Enthalpy: Definition, Difference, Formation and Sample Questions

Enthalpy refers to the energy that is stored in a thermodynamic system. It can be understood as the sum of the internal energy of a system and the products of its volume and pressure. In this article, we will know more about enthalpy, enthalpy change, reaction enthalpy, enthalpy of formation, and Hess’s Law.

What is Enthalpy?

Enthalpy is the summation of the internal energy of a system. It is the energy required to establish a system, having pressure P and volume V, from empty space. Thus, Enthalpy is the total amount of energy of the system. The concept of enthalpy has made understanding of various thermodynamics systems simpler and easier.

Read a brief summary of the Chapter Thermodynamics

Difference between Enthalpy and Entropy

• Enthalpy is the sum of internal energy and flow of energy while entropy value defines the randomness of molecules. 

• Entropy value is valid under controlled conditions only while enthalpy value is valid for normal conditions.

Relation between enthalpy, pressure and volume

The enthalpy is the sum of internal energy and the work done. If there is no change in pressure and volume, it means no work is done, which means that the change in enthalpy is only due to the internal energy change.

• If volume remains constant, then the change in energy during the reaction will be equal to the internal energy of a system.
• If the pressure remains constant, then the change in energy during the reaction will be equal to the enthalpy change of the system.

Example of Enthalpy

When flames of pure hydrogen gas are burnt, Hydrogen(H₂) reacts with oxygen (0₂) presents in the environment and then the water(H₂0) is created. In the above reaction, we are giving the energy to break the bond between the hydrogen-hydrogen atom and oxygen-oxygen atom. Then when a bond is created between oxygen and water, energy is released. The energy released is more than the energy consumed, so the net outcome of this reaction is that it is an exothermic reaction

Enthalpy Change

Enthalpy Change is the change in heat when a reaction takes place, but the change in heat can be of two types - 

1. Increase in heat (Absorption of Heat) 
2. Decrease in heat (Release of Heat) 

The reactions in which heat is absorbed are called endothermic reactions and the reactions in which heat is released are called exothermic reactions.

Δ_rH =(sum of enthalpy of products) - (sum of enthalpy of reactants)

Δ_rH = ∑_ia_iH_p -∑ _ib_iH_r

Here, sigma is used for summation, and a_i and b_i are the stoichiometric coefficients in the reaction.

It is very difficult or literally impossible to calculate the actual value of Enthalpy, So the enthalpy in absolute terms has no real significance. But the concept of enthalpy change is a very important and widely used concept. The Enthalpy change tells you the potential energy produced or absorbed during the reaction. The change in enthalpy during the reaction is hence called reaction enthalpy.

Standard Enthalpy of Reactions

The enthalpy of a reaction varies with the change in conditions in which the reaction takes place. Hence, the standard enthalpy of reaction is the enthalpy of reaction when all the components of a reaction are in their standard state. The standard state of a substance at a specific temperature is purest at 1 Bar. For Example, Ethanol is purest at 298 k and 1 bar. The standard enthalpy of a reaction is represented by the symbol ΔH^θ.

Enthalpy Change during Phase Transformations

Phase transformations refer to the change in the phase or energy when a reaction takes place. The enthalpy change during the phase transformation is the enthalpy change of a reaction for changing one or more substances from one state to another. For example molar enthalpy of vaporization, fusion, and sublimation.

Phase Transformation in Thermodynamics

The change in enthalpy that takes place when one mole of a solid substance is melted is known as the standard or molar enthalpy of fusion. It is represented as Δ_fusH^θ

For example: H₂O(s) → H₂O(l)

The Δ_fusH^θ for the above reaction is 6.00 KJ mol^-1.

The amount of heat that is required to transform one mole of a liquid substance at standard pressure and constant temperature to vapours is known as the standard enthalpy of vaporization. It is represented as Δ_vapH^θ.

For example: H₂O(l) → H₂O(g)

The Δ_vapH^θ for the above reaction is +40.79 KJmol^-1.

The enthalpy change that takes place when one mole of a solid substance sublimes under standard pressure and constant temperature is known as the standard enthalpy of sublimation. It is represented by Δ_subH^θ.

For example, Dry ice or solid CO₂ sublimes with a Δ_subH^θ= 25.2 KJ mol^-1 at a temperature of 195 K.

Standard Enthalpy of Formation

This is the change in enthalpy when one mole of a substance is formed from its constituent reactant elements in their most stable state of aggregation. This is also called as standard molar enthalpy of formation. It is represented as Δ_fH^θ.

H₂(g) + ½ O₂(g) → H₂O(l); Δ_fH^θ= -285.8 KJ mol^-1

The standard molar enthalpy of formation is understood as a special case of reaction enthalpy where one mole of the product is formed from the constituent elements.

Hess’s Law of Constant Heat Summation 

As enthalpy is a state function, the enthalpy change does not depend on the path between the initial to the final state of the reactions. Hess’s Law of Constant Heat Summation states that the change in enthalpy will remain the same, irrespective of the steps involved in the process.

Let us understand this through the given example: 

From the given example of reactions, we can clearly observe that the reaction used between the initial and final stage is irrelevant in the Enthalpy. The Net Enthalpy Change will remain the same, whichever method is used.

Bond Enthalpy

Commonly known as bond energy, the bond enthalpy, Δ_bondH^θ, is the amount of energy required to break the bond between the one mole of that chemical substance. As one bond takes more energy than another bond, it tells you that the first bond is stronger than the second bond. So the concept of bond enthalpy can be used to define the stability and reactivity of chemical substances. This concept of bond energy leads to many chemical inventions and has helped in the creation of many innovative products.

Note: If bond enthalpy is positive then it means that you need to give the energy to break the bond, and if the bond enthalpy is negative, it means that bond will release energy on breaking.

Things To Remember on Enthalpy

• Enthalpy refers to the energy that is stored in a thermodynamic system
• The enthalpy change of energy reaction is the net energy absorbed or net energy released during the whole reaction.
• Δ_rH^θ =(sum of enthalpy of products) - (sum of enthalpy of reactants)
• The strength of bonds can also be defined by the amount of energy required to break them.
• The reactions in which heat is absorbed are called endothermic reactions and the reactions in which heat is released are called exothermic reactions.
• The net enthalpy of the reaction does not get affected by the process or steps that are chosen, it will remain the same. This is known as Hess’s Law of Constant Summation.
• The change in enthalpy that takes place when one mole of a solid substance is melted is known as the standard or molar enthalpy of fusion. It is represented as fusH.
• The sum of the change in enthalpies of step reactions will always remain the same and is always equal to the Net Enthalpy.

Sample Questions on Enthalpy

Ques. A non-ideal gas goes from state 1 ( 2 atm, 3 Liter, 95K) to State 2 (4 atm, 5 Liter, 245k). Find ΔH, if ΔE= 30 atm Liter. 2 marks

Ans. By using the formula ΔH = ΔE + Δ(PV)

ΔH = 30 + (20 - 6)

= 44 atm Liter

Ques. Can a Net Enthalpy of a reaction be exothermic if 90% of its step reactions require energy to happen? 1 mark

Ans. Yes, it can if the rest 10% release more energy than required by the other 90% of the reaction.

Ques. Prove that Noble Gases are very stable with the help of bond energy. 2 marks

Ans. The energy required to break the bonds of noble gases is very high. This means that you have to work very much to break the bonds between them. This means that they are very stable.

Ques. If a reaction takes part in 3 steps, each step has different enthalpy. One step has ΔE = +20 unit, second step has ΔE = -60 unit. The combined enthalpy of the reaction is +90 units. Find the ΔE for 3^rd step. 3 marks

Ans. The sum of ΔE for each reaction is equal to net enthalpy. Hence,

 ΔE_combined = ΔE₁ + ΔE₂ + ΔE₃

Therefore,

+90 = + 20 - 60 + ΔE₃

ΔE₃ = +130 unit.

Ques. In what type of reaction does enthalpy become equal to internal energy. Give a condition when it does not happen? 2 marks

Ans. The enthalpy is equal to internal energy when the reaction is carried out in a closed container, meaning that the number of particles before the reaction and after the reaction is the same. This case is not true when there is a change in pressure during the reaction.

Ques. If work is done by the system, then what is the change in enthalpy? Explain the case when work is done on the system. 3 marks

Ans. If work is done by the system, energy will be released from the system – Then it is will lead to a decrease in the internal energy and energy will be released from the system. This will lead to a reduction in the enthalpy of the system.

If work is done on the system, energy will be absorbed by the system – Then it will lead to an increase in the internal energy and the energy will be absorbed into the system. This will lead to an increase in the enthalpy of the system.