Exothermic and endothermic reactions

Welcome to Energy Changes!

Ever wondered why a campfire feels warm or why some sports injury cold packs get freezing cold the moment you squeeze them? This chapter is all about energy changes in Chemistry. You will learn how energy moves between a chemical reaction and its surroundings, and how we can use diagrams to see exactly what is happening to that energy.

1. Energy Transfer: Exothermic and Endothermic

In every chemical reaction, energy is conserved. This means it cannot be created or destroyed—it just moves from one place to another. We focus on the "system" (the chemicals) and the "surroundings" (everything else, like the beaker or the air).

Exothermic Reactions

An exothermic reaction is one that transfers energy to the surroundings. Because energy is leaving the chemicals and entering the surroundings, the temperature of the surroundings increases.

Real-world examples:
1. Combustion: Burning fuels like wood or gas.
2. Neutralisation: Mixing an acid and an alkali.
3. Oxidation: Such as the reaction used in hand warmers or self-heating cans.

Endothermic Reactions

An endothermic reaction is one that takes in energy from the surroundings. Because energy is being pulled into the chemical reaction, the temperature of the surroundings decreases.

Real-world examples:
1. Thermal decomposition: Heating calcium carbonate to break it down.
2. Citric acid and sodium hydrogencarbonate: Often found in "fizzing" reactions.
3. Sports injury packs: These get cold instantly to help with bruising.

Memory Aid: Simple Tricks

• EXothermic = Energy EXits (think of an Exit sign). It feels hot!
• ENdothermic = Energy goes IN (think of Entering). It feels cold!

Quick Review Box

• Exothermic: Temperature goes UP. Energy is released.
• Endothermic: Temperature goes DOWN. Energy is absorbed.

Takeaway: If it gets hot, it's exothermic. If it gets cold, it's endothermic.

2. Reaction Profiles

Chemists use graphs called reaction profiles to show how the energy of the chemicals changes during a reaction. Don't worry if these look like rollercoasters at first; they are quite simple once you know what the "hump" represents!

Activation Energy

Before any reaction can happen, the particles must collide with enough energy. The minimum amount of energy needed for particles to react is called the activation energy.

Analogy: Think of it like a "gatekeeper." If you don't have enough energy to get over the hill, the reaction won't start.

Drawing the Profiles

• Exothermic Profile: The products have less energy than the reactants because energy was released. The graph starts high and ends low.
• Endothermic Profile: The products have more energy than the reactants because energy was taken in. The graph starts low and ends high.

Common Mistake to Avoid

Students often forget to draw the curved line showing the activation energy. Always include the "hump" between the reactants and products! The activation energy is measured from the reactants to the peak of the curve.

Takeaway: Reaction profiles show the energy difference between reactants and products, plus the "activation energy" hill required to start.

3. Bond Breaking and Bond Making (Higher Tier Only)

During a chemical reaction, we have to break the bonds in the reactants and make new bonds in the products. This is where the energy change actually comes from.

The Two Steps

1. Breaking Bonds: This requires energy. It is an endothermic process. (Think of it like needing energy to pull two LEGO bricks apart).
2. Making Bonds: This releases energy. It is an exothermic process. (The LEGO bricks "click" together and release energy).

Calculating the Overall Energy Change

The overall energy change is the difference between the energy needed to break the bonds and the energy released when making them.

• In an exothermic reaction: The energy released from forming new bonds is greater than the energy needed to break existing bonds.
• In an endothermic reaction: The energy needed to break existing bonds is greater than the energy released from forming new bonds.

The Formula

You may be asked to calculate the total energy change using values provided in a table:
\( \text{Overall Energy Change} = \text{Energy in (breaking)} - \text{Energy out (making)} \)

Mnemonic: BENDO MEXO

• Bond Ebreaking is ENDOthermic.
• Making bonds is EXOthermic.

Takeaway: If more energy is released when making bonds than was used to break them, the reaction is exothermic.

4. Required Practical: Temperature Changes

You need to know how to investigate the variables that affect temperature changes in reacting solutions (like adding a metal to an acid).

Step-by-Step Process:

1. Place a polystyrene cup inside a glass beaker for stability and insulation.
2. Measure a specific volume of a reactant (e.g., Hydrochloric Acid).
3. Record the starting temperature with a thermometer.
4. Add the second reactant (e.g., Magnesium powder) and stir.
5. Record the highest (or lowest) temperature reached.
6. Calculate the temperature change.

Why use a polystyrene cup?

We want to measure the energy change of the reaction, not the energy used to heat up the room! Polystyrene is a good insulator, which means it reduces the amount of heat lost to the surroundings.

Did you know?

Adding a lid to your cup makes the experiment even more accurate because it prevents heat loss through evaporation!

Quick Review Box

• Independent Variable: The thing you change (e.g., the volume of a reactant).
• Dependent Variable: The thing you measure (the temperature change).
• Control Variables: Things you keep the same (e.g., concentration of acid, insulation).

Takeaway: Accurate results in this practical depend on good insulation to minimize heat transfer to the air.