ShmoopTube

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Here’s your Shmoop du jour, brought to you by The Jackson 5. They think everything is [Jackson 5 singing]

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as easy as ABC, 123… Apparently they never took AP Chemistry. [Jackson 5 in a chemistry class]

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Okay, enough singing and dancing…here’s our question:

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Consider the following chemical reaction equation…

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Using this equation, what is the correct expression for the equilibrium constant, Keq?

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And here are the potential answers:

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Okay, okay, maybe we were a little hard on those Jacksons. This one really is as easy

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as ABC… D. Can’t forget D. [Jackson 5 in chemistry class and teacher shows A, B, C, D on board]

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Anyway, let’s get to our question. The equilibrium constant for any chemical

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reaction will always be the concentrations of the products divided by the concentrations

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of the reactants, with each species raised to the power of its stoichiometric

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coefficient. Yeah, where's your ABC, 123 now, Jackson Five? [Person taking photo's of Jackson 5]

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So for this reaction, the equilibrium constant is C times D squared divided by A squared

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times B cubed. But wait, there’s more! Call now, and you’ll [Man selling frying pans]

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get a second full set of banana-shaped frying pans for free, just pay $89.95 processing

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and handling! …Whoops. Sorry for the confusion, that’s

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our other job… tough economic times, right?

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But there actually is more… There’s an important trick to this question:

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we also need to consider the phase of each reactant and product.

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Remember that we only include the reactants and products that are in the gaseous state [Reactants and product put inside a box]

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in the equilibrium constant. We exclude species that are liquids or solids.

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That means we can’t include species C in the equilibrium constant expression.

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So, if we revisit our previous expression for Keq but remove species C, which is a liquid, [Species C removed from formula]

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we’re left with D squared divided by A squared times B cubed. And that’s choice B, the

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correct answer. Now, who wants some banana-shaped blueberry

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pancakes? If we’ve learned one thing today, it’s that this is a deal too good to pass [Man selling banana-shaped blueberry pancakes]

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up – don’t wait, call now! …Sorry. We have to meet a quota.

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