Welcome to Energy Changes!

Have you ever wondered why a campfire feels hot, or why some sports injury cold-packs get freezing cold just by squeezing them? In Chemistry, every single reaction involves a transfer of energy between the chemicals and their surroundings. In this chapter, we are going to explore why some reactions give out heat while others soak it up. This is a fundamental part of Topic 7: Rates of reaction and energy changes for your Edexcel GCSE.

1. The Big Idea: Conservation of Energy

Before we dive in, remember the golden rule: Energy cannot be created or destroyed. It can only be transferred. When we talk about "energy changes" in chemistry, we usually mean heat energy moving between the system (the chemicals) and the surroundings (the beaker, the air, the thermometer).

2. Exothermic Changes

The word "Exo" means "exit" (like an exit door). In an exothermic reaction, heat energy exits the chemicals and moves into the surroundings.

  • What happens? The temperature of the surroundings increases (it gets hotter).
  • The "Feel": If you touch the flask, it feels warm or hot.
Examples you need to know:

1. Combustion: Burning fuels is a classic exothermic reaction.
2. Neutralisation: When an acid reacts with an alkali.
3. Displacement: When a more reactive metal takes the place of a less reactive metal in a compound.
4. Precipitation: When two solutions react to form an insoluble solid.
5. Physical changes: Freezing and condensing (because particles lose energy to slow down).

3. Endothermic Changes

The word "Endo" means "inside" or "into". In an endothermic reaction, heat energy is taken into the chemicals from the surroundings.

  • What happens? The temperature of the surroundings decreases (it gets colder).
  • The "Feel": If you touch the flask, it feels cold.
Examples you need to know:

1. Dissolving certain salts: For example, dissolving ammonium nitrate in water.
2. Thermal decomposition: Breaking down a substance using heat (like heating calcium carbonate).
3. Physical changes: Melting and boiling (because particles need to take in energy to break free from each other).

Quick Tip: If the thermometer reading goes UP, it's EXO. If the thermometer reading goes DOWN, it's ENDO.

4. Bond Breaking and Bond Making

To understand why energy moves, we have to look at the chemical bonds. This is a favorite exam topic!

Chemical reactions happen in two steps:

  1. Breaking bonds: This requires energy. You have to "pull" the atoms apart. Therefore, bond breaking is endothermic.
  2. Making bonds: This releases energy. Atoms "snap" together and give off heat. Therefore, bond making is exothermic.
The Energy Balance:
  • If more energy is released when making new bonds than was used to break the old ones, the reaction is exothermic.
  • If more energy is used to break the old bonds than is released when making new ones, the reaction is endothermic.

5. Activation Energy

Even exothermic reactions (like lighting a match) often need a little "push" to get started. This minimum amount of energy needed for particles to react when they collide is called the activation energy (\(E_a\)). Think of it like a hill you have to climb before you can slide down the other side.

6. Reaction Profiles

A reaction profile is a graph that shows how the energy of the chemicals changes during a reaction. You must be able to recognize and label these!

Exothermic Reaction Profile

In an exothermic reaction, the products have less energy than the reactants because energy was given out.

  • The line starts high (reactants) and ends lower (products).
  • The overall energy change is the difference between the reactant level and the product level (it is a negative value).
  • The "hump" from the reactants to the peak of the curve is the activation energy.

Endothermic Reaction Profile

In an endothermic reaction, the products have more energy than the reactants because energy was taken in.

  • The line starts low (reactants) and ends higher (products).
  • The overall energy change is the difference between the reactant level and the product level (it is a positive value).
  • The activation energy is the distance from the reactant level all the way to the very top of the peak.

7. Higher Tier Only: Bond Energy Calculations

If you are taking the Higher Tier paper, you must be able to calculate the overall energy change using bond energies (measured in \( \text{kJ mol}^{-1} \)).

The Step-by-Step Method:
  1. Calculate "Energy In": Add up the energies of all the bonds in the reactants (this is the energy needed to break them).
  2. Calculate "Energy Out": Add up the energies of all the bonds in the products (this is the energy released when they form).
  3. Calculate Energy Change: Use the formula:
    \( \text{Energy Change} = \text{Total Energy In} - \text{Total Energy Out} \)
How to interpret the result:
  • If the answer is negative (e.g., \( -184 \text{ kJ mol}^{-1} \)), the reaction is exothermic.
  • If the answer is positive (e.g., \( +52 \text{ kJ mol}^{-1} \ )), the reaction is endothermic.

Example: If breaking bonds takes \) 1000 \text{ kJ} \) and making bonds releases \( 1200 \text{ kJ} \):
\( 1000 - 1200 = -200 \text{ kJ} \). Because it is negative, it is exothermic.

Summary Checklist

• Exothermic: Heat given out; temperature rises; \(\Delta H\) is negative; bonds made > bonds broken.
• Endothermic: Heat taken in; temperature drops; \(\Delta H\) is positive; bonds broken > bonds made.
• Activation Energy: The "startup" energy for a reaction.
• Reaction Profiles: Be ready to label Reactants, Products, Activation Energy, and Overall Energy Change.

Don't worry if the bond energy calculations seem tricky at first. Just remember: Break minus Make. If you can do simple subtraction, you can do bond energy!