Nearly all chemical reactions involve a change in energy. This energy can be absorbed from the surroundings or released into them. Understanding these energy changes is crucial for controlling reactions and harnessing their power.

Types of Energy

Kinetic Energy: The energy of motion. In chemistry, this relates to the movement of atoms and molecules. The faster they move, the higher the temperature.
Potential Energy: Stored energy. In chemical systems, this is often called Chemical Energy, which is the energy stored within the bonds of chemical compounds. Breaking bonds requires an energy input, while forming new, more stable bonds releases energy.

Enthalpy (ΔH)

Enthalpy is a measure of the total heat content of a system. The change in enthalpy (ΔH) during a reaction tells us how much heat is released or absorbed. It's the primary way we quantify energy changes in chemistry.

Exothermic vs. Endothermic Reactions

Reactions are classified based on whether they release or absorb energy.

Exothermic Reactions: An exothermic reaction is a chemical process that releases energy, often in the form of heat, light, or sound. In these reactions, the energy stored in the chemical bonds of the reactants is greater than the energy in the bonds of the products. As a result, the excess energy is discharged into the surroundings, causing the temperature of the surroundings to increase.
Key Idea: The change in enthalpy (ΔH) for an exothermic reaction is negative (ΔH < 0), indicating a net loss of energy from the system.
Example: The combustion of methane (natural gas): CH₄ + 2O₂ → CO₂ + 2H₂O + Energy.
Endothermic Reactions: An endothermic reaction is a process that absorbs energy from its surroundings, typically in the form of heat. For these reactions to occur, they must take in more energy to break the bonds of the reactants than is released when the bonds of the products are formed. This results in a net absorption of energy, causing the temperature of the surroundings to decrease.
Key Idea: The change in enthalpy (ΔH) for an endothermic reaction is positive (ΔH > 0), indicating a net gain of energy in the system.
Example: Photosynthesis, where plants absorb light energy to convert CO₂ and water into glucose: 6CO₂ + 6H₂O + Energy → C₆H₁₂O₆ + 6O₂.

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Lesson 4: Building Blocks of Matter

1. Classification of Matter

2. Properties of Matter

3. States of Matter

4. Physical Changes

5. Chemical Changes (Reactions)

6. Energy in Chemical Reactions

7. Physical and Chemical Separation