Enthalpy multiple choice!!

Hello everyone, I just want to tell you that I'm planning to kill myself and the reason is chemistry.I have Paper 1 AS-LEVEL as MCQ, and they have really complicated (well, prob not, but i don't know the trick yet) enthalpy questions. I can solve if you give me like min. 5 mins, but under time constraint, I don't understand how I CAN POSSIBLY solve them in the given time.I know the answer is prob not that difficult, but I want to know what they ACTUALLY expect us to do for solving the question. Surely they don't expect us to draw the full enthalpy change cycle and stuff.I'll give you a question, can someone explain how you'd solve it under time contraint?

Q) The enthalpy change of formation of cyclopropane is +53.3kJmol-1 and the enthalpy change of atomisation of graphite is +717 kJ mol-1.

The bond enthalpy of H-H bond is 436 kJmol -1 and of C-H is 410 kJmol-1.

What value for the average bond enthalpy of the C-C bond in cyclopropane can be calculated from this data?
Options:A) 187kJmol-1B)315 kJmol-1C)351kJmol-1D) 946kJmol-1

Thank you!

Original post by fandom-queen

Hello everyone, I just want to tell you that I'm planning to kill myself and the reason is chemistry.I have Paper 1 AS-LEVEL as MCQ, and they have really complicated (well, prob not, but i don't know the trick yet) enthalpy questions. I can solve if you give me like min. 5 mins, but under time constraint, I don't understand how I CAN POSSIBLY solve them in the given time.I know the answer is prob not that difficult, but I want to know what they ACTUALLY expect us to do for solving the question. Surely they don't expect us to draw the full enthalpy change cycle and stuff.I'll give you a question, can someone explain how you'd solve it under time contraint?

Q) The enthalpy change of formation of cyclopropane is +53.3kJmol-1 and the enthalpy change of atomisation of graphite is +717 kJ mol-1.

The bond enthalpy of H-H bond is 436 kJmol -1 and of C-H is 410 kJmol-1.

What value for the average bond enthalpy of the C-C bond in cyclopropane can be calculated from this data?
Options:A) 187kJmol-1B)315 kJmol-1C)351kJmol-1D) 946kJmol-1

Thank you!

There are three carbon-carbon bonds and 6 carbon-hydrogen bonds in cyclopropane

Formula for formation:

3C(g) + 3H2(g) --> C₃H₆ ..... +53.3 kJ

Working on bonds broken - bonds formed = +53.3 kJ

(3 x 717) + (3 x 436) - (6 x 410) - 3C-C = +53.3

rearrange and solve for C-C

all in all 2 minutes work (max)

Reply 2

Alimaaz

3

There are no bonds being broken in gaseous carbon so why is that energy being used pleasr explain

yeah why???

Original post by Alimaaz

There are no bonds being broken in gaseous carbon so why is that energy being used pleasr explain

To make gaseous carbon you have to break the bonds in the carbon graphite which is solid. This is the enthalpy change of atomisation.

Reply 5

username1445490

9

In an earlier answer this is not correct 3C(g) + 3H2(g) --> C3H6 ..... +53.3 kJ Since the +53.3 kJmol-1 is for is the enthalpy change of formation of C3H6 which is from C(s). So it should be 3C(s) + 3H2(g) --> C3H6 ..... +53.3 kJ
The sum of the bond enthalpies are C3H6 --> 3C(g) + 6H(g)
We need to get from 3C(s) to 3C(g) and from 3H2(g) to 6H(g)
so to get from 3C(s) to 3C(g) 3 x enthalpy change of atomisation is needed and to get from 3H2 (g) to 6H(g) 3 x H-H bond enthalpy is needed.