Equilibria Calculations - ICE Tables method

[liamxrich]

Hi,

Can someone please break-down the use of ICE tables in determining unknown concentrations for equilibria, and explain the concepts that is behind it so I can better apply it to questions, without the need to use examples or expressions to answer each questions (as I have needed to get over the line with answering these questions).

Particularly on those which also have coefficients in the equation.

Thanks!
 

[Elizawei]

Okay so ICE tables are really neat for helping you determine the equilibrium constant of an equation when you're given insufficient information. Usually they'll give you some info on initial mol/concentration and some info on equib mol/concentration and you're meant to set up a little tablet to help you find the other info

I'll do an example with you:
Question- 0.10 mol of N2O4 is allowed to come to equib in a 2.0 L flask according to the following reaction:
N2O4(g) <---> 2NO2(g)
At equib, there are 0.060 mol of NO2. Determine K

Okay so basically we're given incomplete info, and to figure out K we need to set up an ICE table:



Since we're given the initial mol of N2O4 (0.10 mol) and the equib mol for NO2, we can fill them in at their respective spots. Now let's look at the column for NO2. Initially, when the reaction has just begun, there should be no products produced yeah? So the initial mol for NO2 is 0.00. Now at equib, we're given that there are 0.060 mol of NO2. From 0.00, we see a +0.060 mol change. So we pop the +0.060 in the table.

Now let's have a look at the N2O4. This is where the coefficients in the equation comes into play. According to the equation for the reaction, N2O4 produces NO2 in a ratio of 1:2 right? So for every two moles of NO2 made, we use 1 mol of NO2.
Therefore because we see an change of +0.060 mol in NO2, there should be a change of -0.030 in N2O4. (because 2*0.030 = 0.060). Do the simple calculation to find the equib. mol for N2O4 (0.10-0.030=0.07 mol).

Alright so we've got this far, but remember to calculate K you need to convert all the mol into concentration first. Note the 2.0 L container. (If the question doesn't give volume, assume it's 1L)

After we figure out the concentration, we can now construct an equib. constant, sub the concentrations in, and voila we get K.


Haha hope this makes sense, if my explanation is a bit iffy or if you don't get anything ^^ pls ask