### HOW to CALCULATE Kc for A Level Chemistry

Online A Level Chemistry Tutor Paul Morgan with fellow tutor Atul Rana shows how to calculate an equilibrium constant Kc and it's units using the online whiteboard bitpaper
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(00:00)
Hi, I'm Atul and I'm in London right now. Hi, I'm Paul and I'm in Burnley Lancashire.
So we're going to have a look at the equilibrium constant Kc and calculate Kc.
So I'll go straight into the question...
5 moles of A is mixed with 5 moles of B in a volume 12 dm3. When the equilibrium is established it contains 3 moles of C.
Find the equilibrium constant Kc at this temperature.
So the equation Kc is concentration of products over reactants raised to the powers of their moles in the balanced equation. So
[C]^2
•••••••••••••••••••
[A]× [B]^2
ICE method = Initial Change Equilibrium
Initial
5 moles A
5 moles B
0 moles C
Equilibrium
3 moles C (change of +3)
So Atul what are changes of A and B?
(01:30)
Well since number of moles B is the same as the number of moles of C. I expect the change of B to be 3 as well.
(01:40)
It'd be a change of 3. So our reactants go down. And the change in A is half that so 1.5.
(01:55)
Excellent so what would be the equilibrium moles.
(02:02)
B 5 - 3 = 2 moles
A 5 - 1.5 = 3.5 moles
Equilibrium
3.5 moles A
2.0 moles B
3.0 moles C
(02:21)
We need to find the concentrations. So concentration = moles ÷ volume. So
Concn C 3/12 = 1/4
Concn A 3.5/12 = 7/24
Concn B 2/12 = 1/6
And if we put all that into the calculator,
(1/4)^2
•••••••••••••••••••••••• = 7.71
(7/24) x (1/6)^2
= 7.7 ( 2 sig fig)
(03:35)
And the units? Instead of putting numbers in the equation, we put the units.
(molesdm-3)^2
••••••••••••••••••••••••••••••••••••••••
(molesdm-3) x (molesdm-3)^2
So the squared terms cancel. So Atul with your maths head on could you tell me what the units would be for Kc?
(04:19)
Right so all we have left is the reciprocal of (molesdm-3) which is
1 ÷ (moldm-3)
So when you move (moldm-3) to the top that is moles to the -1 and dm to the +3 so (mol-1dm+3)
(04:40)
So Kc is 7.7 mol-1dm3.
(04:53)
Great and the units of Kc depend on the number of moles of the reactants and the products in the equation.
(05:08)
Yes they do. And so you can have different values of units for Kc and you can even have no units if the concentrations cancel.
(05:17)
Of course. Can be dimensionless. And just to sum up trying to get a tangible meaning of Kc.
I suppose it's how much towards the right the equilibrium is. The bigger that number the bigger the rightness to use an unscientific language.
(05:42)
Yes the bigger value (greater than 1) tells us the equilubrium lies to the right and the bigger the value the more to the right the equilibrium lies.
And a value less than 1 lies to the left of the equilibrium.
(05:58)
And chemists interested in yield will be wanting a big value of Kc
(06:03)
Mm, that's pretty smart. And I suppose it's like an extension from GCSE where they look at yield in reversible reactions where the yield is a percentage but here you're looking at it more like an absolute number rather than percentage.
(06:21)
Yeah and at GCSE they look at what cause shifts in equilibrium.
(06:26)
Yeah. Perfect. That sounds great. Thanks very much for that. And I look forward to the next video.
(06:34)
Ok. Bye.
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More information
Paul Morgan - A Level Chemistry Tutor
Website
https://www.alevelchemistrytuition.com
LinkedIn
https://www.linkedin.com/in/onlinealevelchemistrytutor
Atul Rana - Maths and Science Tutor
http://www.atultanatutors.co.uk
#OnlineALevelChemistryTutor #PaulMorgan #EquilibriumConstant
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