How to Calculate Reaction Enthalpy with Simple Animations

Enthalpy, represented by the letter H, is a measure of the total energy content in a chemical system at constant pressure.This energy includes both the internal energy of the molecules and the energy from pressure and volume.To understand enthalpy changes, let's look at an energy diagram. The vertical axis shows enthalpy, while the horizontal axis shows the progress of the reaction.As a reaction proceeds from reactants to products, the enthalpy changes. The difference between the final and initial enthalpy is called delta H.Delta H is calculated by subtracting the initial enthalpy from the final enthalpy. In this case, the negative delta H indicates an exothermic reaction.In an exothermic reaction, energy is released from the reactants to the surroundings.In contrast, an endothermic reaction absorbs energy from the surroundings.Remember, a negative delta H means the reaction releases energy, while a positive delta H means the reaction absorbs energy.Bond energy represents the energy required to break or form a chemical bond.Let's examine the reaction between hydrogen and chlorine to form hydrogen chloride.First, we need to break the bonds in the reactant molecules. This requires energy.Let's look at the specific bond energies involved in this reaction.To break the H-H and Cl-Cl bonds, we need to input a total of 678 kilojoules per mole of energy.When we form two new H-Cl bonds, each bond releases 431 kilojoules per mole, for a total of 862 kilojoules per mole.The net energy change is negative 184 kilojoules per mole, indicating this is an exothermic reaction.Now that we understand how to calculate energy changes using bond energies, let's move on to Hess's Law.Hess's Law tells us that the total enthalpy change is independent of the pathway taken.Let's look at the formation of carbon dioxide from its elements.Instead of this direct path, we can break the reaction into two steps.First, carbon reacts with half a mole of oxygen to form carbon monoxide.Then, carbon monoxide reacts with the remaining oxygen to form carbon dioxide.According to Hess's Law, we can add the enthalpy changes of these steps to find the total enthalpy change.Negative one hundred ten point five plus negative two hundred eighty three point zero kilojoules.Gives us a total enthalpy change of negative three hundred ninety three point five kilojoules.Let's review the key points about Hess's Law.The total enthalpy change is independent of the pathway taken.We can break complex reactions into simpler steps.And the sum of the step enthalpy changes equals the overall enthalpy change.Thanks for learning about Hess's Law with Spark.E!