GS-1 Physical Sciences Question 39

[cbaha847962]

Does anyone know exactly how this decrease in volume favors diamond formation? They refer to Le Chatelier's, but I can't figure it out how they apply it in a chemical equation.

[mcat_premed3832]

You are right to suggest that when applying Le Chatelier's Principle, it's often useful to write and equation. We are told:

graphite <--> diamond

We are also given that diamond has a greater density meaning that it occupies less volume than the same mass of graphite, so the balanced equation could be written as:

graphite <--> diamond + volume

Now think of it this way: if you were to add volume, you would shift to the left according to Le Chatelier's Principle. Therefore, an increase in pressure (= decrease in volume) must shift to the right.

[manmwila1143]

I thought that increase or decrease in pressure/volume in Le chatelier's principle applies only to gases, which is why I thought the answer was D.

[jellywing_2058]

No, it essentially applies to most systems at equilibrium. Moreover, a temperature increase would also affect the vapor pressure of a solid. In which the equilibrium would shift to the right increasing the vapor pressure of the solid.

[bisepourto1364]

"higher temperatures shift in the direction of the endothermic reaction"

got this question right but out of curiosity, how would we know that producing graphite is endothermic?

thx!

[mohsinalam5969]

Since the density of diamond is greater than that of graphite, the same number of carbon atoms occupy less volume in diamond than in graphite (density = mass/volume). An increase in pressure leads to a decrease in volume, therefore diamond formation will be facilitated (Le Chatelier's Principle). Although these rules usually apply to gases, extremely high pressures make the process possible with solids. Lowering the temperature would shift to the production of graphite (higher temperatures shift in the direction of the endothermic reaction).
Well first of all C(gr) is considered natural state so it's DHf=0
DHf (DIamond) = 1.9 kJ/mol => the formation of diamond from graphite is an endothermic reaction so it is favored by high temperature

The molar volume of diamond is smaller than graphite so it is favored by high pressure (high pressure tends to compress matter , thus increasing density and decreasing molar volume)