Chemistry - anybody good with Kc's?

Brianz

Hey guys, as the title goes, anybody a wiz at Kc's?

Teacher thinks we should be all able to do these "difficult questions if you all have any hope at the honours paper". How do these work out??

Q1 In an experiement to study the formation of HI
H2 + I2 ----> 2HI
<----

H2 and I2 were placed in a sealed container at a certain temperature. At equilibrium, [H2] =0.650 M, [I2] =0.106 M, [HI] =0.187 M. Calculate the Kc for this reaction.


Q2 Hydrogen fluoride, HF, can be made from the reaction

H2 + F2 ----> 2HF
<----

In one experiement, 0.05 mol of H2 and 0.025 mol of F2 are introduced into a 0.25 L container. At equilibrium there is 0.005 M of F2 present. Calculate the Kc for this reaction

lemansky

Brianz wrote:

Hey guys, as the title goes, anybody a wiz at Kc's?

Teacher thinks we should be all able to do these "difficult questions if you all have any hope at the honours paper". How do these work out??

Q1 In an experiement to study the formation of HI
H2 + I2 ----> 2HI
<----

H2 and I2 were placed in a sealed container at a certain temperature. At equilibrium, [H2] =0.650 M, [I2] =0.106 M, [HI] =0.187 M. Calculate the Kc for this reaction.


Q2 Hydrogen fluoride, HF, can be made from the reaction

H2 + F2 ----> 2HF
<----

In one experiement, 0.05 mol of H2 and 0.025 mol of F2 are introduced into a 0.25 L container. At equilibrium there is 0.005 M of F2 present. Calculate the Kc for this reaction

Q1 (.187)^2 /(.106)(.650)
Q2 i'm not sure of...(.04/.25)^2/(.03/.25)(.005/.25)
Thats probably wrong though...i did the figures in my head and i'm not so hot at that!

seinstein

It's so easy when you just look at an example from your chemistry book. The formulae are even in there (usually).

cocoa

the second Q:

In one experiement, 0.05 mol of H2 and 0.025 mol of F2 are introduced into a 0.25 L container. At equilibrium there is 0.005 M of F2 present. Calculate the Kc for this reaction
[H]=0.05 - 0.02 = 0.03
[F]=0.005
[HF]=0.02
(the size of the container V molarity just cancles out so i ignored it...)
Kc=[HF]^2/[H][F] = 0.02^2/(0.03)(0.005)= 2.666

unless i've mixed it up and it should be [H][F]/[HF]^2 in which case ans=3/8

JC 2K3

lemansky wrote:

Q1 (.187)^2 /(.106)(.650)
Q2 i'm not sure of...(.04/.25)^2/(.03/.25)(.005/.25)
Thats probably wrong though...i did the figures in my head and i'm not so hot at that!

Looks right, the volume(.25) cancels in the second one though(same amount of molecules on both sides means volume doesn't make a diference)

lemansky

JC 2K3 wrote:

Looks right, the volume(.25) cancels in the second one though(same amount of molecules on both sides means volume doesn't make a diference)

Can you still leave the volume in though for the sake of completion?Cos all the books put it in even when its not necessary....i suppose it doesn't make a difference

seinstein

lemansky wrote:

Can you still leave the volume in though for the sake of completion?Cos all the books put it in even when its not necessary....i suppose it doesn't make a difference

what do you exactly mean 'leave the volume in'? - do u mean the units, or something else? Because it's standard to leave in units to any calculation, as if you get the wrong units at the end you know you've made a mistake somewhere. (Units cancel algebraically) :rolleyes:

lemansky

seinstein wrote:

what do you exactly mean 'leave the volume in'? - do u mean the units, or something else? Because it's standard to leave in units to any calculation, as if you get the wrong units at the end you know you've made a mistake somewhere. (Units cancel algebraically) :rolleyes:

the values....divide by them even if its not necessary.

seinstein

you might need to leave them in for completeness' sake. Not sure!

lemansky

seinstein wrote:

you might need to leave them in for completeness' sake. Not sure!

Ah right!No thats what I was wondering.I didn't word any of the posts dealing with my question right at all.I knew that from an algebraic point of view,in this example it was okay to leave out the division,but thats only courtesy of algebra......I was thinking that since they gave you the volume and you're technically meant to divide by it, that if you didn't, the marking scheme might say that you lose a few marks-I can't see them giving you the benefit of the doubt as to whether you left it out knowing it wasn't needed or whether you just forgot!

Thanks for the help on that one though:)

JC 2K3

You should demonstrate that you're cancelling it out to be sure.

lemansky

JC 2K3 wrote:

You should demonstrate that you're cancelling it out to be sure.

Thats what i was thinking.

Cheers:)