Enthalpy Changes and Exothermic Reactions

Study Notes

Enthalpy Changes

Enthalpy is the total chemical energy within a substance also known as heat content
Chemical reactions involve changes in chemical energy resulting in enthalpy changes.
Enthalpy change is represented by ΔH where Δ denotes change and H denotes enthalpy.
Enthalpy change can be positive or negative.

Exothermic Reactions

Exothermic reactions release heat energy into the surroundings when products have less energy than reactants.
This results in an increase in environmental temperature which can be measured by a thermometer.
The system's temperature decreases.
ΔH is negative as there is an enthalpy decrease during the reaction.
Exothermic reactions are thermodynamically feasible because reactants have higher enthalpy than products.
Although being thermodynamically feasible, the rate of the reaction may be too slow for observation.
Such reactions are kinetically controlled as they might have a high activation energy hindering the reaction.

Physical Chemistry

Branch of chemistry focused on the physical structure of chemicals, forces between particles, and energy changes in reactions.
Key Concepts:
- Thermodynamics: Study of energy, heat, and work in chemical processes.
  - Laws of Thermodynamics:
    - Zeroth Law: Two systems in thermal equilibrium with a third system are also in equilibrium with each other (defining temperature).
    - First Law: Energy cannot be created or destroyed, only transformed (conservation of energy).
    - Second Law: Entropy (disorder) of an isolated system always increases over time (spontaneous reactions increase entropy).
    - Third Law: Entropy of a perfect crystal at absolute zero (0 kelvin) is zero.
- Kinetics: Studies reaction rates and factors affecting them.
  - Rate Laws: Equations that describe the relationship between the rate of reaction and the concentration of reactants.
  - Activation Energy: The minimum energy required for a reaction to occur.
  - Catalysts: Substances that increase reaction rates without being consumed in the reaction.
- Equilibrium: A state where the rates of forward and reverse reactions are equal.
  - Le Chatelier's Principle: A system at equilibrium will adjust to relieve stress (change in concentration, temperature, or pressure).
  - Equilibrium Constant (K): A ratio of products to reactants at equilibrium, indicating the relative amount of each present.

Energetics

Study of energy changes in chemical reactions, including heat transfer and thermodynamic properties.
Types of Energies
- Internal Energy (U): Total energy of a system (kinetic and potential energy).
- Enthalpy (H): Heat content of a system at constant pressure.
  - ΔH: Change in enthalpy; positive for endothermic reactions (heat absorbed), negative for exothermic reactions (heat released).
- Gibbs Free Energy (G): Determines the spontaneity of a process (negative ΔG = spontaneous).
  - ΔG = ΔH - TΔS (where T = temperature, ΔS = change in entropy)
- Entropy (S): Degree of disorder or randomness in a system (higher entropy = more disorder).
Heat Transfer
- Endothermic Reactions: Absorb heat from the surroundings, increasing the system's internal energy (ΔH > 0).
- Exothermic Reactions: Release heat to the surroundings, decreasing the system's internal energy (ΔH < 0).
- Calorimetry: Measuring heat changes in chemical or physical processes.
  - Types: Coffee cup calorimeter (constant pressure) and bomb calorimeter (constant volume).

Description

This quiz covers the concepts of enthalpy changes in chemical reactions, focusing on exothermic reactions. You will explore the definitions, characteristics, and implications of enthalpy changes, including the thermodynamic and kinetic aspects influencing these reactions.