BR Chem Question (Le Chatelier's)

[iololiol]

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Example 3.12 Which of the following reactions at equilibrium would NOT shift as the result of an increase in pressure?
A. CaO(s) + C02(g) to CaC03(s)
B.PCl3(g) +Cl2(g) to PCl5(g)
C. N2(g) + 3H2(g) to 2NH3(g)
D.H2(g)+I2(g) to 2HI(g)

the answer is D but I do not understand why.

Here is the solution they provided:
The equilibrium reaction does not shift after an increase in pressure when there is an equal number of gas molecules on each side of the reaction. This is observed inchoiceD. Asapointofinterest,CaC03cannotbestoredinanopenspacefor this reason, because it will fully dissociate into CaO and C02 over time, if the carbondioxidepartialpressureisnothighenough. AlsotakenotethatI2isagas in choice D, which means that the reaction must be at a temperature above ambient conditions.

 

[Flapjacks]

A) 1 -> 0
B) 2 -> 1
C) 4 -> 2
D) 2 -> 2

Notice how D is the only one with the same amount of molecules before and after? Increasing pressure tends to favor less randomness of the two possible outcomes; however in D, the increased pressure does not influence the randomness of the molecules.

 

[pm1]

Example 3.12 Which of the following reactions at equilibrium would NOT shift as the result of an increase in pressure?
A. CaO(s) + C02(g) to CaC03(s)
B.PCl3(g) +Cl2(g) to PCl5(g)
C. N2(g) + 3H2(g) to 2NH3(g)
D.H2(g)+I2(g) to 2HI(g)

the answer is D but I do not understand why.

Here is the solution they provided:
The equilibrium reaction does not shift after an increase in pressure when there is an equal number of gas molecules on each side of the reaction. This is observed inchoiceD. Asapointofinterest,CaC03cannotbestoredinanopenspacefor this reason, because it will fully dissociate into CaO and C02 over time, if the carbondioxidepartialpressureisnothighenough. AlsotakenotethatI2isagas in choice D, which means that the reaction must be at a temperature above ambient conditions.

Reactions shift in order to restore equilibrium in order to relieve the stress that was applied, according to LeChat.
When there is an increase in pressure (dec in volume) the reaction shifts to the side where there is a lower number of gas molecules, conversely, it shifts to the side with a higher number of gas mols when there is a decrease in pressure (inc in volume).
In choice D there is a equal number of gas moles, thus no shifting should happen in response to pressure change.

 

[Flapjacks]

Also forget about everything else in that solution other than the first sentence. This question was just testing your concept knowledge.

 

[iololiol]

I SEE IT!!! 😀

Thanks, lol I wasnt reading correctly