Enthalpy: Overview, Questions, Preparation

Updated on May 11, 2021 02:23 IST

Table of Contents

Introduction
Enthalpy
Illustrative Examples
FAQs on Enthapy

Introduction

Enthalpy is the thermodynamic quantity equivalent to the total heat content of the system. It is the measure of energy. So every chemical change has a change in enthalpy(energy) associated with it.

Enthalpy

Most of the heat changes in chemical reactions take place at constant atmospheric pressure. So when the process occurs at constant pressure, the heat absorbed or released is equal to the change in enthalpy.

Enthalpy is the sum of internal energy (U) and the product of pressure and volume(PV).

H = U+ PV

Explanation

Heat absorbed at constant volume is equal to the change in the internal energy.

∆U = qᵥ.

But as we know the chemical reactions carried at constant pressure, the equation is written as ∆U = qₚ- p∆V, where

qₚ = heat absorbed by the system

p∆V = Expansion work done by the system

We can rearrange the above equation as

Here we introduce Enthalpy function H = U+PV. so, the above equation becomes

qₚ = H₂ - H₁ = ∆H

Although q is a path-dependent function and H is a state function because it depends on U, P and V. Therefore, ∆H is independent of path. Hence, qₚ is also independent of path.

For finite changes at constant pressure,

Reference: NCERT

The heat absorbed by the system at constant pressure is ∆H = qₚ

∆H is negative for exothermic reactions which evolve heat during the reaction. ∆H is positive for endothermic reactions which absorb heat from the surroundings.

What is the enthalpy for class 11?

Enthalpy is a state function. In this chapter, we learn how enthalpy measured, enthalpy changes during transformations and standard enthalpy of reactions. For class 11, it carries a weightage of below 10 marks.

Illustrative Examples

2.9 kJ of heat produced, when 0.0500 mol of HCl(aq) reacted with 0.0500 mol of NaOH(aq) to form 0.0500 mol of NaCl(aq). What is ΔH, the enthalpy change per mole of acid reacting?

Hcl+ NaOH →Nacl +H20

For reaction 0.0500 mol of Hcl, q = -2.9KJ

For 1 mol of HCl reacts = 1(-2.9/0.0500) = -58KJ

Enthalpy of change ∆H =-58KJ

The enthalpy of, combustion of methane, graphite and dihydrogen at 298K is -890.3KJ/mol, -393.5 KJ/mol and -285.8 KJ/mol. What is the enthalpy of formation of CH4?

Reference: Learn CBSE

We need C + 2H2 → CH4 = -393.5 + 2(-285.8) -(-890.3) = -74.8KJ/mol.

Given : N2(g) + 3H2(g)→ 2NH3(g); ∆rH–(enthalpy of reaction) = -92.4 kj mot-1 What is the standard enthalpy of formation of NH3 gas?

∆H– NH3 (g) = – (92.4)/2 = – 46.2 kj mol-1

FAQs on Enthapy

1. What is the effect of a catalyst on the enthalpy?

The change of enthalpy is not affected by the catalyst. It depends only on the chemical composition of reactants and products.

2. How do pressure and temperature affect enthalpy?

With the increase in temperature, the internal energy of the molecules increases. So the ∆H increases.
Enthalpy increases as pressure increases. At constant pressure, change in enthalpy is equal to the heat released/absorbed by the reaction.

3. What is the use of enthalpy?

Enthalpy used to calculate the heat of reaction of a chemical process. Change in enthalpy used for measurement of heat flow in calorimetry, minimum power of the compressor.

4. How to calculate the enthalpy of a chemical reaction?

Change in enthalpy is the change in temperature over time.

∆H = m x s x ∆T, m= mass of reactants, s = specific heat of product ,∆T= change in temperature.
If the sign is negative, we consider it as an exothermic reaction.
We can estimate enthalpy of the reactions by using bond energies or by using enthalpies of formation.

5. What are examples of endothermic and exothermic reactions?

Exothermic reactions are combustion, neutralisation, dissolving anhydrous salts.
Endothermic reactions are atomisation, photosynthesis and thermal decomposition.