Bond Enthalpy and Energy

[blazinfury]

I am confused about what the difference is between Bond Energy, Enthalpy of Formation, Hess's Law and Enthalpy? The reason being that they all involve heat (ie enthalpy) as a measurement. Enthalpy I know is the heat present in a system. Enthalpy of Formation is the formation of a compound from elements in their most stable/standard state. Bond energy is the heat added to a system to break bonds and then heat is released when new bonds are formed. But the answer that you get in the end of the day is still in terms of enthalpy, correct?

 

If so, then does that mean that if a reaction is exothermic, heat is released, surroundings get hotter, and the bonds formed are stronger than the bonds broken because more heat/energy is released than was used to initially break the bonds?

 

Also, if one compares bond strength (such as in sp>sp2>sp3), does that mean that the stronger bond is more exothermic, the products are at a lower energy and are more stable than the reactants? To make this claim, we would use Hrxn = E(bonds broken) - E(bonds formed), correct? Then when do we use delt H= products - reactants?

 

I would greatly appreciate it if someone could please clarify my confusion. Many thanks.

Edited March 1, 2012 by blazinfury

[CaptainPanic]

Bond energy is the energy to make/break a single bond between two atoms. I think this is only used for covalent bonds.

If you have a strong bond, you need to put in more energy to break it.

 

The "enthalpy changes" are the sum of breaking and creating all the bonds in a reaction. In a chemical reaction, usually more than 1 bond gets broken at the same time (or in the same blink of an eye - not exactly the same time, but nearly).

 

The enthalpy itself (not the change) is just the state of the material at a certain time. It's like an altitude: you are now at 245 meters above sea level. You can have so many J/kg of enthalpy. You can change the enthalpy of a material by adding heat (which can cause a reaction, or melting, or evaporation, or nothing except just heating it up), or removing heat.

 

And Hess's Law says that it doesn't matter by which route you arrived at a certain place. Using the metaphor of the altitude. It doesn't matter if you first climbed to the peak of a mountain, and then back down, or straight to a certain place. If you arrived at 245 m above sea level, the route doesn't matter: your potential energy is the same. Same for enthalpy.

 

The enthalpy of formation is a specific type of change-of-enthalpy. It's a very specific reaction. It's the reaction from the elements to the material. So, the enthalpy of formation of methanol is:

 

Carbon (graphite) + Oxygen (gas) + hydrogen (gas) --> methanol.

 

Of course, this is an unlikely reaction to happen... but it's just a practical definition.

 

Hope that helps.