form in icecaps and also, how it exists in its gaseous form when water is boiled on the stove. Therefore, with the differences in the phases of a material, the respective heat of formation also gets altered.
What is Heat of Formation?
As has been mentioned earlier, the standard enthalpy of formation is used to refer to the heat amount that is absorbed or even, evolved during the process of formation of one mole of any compound from the constituent elements of the compound. Thus, as is to be understood, in order to deduce the amount of heat, whether evolved or absorbed, there is a standard formula which can be utilised to infer the amount.
The Heat of Formation Formula
As has been discussed thus far, the standard heat of the formation of a mole of a compound is the combined heat of the sum of its internal energy and the product of volume and pressure. Since assumptions and deductions are necessary for the formation of any and every mole, it has been observed that generally, a compound initiates its formation at 77- degree Fahrenheit temperature, where the necessary pressure is of one atmosphere.
So, with regards to the question of “What is standard heat of formation?”, it can be answered by stating the formula of the same. The formula is thereby, followed-
[Image will be Uploaded Soon]
Did You Know?
For the calculation of heat absorbed or even evolved during any chemical reaction, the heats of that step’s formation and/ or combustion are to be summed up.
In order to calculate the heat absorbed or heat evolved for a reaction, the theory that is used is known as Hess's Law of Heat Summation.
The name for the law of Heat Summation belongs to Germain Henri Hess, who first proposed the law in Russia in 1840.
FAQs on Heat of Formation in Thermochemistry
1. What is heat of formation in chemistry?
Heat of formation (or enthalpy of formation) is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. It is usually written as ΔHf° and expressed in kJ/mol. For example, the standard heat of formation of water is based on the reaction:
H2(g) + 1/2 O2(g) → H2O(l)
It measures the energy absorbed or released during compound formation.
2. What is the standard heat of formation (ΔHf°)?
Standard heat of formation (ΔHf°) is the enthalpy change when one mole of a compound is formed from its elements in their most stable forms at 1 bar pressure and 298 K. Key conditions include:
- Pressure = 1 bar
- Temperature = 298 K (25°C)
- Elements in their standard states
C(s, graphite) + O2(g) → CO2(g)
3. Why is the heat of formation of an element zero?
The standard heat of formation of an element in its standard state is zero because no energy change occurs when an element forms from itself. For example:
- O2(g), H2(g), and C(s, graphite) all have ΔHf° = 0.
4. What is the formula to calculate enthalpy change using heats of formation?
The enthalpy change of a reaction using heats of formation is calculated using the formula ΔH°reaction = ΣΔHf°(products) − ΣΔHf°(reactants). Steps include:
- Multiply each ΔHf° value by its stoichiometric coefficient.
- Add values for products.
- Subtract values for reactants.
5. How do you calculate the heat of reaction from heats of formation?
To calculate the heat of reaction from heats of formation, use ΔH°reaction = ΣΔHf°(products) − ΣΔHf°(reactants) after balancing the equation. Example for combustion of methane:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Since ΔHf° of O2(g) is 0, substitute tabulated ΔHf° values of CH4, CO2, and H2O to obtain ΔH°reaction.
6. What is the difference between heat of formation and heat of reaction?
The heat of formation refers to forming one mole of a compound from its elements, while the heat of reaction refers to the enthalpy change for any balanced chemical reaction. Key differences:
- ΔHf° forms one mole of a compound.
- ΔH°reaction applies to the entire balanced equation.
- ΔHf° values are used to calculate ΔH°reaction.
7. Can the heat of formation be positive or negative?
Yes, the heat of formation can be positive (endothermic) or negative (exothermic) depending on energy change.
- Negative ΔHf°: energy is released (e.g., formation of H2O(l)).
- Positive ΔHf°: energy is absorbed (e.g., formation of NO(g)).
8. What are standard states in heat of formation?
Standard states are the most stable physical forms of elements at 1 bar and 298 K used when defining ΔHf°. Examples include:
- O2(g) for oxygen
- H2(g) for hydrogen
- C(s, graphite) for carbon (not diamond)
9. How is heat of formation related to Hess's Law?
Heat of formation values are used in Hess's Law to calculate enthalpy changes of complex reactions. Hess's Law states that the total enthalpy change depends only on initial and final states. By applying:
ΔH°reaction = ΣΔHf°(products) − ΣΔHf°(reactants)
we indirectly apply Hess's Law because formation reactions are combined algebraically.
10. Can you give an example of a heat of formation calculation?
Yes, an example of a heat of formation calculation is determining ΔH° for:
C(s) + O2(g) → CO2(g)
Since this reaction forms one mole of CO2 from its elements in their standard states, the enthalpy change equals ΔHf° of CO2(g). From thermodynamic tables, ΔHf° ≈ −393.5 kJ/mol, meaning the reaction is exothermic.
