Thermodynamics and Thermochemistry: An Introduction
Everything around us requires reaction to form the end product. There is always an initiator in the reaction which occurs in this universe. The universe consists of the two things - System and Surroundings. The System can be defined as the place where the reaction is happening or everything else is considered as the surroundings. The reaction can happen naturally or can be artificially done. There is always an initiator or catalyst which begins the reaction or helps the reaction to run. The catalyst can be anything which helps the reaction, it can be heat, light, water, electricity, or some bacteria. In this article, we will discuss the heat as the initiator or catalyst in the chemical reaction.
Thermodynamics and thermochemistry deal with the rate of flow of the heat in the chemical reaction or the chemical reaction which is produced by the presence of heat.
What is Thermodynamics?
In Thermodynamics, thermo means heat and dynamics means rate which means the rate of flow of the heat. The thermodynamics came into existence to increase the efficiency of the heat engine, to convert all the energy into work and produce less heat as the product in the engine because of wastage of energy and wear-tear in the engine.
Thermodynamics can be defined as the transformation of one energy into the other energy. In this article, we will talk about the transformation of heat energy into some useful energy to do work. Thermodynamics can be defined by the flow of heat which can be described by the internal energy, entropy, and enthalpy.
Thermochemistry Equation
Thermochemical equation is the equation which describes the change in the heat during the reaction.
Here, exothermic reaction can be defined as the balanced chemical reaction which occurs due to the heat.
\[C{H_4} + {\rm{ }}{O_2} \to C{O_2} + {\rm{ }}{H_2}O{\rm{ }} + {\rm{ }}Heat\]
In the above reaction, the methane reacts with oxygen gas to provide the carbon dioxide, water molecule, and heat as bi-products.
Endothermic reaction can be defined as the balanced chemical reaction which gives heat as a product.
\[C\left( s \right){\rm{ }} + {\rm{ }}{H_2}O\left( l \right){\rm{ + Heat}} \to C{O}\left( g \right){\rm{ }} + {\rm{ }}{H_2}\left( g \right)\]
In the above reaction, the carbon reacts with water molecules in the presence of heat to provide the carbon monoxide, hydrogen gas molecule.
What is Thermochemistry?
In thermochemistry, thermos means heat and chemistry means chemical reaction which means the heat is responsible for the chemical reaction to occur. The French chemist Antoine Laurent Lavoisier and Pierre Simon de Laplace made this different field of chemistry in the year 1780. They study about the reaction happening around as most of the reaction takes place by the absorption of the heat or releasing heat during the reaction or as the end product.
Thermochemistry is the branch of chemistry which describes the changes occurring in the chemical reaction due to the presence of heat.
According to the flow of heat, the chemical reaction can be defined as the exothermic and endothermic reaction. Exothermic reactions are those reactions which occur when the heat is provided to the reactants whereas the endothermic reactions are those which releases heat energy after the chemical reaction occurs.
Laws of Thermochemistry
Thermochemistry can be defined as the change in the heat of the chemical reaction. Here, change in heat can be defined by the equations.
Thermochemistry can follow the law:
Change in the heat is directly proportional to how the moles of substance reacts to form the balanced moles of product.
Magnitude of change in heat in the reaction is equal to the opposite of the sign of the reverse reaction.
The change in heat in the reaction can be defined by Hess's law which states that the change in heat is equal to the resultant reaction to the magnitude of the change in heat of the small reaction involved.
$\text{Resultant} \ \text{reaction} \ = \ \text{Reaction} \ 1 + \text{Reaction} \ 2$
$\Delta \ H_R \ = \Delta H_1 + \Delta H_2$
Difference Between Thermodynamics and Thermochemistry
Both thermodynamics and thermochemistry are the study of heat in the reaction. There are some differences in thermodynamics and thermochemistry because of the purpose. Thermodynamics tells about the rate of the flow of heat whereas thermochemistry can be defined as the type of chemical reaction which happens due to the absorption heat and releasing heat.
Key Features
Chemical reaction can be defined as the reactants react to form products with the help of the initiator or catalysts.
Thermodynamics can be defined as the transformation of one energy into the other energy.
Thermochemistry is the branch of chemistry which describes the changes occurring in the chemical reaction due to the presence of heat.
Exothermic reactions are those reactions which occur when the heat is provided to the reactants whereas the endothermic reactions are those which releases heat energy after the chemical reaction occurs.
Hess’s Law states that the change in heat is equal to the resultant reaction to the magnitude of the change in heat of the small reaction involved.
Interesting Facts
In cold countries, the thermometer is more filled with alcohol than mercury because alcohol has a lower freezing point than mercury.
The only perfect body which is called isolated is our Universe.
The energy absorbed and released by any object can be determined by thermochemistry.
FAQs on Thermodynamics and Thermochemistry
1. What is the precise relationship between Thermodynamics and Thermochemistry in the context of JEE Advanced?
Thermodynamics is a broad branch of physics and chemistry that deals with energy transformations, heat, work, and their relation to physical properties of matter. For JEE Advanced, it covers the First, Second, and Third Laws. Thermochemistry, on the other hand, is a sub-branch of thermodynamics that specifically focuses on the heat changes accompanying chemical reactions, such as enthalpy of reaction (ΔH), formation, combustion, and Hess's Law. Essentially, all thermochemistry is thermodynamics, but not all thermodynamics is thermochemistry.
2. What are the essential sign conventions for heat (q) and work (w) under the First Law of Thermodynamics for JEE problems?
For JEE Advanced, the IUPAC sign convention is standard. It is crucial for correctly applying the first law equation, ΔU = q + w, where ΔU is the change in internal energy. The conventions are:
- q (heat): Positive (+) when heat is absorbed by the system (endothermic), and Negative (-) when heat is released by the system (exothermic).
- w (work): Positive (+) when work is done on the system (compression), and Negative (-) when work is done by the system (expansion).
3. In what specific scenarios for a chemical reaction can the change in enthalpy (ΔH) be equal to the change in internal energy (ΔU)?
The relationship is given by ΔH = ΔU + Δn_gRT. Therefore, ΔH can equal ΔU under two primary conditions:
- When the reaction involves only solids and liquids, or the number of moles of gaseous reactants equals the number of moles of gaseous products. In this case, the change in moles of gas (Δn_g) is zero.
- When the reaction is carried out in a closed, rigid container (like a bomb calorimeter), where the volume is constant. In this case, the work done is zero, making the heat exchanged at constant volume (q_v) equal to ΔU.
4. How is Hess's Law of Constant Heat Summation applied to find the enthalpy of reactions that are difficult to measure experimentally?
Hess's Law states that the total enthalpy change for a reaction is the same, regardless of whether it occurs in one step or multiple steps. For JEE problems, this principle is used to calculate the enthalpy of a target reaction by algebraically manipulating known thermochemical equations of other reactions. The process involves:
- Identifying the target equation.
- Arranging the given step-reactions (reversing or multiplying them as needed) so they sum up to the target equation.
- Applying the same algebraic operations to their corresponding ΔH values to find the final enthalpy change.
5. Why is the work done in a reversible isothermal expansion always greater than in an irreversible isothermal expansion between the same initial and final states?
This is a key concept in thermodynamics. In a reversible expansion, the external pressure is only infinitesimally smaller than the internal pressure at every stage, allowing the maximum possible work to be extracted from the system. In contrast, an irreversible expansion occurs against a constant, lower external pressure. Since work done by the system is calculated as the integral of P_ext dV, the continuously adjusting (and higher) external pressure in the reversible process results in a greater area under the P-V curve, and thus, more work is done.
6. What is the fundamental difference between bond enthalpy and bond dissociation enthalpy, and which is more relevant for JEE calculations?
Bond Dissociation Enthalpy is the energy required to break one mole of a specific bond in a gaseous molecule to form gaseous atoms or radicals (e.g., breaking the C-H bond in CH₄). Bond Enthalpy (or average bond enthalpy) is the average energy required to break one mole of a particular type of bond over a range of different gaseous compounds. For calculations involving specific reactions, bond dissociation enthalpy is more accurate, but for general estimations in JEE problems, average bond enthalpies are commonly used.
7. How does Gibbs Free Energy (ΔG) predict the spontaneity of a reaction, and what is the common trap related to ΔG vs. ΔG°?
The sign of Gibbs Free Energy (ΔG) at constant temperature and pressure determines spontaneity:
- ΔG < 0: The process is spontaneous.
- ΔG > 0: The process is non-spontaneous.
- ΔG = 0: The system is at equilibrium.
8. What key formulas from Thermodynamics and Thermochemistry are most frequently used in JEE Advanced?
For success in JEE Advanced, a firm grasp of these formulas is essential:
- First Law: ΔU = q + w
- Work Done (isothermal reversible): w = -2.303 nRT log(V₂/V₁)
- Work Done (adiabatic): w = (P₂V₂ - P₁V₁)/(γ-1) = nR(T₂ - T₁)/(γ-1)
- Enthalpy-Internal Energy Relation: ΔH = ΔU + Δn_gRT
- Gibbs Free Energy: ΔG = ΔH - TΔS
- Relation to Equilibrium Constant: ΔG° = -2.303 RT log K_eq
- Kirchhoff's Equation: ΔH₂ - ΔH₁ = ΔC_p(T₂ - T₁)
