Introduction to Reaction Profiles

In Chemistry, reactions aren't just about substances changing; they are also about energy moving in or out. To visualize this, we use reaction profiles (sometimes called energy level diagrams). These graphs show us how the energy of the chemicals changes from the moment they start reacting until they become products.

Understanding these profiles helps us explain why some reactions happen instantly, why some need a spark to start, and why others don't happen at all! Don't worry if it seems like a lot of lines and arrows at first—we will break it down step-by-step.

1. Exothermic and Endothermic Changes

Before we look at the graphs, let's recap the two ways energy moves in a reaction:

Exothermic Reactions: These reactions release heat energy to the surroundings. Because the chemicals are "giving away" energy, the products end up with less energy than the reactants started with. Examples include neutralisation, displacement, and combustion.

Endothermic Reactions: These reactions absorb heat energy from the surroundings. Because the chemicals are "taking in" energy, the products end up with more energy than the reactants started with. Examples include thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate.

Quick Tip: Think of "Exo" as "Exit" (energy leaves) and "Endo" as "Entrance" (energy enters).

2. What is Activation Energy?

Have you ever wondered why a piece of paper doesn't just burst into flames on its own, even though the reaction with oxygen is exothermic? It’s because reactions need a "starting push."

Activation Energy (\(E_a\)) is the minimum amount of energy that particles must have when they collide in order to react.

The Hill Analogy: Imagine you are trying to push a heavy boulder over a hill to reach the valley on the other side. The "Activation Energy" is the height of that hill. If you don't push hard enough to get the boulder to the very top, it will just roll back down, and you won't reach the other side. In Chemistry, if the particles don't collide with enough energy to overcome this "energy barrier," they just bounce off each other and no reaction happens.

3. Drawing Reaction Profiles

On a reaction profile, we plot Relative Energy on the vertical axis (\(y\)-axis) and the Progress of Reaction on the horizontal axis (\(x\)-axis).

Exothermic Reaction Profiles

In an exothermic reaction, the energy of the products is lower than the energy of the reactants.

  • The Start: The line for reactants is higher up.
  • The Peak: The curve goes up first—this represents the activation energy being supplied.
  • The End: The line for products is lower than the reactants.
  • Overall Energy Change: The difference between the reactant level and the product level shows that energy was released to the surroundings.

Endothermic Reaction Profiles

In an endothermic reaction, the energy of the products is higher than the energy of the reactants.

  • The Start: The line for reactants is lower down.
  • The Peak: The curve goes very high up.
  • The End: The line for products is higher than the reactants.
  • Overall Energy Change: The difference between the reactant level and the product level shows that energy was absorbed.

4. How to Label a Reaction Profile Correctly

When you are asked to label these diagrams in an exam, there are three specific things you must get right:

1. The Activation Energy (\(E_a\)): This is an arrow pointing upwards from the reactant energy level to the highest point (the peak) of the curve.

2. The Overall Energy Change (\(\Delta H\)): This is the vertical difference between the reactants and the products.
For exothermic: The arrow points down from reactants to products.
For endothermic: The arrow points up from reactants to products.

3. The Reactants and Products: Always write the names or formulas of the substances on the horizontal lines.

Common Mistake to Avoid: Many students accidentally draw the activation energy arrow starting from the bottom of the graph. Always start the \(E_a\) arrow from the reactant line.

5. Summary Table

Use this table to quickly check the differences between the two profiles:

Feature: Energy level of products
Exothermic: Lower than reactants
Endothermic: Higher than reactants

Feature: Temperature of surroundings
Exothermic: Increases (gets hotter)
Endothermic: Decreases (gets colder)

Feature: Overall energy change arrow
Exothermic: Points downwards
Endothermic: Points upwards

Key Takeaways

  • Reaction profiles show the energy changes during a chemical reaction.
  • Activation energy (\(E_a\)) is the "energy barrier" that must be overcome for a reaction to start.
  • In exothermic profiles, the products are below the reactants because energy is released.
  • In endothermic profiles, the products are above the reactants because energy is absorbed.
  • The peak of the curve represents the transition state where bonds are breaking.

Note: For Higher Tier students, you will also learn how to calculate these energy changes using bond energies in a separate lesson. For now, focus on being able to identify, draw, and label these two types of graphs!