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Tips for Writing Equilibrium Constant Expressions - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are some tips, and tricks for writing equilibrium constant expressions. So here is four basic things that will make your life easier. Make sure you balance the equation first. So don't forget, always make sure your equation is balanced. Then make sure you write K= because that would be part of your equilibrium constant expression. That stands that capital K stands for the constant.

Then number 3; make sure you omit solids, and liquids, because they don't count. Their concentrations stay constant. Then remember the coefficients become the exponents. So let's take a look at a few examples.

So in our first example, we have N2 plus H2 gas yields NH3 gas. All of them are gases. So this is homogenous equilibrium that we have going on here.

So when we balance it, I'll put 2 coefficients here. 2 and the 3. Then we'll write K= and then remember, always products over reactants. So don't forget.

So we have NH3 gas. If we want to make it concentration we put brackets. So we put [NH3]. Since the coefficient's 2, we'll put squared there. That's all my products. So now we go to the reactants.

So we have N2 gas. Since it has concentration, we put in brackets. Then there's only 1, so no coefficient times [H2]3, so it's cubed. That's the equilibrium constant expression.

That K could be the same as Kc, or Keq. The C stands for concentration. So like the equilibrium constant with relation to concentration. 'Eq' obviously would stand for equilibrium. That's exactly the same as the K that you have here. So these are all the same. They mean the same thing.

You might see Kp where p stands for pressure, so you might have gases. So let's say for example if they ask you to write the Kp expression, for the same equation that we're going to do. You follow the same method; products over reactants. So instead of brackets, you will put parenthesis, because that stands for atmospheres per units. So we put P, and then we'll put the NH3 as a subscript, and then we'll square that just like before. We'll put P, N2, and no coefficient. So there's still 1 times (PH2), then we'll cube that.

So it's identical to the K or the Kc, or the Keq, except it deals with pressures since you have this here, and you have the parenthesis. So you can have Kp.

Let's take a look at our second example. So first thing to do; make sure you balance the equation first. So we're going to balance the equation. So if you balance the equation you would find that you need 3 in front of the H2+, and 3 in front of this Silver.

Then write K=. Then we're going to omit the solids and liquids. So we have a couple of solids here. So I'll cross them out just to make them easier. Then coefficients become the exponents, then products over reactants. So we have products. So we have [Fe3+], that has the coefficient of 1. So the exponent is 1 over [Ag+], the coefficient is 3, so the exponent becomes 3. That's how you write the K expression for this particular equation.