Take the reaction A +B ==> Z and you mix 0.1M A and 0.1M B then you KNOW Z is zero to start so you put down
...................A + B ==> Z
I................0.1..0.1.........0, then
C...............-x......-x.........+x
If you start with A at 0.1 and B at 0.1 then some of A and some of B will react. You don't know how much so that is x. Thus -x for A and -x for B are leaving A and B. Whatever A and B lose must go to Z so C line is +x for Z. Then E line is the sum of I and C so the E line is
............0.1-x....0.1-x .........x
If one side has zero the +x goes there but I think it's better to work it through logically as I've done above.
For the case where both sides have a quantity listed for initial as in above
......................A + B ==> Z
I................0.1......0.1.......0.1
Now what? Some problems will tell you with the E line (equilibrium) is. Say
E................0.09. So you can calculate that change for A must be 0.1-0.09 = 0.01. If change for A is -0.01 then change for B must be 0.01 and change for Z must be +0.01. That finishes the E line to look like this.
.................0.09.........0.09...........0.11
What about the case where the E line is NOT given. Then it looks like this
..............A + B ==> Z
I...........0.01..0.01.....0.01
C..........
E............
In this case you don't know which way the reaction will shift (move) in order to reach equilibrium. So you will have the Keq given. Let's say it is 100. Now you compare Qeq with Keq like this.
Keq = 100 = (Z)/(A)(B)
Qeq = ;(0.1)/(0.1)(0.1)] = 10
To reach equilibrium, Q must be the same as K. It isn't so the reaction will move to the left or the right to make Q = K. You can see that Q at 10 is smaller than K at 100. How do we make Q larger. Easy. If the numerator is larger and the denominator is smaller that will make Q larger than 10. So the reaction will move to the right to make Z larger and A and B smaller so now you know the -x goes on the left and +x goes on the right.
Hope this helps. It's a little tough to do this without a blackboard and having you here in person.
Good day,
Regarding ICE tables, how would you know which x's would be positive or negative for the Change row?
I've heard to "always put the positive x's on the side with 0/no values"; is this correct? Also, what if both reactants and products were given a value -- which side would be positive or negative then? Thank you
2 answers
No, that was a wonderful explanation! Thank you so much!
3 answers