Heat and Thermodynamics

Heat and thermodynamics are fundamental branches of physics that deal with the study of energy, heat transfer, and the relationship between different forms of energy. These concepts are not only pivotal in understanding the physical world around us but also have broad applications across various industries, from engines and refrigeration systems to environmental science and even biological processes.

1.0What is Heat?

Heat is a form of energy that is transferred between two substances or systems due to a difference in temperature. When two objects are at different temperatures, heat flows from the hotter object to the cooler one. This process continues until both objects reach thermal equilibrium (the same temperature).

Heat is defined as the energy transferred between a system and its surroundings due to a temperature difference. The direction of heat flow is important and follows a convention:

• Heat lost by the system is assigned a negative sign (– iv).
• Heat gained by the system is assigned a positive sign (+ iv).

This convention helps in thermodynamic calculations to clearly indicate whether the system is losing or gaining energy.

2.0Concepts of Heat

Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance. It is commonly measured in Celsius, Fahrenheit, or Kelvin.

Heat Transfer: Heat can be transferred through three methods:

• Conduction: Direct transfer of heat through a material, from one particle to another.
• Convection: Transfer of heat in fluids (liquids and gases) by the movement of the fluid itself.
• Radiation: Transfer of heat through electromagnetic waves, such as the heat from the Sun reaching the Earth.

3.0Types of Heat

Heat can be classified into two major types:

• Sensible Heat: This is the heat that causes a change in temperature without changing the state of the substance. For example, heating water from 25°C to 80°C.
• Latent Heat: This is the heat required to change the state of a substance without changing its temperature. For example, the heat needed to melt ice into water at 0°C.

4.0Heat Capacity and Specific Heat

Heat capacity in Thermodynamics is the amount of heat required to raise the temperature of an entire object or system by 1°C, while specific heat capacity refers to this heat requirement per unit mass. The difference is that heat capacity is an extensive property (dependent on the amount of substance), while specific heat capacity is an intensive property (independent of the amount).

5.0Heat at Constant Volume and Constant Pressure

In thermodynamics, heat transfer can occur under constant volume or constant pressure conditions, which are described by specific heat capacities.

1. Heat Transfer at Constant Volume:

Formula: q_v = nC_v,mΔT

Where:

• n = number of moles of the substance
• C_v,m = molar heat capacity at constant volume
• ΔT = change in temperature

In a constant volume process, the system does no work (since volume doesn't change), and all the heat goes into changing the internal energy of the system.

2. Heat Transfer at Constant Pressure:

Formula: q_p = nC_p,mΔT_q

Where: C_p,m= molar heat capacity at constant pressure

In this process, heat transfer occurs at constant pressure, and part of the energy goes into doing work by expanding or compressing the gas, while the rest increases the internal energy.

Relationship Between C_p​ and C_v for Ideal Gases

For ideal gases, the relationship between the heat capacities at constant pressure (C_p​) and constant volume (C_v​) is given by: C_p,m−C_v​,m=R

Where R is the universal gas constant. This equation shows that for an ideal gas, the heat capacity at constant pressure is always greater than the heat capacity at constant volume by an amount equal to the gas constant.

Temperature Dependence of Heat Capacities

The specific heat capacities (C_p​ and C_v​) are not always constants and can depend on temperature. For an ideal gas, the heat capacity can be expressed as a function of temperature in the form:

                                     C = a+bT+cT²+…

Where a, b, and c are constants. This equation shows how the heat capacity varies with temperature.

Heat Capacities and Atomicity

For different types of gases (monoatomic, diatomic, or polyatomic), the heat capacities and degrees of freedom differ. These are important when calculating heat capacities for real gases based on atomic structure.

• Monoatomic gases: Have 3 translational degrees of freedom, contributing to their heat capacity.
• Diatomic gases: Have translational, rotational, and vibrational degrees of freedom, which contribute to their heat capacities.
• Triatomic gases: Can be linear or non-linear, having additional rotational and vibrational degrees of freedom.

Units of Heat and Work L-atm

Thermodynamics uses various units for heat and work, including Joules, calories, and ergs. Here are some important unit conversions:

• 1 Joule (J) = 1 Newton meter (Nm) = 10⁷ erg
• 1 calorie (Cal) = 4.184 J = 4.2 J
• 1 L-atm = 101.3 J = 24.206 Cal

These conversions are essential when calculating the energy transfer in different units.

6.0Key Points of Thermodynamic

Cyclic Processes: 

In a cyclic process, the system returns to its initial state after completing a cycle. As a result, all state functions (such as internal energy, enthalpy, pressure, and temperature) return to their original values:

ΔE=0 ; ΔH=0 ; ΔP=0 ; ΔT=0

Reversibility and Irreversibility:

• All natural processes are irreversible.
• Reversible processes are idealized and quasi-static, meaning they occur infinitely slowly, ensuring that the system remains in equilibrium throughout.

Sign Conventions for Heat and Work:

• Heat and work are considered positive if they are added to the system (input).
• Heat and work are considered negative if they are removed from the system (output).

Free Expansion:

• In a vacuum (where external pressure is zero), the work done by the system during free expansion is zero, as there is no opposing force.

7.0Molar Heat Capacity

The molar heat capacity is the amount of heat required to raise the temperature of one mole of a substance by 1°C (or 1 K). Its units are J/mol°C. Molar heat capacity can vary depending on whether the process occurs at constant volume (Cv) or constant pressure (Cp). It provides insight into how different substances absorb heat and can be crucial in chemical engineering and physical chemistry.