ICE table

[Miranda 1J]

When doing ICE table problems how do we know when to assume that x is 0?

[nickjadidian 1A]

If x is zero, it means that there is no change in the concentrations of the products or reactants. In this case, x will be zero for both sides. This also implies that the forward and backward reactions have already achieved dynamic equilibrium. The whole point of the ice table is to determine the equilibrium concentrations of all the products and reactants. When x is zero, the chemical concentrations given in the problem are the equilibrium concentrations of the chemical equation.

[JennyCKim1J]

Does x=0 mean that the amounts of both reactants and products stay the same?

[Scott Chin_1E]

Well we can't assume x=0 because then that would mean there was no change and we know that in a weak acid/base reaction, equilibrium is in constant motion. Instead, we assume x is a very small number, so much so that subtracting it from the initial concentration is negligible and wouldn't affect the overall initial concentration.

For example, if the initial concentration is 0.15M, subtracting that number by 0.000000000000001 would not make a difference to 0.15 at all, thus we can omit x when subtracting it from the initial concentration.

A general rule of thumb is that for any or that is less than we can assume that the change in concentration for the initial concentration given will be so small, it will not affect the overall answer at all.

[Vincent Kim 2I]

Basically when the Q and K values in the problem are very close, then the changes in concentration (x) will be extremely small, even negligible in most cases.

[Clarissa Molina 1D]

Can somebody explain the 5% rule when approximating x please? Is K being less than 10^-3 enough to be able to approximate or does it also have to follow the 5% rule?

[Angel R Morales Dis1G]

Can somebody explain the 5% rule when approximating x please? Is K being less than 10^-3 enough to be able to approximate or does it also have to follow the 5% rule?

The 5% rule and K being less than 10^-3 go hand in hand. The concentrations of the acids/bases are so little, that it should end up being less than 5% the initial concentration. So yeah, K being less than 1.00 x 10-3 is reason enough to approximate.

[Guadalupe T 1E]

Can somebody explain the 5% rule when approximating x please? Is K being less than 10^-3 enough to be able to approximate or does it also have to follow the 5% rule?

It was mentioned that 10^-3 should be enough to use approximation but you can be asked to prove that it follows the 5% rule.