Thermochemistry is the study of heat and energy changes during chemical reactions.
It primarily focuses on thermal changes (enthalpy) and entropy changes.
Organization of the Universe in Chemistry
In chemistry, the "universe" is defined as the system and the surroundings.
System: The part of the universe where the reaction occurs.
Surroundings: Everything around the system that may be influenced by it. The surroundings do not usually influence the system.
Example: Football Game
System: Players and the ball.
Surroundings: Stadium, fans, workers, etc.
The system (e.g., a player scoring) can influence the surroundings (e.g., cheering in the stands).
Example: Precipitation Reaction
Universe: Beaker or test tube containing the reaction.
Reaction: Ions coming together.
Surroundings: Water and the container.
First Law of Thermodynamics: Law of Conservation of Energy
Energy cannot be created nor destroyed during a chemical process.
Energy can change forms (e.g., chemical potential energy to thermal energy, light energy, or sound energy).
Thermal energy can initiate other chemical reactions.
Work done by a system (expansion, contraction, pushing, or pulling) is a form of energy exchange.
Second Law of Thermodynamics: Entropy
The entropy of the universe is always increasing.
The universe is considered one giant isolated system.
Red shift supports the idea of an expanding universe, aligning with entropy increasing as things spread out.
Entropy increases from solid to liquid to gas.
Reactions can have decreased entropy within the system (e.g., gas to liquid to solid), but the entropy of the surroundings increases, ensuring the overall entropy of the universe still increases.
Third Law of Thermodynamics: Zero Entropy
A perfect crystalline substance at zero Kelvin has zero entropy.
At absolute zero, all motion stops, and each atom has only one possible location.
Perfect crystals do not exist.
Diamonds and graphite are close to perfect crystals.
Examples of Crystalline Structures
Diamond: Tetrahedral carbon atoms connected in a rigid structure.
Graphite: Hexagons connected in a thin, rigid sheet; also has low entropy, but higher than diamond.
Solid Carbon Dioxide: Does not form a good crystalline structure and has higher entropy than diamond or graphite.
Gaseous Carbon Dioxide: Higher entropy compared to solid forms.
Zeroth Law of Thermodynamics: Thermal Equilibrium
If two systems are in thermodynamic equilibrium with a third system, the two original systems are in thermodynamic equilibrium with each other.
Transitive property in math: if A = B and A = C, then C = B.
Thermal equilibrium means the systems have the same temperature and disorder.
Analogy: The mole concept connects particles, volume, and mass in chemistry.