The enthalpy of an exothermic reaction
Is always negative
Understanding the Enthalpy of an Exothermic Reaction
Let's analyse the question about the enthalpy of an exothermic reaction. Enthalpy is a thermodynamic property that is the sum of the internal energy of a system plus the product of its pressure and volume. In chemistry, we often focus on the change in enthalpy ($\Delta H$) during a process, especially a chemical reaction. This enthalpy change tells us whether a reaction absorbs or releases energy.
What is an Exothermic Reaction?
An exothermic reaction is a chemical reaction that releases energy into its surroundings, usually in the form of heat. This means the system (the reactants) loses energy, and the surroundings gain energy. Think of burning fuel or the reaction in a hand warmer – these are common examples of exothermic reactions.
Enthalpy Change ($\Delta H$) and Exothermic Reactions
The enthalpy change ($\Delta H$) for a reaction is defined as the enthalpy of the products minus the enthalpy of the reactants:
$$\Delta H = H_{products} - H_{reactants}$$
In an exothermic reaction, energy is released. This release of energy means that the total enthalpy of the products is lower than the total enthalpy of the reactants. Since $H_{products}$ is less than $H_{reactants}$, the difference ($H_{products} - H_{reactants}$) will be a negative value.
Therefore, the enthalpy change ($\Delta H$) for an exothermic reaction is always negative.
Here's a summary of the relationship between enthalpy change and reaction types:
| Reaction Type | Energy Flow | Relationship of Enthalpy | Enthalpy Change ($\Delta H$) |
|---|---|---|---|
| Exothermic | Energy released to surroundings | $H_{products} < H_{reactants}$ | Negative ($\Delta H < 0$) |
| Endothermic | Energy absorbed from surroundings | $H_{products} > H_{reactants}$ | Positive ($\Delta H > 0$) |
Based on this, for an exothermic reaction, the enthalpy of the system decreases, leading to a negative enthalpy change.
Conclusion on Exothermic Enthalpy
The question asks about the enthalpy of an exothermic reaction. Specifically, it is asking about the change in enthalpy that occurs during the reaction. As established, the enthalpy change ($\Delta H$) for an exothermic reaction is always negative because the products have lower enthalpy than the reactants, and energy is released.
Options analysis:
- Option 1: Is always Positive - Incorrect, this describes endothermic reactions.
- Option 2: Is always negative - Correct, as explained above.
- Option 3: Is always zero - Incorrect, this would mean no energy change, which is not the case for exothermic reactions.
- Option 4: May be positive or negative - Incorrect, for a specific type of reaction (exothermic), the enthalpy change has a consistent sign.
Thus, the enthalpy change for an exothermic reaction is always negative. The concept of enthalpy is fundamental to understanding energy transformations in chemical processes.
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