How Do Enthalpy Changes Impact Chemical Reactions?
Reactions are the result of chemical compositions. To make the reaction possible, reactants are necessary. They collage together to form new products. Every reactant absorbs energy during its chemical collages.
Some of the reactions absorb energy, whereas others take part in the evolution of energy. We know that the change in enthalpy is obvious in many chemical reactions. Without it, the process is incomplete.
You can describe the change of enthalpy as the enthalpy of reaction. This article is all about how you should describe standard enthalpy of formation, standard enthalpy of combustion, and enthalpy of bond dissociation.
Define Enthalpy of Formation
We can define standard enthalpy of formation just by mentioning the enthalpy change. It is possible when a compound’s one mole is created from its associated elements within their stable state of aggregation state.
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The stable state of aggregation is considered when the temperature is at 298.15 K, and atmospheric pressure is at 1 atm.
What Is the Enthalpy of Formation?
The enthalpy of formation definition can be understandable with the examples. Let’s take an example to elaborate it briefly. We can consider the formation of methane from hydrogen and carbon:
C(graphite, s) + 2H2(g)→ CH4(g); ΔfHo = - 74.81kJmol-1
Can you answer What Is Standard Enthalpy Of Formation? Enthalpy of formation comes under the category of a special case of standard enthalpy of reaction. In this process, two or more reactants are involved. They combine together to create one mole of the product.
The example of the formation of hydrogen bromide from bromine and hydrogen can be the best example. Here is the expression:
H2(g) + Br2(l) ⟶ 2HBr(g) ; ΔrHo = - 72.81kJmol-1
As per the above expressions, it is clear that two moles of hydrogen bromide are available. Therefore, standard enthalpy of formation can be taken as the enthalpy of reaction, and not as the enthalpy of formation of hydrogen bromide.
We can say that
ΔfHo = 2 ΔrHo
ΔfHo = Enthalpy of formation
ΔrHo = Enthalpy of reaction
Enthalpy of Combustion
We can say that the enthalpy of combustion is only possible when one mole of a compound is burnt completely to give rise to oxygen at the end. All of the processes are taken into consideration when all the reactants and products are in the standard state and under standard conditions (1 bar pressure and 298K).
For example:
H2(g) + 1/2 O2(g) ➝ H2O(l) ; ΔcHo = - 286 kJmol-1
C4H10(g) + 13/2 O2(g) ➝4CO2(g) + 5H2O(l) ; ΔcHo = -2658 kJmol-1
Standard enthalpy of combustion is a positive value as combustion is always exothermic. When a chemical substance comes under the process of combustion, it generates energy to outside. So, the change in enthalpy for the exothermic reactions is negative.
However, in the convention process, the molar heat of combustion (also molar enthalpy of combustion) is considered as a positive value.
We can calculate the enthalpy of combustion with ease. The process is very simple. We do it by calculating the difference between the mass of the fuel before the boiled water and the mass of the fuel.
The workout energy of a substance can be given as 1 mole. The unit that stands to show the measurement of enthalpy of combustion is known as Joule per mole (or Kilojoule per mole).
If you need your answer in KJ (kilojoule) format, you just need to devise the result by 1000.
Bond Dissociation Enthalphy
This is a type of change in enthalpy where one mole of covalent bonds of a gaseous compound is taken apart to manufacture different gaseous phase products.
In general, the enthalpy of bond dissociation is always different from the bond enthalpy values. In a molecule, it is the average of some of all the bond dissociation energy.
A few examples of diatomic molecules that come under the bond dissociation enthalpy process:
Cl2(g) ➝2Cl(g)
ΔCl-ClHo = 242kJmol-1
FAQs on Standard Enthalpy of Formation, Combustion, and Bond Dissociation Explained
1. What is the standard enthalpy of formation (ΔfH°)?
The standard enthalpy of formation (ΔfH°) is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements, with all substances in their standard states. The standard state is the most stable form of a substance at a pressure of 1 bar and a specified temperature, usually 298 K (25°C).
2. What does the standard enthalpy of combustion (ΔcH°) represent?
The standard enthalpy of combustion (ΔcH°) is the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions (1 bar pressure, 298 K). Since combustion is an exothermic process, the value of ΔcH° is always negative, indicating the release of heat.
3. Can you explain bond dissociation enthalpy with an example?
Bond dissociation enthalpy is the energy required to break one mole of a specific type of bond between atoms in the gaseous state. It is always a positive value as energy is needed to break bonds. For example, the bond dissociation enthalpy of the H-H bond in a hydrogen molecule (H₂(g)) is the energy needed for the reaction: H₂(g) → 2H(g), which is +435.8 kJ/mol.
4. Why is the standard enthalpy of formation of an element like O₂(g) or Na(s) taken as zero?
The standard enthalpy of formation for an element in its most stable form (its standard state) is defined as zero by convention. This is because there is no enthalpy change involved in forming an element from itself. This convention provides a baseline or a reference point from which the enthalpies of formation of compounds can be measured and calculated.
5. How are standard enthalpy of formation and enthalpy of combustion related?
These two concepts are linked through Hess's Law. The enthalpy of a reaction can be calculated using either value. For a combustion reaction, the standard enthalpy of combustion (ΔcH°) can be calculated using the standard enthalpies of formation (ΔfH°) of the reactants and products. The formula is:
ΔrH° = ΣΔfH°(products) - ΣΔfH°(reactants).
This shows that if you know the formation enthalpies of all substances in a combustion reaction, you can determine the heat of combustion.
6. What is the difference between bond dissociation enthalpy and mean bond enthalpy?
There is a subtle but important difference:
Bond Dissociation Enthalpy refers to the energy required to break a specific bond in a specific molecule. For example, breaking the first O-H bond in H₂O has a different energy value than breaking the second O-H bond in the remaining OH radical.
Mean Bond Enthalpy is the average energy required to break a particular type of bond over a range of different compounds. For the O-H bond, it is the average value from molecules like water, alcohols, etc. It is used when specific dissociation data is unavailable.
7. How can you calculate the enthalpy of a reaction using bond dissociation enthalpies?
The overall enthalpy change of a reaction (ΔrH°) can be estimated by considering the energy required to break all the bonds in the reactants and the energy released when new bonds are formed in the products. The formula is:
ΔrH° = Σ(Bond enthalpies of reactants) - Σ(Bond enthalpies of products).
This method is particularly useful for reactions in the gaseous phase where bond breaking and formation are the primary energy changes.
8. What is the practical importance of knowing the enthalpy of combustion for different fuels?
Knowing the enthalpy of combustion is crucial for practical applications. It helps determine the calorific value of fuels like petrol, diesel, LPG, and hydrogen. A higher negative value for ΔcH° means more energy is released per mole. This information is vital for:
Designing efficient engines and power plants.
Comparing the energy output and efficiency of different fuels.
Calculating the energy requirements for industrial processes and rocket propulsion.
