How to Calculate and Apply Heat of Formation in Chemistry
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 Explained for Students
1. What is meant by the standard enthalpy of formation in Chemistry?
The standard enthalpy of formation (ΔH°f) is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable states under standard conditions. These conditions are typically 298 K (25°C) and 1 atm pressure. It helps measure the stability of a substance.
2. How is the heat of formation of a compound generally calculated?
The heat of formation is usually calculated indirectly using Hess's Law. This involves using the known enthalpy changes of other reactions. The formula used is:
ΔH°reaction = ΣΔH°f (products) - ΣΔH°f (reactants).
Direct measurement is often difficult or impossible.
3. Can the heat of formation have a positive value? What does that signify?
Yes, the heat of formation can be positive. A positive value indicates an endothermic process, meaning that energy is absorbed from the surroundings to form the compound. This usually means the resulting compound is less stable than its constituent elements.
4. Why is the standard enthalpy of formation for an element like O₂(g) or C(graphite) taken as zero?
The enthalpy of formation for an element in its most stable, standard state is defined as zero because no energy change is required to form an element from itself. This value acts as a fundamental baseline or reference point from which the enthalpies of formation for all compounds are measured.
5. What is the difference between standard enthalpy of formation and standard enthalpy of reaction?
There is a key difference between the two:
- Standard Enthalpy of Formation (ΔH°f): Refers specifically to forming one mole of a compound from its elements in their standard states.
- Standard Enthalpy of Reaction (ΔH°rxn): Is the total enthalpy change for any chemical reaction carried out under standard conditions, which can involve compounds reacting, not just elements.
6. How does the physical state (solid, liquid, or gas) of a substance influence its heat of formation?
The physical state is crucial because energy is needed to change states (e.g., from liquid to gas). For example, the standard enthalpy of formation for liquid water (H₂O(l)) is different from that of water vapour (H₂O(g)). The difference between these two values is equal to the enthalpy of vaporization of water.
7. What does a negative value for the heat of formation, like that of CO₂, imply?
A negative heat of formation indicates an exothermic process. This means that energy is released when the compound is formed from its elements. A large negative value, such as for carbon dioxide (CO₂), signifies that the compound is very stable compared to its elements.
