Energy and Chemical Reactions

Radiant Energy: Energy from light sources.
Thermal Energy: Heat; energy transferred between objects of different temperatures.
Solar Energy: Electromagnetic radiation from the sun.
Nuclear Energy: Energy released when atoms combine or split, involving a change in mass.
Kinetic Energy: Energy of motion (e.g., dancing, roller coaster).
Potential Energy:
- Energy of position (e.g., a lifted marker possessing potential energy due to gravity).
- Energy stored within chemical bonds (focus of the lecture).

Enthalpy (H) is the heat change during a chemical reaction.
Symbol: $H$
Units: Kilojoules per mole (kJ/mol).
Example: Combustion of octane has an enthalpy value of $-5470$ kJ/mol (negative because energy is released).

Entropy (S) is a measure of the disorder or randomness in a system, or the ability of particles to spread out.
Symbol: $S$
Entropy increases when a room becomes messy because it takes more energy to organize things than to let them spread out.
- Example: cleaning a room. Expending energy organizes the space. Over time, the room becomes messy again because it is easier for things to spread out.
Entropy increases as matter changes from solid to liquid to gas, since there are more places for the atoms to exist.

Energy is stored inside chemical bonds.
The size of atoms increases down the periodic table, leading to longer bond lengths and weaker bonds.
Halogen examples: Chlorine (Cl-Cl), Bromine (Br-Br), Iodine (I-I).
- Bond length increases from Cl-Cl to Br-Br to I-I.
- Bond strength decreases from Cl-Cl to Br-Br to I-I.
Breaking a bond requires energy input.
Forming a bond releases energy.
Reaction Diagram:
- Breaking a chlorine-chlorine bond requires energy input, increasing the potential energy.
- If chlorine atoms form a new bond, energy is released, decreasing potential energy.
- The difference in energy between breaking and forming bonds dictates the enthalpy of the reaction.