B54321
Badges: 10
Rep:
?
#1
Report Thread starter 2 years ago
#1
UPDATE: NVM I figured it out.

How can I calculate the enthalpy change of formation of CH3Br (g)?


C(s) → C(g) ∆H = 715 kJ mol-1
Br2(l) → Br2(g) ∆H = 15 kJ mol-1



Bond enthalpies:
H-H : 436
Br-Br: 193
C-H: 412
C-Br: 276
Last edited by B54321; 2 years ago
0
reply
Kian Stevens
Badges: 16
Rep:
?
#2
Report 2 years ago
#2
The enthalpy change of formation requires one mole of product to be formed from its constituent elements, which are all in their standard states.
In this reaction, this can be shown using the balanced equation: C(s) + \frac{3}{2}H2(g) + \frac{1}{2}Br2(l) \rightarrow CH3Br(g)

You want to start off by drawing a Hess cycle. Here's my extremely poorly drawn one, done using Paint...:
Name:  Untitled.png
Views: 209
Size:  21.9 KB
'H1', 'H2' and 'H3' correspond to \Delta H_{1}, \Delta H_{2} and \Delta H_{3} respectively.
The important thing to note is that you'll be starting from the bottom rectangle, as you're going from a bunch of elements in their standard states to form the starting reactants, and these reactants then form bonds which results in the formation of bromomethane.
This process' enthalpy change is \Delta H_{3}, and is calculated using: \Delta H_{3} = \Delta H_{1} + \Delta H_{2}.

You're given some bond enthalpies, so you can work out some values for \Delta H_{1} and \Delta H_{2}:
  • \Delta H_{1} is the sum of each average bond enthalpy which is present. However, with the way average bond enthalpies are determined, the result will be negative. There are three C-H bonds and one C-Br bond, hence this is equal to: (412 x 3) + 276 = -1512 kJ mol-1.
  • \Delta H_{2} is the sum of each enthalpy change from elements to starting materials. The trick here is that hydrogen is also being converted into something too, and this will have its own enthalpy change which isn't given, unlike the ones for carbon and bromine. This enthalpy change is just a sum of each elementary enthalpy change, based on the stoichiometry in the balanced equation right at the start of this reply. Hence, this will be equal to: 715 + (\frac{3}{2}436) + (\frac{1}{2}15) = 1376.5 kJ mol-1.

Now we have everything to work out the enthalpy change of formation for bromomethane.
As said before, this is calculated using: \Delta H_{3} = \Delta H_{1} + \Delta H_{2}. So, it's just a case of plugging some numbers in.
Hence, \Delta H_{3} = -1512 + 1376.5 = -135.5 kJ mol-1.
Last edited by Kian Stevens; 2 years ago
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

If you haven't confirmed your firm and insurance choices yet, why is that?

I don't want to decide until I've received all my offers (36)
40%
I am waiting until the deadline in case anything in my life changes (21)
23.33%
I am waiting until the deadline in case something in the world changes (ie. pandemic-related) (5)
5.56%
I am waiting until I can see the unis in person (8)
8.89%
I still have more questions before I made my decision (5)
5.56%
No reason, just haven't entered it yet (6)
6.67%
Something else (let us know in the thread!) (9)
10%

Watched Threads

View All