TBR Gen Chem Equilibrium section 3 Passage 3.. confused over a stupid question..

[dchasse]

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guys I am getting rocked with this tbr equilibrium passage because i dont get a simple math concept.

Compound 1: PCl3(g) + Cl2(g) <-->PCl5(g) Keq@ 100C= 1.95atm^-1

Compound 2: 2NO (g)+ Cl2(g) <--> 2 NOCl (g) Keq @ 100C= 2.1 *10^4 atm^-1

which compound is most readily oxidized by chlorine?
- The passage says that the greater the value of the equlib. const., the more favorable the rxn. I agree.

Then, the compound that is most readily oxidized will be the one that has the greater Keq? yes?

ok, now someone explain to me why compound 2 has a higher Keq than compound 1... I am getting messed up because of the atm^-1 term, i think..

Compound 1's Keq is basically 0.5 ---> 1/1.95
Compound 2's Keq will be 4.8e-5 ------> 1/2.1e4

HOW is compound 2 more favorable towards products???


EDIT: I know this seems like a dumb question because you can just see that both Keq's are the same units therefore you just need to look at the numbers but whats with the atm^-1??

 

[V5RED]

guys I am getting rocked with this tbr equilibrium passage because i dont get a simple math concept.

Compound 1: PCl3(g) + Cl2(g) <-->PCl5(g) Keq@ 100C= 1.95atm^-1

Compound 2: 2NO (g)+ Cl2(g) <--> 2 NOCl (g) Keq @ 100C= 2.1 *10^4 atm^-1

which compound is most readily oxidized by chlorine?
- The passage says that the greater the value of the equlib. const., the more favorable the rxn. I agree.

Then, the compound that is most readily oxidized will be the one that has the greater Keq? yes?

ok, now someone explain to me why compound 2 has a higher Keq than compound 1... I am getting messed up because of the atm^-1 term, i think..

Compound 1's Keq is basically 0.5 ---> 1/1.95
Compound 2's Keq will be 4.8e-5 ------> 1/2.1e4

HOW is compound 2 more favorable towards products???


EDIT: I know this seems like a dumb question because you can just see that both Keq's are the same units therefore you just need to look at the numbers but whats with the atm^-1??

The K's are1.95 and 2.1E4.

You are being confused by the units which are in atm^-1 because the equation for K in this situation is gas/(gas*gas) because 2 gas reactants combine to form a gas product. This will lead to units of inverse atm. You don't apply that inversion to the K values.

 

[member232]

I'm pretty sure you don't have to divide the Keq by 1.. by doing so, you're calculating the Keq for the reverse reactions...

don't let the "atm-1" throw you off... its only "-1" because of the way the units cancel out:

For compound 1: Keq: [PCl5] / [PCl3][Cl2] = atm/ (atm) ^2 = atm ^-1
compound 2: Keq = [NOCl2]^2 / [Cl2][NO]2 = atm^-1

therefore, those ARE the Keq... dont divide and you see that just as they are stated, compund 2 has the greater Keq 🙂

 

[johnnybravo19]

Find the x-value . Compound 1 has 5 Cl while Compound 2 has 2 Cl.

 

[dchasse]

The K's are1.95 and 2.1E4.

You are being confused by the units which are in atm^-1 because the equation for K in this situation is gas/(gas*gas) because 2 gas reactants combine to form a gas product. This will lead to units of inverse atm. You don't apply that inversion to the K values.

thanks!
its been about 3 years since gen chem.. is there a chart the lists how the units change? if i remember correctly the units change based on the order of the reaction or something like that right? are knowing these units of any testing value?

i literally spent like 15 mins on this getting frustrated hahahha i thought the book was wrong and almost convinced myself with my ego but tbr is rarely wrong...

 

[LoLCareerGoals]

Units can be a lot of things. In reactions where:
1) coefficients are vastly different on either side
2) gases and non-gases are present (here you'll have atms mixed with M)

you will have pretty weird units for Keq. Just remember to use partial pressure raised to the coef power for gas and molarity raised to the coef power for other (non solid/non liquid) compounds.

 

[V5RED]

thanks!
its been about 3 years since gen chem.. is there a chart the lists how the units change? if i remember correctly the units change based on the order of the reaction or something like that right? are knowing these units of any testing value?

i literally spent like 15 mins on this getting frustrated hahahha i thought the book was wrong and almost convinced myself with my ego but tbr is rarely wrong...

You don't need a chart.

Just set up your law of mass action to find K(products/reactants) and cancel out the units until you get the proper units.

Things to remember are:
Stoichiometric coefficients become exponents
Ignore Solids and Liquids(only gasses and solutes count)

Example for molarity
A+2B<->C
[C]/([A]^2)=K
M/(M*M^2)=K
M^-2=K

The actual truth is that K is unitless and depends on activites, but that is beyond the scope of the MCAT.