First Law of Thermodynamics in Chemistry

The First Law of Thermodynamics is a universal principle that governs all processes involving energy exchange. It reinforces the concept that energy is conserved and that energy in one form (e.g., heat) can be converted into another form (e.g., work) without any loss or creation of energy. This foundational law is important for understanding engines, refrigerators, chemical reactions, and even biological processes.

1.0Explanation of  First Law of Thermodynamics

The First Law of Thermodynamics deals with the conservation of energy. According to it energy neither can be created nor destroyed; it can only be transformed from one form to another, which keeps total energy of an isolated system constant, although it can change forms (e.g., from heat to work).

In mathematical terms, the First Law is expressed as:

ΔU = Q + W

Where:

• ΔU is the change in the internal energy of the system.
• Q is the heat added to the system.
• W is the work done on the system.

2.0Concepts involved in Thermodynamics

1. Internal Energy (U):

• Internal energy is the total energy of a system, which includes the kinetic and potential energies of all the molecules or particles making up the system.
• Internal energy is a state function, meaning it depends only on the current state of the system, not how the system reached that state.

2. Heat (Q):

• Heat is a form of energy transfer, reason behind this, a temperature difference between the system and its surroundings. It flows from a hotter body to a cooler one.
• If heat is added to the system, Q is positive.
• If heat is removed from the system, Q is negative.

3. Work (W):

• Work is the energy transfer with a force acting over a distance. In thermodynamics, work is often done by the system on its surroundings or vice versa, such as when a gas expands or compresses.
• If work is done on the system (compression), W is positive.
• If work is done by the system (expansion), W is negative.

3.0Understanding the First Law in Different Processes

1. Isothermal Process (Constant Temperature):

• In an isothermal process, the temperature of the system stays constant, meaning the internal energy change (ΔU) is zero.
• The First Law becomes: Q = −W 

This means the heat added to the system is entirely converted into work done by the system.

2. Adiabatic Process (No Heat Exchange):

• In an adiabatic process, no heat is exchanged with the surroundings (Q = 0).
• The First Law becomes: ΔU = W 

Any change in internal energy is due to the work done on or by the system.

3. Isochoric Process (Constant Volume):

• In an isochoric process, the volume of the system stays constant, so no work is done (W = 0).
• The First Law simplifies to: ΔU = Q 

Any heat added to the system changes its internal energy.

4. Isobaric Process (Constant Pressure):

• In an isobaric process, the pressure remains constant. The work done by the system is: W = PΔV, where P represents the pressure and ΔV is the change in volume. In this case, both heat and work affect the internal energy.

4.0Solved examples

Example.1 Represent the following observations using proper thermodynamic notations for heat (q) and work (w):

(a) The system absorbs 20 Joules of heat. 

(b) The system does 40 Joules of work on its surroundings. 

(c) The surroundings perform 5 Joules of work on the system. 

(d) The system releases 50 Joules of heat.

Solution:

It is standard practice to represent heat and work (whether entering or leaving the system) by the symbols q for heat and w for work:

(a) q = +20 Joule (since heat is absorbed by the system) 

(b) w = −40 Joule (since work is done by the system) 

(c) w = +5 Joule (since work is done on the system) 

(d) q = −50 Joule (since the system releases heat)

5.0Applications of the First Law

1. Heating and Cooling Systems: The First Law explains how energy input as heat in heating systems or energy removed in cooling systems affects the internal energy and work performed by the system.
2. Engines and Refrigerators: Heat engines operate by converting heat into work, while refrigerators use work to transfer heat. The First Law governs the energy transformations in these devices, ensuring that energy input equals energy output.
3. Expansion and Compression of Gases: When gases expand, they do work on their surroundings, often cooling down in the process, as seen in adiabatic expansions. Conversely, compressed gases heat up, as observed when work is done on them.

6.0Conservation of Energy

The First Law emphasizes the conservation of energy. In any process, the total energy remains constant, but it can change forms. For example, when gasoline burns in an engine, chemical energy is converted into heat and mechanical work. While energy transforms, the total energy of the system and its surroundings remains constant.

7.0Limitations of the First Law

• Direction of Energy Flow: 

The First Law does not tell us the direction in which processes occur. For example, it does not explain why heat flows from hot objects to cold objects, which is addressed by the Second Law of Thermodynamics.

• No Distinction Between Work and Heat: 

The First Law treats work and heat as interchangeable, but in practice, heat and work have different implications in processes.

8.0Example of First Law in Action

First consider a gas in a cylinder with a movable piston. If the gas is heated, it expands, pushing the piston up. In this case:

• The heat energy added to the gas (Q) increases its internal energy (ΔU).
• Some of the internal energy is used to do work by pushing the piston upward (W).

Thus, the First Law balances the energy input (heat) with the change in internal energy and the work done by the gas.