Exothermic and endothermic reactions

Introduction to heat in reactions

In Chemistry, heat energy is always moving. When you strike a match or use a gas cooker to boil water, heat is involved. Every chemical reaction either releases heat to the environment or takes heat away from it.

This movement of heat is what we call Enthalpy Change. We use the symbol delta H to represent this change. Depending on whether heat is going out or coming in, we classify reactions into two types: Exothermic and Endothermic.

Exothermic reactions

Think of a generator running for hours. If you touch it, it feels very hot. This is because the fuel is burning and releasing heat. In Chemistry, reactions that give off heat to the surroundings are called exothermic reactions.

Simply put, heat leaves the chemicals and enters the air, making everything around it feel hot.

Common examples include the burning of kerosene, the reaction between an acid and a base (neutralization), and the way your body breaks down food during respiration. In these reactions, the heat change delta H is always negative because the system is losing energy.

Endothermic reactions

Have you ever noticed how a glucose powder feels cold on your tongue? Or how a rehydration salt (ORS) feels cool when mixed with water? These are endothermic processes. They 'suck' heat from their surroundings.

In simple terms, these reactions take in heat from the environment, making the container feel cold to your hand.

Examples include dissolving ammonium chloride in water and the process of photosynthesis where plants take in light energy. Because these systems are gaining energy, their heat change delta H is always positive.

Worked example of heat calculation

Let us look at a typical JAMB question involving heat change. If a reaction releases 500 Joules of heat, we say the enthalpy change is -500 J. The minus sign tells us it is exothermic.

Question: Calculate the heat change when 2 moles of a substance react, given that the heat of reaction is -250 kJ mol^-1.

1. Identify the molar heat: -250 kJ for every 1 mole.

2. Identify the number of moles: 2 moles.

3. Multiply them: Heat change = 2 moles x (-250 kJ/mole).

4. Final answer: -500 kJ. This is an exothermic reaction because the value is negative.

Summary of signs

If heat is released: The reaction is Exothermic and delta H is negative (-).

If heat is absorbed: The reaction is Endothermic and delta H is positive (+).