Hey guys,
Here is a multiple choice question that is giving me a headache. I have tried to reason it out, but I am not too sure if my reasoning is correct. The mark scheme says that all statements are correct. I am just not too sure why they are correct.
The conversion of graphite into diamond is an endothermic reaction
C(graphite) --> C(diamond) Enthalpy change is +3kJ/mol
Which statements are correct?
1 The enthalpy change of atomisation of diamond is smaller than that of graphite.
2 The bond energy of the C–C bonds in graphite is greater than that in diamond.
3 The enthalpy change of combustion of diamond is greater than that of graphite.
Now I examine each statement:
1. Enthalpy change of atomization is the enegy change when one mole of gas atoms are formed from the element in its standard state.
If the reaction is endothermic, then it means that on a reaction pathway graph, diamond will be higher than graphite. So it means that diamond has a higher energy than graphite. So this means that, in a race, where diamond and graphite starts in their standard states and finish as gaseous atoms, then diamond will win because it has a shorter distance to reach the gaseous finish line(it is already at a higher energy).
So, following that reasoning, statement one is true?
2. If the enthalpy change of atomization is smaller for diamond, then diamond must require less energy for it to transform into gaseous state. This must mean that its bonds aren't that strong as compared to graphite. So graphite must have a greater bond energy (more energy is required to pull all the carbon atoms from eachother). This fits in with the reasoning for 1 above.
Is the reasoning for 2 true?
3. All combustion reactions are exothermic (give off heat). Now, diamond and graphite are both carbon atoms; when each fully reacts with oxygen we get the same product, carbon dioxide. So, from 1., as diamond is on a higher level on a reaction pathway graph, it will have to drop down a lot more to get to its products, i.e diamond will have to give out more heat than graphite to reach combustion products.
I am still confused and fuzzy about the type of reasoning I have applied. How would you go about figuring out a problem like this?
Thanks for the help mates!
Much love.