## Title

Enthalpy Changes
4 posts / 0 new
10042
Offline
Joined: 31/10/2008 - 12:57
Posts: 23
Enthalpy Changes
10042
Offline
Joined: 31/10/2008 - 12:57
Posts: 23
Enthalpy Changes

Hi, we've begun module 2 doing energetics. to begin with the questions we were given were pretty self explanatory but some of the questions now seem a lot more confusing. my teacher says that we have to use the "cycle method" (i dont know if you know what im talking about), which i find difficult enough to use even with the basic questions. We were recently given a sheet for homework and i wondered if you could talk me through a few of the questions:

1)Calculate the average S-F bond energy in SF6(g) using the bond data
S(s)-> S(g)=223kJ/mol

and the (symbol delta H of formation) of SF6 which is -1100kJ/mol.

2) Calculate the (symbol for Delta H of formation) for bromomethane, CH3Br(g) given the bond data
C(s)->C(g) =715kJ/mol
Br2(l)->Br(g)=15kJ/mol
C-H = 412kJ/mol.

I hope you understand what i mean.
Thx

chemistrysubjec...
Offline
Joined: 10/06/2007 - 22:31
Posts: 347

I will have a think about this and get back to you.

chemistrysubjec...
Offline
Joined: 10/06/2007 - 22:31
Posts: 347

The secret is to be clear about individual statements.

Understand the definitions of the DH formation and DH combustion.

Understand the meaning of Hess's Law.

Understand the meaning of bond enthalpies.

Believe in what you are writing on the paper.

Whichever questions you are given, show your working either in the form of a diagram or in clear mathematical expressions.

If necessary, say the equations in words ike, "The sulfur to fluorine bond is breaking and requires XXXkJ per mole".

State symbols are vitally impoortant too because there is an energy change associated with change of state.

Hope this helps. If not, get back online and ask again.