What is Thermochemistry?
Thermochemistry - The branch of Physical Chemistry that deals with the study of the exchange of heat in the reaction. A brief discussion of Thermochemistry is given in the article.
What is Thermochemistry?
This branch of Chemistry describes the phenomena of thermal energy conversion from one form to another form of energy. In this branch, the effects of heat on the matter are also being studied. When we discuss thermodynamics, the particular item or collection of items that we are interested in is called the system, while everything that is not included in the system we have defined is called the surroundings. System and surroundings are separated by the boundary.
For example, If the system is one mole of gas in the container, then the system is the one mole of the gas, the inner wall of the container is known as the boundary (separates the system and surrounding), and everything that is present outside of the boundary is considered the surroundings, which would include the container itself.
Types of System
In order to understand thermodynamics in physical chemistry and the impact of different effects of thermal energy on any particular chemical change, we need to define
1. Open System
An open system can exchange both energy and matter with its surroundings. The stovetop is a good example of the open system. As the water vapour and heat can be lost to the atmosphere.
2. Closed System
Closed system can exchange only energy with its surroundings, not matter. When we put a very tightly fitting lid on the pot, it would be considered as a closed system.
3. Isolated System
An isolated system cannot exchange either matter or energy with its surroundings. A perfect isolated system is hard to come by, but an insulated drink cooler with a lid is conceptually similar to a truly isolated system.
What is Thermochemical Reaction?
The balanced chemical reaction indicates the physical state of all the reactants and products and also indicates the heat change known as a thermochemical reaction.
The thermochemical reaction is of two types:
1. Endothermic Reaction
Those thermochemical reactions in which heat is absorbed. Change in enthalpy for this reaction is positive. A compound formed in the endothermic reaction is known as an endothermic compound. If more heat is absorbed then the product formed will be less stable. Example - decomposition reaction, fusion reaction, vapourisation reaction, sublimation reaction, and photosynthesis.
2. Exothermic Reaction
Exothermic reactions are the reaction in which the heat or the energy is evolved during the reaction. The change in enthalpy for the exothermic reactions is negative. A compound formed in the exothermic reaction is known as an exothermic compound. If more heat is evolved then the product formed will be more stable. Example- combustion reaction, neutralization reaction, respiration, and fermentation.
Some Important Point Related to Thermochemical Reaction
In thermochemical reaction, if conditions are not given then change in enthalpy is considered to be ΔH0.
If the thermochemical reaction is multiplied by some coefficient then the change in enthalpy is also multiplied by the cell coefficient.
If the thermochemical reaction is reversed then the numerical value of change in enthalpy remains the same but the sign is changed.
The Heat of Reaction or Enthalpy of Reaction
The amount of heat change when moles of reactant present in the thermochemical reaction has completely reacted is called the heat of reaction or enthalpy of reaction.
Types of Heat of Reaction
1. The Heat of Combustion or Enthalpy of Combustion
The amount of heat evolved by the complete combustion of one mole of a compound is known as the heat of combustion.
2. The Heat of Formation or Enthalpy of Formation-
The amount of heat evolved or absorbed when one mole of a compound is formed from its constituent elements which are in their stable standard or stable state.
The standard heat of formation of all the stable and free elements is taken to be zero.
3. The Heat of Neutralization or Enthalpy of Neutralization
Amount of heat evolved when 1 gram equivalent of acid is completely neutralised by 1 gram equivalent of the base in dilute solution.
The heat of neutralisation of strong acid and strong base always remains constant and its value is -13.7 Kcaleq-1 because some amount of heat is used in the dissociation of weak acid or base and this difference amount of heat is known as the heat of dissociation. Exception: HF (weak acid) heat of neutralisation of HF is more than -13.7 KCaleq-1 because of the high hydration energy of fluoride ions.
4. The Heat of Solution or Enthalpy of the Solution
The amount of heat evolved or absorbed when 1 mole of a compound is dissolved in such an excess amount of solvent that further dilution does not involve any more heat change known as the heat of solution.
5. The Heat of Hydration or Enthalpy of Hydration
Amount of heat released or absorbed when 1 mole of anhydrous or partially hydrated salt reacts with a required number of a water molecule to form hydrated salt.
6. The heat of Transition or Enthalpy of Transition
The amount of heat changes when one allotropic form of 1 mole of compound converts into another allotropic form known as the heat of transition.
7. The Heat of Fusion or Enthalpy of Fusion
Amount of heat required to convert 1 mole of solid substance into a liquid at its melting point known as enthalpy of fusion.
8. The Heat of Vaporisation or Enthalpy of Vaporization-
The amount of heat required to convert 1 mole of liquid into vapour form is called enthalpy of vaporisation.
9. The Heat of Sublimation or Enthalpy of Sublimation
The amount of heat required to convert 1 mole of solid into gaseous form is called enthalpy of sublimation.
10. Lattice Energy
Amount of heat released when 1 mole of ionic solid is performed from its gaseous ion known as lattice energy.
11. The Heat of Hydrogenation or Enthalpy of Hydrogenation
The amount of heat evolved when 1 mole of unsaturated organic saturated compound reacts with hydrogen to form a saturated organic compound known as the heat of hydrogenation.
12. The Heat of Atomisation or Heat of Atomisation
Amount of energy required to dissociate 1 mole of the stable molecule into a gaseous atom known as the heat of atomisation.
13. Bond Dissociation Energy or Enthalpy of Bond Dissociation
Amount of energy required to dissociate 1 mole of a particular type of bond to separate the atoms in a gaseous state known as bond dissociation energy.
In the case of the diatomic molecule, bond dissociation energy and heat of atomisation is the same.
14. Bond Energy
Bond energy is the average bond dissociation energy in the same type of molecule it is always positive.
Laws of Thermochemistry
1. Lavoisier and Laplace Laws
C(s) + O2(g)→CO2 ΔH=-393 kJ
CO2(g)→C(s) + O2(g) ΔH= +393 KJ
2. Hess Law of Constant Heat Summation
In physical or Chemical processes heat of reaction/enthalpy of reaction remains the same whether it takes place in one step or multistep.
Thermochemistry and Calorimetry
The only thermal quantity that can be measured directly is the heat denoted by q that flows into or out of a reaction vessel (system), and that q is numerically equal to ΔH° only under the special condition of constant pressure. Moreover, q is equal to the standard enthalpy change only when the reactants and products are both at the same temperature, normally 25°C. The measurement of heat (q) is known as calorimetry.
An indirect Calorimeter determines heat (q) which is produced by living bodies by measuring the production of nitrogen compounds and carbon dioxide or from the amount of oxygen taken. A direct Calorimeter can be used to determine heat that is produced by living bodies.
Did You Know?
If heat required to dissociate bonds is more than the heat evolved in bond formation, then the stability of the reactant is more than the stability of the product.
The formation of an explosive compound is an endothermic reaction.
NaCl, KCl, and NH4Cl do not form hydrated salts.
Conclusion
The main distinction between thermochemistry and thermodynamics is that thermochemistry is the quantitative study of the relationship between heat and chemical reactions, whereas thermodynamics is the study of laws relating to the same.
FAQs on Thermochemistry
1. What is thermochemistry and why is it important in Chemistry?
Thermochemistry is a branch of physical chemistry that studies the heat energy changes associated with chemical reactions and phase transitions. It is crucial because it helps us understand and predict whether a reaction will release heat (exothermic) or absorb heat (endothermic), which is fundamental to controlling chemical processes in industries, understanding energy production in biological systems, and designing new materials.
2. How does thermochemistry differ from thermodynamics?
While related, they have a different focus. Thermodynamics is a broader field that studies all forms of energy transfer and transformations and the laws governing them. Thermochemistry is a specific subset of thermodynamics that concentrates exclusively on the heat changes (enthalpy) that occur during chemical reactions. In simple terms, all thermochemistry is thermodynamics, but not all thermodynamics is thermochemistry.
3. What are exothermic and endothermic reactions? Provide an example for each.
Exothermic and endothermic reactions are two types of thermochemical reactions classified by their heat exchange with the surroundings.
- An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat, into its surroundings. The enthalpy change (ΔH) is negative. A common example is the combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), ΔH < 0.
- An endothermic reaction is a chemical reaction that absorbs energy, usually heat, from its surroundings. The enthalpy change (ΔH) is positive. An example is the decomposition of calcium carbonate: CaCO₃(s) → CaO(s) + CO₂(g), ΔH > 0.
4. Can you explain the different types of systems studied in thermochemistry?
In thermochemistry, a 'system' is the part of the universe being studied. There are three types:
- Open System: Can exchange both energy (heat) and matter with its surroundings. For example, a pot of boiling water without a lid.
- Closed System: Can exchange only energy, not matter, with its surroundings. For example, a sealed bottle of warm water.
- Isolated System: Cannot exchange either energy or matter with its surroundings. A perfect isolated system is theoretical, but a well-insulated thermos flask is a close approximation.
5. What is the enthalpy of reaction (ΔH), and what do its positive and negative signs signify?
The enthalpy of reaction (ΔH) represents the total heat content change that occurs when reactants are converted into products in a chemical reaction at constant pressure. The sign of ΔH indicates the nature of the reaction:
- A negative ΔH (ΔH < 0) signifies an exothermic reaction, where the system loses heat to the surroundings.
- A positive ΔH (ΔH > 0) signifies an endothermic reaction, where the system absorbs heat from the surroundings.
6. What are the key laws governing thermochemistry?
Thermochemistry is primarily governed by two fundamental laws:
- Lavoisier-Laplace Law: This law states that the enthalpy change for a given reaction is equal in magnitude but opposite in sign to the enthalpy change for the reverse reaction. For example, if A → B has ΔH = +50 kJ, then B → A will have ΔH = -50 kJ.
- Hess's Law of Constant Heat Summation: This law states that the total enthalpy change for a chemical reaction is the same, regardless of whether the reaction occurs in one step or in a series of steps. This principle is extremely useful for calculating the enthalpy of reactions that cannot be measured directly.
7. What are some of the different types of enthalpy of reaction studied in Class 11?
Several specific types of enthalpy of reaction are important for understanding chemical changes. Key examples include:
- Enthalpy of Formation (ΔfH): The heat change when one mole of a compound is formed from its constituent elements in their standard states.
- Enthalpy of Combustion (ΔcH): The heat released when one mole of a substance is completely burned in excess oxygen.
- Enthalpy of Neutralisation (ΔneutH): The heat change when one mole of water is formed from the reaction of an acid and a base.
- Enthalpy of Solution (ΔsolH): The heat change when one mole of a substance dissolves in a specified amount of solvent.
- Bond Enthalpy: The energy required to break one mole of a specific type of bond in gaseous molecules.
8. How is the standard enthalpy of formation used to calculate the enthalpy of a reaction?
The standard enthalpy of formation (ΔfH°) of various compounds is a powerful tool for calculating the enthalpy of any reaction (ΔrH°). According to Hess's Law, the enthalpy of a reaction can be calculated by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products. The formula is:
ΔrH° = ΣΔfH°(Products) - ΣΔfH°(Reactants). This method allows us to find reaction enthalpies without performing the experiment directly.
9. What is the practical significance of bond enthalpy in predicting reaction outcomes?
Bond enthalpy, or bond energy, is the average energy required to break a particular type of bond. Its practical significance lies in estimating the enthalpy change of a reaction. A reaction involves breaking bonds in reactants (which requires energy) and forming new bonds in products (which releases energy). By summing the energies of bonds broken and subtracting the sum of energies of bonds formed, we can approximate the overall enthalpy of the reaction. This helps predict whether a reaction will be exothermic or endothermic before it is even carried out.
10. Is thermochemistry a part of the CBSE Class 11 or Class 12 syllabus for 2025-26?
Yes, for the academic year 2025-26, thermochemistry is a significant topic within the chapter on 'Thermodynamics' in the CBSE Class 11 Chemistry syllabus. It is not a separate topic in the Class 12 syllabus, so a strong understanding from Class 11 is essential.
