PHD ↓
Physical Chemistry
Physical chemistry is the branch of chemistry that studies how matter behaves at the molecular and atomic level and how chemical reactions occur. By understanding these principles, physical chemists can develop new theories and models that explain the properties of gases, liquids, and solids and how substances behave under different conditions.
Thermodynamics in physical chemistry
One of the cornerstones of physical chemistry is thermodynamics, which is the study of energy transfer and transformation. Thermodynamics deals with concepts such as temperature, energy, heat, work, entropy, and the laws governing the transfer of energy.
Consider the following example of the first law of thermodynamics, also called the law of conservation of energy: In any chemical reaction or physical process, the total energy of the system and its surroundings is conserved.
ΔU = Q - W
Here, ΔU
represents the change in the internal energy of the system, Q
is the energy added to the system as heat, and W
is the work done by the system. This equation shows how energy flows through a system and is fundamental to understanding physical processes.
The diagram above may represent a simplified version of a system. You can imagine that within the circle the system is interacting with its surroundings, exchanging energy in the form of heat and work.
Entropy and the second law of thermodynamics
Entropy is a measure of the disorder or randomness of a system. The second law of thermodynamics states that in any natural thermodynamic process, the total entropy of a system and its surroundings always increases. This is important because it predicts the direction of processes and the feasibility of reactions.
An example of this is that heat automatically flows from a hotter object to a colder object, rather than the other way around.
ΔS = Q_rev / T
In the above formula, ΔS
represents the change in entropy, Q_rev
is the reversible heat exchange, and T
is the absolute temperature. Entropy calculations are necessary to determine reaction spontaneity.
In the given diagram, you can see heat flow from a hotter region to a colder region. This is an example of how entropy increases, which shows the second law of thermodynamics.
Quantum chemistry
Quantum chemistry is a branch of chemistry that focuses on the application of quantum mechanics to chemical systems. It helps to understand how atoms and molecules interact and bond, how chemical reactions occur, and what electronic structures are signed by atoms and molecules.
In quantum chemistry, the Schrödinger equation is often used to describe how the quantum state of a physical system changes over time.
Ĥψ = Eψ
Here, Ĥ
is the Hamiltonian operator, ψ
is the wave function, and E
is the energy of the system. This equation allows chemists to estimate the probability of finding an electron at a particular location inside an atom.
The figure above shows a simplified model of the hydrogen atom: the nucleus at the center and the possible region where the electron can exist.
Kinetics and reaction dynamics
Chemical kinetics investigates the rates at which chemical reactions occur and the factors that affect these rates. Reaction rates are affected by the concentration of reactants, temperature, and the presence of catalysts.
A fundamental equation in kinetics is the rate law, which expresses the reaction rate with respect to the concentration of the reactants:
Rate = k[A]^m[B]^n
In this equation, Rate
is the reaction rate, k
is the rate constant, and [A]
and [B]
are the concentrations of the reactants, with m
and n
being their respective orders. Studying kinetics helps to understand how quickly a reaction proceeds and what steps follow.
This graph can display a progress line showing a reactant continually being converted into products over time.
Equilibrium and chemical potential
Chemical equilibrium occurs when a chemical reaction is reversible, and the rate of the forward reaction is equal to the rate of the reverse reaction. The concept of equilibrium is closely related to the idea of chemical potential, which determines the direction of material and thermal quantity transport.
Understanding equilibrium involves expressions such as the equilibrium constant K
, which is defined for the reaction aA + bB ⇌ cC + dD
as:
K = ([C]^c[D]^d)/([A]^a[B]^b)
At equilibrium the concentration remains constant, and K
provides insight into the equilibrium state of a reaction.
This line diagram shows the state of equilibrium between reactants and products, where the orange and blue lines meet, representing the equilibrium point.
Statistical mechanics
Statistical mechanics serves as a bridge between the macroscopic and microscopic properties of matter. It uses statistics to model systems composed of large numbers of particles, allowing scientists to predict thermodynamic and thermochemical behavior based on molecular and atomic perspectives.
An example of the use of statistical mechanics is the Boltzmann distribution, which describes the distribution of energy states in a system:
P_i = (g_i e^(-E_i/kT))/Z
In this formula, P_i
is the probability of state i
, g_i
is the decay, E_i
is the energy of state i
, k
is the Boltzmann constant, T
is the temperature, and Z
is the partition function.
This illustration shows the different energy states of a system, where each circle represents a different state the particle can be in.
Applications of physical chemistry
Physical chemistry provides powerful tools and concepts that are essential for developing modern technology and solving scientific challenges. It is important in fields such as materials science, pharmaceuticals, environmental science, and nanotechnology.
Together with materials science, the principles of physical chemistry enable the development and understanding of novel materials such as superconductors, polymers, and advanced ceramics with unique physical properties.
In pharmaceuticals, kinetics and thermodynamics help understand drug delivery mechanisms by assessing reaction rates and energy changes within biological systems.
Environmental chemistry uses the principles of physical chemistry to analyze and innovate solutions to pollution, climate change and sustainable development. An understanding of particulate matter, the chemical properties of pollutants and their interaction with the environment is crucial.
Nanotechnology is another field where physical chemistry is essential. The manipulation and use of atoms in nanomaterials and devices is made possible through quantum chemistry and nanoscale kinetic analysis.