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I hope this isn't a dumb question but how do we know when to use ICE as opposed to just plugging in values into: Kc = [products]/[reactants]? I've been doing problems the latter way when the problem is solved through ICE in the answer key.
Hi, generally you would use ICE charts to help when you have an initial concentration and an equilibrium concentration. When you solve for the change in concentration, you are able to get the equilibrium concentrations and use Kc accordingly. Typically, when the question gives or states that the concentrations are all at equilibrium or such then you could simply plug in these values and use Kc directly. However, I think it's important to keep in mind that you don't have to use an ICE chart either, but it could help with organization and for visuals. Hope this helps!
ICE is never necessary. It is a way of organizing the information you're given and finding your answer in a clear way. Now, you do have to use the placeholder of x to find out exactly how much the concentration or partial pressure changed. I'm not a hundred percent sure exactly what you mean by just plugging in to the expression: if you are just plugging in the values given in the problem, or if you are doing the ICE chart in your head and putting the proper expressions for each concentration. Assuming you are doing the prior, you are probably using the initial values, which aren't correct because the question is asking for the value at equilibrium. The easiest way to tell if you need to use ICE is if they give you the values of the concentration at equilibrium or initially. If they give you initial concentrations, you have to use ICE and solve for x.
I think it's especially important to pay attention to the words "equilibrium" and "initial". If only equilibrium concentrations are present, then I think it's safe to assume that the ICE tables aren't necessary. However, if initial concentrations are present, then using an ICE table is necessary.
It's just a means of organization to help you better see your known and unknown values and how values find relation to each other (such as x-something is one substance's concentration at equilibrium while x or coefficient*x is another substance's concentration).
To figure out whether to use a ice table or the equilibrium constant equation you must pay attention to what is given in the problem. If you are given the initial concentrations of the reactants and an equilibrium concentration, you should use the ICE table to find the concentrations of the products and reactants at equilibrium. If you are given all the concentrations at equilibrium already and are asked to find Kc, then you should plug the values into the equilibrium constant equation to find Kc.
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