ShmoopTube

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And here's your Shmoop du jour, brought to you by equilibrium, the human rights movement [People protesting outside a library]

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that insists librarians should be treated equally.

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Okay, here's our question.

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Which of the following is true for a reaction at equilibrium?

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And here are your potential answers.

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Okay, don’t get all “one with nature” on us here…we’re talking about molecular [Boy balancing books on his head]

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equilibrium here.

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That is, the point at which the forward rate of a reversible chemical reaction equals the

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reverse rate. [Equilibrium of reaction depicted on a graph]

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The concentrations of reactants and products don’t change, even though reactions are

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still taking place.

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So A and B make C and D at the same rate that C and D make A and B.

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You thinking what we’re thinking? [Two guys jumping up and down]

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Great, let's say it together.

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3, 2, 1…Bananas!

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…Oh.

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Or example time.

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That's good, too.

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Okay, imagine that there’s a team of mathletes who decide that they should socialize more. [Mathletes socialising with each other]

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So they schedule a hang-out sesh with the science Olympiad team studying next door.

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We're sensing this is going to end in a rager. [Mathletes and Science Olympians together]

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…Just us?

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At some point, the students are split among the rooms, and the rate at which a mathlete

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leaves his room is the same as the rate at which a different mathlete enters. [Mathletes leaving and entering the rooms]

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The same is true for the science Olympians.

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This smarty-swap can be thought of as our “reaction.”

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The relative “concentration” of mathletes and science Olympians remains the same, despite

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the fact that there’s still movement and different individuals might be present at [Olympians and Mathletes interchanging between rooms]

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any given time.

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And here we have achieved the ultimate nerdy equilibrium.

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Hm.

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Really thought it would end in a rager.

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So let’s take another look at our question. [Boy using binoculars]

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What do all these variables mean?

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Well, DeltaH - or change in enthalpy - tells us if a reaction absorbs or releases

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heat.

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DeltaG, or the change in Gibbs free energy, tells us if a reaction is spontaneous

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or non-spontaneous.

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And DeltaS, or the change in entropy, tells us if a reaction leads to more or less [Delta H, G and S definitions]

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disorder.

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Of the science-y kind, not the mental kind. [Boy spinning a basketball on his finger]

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Though too much chemistry will give the best of us anxiety.

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Option A just means the reaction releases heat. [Girl cooking marshmallows on a fire]

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It doesn’t tell us anything about equilibrium.

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Option C tells us that there is no entropy change.

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This just means that the degree of disorder remains the same.

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Option D implies that there is no heat absorbed or released from the reaction.

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All right, Option B. Not to Obi Wan you, but you’re our only hope. [Obi Wan appears and prods option B]

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This option is all about spontaneity.

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We know that if DeltaG<0, the reaction is spontaneous, and if DeltaG>0, the reaction is not spontaneous.

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But what if DeltaG equals zero? [Boys walking a tight rope together]

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This means the reaction is at equilibrium, and forward and reverse rates are the same.

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So B is the right answer.

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And there you have it folks, equilibrium.

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Now let’s check on our smarty-pants party before somebody blows up a Bunsen burner. [Olympians and Mathletes socialising together in classrooms]

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