[INeedSerotonin]

Hello

My book gave me the following bond enthalpy values:

H-H = 470,7 kJ / mole
Cl-Cl = 242,5 kJ / mole
H-Cl = 431,5 kJ / mole

And then it asked me to tell whether this reaction is endothermic or exothermic:

H₂ + Cl₂ -----> 2 HCl

I know that the bond enthalpy is calculated as H_reagents - H_products, but I'm still confused.

Isn't the bond enthalpy the energy needed to break the bonds between the ions? Then how exactly can this tell me whether this is endothermic or exothermic?

For example, I found a negative value for the aforementioned formula. But I don't know what this means. I'm confused.

[Corribus]

Most heat energy liberated or absorbed during a chemical reaction is due to breaking and forming bonds. The bond enthalpy is a measure of how much energy is released when a bond is formed, or how much energy it takes to break a bond. This reaction involves (per mole) breaking one hydrogen-hydrogen and one chlorine-chlorine bond, and forming two hydrogen-chlorine bonds.

Does this background information help?

[INeedSerotonin]

Most heat energy liberated or absorbed during a chemical reaction is due to breaking and forming bonds. The bond enthalpy is a measure of how much energy is released when a bond is formed, or how much energy it takes to break a bond. This reaction involves (per mole) breaking one hydrogen-hydrogen and one chlorine-chlorine bond, and forming two hydrogen-chlorine bonds.

Does this background information help?

So, in order to form 2 HCl, I have to break the H-H bond and the Cl-Cl bond, right? This means that I will obtain 470,7 kJ + 242,5 kJ = 713,2 kJ from breaking it.

On the other hand, I need 2 . (H-Cl) = 2 . 431,5 kJ = 863 kJ to break the product's bond.

So I need more energy to break the products than to break the reagents.

But how does this lead me to "endothermic" or "exothermic"? If I need more energy to break the product, doesn't it mean that the product absorbed energy to be formed? But my book says, if I'm not mistaken, that the reaction is an exothermic one.

Or perhaps I should consider just one mole of the H-Cl? Then we need 470,7 kJ + 242,5 kJ = 713,2 kJ to break the reagents, and just 431,5 kJ to break one mole of the product, which would mean it is an exothermic reaction.

 

[Corribus]

You're not breaking bonds in the product, you're forming bonds - same quantity of energy, but opposite in sign. Breaking bonds takes energy, and forming bonds releases energy. You're looking for the net difference, and then you need to decide whether the balance is energy released or energy absorbed. This will tell you whether the reaction is exothermic or endothermic (i.e., the sign on delta-H).

[INeedSerotonin]

You're not breaking bonds in the product, you're forming bonds - same quantity of energy, but opposite in sign. Breaking bonds takes energy, and forming bonds releases energy. You're looking for the net difference, and then you need to decide whether the balance is energy released or energy absorbed. This will tell you whether the reaction is exothermic or endothermic (i.e., the sign on delta-H).

Hey! I think I got it now! I hadn't noticed how breaking and forming bonds are the same energy, but opposite in sign!

So it takes 470,7 kJ + 242,5 kJ = 713,2 kJ to break the reagents, and when HCl is formed, it releases 863 kJ. This means there are more releasing of energy than using it, which means it is exothermic.

I think things are starting to make sense to me!

Thank you!