Why does a negative ΔH indicate an exothermic reaction?

[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: ΔH_products-ΔH_reactants.
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

 

[Chestermiller]

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.

 

[chemisttree]

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: ΔH_products-ΔH_reactants.
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 = ΔH_products-ΔH_reactants. 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.