Energy changes are very important when we group chemical reactions. We usually classify these reactions based on how energy moves in and out during the process. Here are the main types of reactions affected by energy changes:

1. Exothermic Reactions:

   • These reactions give off energy, usually as heat, to the surrounding area.
   • A common example is when hydrocarbons burn. The energy released can be measured.
   • For example, burning methane can release about -890 kJ/mol of energy.

2. Endothermic Reactions:

   • In contrast, endothermic reactions take in energy from their surroundings.
   • Photosynthesis is a great example of this. Plants absorb energy from sunlight to turn carbon dioxide and water into glucose and oxygen. This process needs about 2800 kJ/mol of energy.

3. Energy Profile Diagrams:

   • We can visualize energy changes using energy profile diagrams. These diagrams show the energy levels of the starting materials (reactants), the final products, and a stage in between called the transition state.
   • In exothermic reactions, the end products have less energy than the starting materials. In endothermic reactions, the products have more energy than the starting materials.

4. Activation Energy:

   • The energy needed to start a chemical reaction is called activation energy. This energy level can be different for each reaction.
   • The activation energy can greatly affect how fast a reaction happens and how it takes place.

In summary, understanding energy changes helps chemists organize reactions, predict how likely a reaction is to happen, and find the best conditions to get the results they want.