Question

6.54 Does a negative \(\Delta H\) mean that the heat should be treated as a reactant or as a product?

Short answer

Heat should be treated as a product.

Step-by-step solution

Understand \( \Delta H \)

The symbol \( \Delta H \) represents the change in enthalpy of a system during a reaction.

Identify the sign of \ \Delta H \)

A negative \( \Delta H \) indicates that the reaction releases heat. This type of reaction is called exothermic.

Determine the role of heat

In an exothermic reaction, heat is released, meaning it is produced in the reaction. Therefore, heat should be treated as a product.

Key concepts

Exothermic Reactions

When we talk about chemical reactions, they often involve the transfer of energy. If a reaction releases energy in the form of heat, it is called an exothermic reaction.

Energy release can make the surroundings warmer. This happens because energy stored in the reactants is turned into heat, which then exits the system.

Simplified, exothermic means 'giving out heat'. Typical examples include burning wood and the reaction between acids and bases.

Negative Delta H

The term \( \Delta H \), or delta H, represents the change in enthalpy during a reaction. Enthalpy is a fancy word for the total energy in a system, including heat.

When \( \Delta H \) is negative, it tells us that the system is releasing energy. This is a key indicator of an exothermic reaction.

In simpler terms:

• Negative \( \Delta H \) = Exothermic reaction
• Energy leaves the system

Think of it as losing weight; the system 'loses' energy.

Heat as a Product

In the context of an exothermic reaction, heat is 'produced'. This means it should be on the product side of the equation.

For example, consider the combustion of methane:

• CH_4 + 2O_2 → CO_2 + 2H_2O + \text{Heat}
• Notice 'heat' as a product

Whenever you see a negative \( \Delta H \), always remember heat is a product.

This shows us that the reaction provides energy to the surroundings.