Equilibria, effect on rate reactions

I dont know how to work out the units for calculations of equilibrias.

Also, I just cannot get my head around the effect of concentration on rate graphs and just the whole of that topic.

All help is appreciated. :dh:

Thank you

Reply 1

Irrelevance

16

Unit for equilibria - if i'm right, you just cancel them out

So if you have 3A + B -> 2C + D And the units are in mol-1 dm3 for example.

You do products over reactants:

2A x B/3C x D

And then cancel out:

So, B and D cancel each other out because they are 1:1. Leaving you with:

2A/3C

3C cancels out 2A, leaving you with just C. Making your units, mol-1 dm3. Sorry for the really crappy explanation, hope it helps :s-smilie:

Reply 2

dream123

OP

2

Irrelevance

Unit for equilibria - if i'm right, you just cancel them out

So if you have 3A + B -> 2C + D And the units are in mol-1 dm3 for example.

You do products over reactants:

2A x B/3C x D

And then cancel out:

So, B and D cancel each other out because they are 1:1. Leaving you with:

2A/3C

3C cancels out 2A, leaving you with just C. Making your units, mol-1 dm3. Sorry for the really crappy explanation, hope it helps :s-smilie:

Its not crappy, thanks for replying.
But, this is the bit Im really stuck on...cancelling the units down to get a final unit. :frown:

Reply 3

Irrelevance

16

dream123

Its not crappy, thanks for replying.
But, this is the bit Im really stuck on...cancelling the units down to get a final unit. :frown:

Units are all related to how many moles there are of it. So, 1 mole is equal to 1 unit. We'll use mol-1 dm3 again.

If there's one mole i.e. it's by itself in the reaction equation, it's simply mol-1 dm3

If there's 2 moles, there are two units, so it becomes, mol-2 dm6

3 moles - mol-3 dm9 etc.

Back in the example of 3A + B -> 2C + D. We have:

A - which is 3 moles, so mol-3 dm9

B - mol-1 dm3

C - 2 moles, so mol-2 dm6

D - mol-1 dm3. Essentially making our equation:

(mol-2 dm6) x (mol-1 dm3)
--------------------------
(mol-3 dm9) x (mol-1 dm3)

Now just divide away! Crossing out the ones which have cancelled each other out:

Step 1: Clear out the 1:1 ratios -

(mol-2 dm9) x (mol-1 dm3) (mol-2 dm9)
-------------------------- Becomes ------------
(mol-3 dm6) x (mol-1 dm3) (mol-3 dm6)

mol-1 dm3 have cancelled each other out of the equation. Leaving us with the above numbers.

Step 2: Cancel out what's left as far as you can

(mol-2 dm9)
------------ becomes 1/mol-1 dm3 due to the mol-2 dm9 cancelling out two units
(mol-3 dm6)

3. Invert signs if 1/

1/mol-1 dm3 becomes mol dm-3

Hope that's a little clearer. Edited.

Reply 4

dream123

OP

2

I thought its product over reactants?

D - mol-1 dm3. Essentially making our equation:

(mol-3 dm9) x (mol-1 dm3)
--------------------------
(mol-2 dm6) x (mol-1 dm3)

So, wouldnt it be:

(mol-2 dm6) x (mol-1 dm3)
--------------------------
(mol-3 dm9) x (mol-1 dm3)

Reply 5

Irrelevance

16

dream123

I thought its product over reactants?

D - mol-1 dm3. Essentially making our equation:

(mol-3 dm9) x (mol-1 dm3)
--------------------------
(mol-2 dm6) x (mol-1 dm3)

So, wouldnt it be:

(mol-2 dm6) x (mol-1 dm3)
--------------------------
(mol-3 dm9) x (mol-1 dm3)

Yeah, products over reactants, i just wrote it the wrong way round lol.

So actually it becomes

1
------
mol-1 dm3

If it's ever 1 over something, you just invert the signs. Making it turn from mol-1 dm3 into mol dm3

Reply 6

dream123

OP

2

So, from here...how does the cancelling go? and thanks for your help so far!

(mol-2 dm6) x (mol-1 dm3)
--------------------------
(mol-3 dm9) x (mol-1 dm3)

do you do:

-2 * -1 and 6*3 (top half)
-3 * 9 and -1 * 3 (bottom half)

Reply 7

Irrelevance

16

dream123

So, from here...how does the cancelling go? and thanks for your help so far!

(mol-2 dm6) x (mol-1 dm3)
--------------------------
(mol-3 dm9) x (mol-1 dm3)

do you do:

-2 * -1 and 6*3 (top half)
-3 * 9 and -1 * 3 (bottom half)

If they're on the same side, they can't cancel each other out. So A can cancel out C or D but not B. Or, if you prefer, you can only cancel diagonally or downwards/upwards

Cancelling out would go like:

(mol-1 dm3) cancels out (mol-1 dm3) completely and removing these two from the units equation.

We'll do the next on in two parts. First:

(mol-2 dm6) cancels out (mol-3 dm9)

to make

(mol-1 dm3) and (mol-2 dm6)

And then again, because you need to completely cancel out 1 side or all sides to determine rates:

(mol-1 dm3). And no probs about the help, as you can see, chemistry isn't really my forte lol

Equilibria, effect on rate reactions

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