Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → General chemistry → Thermodynamics ↓


Thermodynamic cycles (Hess's law, Carnot cycle)


Thermodynamics is a branch of physics that deals with heat and temperature and their relation to energy and work. In the field of chemistry, especially general chemistry at the undergraduate level, two important concepts are Hess's Law and the Carnot cycle. These concepts help in understanding how energy is conserved and transferred in chemical reactions and mechanical processes.

Hess's law

Hess's law is a powerful principle that states that the total enthalpy change in a chemical reaction is the same whether the reaction occurs in a single step or in multiple steps. In simple terms, the enthalpy change of a reaction is independent of the pathway, provided that the initial and final conditions are the same. This principle is useful for calculating enthalpy changes that are difficult to measure directly.

Understanding Hess's law through an example

Consider a reaction in which carbon reacts with oxygen to form carbon dioxide:

C(s) + O 2 (g) → CO 2 (g)

This reaction can be divided into two steps:

  1. Carbon reacts with half a mole of oxygen to form carbon monoxide:
    2. C(s) + 1/2 O 2 (g) → CO(g) ΔH 1
  2. Carbon monoxide reacts with half a mole of oxygen to form carbon dioxide:
    3. CO(g) + 1/2 O 2 (g) → CO 2 (g) ΔH 2

According to Hess's law, the overall enthalpy change for a direct reaction is equal to the sum of the enthalpy changes of the two steps:

ΔH = ΔH 1 + ΔH 2

Practical application of Hess's law

In practice, chemists often use Hess's law to determine enthalpy changes of reactions that are challenging to measure directly. For example, consider the enthalpy change for the reaction of graphite with oxygen to form carbon dioxide. Direct measurement is difficult, but using Hess's law, the reaction can be estimated from the enthalpy of combustion of graphite and carbon monoxide.

Carnot cycle

The Carnot cycle is a theoretical cycle that represents the most efficient possible heat engine. Named after Sadi Carnot, this cycle focuses on converting heat energy into work and sets a benchmark for engine efficiency. The Carnot cycle is composed of four reversible steps: two isothermal processes and two adiabatic processes.

Four steps of the Carnot cycle

Let's explore each step with visual examples:

1. Isothermal expansion

In the first stage, the gas is allowed to expand isothermally at a higher temperature. During this expansion, the gas absorbs heat, keeping the temperature constant. The substance does work while expanding.

2. Adiabatic expansion

The gas continues to expand, but this time without heat exchange, called adiabatic expansion. The gas does work on the surroundings, causing the temperature to drop.

3. Isothermal compression

Subsequently, the gas undergoes isothermal compression at a lower temperature, the medium gives off heat to the surrounding environment. The internal energy of the system decreases and the substance cools down.

4. Adiabatic compression

Finally, the gas is compressed adiabatically. No heat is transferred during this process, so that the temperature of the gas increases as work is done on it, making it ready for another cycle.

Efficiency of the Carnot cycle

The efficiency of a Carnot engine is determined by the temperatures between which it operates. It can be expressed as:

Efficiency = 1 - (T c /T h )

where T c is the temperature of the cold reservoir, and T h is the temperature of the hot reservoir, both in Kelvin.

Importance in real-world applications

The Carnot cycle provides a model for understanding the limits of efficiency in real-world engines. Although no real engine can achieve Carnot efficiency due to irreversibility and friction, concepts derived from this theoretical cycle influence the design and improvement of real engines.

Conclusion

In summary, both Hess's law and the Carnot cycle are fundamental concepts in thermodynamics that contribute to our understanding of energy transformations and efficiency in chemical and mechanical systems. Hess's law allows chemists to calculate enthalpy changes indirectly, while the Carnot cycle sets the standard for the maximum theoretical efficiency of heat engines. Together, these principles not only deepen our understanding of thermodynamic systems but also help advance technological advances in energy conversion and chemical engineering.


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Thermodynamic cycles (Hess's law, Carnot cycle)

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