5.2. Calculating Enthalpy Change

The enthalpy change (ΔH) of a reaction can be calculated using the following formula:

ΔH = Total Energy of Products − Total Energy of Reactants

To calculate the change in enthalpy (ΔH) for a chemical reaction, we use a simple formula. This formula compares the total energy of the products of the reaction with the total energy of the reactants. If you subtract the total energy of the reactants from that of the products, you will find out how much energy was absorbed or released during the reaction:

1. Identify the Total Energy of the Products: Look at the products formed after the reaction.
2. Identify the Total Energy of the Reactants: Look at the energy of the substances that react.
3. Calculate ΔH: Use the formula ΔH = Total Energy of Products - Total Energy of Reactants. If the result is positive, the reaction absorbed energy (endothermic). If it's negative, the reaction released energy (exothermic).

Think of ΔH like changes in your bank account. If you deposit more money (energy) than what you start with, your account balance goes up (endothermic reaction). Conversely, if you withdraw more money than you deposit, your balance decreases (exothermic reaction). Just as you can see how your balance changes with each transaction, you can track how the energy changes during a chemical reaction.

If the products have less energy than the reactants, the reaction is exothermic.

In an exothermic reaction, we see that the energy of the products is lower than that of the reactants. This indicates that during the reaction, energy is released into the surrounding environment. This release of energy is often felt as an increase in temperature or can be observed in the form of light or sound.

• Key Point: A decrease in energy (products < reactants) gives a negative ΔH, showing that energy is released (e.g., burning wood).

Consider a campfire. When you burn the wood (reactant), it releases heat and light, making it warm and bright. That heat is energy being released into the atmosphere. Just like in a chemical reaction when energy is released, the products (like ash and smoke) have less energy than the original wood.

If the products have more energy than the reactants, the reaction is endothermic.

In an endothermic reaction, the energy of the products is higher than that of the reactants. This means that the reaction absorbs energy from its surroundings. The result is a decrease in the temperature of the environment, as energy is pulled in to accommodate the formation of the products.

• Key Point: An increase in energy (products > reactants) gives a positive ΔH, indicating that energy is absorbed (e.g., the process of photosynthesis).

Imagine using an ice pack that you place on an injury. The chemicals inside absorb heat from your skin, making the pack feel cold. This is similar to an endothermic reaction where the products absorb energy (heat) from their surroundings, resulting in their energy being greater than that of the reactants.

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