silly, ooshy gooshy gen chem questions

[liveoak]

berkely review inventing new ways to question gen chem. btw, they should change their name.

anyway. question 1 is a goodie, 2 and three are pretty lame, i understand them 60%, but 60% is a D, and I am not a D!


1)If you increases temperature, for an exothermic reaction, the reaction is supposed to turn towards the reactants (Left). However, intuitivly, increasing temperature always is supposed to increases the Keq. Which wins out here?

2) Is partial pressure related to mole fraction? Is partial pressure concentration?

3) how does keq change with new mole values?

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[liveoak]

adding to the first question, here is a sample Q that directly tests this concept.

you'd think since it's exothermic, CO2 would drop, but because you R adding heat Keq rises. But, alas, berkeely review says they both drop.

 

[BerkReviewTeach]

However, intuitivly, increasing temperature always is supposed to increases the Keq.

...but because you R adding heat Keq rises.

What makes you think that adding heat raises Keq? That is not true and seems to be the root of your troubles.

Think about it this way. If we write a reaction in the reverse direction, then it will have a new Keq that is the inverse of the original Keq. So if the addition of heat makes Keq for the original reaction larger, then it MUST make the Keq for the reverse reaction less. Whether Keq increases or decreases with the addition of heat depends on the way a reaction is written.

 

[liveoak]

i always thought an increase in temperature meant more dissociation, which means a bigger Keq.

so is the actual rule, **changing temperature means a change in Keq, but not necessary an increase***

good? thanks mr. berkeley review himself!

 

[music2doc]

What makes you think that adding heat raises Keq? That is not true and seems to be the root of your troubles.

Think about it this way. If we write a reaction in the reverse direction, then it will have a new Keq that is the inverse of the original Keq. So if the addition of heat makes Keq for the original reaction larger, then it MUST make the Keq for the reverse reaction less. Whether Keq increases or decreases with the addition of heat depends on the way a reaction is written.

OP is [most likely] confusing equilibrium and kinetics. OP, you want to make sure you understand the difference b/w those two concepts as they will be tested (simply too common an area misunderstanding to not test).

Kinetics -- corresponds to how frequently the molecules collide/interact (energy of activation, etc.)
Thermodynamics -- stability of the state

And yes...sometimes they do conflict (e.g., thermodynamic vs. kinetic controls in ochem -- which you should UNDERSTAND for the MCAT, but not necessarily have memorized; chances are they're not going to throw something you've seen at you and ask which is the kinetic control but something exotic that only your ochem prof would be familiar with...and even s/he only vaguely without reference to a graduate textbook)

 

[GreenRabbit]

1)If you increases temperature, for an exothermic reaction, the reaction is supposed to turn towards the reactants (Left). However, intuitivly, increasing temperature always is supposed to increases the Keq. Which wins out here?

2) Is partial pressure related to mole fraction? Is partial pressure concentration?

3) how does keq change with new mole values?

2) Partial pressure is the mole fraction of the gas times the total pressure. When you write an equation for K that involves gases, yes, you use partial pressure instead of concentration.

3) Keq does not change with new mole values.

With regards to Keq, BRTeach gives a great example. I also noticed you said dissociation increases with heat, maybe you're referring to solubility? Ksp often increases with temperature, but not always.

 

[liveoak]

2) Partial pressure is the mole fraction of the gas times the total pressure. When you write an equation for K that involves gases, yes, you use partial pressure instead of concentration.

3) Keq does not change with new mole values.

With regards to Keq, BRTeach gives a great example. I also noticed you said dissociation increases with heat, maybe you're referring to solubility? Ksp often increases with temperature, but not always.

regarding #3, i mean something like

1 H2 and 1 N2 = 1 H2N2 keq = 1

versus

2 H2 and 2N2 = 2 H2N2 keq = ??

also, brteach gave a GREAT example. I am just stuck on the dissociation of water. i know it increases its kw w/temp.

 

[GreenRabbit]

Keq for both of those equations is the same.

 

[gunj122]

for #3:

if you add more moles, let say of reactant, to the solution, what's going to happen? well we know that Keq = [Products]/[Reactants]. Therefore, INITIALLY you'll think Keq will decrease. however that is completely wrong. According to Le Chateliers Principle, an increase in stress will result in driving the reaction towards the side that reduces the new stress. In this situation, the stress is excess reactant; therefore the reaction will shift towards the product side and result in an increase in Products.

going back to the Keq equation where it equals [Products]/[Reactants], a proportional increase in reactant will ALSO result in a proportional increase in product to return the state of the system back to its original equilibrium. hope this helped!