# Why does a negative ΔH indicate an exothermic reaction?

• Chemistry
• cookiemnstr510510
In summary, the conversation discusses the concept of heat of reaction and how it relates to the exothermic nature of a reaction. It is explained that the negative value of ΔH indicates an exothermic reaction, as it represents the amount of heat that needs to be removed from the system to maintain a constant temperature. The reasoning behind this is also clarified, with an analogy to the convention of defining the charge of an electron as negative.
cookiemnstr510510
Homework Statement
In your own words, explain why Delta_H^0 is negative for exothermic reactions and positive for endothermic reactions.
Relevant Equations
Potentially equilibrium equations
Hello All,
I wanted some insight on my answer to this problem.
Lets say we have the reaction PCl3(g) + Cl2(g) ↔PCl5(g) ΔH0=-111KJ

So for this reaction we know it is exothermic (because my textbook told me). But I want to make sure I understand why it is. If I were to look at this reaction and try to find the ΔH I would do: ΔHproducts-ΔHreactants.
If this answer is negative then this means that our reactants initially had more energy than our products. And this makes sense with the above reaction being exothermic. Exothermic, to give off heat, means that the reactants had energy to give off, and the ΔH0 is negative is because the reactants had more energy than the products?

I am just a bit confused with how a negative ΔH means exothermic, unless I am correct above.

Thanks so much!
Happy Sunday

What the heat of reaction means is that, in a process that starts with pure reactants in separate containers at 1 bar and ends up with pure products in separate containers in separate containers at 1 bar, ##\Delta H^0## is equal to the amount of heat Q you must add to the system to hold the temperature constant at 298K. If the reaction is exothermic, it means that Q is negative, meaning that you need to remove heat to hold the temperature unchanged from the initial value of 298 K.

cookiemnstr510510 said:
So for this reaction we know it is exothermic (because my textbook told me). But I want to make sure I understand why it is. If I were to look at this reaction and try to find the ΔH I would do: ΔHproducts-ΔHreactants.
If this answer is negative then this means that our reactants initially had more energy than our products. And this makes sense with the above reaction being exothermic. Exothermic, to give off heat, means that the reactants had energy to give off, and the ΔH0 is negative is because the reactants had more energy than the products?

I am just a bit confused with how a negative ΔH means exothermic, unless I am correct above.
You are correct. Its negative because of the definition ΔH = ΔHproducts-ΔHreactants. If we chose it reactants - products, and if everyone agreed to that, it would be positive for exothermic reactions.

Kind of the same thing for the charge of an electron. We chose the charge to be negative but could have just as easily chosen it to be positive. Everyone agrees to call it “something” and that’s what it is.

jim mcnamara

## 1. What is the heat of formation?

The heat of formation, also known as the enthalpy of formation, is the change in enthalpy that occurs when one mole of a substance is formed from its constituent elements in their standard states at a given temperature and pressure.

## 2. How is the heat of formation measured?

The heat of formation is typically measured using calorimetry, where the heat released or absorbed during a chemical reaction is measured. The heat of formation can also be calculated using Hess's Law, which states that the overall enthalpy change of a reaction is equal to the sum of the enthalpy changes of its individual steps.

## 3. What is the significance of the heat of formation?

The heat of formation is an important thermodynamic property that helps us understand the stability and reactivity of a substance. It is also used in various industries, such as in the production of fuels and in the design of chemical processes.

## 4. How does the heat of formation relate to bond energies?

The heat of formation is related to bond energies in that it represents the sum of the bond energies broken and formed during a reaction. If the bonds formed are stronger than the bonds broken, the heat of formation will be negative, indicating an exothermic reaction. If the bonds formed are weaker than the bonds broken, the heat of formation will be positive, indicating an endothermic reaction.

## 5. Can the heat of formation be negative?

Yes, the heat of formation can be negative, which indicates that the reaction is exothermic. This means that more energy is released during the formation of the product than is required to break the bonds in the reactants. On the other hand, a positive heat of formation indicates an endothermic reaction, where more energy is required to break the bonds in the reactants than is released during the formation of the product.

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