dH of formation vs dH of dissociation

[prsndwg]

---

Members do not see this ad.

What is the difference between them? I get the meaning but why is the formula different? if the values of bond energy is positive, do we have to change them to negative when dealing with making bonds, since its exothermic? I am all confused 😕

Standard Enthalpy (Heat) of Reaction,

Ho, is the difference between the standard enthalpies (heats) of formation of the products and the reactants

Ho(reaction) =

Hof(products) -

Hof(reactants)

"Bond dissociation" is the standard enthalpy change when a bond is broken.

Ho(reaction) =

H of bond broken -

H of bond formed

 

[AmpedUp]

Came across a problem today and couldn't really distinguish between both rules. So can anyone help? Is there a way to read a problem and automatically know which formula to use?

 

[Maygyver]

dH of bond formation will be the dH of one mol of a compound from its corresponding pure elements:
H2 + 1/2 O2 ----> H20



dH of bond dissociation will be more specifically the enthalpy change due to the breaking or making of a bond:
Cl-Cl
H-C



Essentially, if you wanted to find the enthalpy change for a reaction, you can use either. However, if you are given the dH(formation) it is easier because you do not need to sum up all of the individual bonds broken and formed.

Hopefully this helps. Let me know if I need to be expand.

 

[UCB05]

What is the difference between them? I get the meaning but why is the formula different? if the values of bond energy is positive, do we have to change them to negative when dealing with making bonds, since its exothermic? I am all confused 😕

Standard Enthalpy (Heat) of Reaction,

Ho, is the difference between the standard enthalpies (heats) of formation of the products and the reactants

Ho(reaction) =

Hof(products) -

Hof(reactants)

"Bond dissociation" is the standard enthalpy change when a bond is broken.

Ho(reaction) =

H of bond broken -

H of bond formed

You'll get the same answer given the correct info with both equations. Remember the additive property of heats of formation. You can get any theoretical head of formation given the basic bond dissociation reactions. The heat of formation equation can be viewed as the sum of bond dissociation energies of the products minus the bond dissociation energies of the reactants. Using the bond dissociation energy calculations in the second equation gets you the heat of formation values used in the first equation. you can think of the standard enthalpy equation as shorthand.

It would be easy to understand bond dissociation energy as reactants - products, except that the sign convention is for energy required to break the bond, so there's a sign flip in the actual equation
I hope i'm getting the idea across correctly.

Also, bond dissociation energies assume homolytic reaction (both elements contribute an electron to the bond formed)

 

[prsndwg]

You'll get the same answer given the correct info with both equations. Remember the additive property of heats of formation. You can get any theoretical head of formation given the basic bond dissociation reactions. The heat of formation equation can be viewed as the sum of bond dissociation energies of the products minus the bond dissociation energies of the reactants. Using the bond dissociation energy calculations in the second equation gets you the heat of formation values used in the first equation. you can think of the standard enthalpy equation as shorthand.

It would be easy to understand bond dissociation energy as reactants - products, except that the sign convention is for energy required to break the bond, so there's a sign flip in the actual equation
I hope i'm getting the idea across correctly.

Also, bond dissociation energies assume homolytic reaction (both elements contribute an electron to the bond formed)

can you please explain this again?