Question

State Hess’s law. How is it used?

Short answer

Hess’s law states that the total enthalpy change is the same regardless of the reaction steps. It is used to find the reaction enthalpy by summing the enthalpy changes of individual steps.

Step-by-step solution

State Hess’s Law

Hess's law states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in, provided the initial and final conditions are the same.

Understanding Enthalpy Changes

Enthalpy change (ΔH) represents the heat absorbed or released during a chemical reaction at constant pressure. Hess's law allows for the determination of ΔH for reactions where it is difficult to measure directly by using known enthalpy changes of other reactions.

Use of Hess’s Law

Hess's law is used by breaking down a complex reaction into a series of simpler steps for which enthalpy changes are known. These steps are then added algebraically to find the total enthalpy change of the original reaction.

Applying Hess’s Law in Practice

To apply Hess’s law, write each step required to transform reactants into products. Ensure that all intermediates cancel out. Sum the enthalpy changes for each step to find the enthalpy change for the overall reaction.

Key concepts

Enthalpy Change

Enthalpy change (\( \triangle H \)) represents the total amount of heat absorbed or released during a chemical reaction at constant pressure. It essentially tells you how much heat energy is involved in the process.
When a reaction absorbs heat from its surroundings, it is called an endothermic reaction (\( \triangle H > 0 \)).
Conversely, when a reaction releases heat, it’s known as an exothermic reaction (\( \triangle H < 0 \)).
Understanding enthalpy change is crucial because it allows us to predict whether a reaction will require heat input or generate heat. It helps explain why certain reactions occur naturally and how we can control them in industrial or laboratory settings.

Heat Absorbed or Released

Heat absorbed or released during a reaction is directly linked to the enthalpy change.
If you've ever mixed vinegar with baking soda and felt the cool sensation, you experienced an endothermic reaction where heat was absorbed.
Boiling water on a stove, where water absorbs heat and turns into steam, is another example of heat absorption.
On the other hand, if you've felt the warmth from a hand warmer, that's an exothermic reaction releasing heat.
Knowing whether a reaction absorbs or releases heat can influence how we use these reactions practically.

• Endothermic Reactors: Often need a continuous external heat source.
• Exothermic Reactors: Can be self-sustaining once initiated.

By understanding this concept, we can harness these reactions effectively.

Chemical Reaction

A chemical reaction is a process where reactants are transformed into products.
This transformation involves breaking old bonds and forming new ones, which often involves an energy change in the form of heat.
Reactions can be simple or complex. Hess's Law is particularly useful for studying complex reactions.
By breaking them down into simpler steps with known enthalpy changes, we can determine their overall enthalpy change.
Imagine baking a cake: you may not know the total calories (enthalpy) of the final cake, but if you know the calories of the ingredients and their interactions, you can calculate it.

• Determine the initial state (reactants).
• Identify the final state (products).
• Break down into simpler steps.
• Sum the enthalpy changes of these steps.

Using these principles helps simplify and solve complex reaction problems.