ShmoopTube

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And here's your Shmoop du jour, brought to you by spontaneity.

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Otherwise known as why we're not going to finish this Shmoop du jour. [Shmoop employee at a desk and disappears]

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We're going to Mexico, instead.

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Vamos! [Car driving to Mexico]

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Okay…maybe our spontaneous trip can wait.

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Here's our question.

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Consider the following reaction…right here…

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Done checking it out?

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Okay, moving right along…

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The enthalpy of formation for the reaction is -92.0 kJ, and the change in

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entropy is -55.4 J/K at 30 °C. What is the DeltaG of the reaction, and

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what is the spontaneity of the reaction, respectively?

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And here are your potential answers. [Potential answers to the reaction]

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So we’re given a few different pieces of information in this problem. [Puzzle pieces separated]

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We’ve got values for the enthalpy or heat of formation,..

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…the entropy change,..

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…and the temperature.

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And we’re asked for the ?G, or the change in Gibbs free energy.

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And no, by free energy, we don’t mean a blow-out sale on Red Bull. [Girl shopping for packs of Red Bull]

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The Gibbs free energy tells us about the spontaneity of a reaction.

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We need to know the value, and specifically the sign, of DeltaG.

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We define DeltaG using the Gibbs free energy equation, DeltaG=DeltaH-T DeltaS

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Anybody else craving alphabet soup? [Dinner lady serves boy alphabet soup]

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We have values for all of the variables on the right hand side of this expression.

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But before we plug them in, we have to consider their units.

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There’s a few sneaky little guys that we need to convert before we plug them in, plug [Person plugs B into a wall socket]

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them in.

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The answer needs to be in kilojoules, so first convert DeltaS to kilojoules by multiplying

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by the number of kilojoules in a joule.

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And let’s not forget about temperature — DeltaS includes a kelvin unit and our given

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temperature is in Celsius.

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The kelvin scale is an absolute scale––it has a value of zero at absolute zero––so [Woman stood on a weighing scale]

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we typically choose to use Kelvin in our equations.

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Are we sure that’s better?

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Absolutely.

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Converting our temperature units, we find that our temperature is 303.15 Kelvin.

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Now we just have to do the math…

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Chugga-chugga-chugga, bing-bang-boom, and…

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DeltaG= -75.2 kJ [Answer for the reaction]

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Looking again at our choices, we see that choice D is what we want.

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But we’re not done yet!

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Is this reaction actually spontaneous? [Couple talking beside a stream]

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Spontaneity depends on the sign of DeltaG

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A negative DeltaG, a change in Gibbs free energy means energy is released and a reaction is

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spontaneous, and a positive DeltaG means energy is added and

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the reaction is not spontaneous.

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So the negative DeltaG we calculated does correspond to a spontaneous reaction.

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High five. [Boy holding out a hand for a high five]

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Seriously.

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Slap your screen. [Person slaps boys hand for high five]

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Don't leave us hanging…

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Now come on!

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Mexico awaits!

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…No?

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You guys have responsibilities?

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Drat.

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Spontaneity looked awfully good in the movies… [Car spontaneously drives to Mexico]

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