Laws of Thermodynamics
The thermodynamic laws can seem simple, but they have many applications across chemistry.
The first law basically states that the total energy in the universe will always remain constant, no matter what. It states that, with heat recognized as energy, the total energy of a system and its surroundings remains constant.
Picture a cylindrical chamber with a movable piston enclosing it. The gas inside of the chamber will do work as it expands the piston and its room inside the chamber. While doing so, the gas will absorb energy from outside the chamber via the walls of the cylinder. The internal energy of the gas must increase due to the lost energy from the surroundings. The energy change can be found with this equation:
ΔU = Q - W
where U is energy, Q is heat, and W is work
The 2nd law of thermodynamics says that entropy will always increase over time. In other words, heat can only flow from a hotter system to a colder one. To fully understand the 2nd law of thermodynamics, you have to understand the impossibility of perpetual motion.
In theory, a perpetual motion machine is a machine that manufactures its own energy and thereby runs forever. These obviously cannot exist. Even though such a machine is impossible, we can create machines that extract seemingly infinite amounts of energy from a nearby reservoir, such as extracting energy from an ocean or wind. These machines do not violate conservation of energy, which is how the 2nd law was discovered.
The 2nd law was mathematically tested using heat flow into a heat reservoir. The equation was found that:
ΔS = Q/T
where S is entropy (a concept that basically shows possible disorder), Q is heat, and T is temperature.
When heat flows from one reservoir to another, the equation found is:
ΔS = Q/T₁ - Q/T₂
where T₁ > T₂. ΔS is always positive, unless the two temperatures are the same, where ΔS = 0.
The 2nd law of thermodynamics creates the theory known as heat death. If the universe is an isolated system, where there is nothing outside of it, then all systems will reach maximum entropy, leading to a uniform temperature across all things. This is one of the theorized ends of the universe.
The third law of thermodynamics states that the entropy of any closed system will approach a constant value as temperature approaches absolute zero. Crystalline structures that are 100% pure will have zero entropy at absolute zero. This means that all the particles making up the crystal are in their lowest energy state, and there is only one possible formation of particles to put the crystal into this state.
The final law, known as the zeroth law, is similar to the transitive property in mathematics. It states that if each of two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.