Why Heat of formation can be both positive and negative?

[kfsa1]

Hi guys,
I was just wondering how heat of formation could be positive? I thought that in order to make compounds, energy needs to be released thus being exothermic and negative. I think I'm most confused about maybe the wording of the term heat of formation bc I associate it with making new bonds.

Or, can the heat of formation be used to describe a whole reaction that takes place to form a compound? As in, in order to make A + B -> C we need to break bonds of A and B which is energy consuming and then the formation of bonds to make C releases less energy than it took to break bonds of A and B so the overall heat of formation is positive? If this is the case, I can't really see what the difference between Bond disassociation E and heat of formation is?

Thanks in advance guys

---

Members don't see this ad.

 

[RogueUnicorn]

the heat of formation is the SUM of all the bonds broken and all the bonds made, from the elemental forms of whatever elements are in the compound.

 

[ema143]

The "heat of formation" of a compound is the enthalpy change for the reaction where the compound is made from the elements in their standard states at 25C and 1 atm. And usually given in a table this are some of the values.
[SIZE=-1]Compound[/SIZE][SIZE=-1]ΔHf (kJ/mol)[/SIZE][SIZE=-1]Compound[/SIZE][SIZE=-1]ΔHf (kJ/mol)[/SIZE][SIZE=-1]AgBr(s) [/SIZE][SIZE=-1]-99.5 [/SIZE][SIZE=-1]C2H2(g) [/SIZE][SIZE=-1]+226.7 [/SIZE][SIZE=-1]AgCl(s) [/SIZE][SIZE=-1]-127.0 [/SIZE][SIZE=-1]C2H4(g) [/SIZE][SIZE=-1]+52.3 [/SIZE][SIZE=-1]AgI(s) [/SIZE][SIZE=-1]-62.4 [/SIZE][SIZE=-1]C2H6(g) [/SIZE][SIZE=-1]-84.7 [/SIZE][SIZE=-1]Ag2O(s) [/SIZE][SIZE=-1]-30.6 [/SIZE][SIZE=-1]C3H8(g) [/SIZE][SIZE=-1]-103.8 [/SIZE][SIZE=-1]Ag2S(s) [/SIZE][SIZE=-1]-31.8 [/SIZE][SIZE=-1]n-C4H10(g) [/SIZE][SIZE=-1]-124.7 [/SIZE][SIZE=-1]Al2O3(s) [/SIZE][SIZE=-1]-1669.8 [/SIZE][SIZE=-1]n-C5H12(l) [/SIZE][SIZE=-1]-173.1 [/SIZE][SIZE=-1]BaCl2(s) [/SIZE][SIZE=-1]-860.1 [/SIZE][SIZE=-1]C2H5OH(l) [/SIZE][SIZE=-1]-277.6 [/SIZE][SIZE=-1]BaCO3(s) [/SIZE][SIZE=-1]-1218.8 [/SIZE][SIZE=-1]CoO(s) [/SIZE][SIZE=-1]-239.3 [/SIZE][SIZE=-1]BaO(s) [/SIZE][SIZE=-1]-558.1 [/SIZE][SIZE=-1]Cr2O3(s) [/SIZE][SIZE=-1]-1128.4 [/SIZE][SIZE=-1]BaSO4(s) [/SIZE][SIZE=-1]-1465.2 [/SIZE][SIZE=-1]CuO(s) [/SIZE][SIZE=-1]-155.2 [/SIZE][SIZE=-1]CaCl2(s) [/SIZE][SIZE=-1]-795.0 [/SIZE][SIZE=-1]Cu2O(s) [/SIZE][SIZE=-1]-166.7 [/SIZE][SIZE=-1]CaCO3 [/SIZE][SIZE=-1]-1207.0 [/SIZE][SIZE=-1]CuS(s) [/SIZE][SIZE=-1]-48.5 [/SIZE][SIZE=-1]CaO(s) [/SIZE][SIZE=-1]-635.5 [/SIZE][SIZE=-1]CuSO4(s) [/SIZE][SIZE=-1]-769.9 [/SIZE][SIZE=-1]Ca(OH)2(s) [/SIZE][SIZE=-1]-986.6 [/SIZE][SIZE=-1]Fe2O3(s) [/SIZE][SIZE=-1]-822.2 [/SIZE][SIZE=-1]CaSO4(s) [/SIZE][SIZE=-1]-1432.7 [/SIZE][SIZE=-1]Fe3O4(s) [/SIZE][SIZE=-1]-1120.[/SIZE]
As you can see, most heats of formation are negative quantities, which implies that the formation of a compound from its elements usually is an exothermic process.
In the other hand 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

And you are right in genaral: since it takes(absorb) energy to pull to atoms apart(breaking a bond) then this process going to be endothermic (+) and bond formation does realse energy therefire exothermic (-)

 

[kfsa1]

the heat of formation is the SUM of all the bonds broken and all the bonds made, from the elemental forms of whatever elements are in the compound.

If heat of formation is the sum of all bonds broken and made, how is it different from bond disassocation energy? BDE is the sum of energy taken to break bonds in the reactants minus the sum of energy released when they are formed in the final product isn't it?

Sorry about not understanding you correctly.

 

[RogueUnicorn]

If heat of formation is the sum of all bonds broken and made, how is it different from bond disassocation energy? BDE is the sum of energy taken to break bonds in the reactants minus the sum of energy released when they are formed in the final product isn't it?

Sorry about not understanding you correctly.

no it's not. BDE is energy needed to break a single bond.