ShmoopTube

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And here's your Shmoop du jour, brought to you by the smoke machine, your weird uncle [Man smoking outside]

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with a pack-a-day habit.

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Okay, here's our question.

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Solid CO2 is known as dry ice and is used in smoke machines and to keep food cold.

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For the process at 25 °C: CO2 (s) ? CO2 (g)

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What are the signs of ?G, ?H, and ?S

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And here are our potential answers.

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All right, what’s this question asking us? [Girl thinking about the question]

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Well, it requires us to think about the thermodynamics of the solid to gas phase change for CO2.

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Let’s think about what the sign of ?G, ?H, and ?S mean

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qualitatively.

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First up - ?G, or the change in Gibbs free energy. [Gibb's free energy table]

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No relation to Barry, Robin, or Maurice.

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We know that ?G tells us about the spontaneity of a chemical reaction.

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Like whether or not the chemical reaction will take a spur of the moment trip to Disneyland [Man on a rollercoaster in DisneyLand]

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with you.

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We hope so…we've been waiting to wear our mouse ears for years now…

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Anyway, if the ?G is negative, the reaction is spontaneous.

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Now what about ?H, or the change in enthalpy?

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The sign of ?H tells us if a reaction is endothermic or exothermic. [DeltaG sign with exothermic or endothermic]

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A positive change in enthalpy implies that the reaction is endothermic and absorbs heat,

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while a negative change in enthalpy implies that the reaction is exothermic and releases

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heat.

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As far as blankets go, you want the endothermic ones. [A dog wrapped in a blanket]

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Anyone selling you an exothermic blanket is a bad guy..

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And finally ?S, or the change in entropy.

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Entropy tells us about the degree of dispersal, or disorder.

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A positive change in entropy means that entropy has increased, and thus disorder has increased.

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Your messy room? [Boy in a messy bedroom]

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That’s a very positive change in entropy.

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Now that we have this handy dandy chart, the problem becomes a lot easier.

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That’s right, take it all in. [Man studying the chart]

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We're pretty sure this chart is in contention to be the eighth wonder of the world…

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… it's okay to cry.

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Okay, so let’s consider our specific “reaction,” which is just a phase change: [Scientist drops substance into a beaker]

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CO2 (s) ? CO2 (g)

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We want to decide if each variable in our chart is positive or negative for this reaction. [Girl sat waiting]

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Let’s walk through it.

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And yes, we'll do it hand in hand.

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Safety first.

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Is our reaction spontaneous?

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Yes—it happens at 25°C without any effort from us. [Pregnancy stick with 25 degrees shown]

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Easy, breezy, beautiful.

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Chemical reactions.

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Is our reaction endothermic or exothermic?

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AKA does it absorb or release heat? [fire burning]

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Well, if it’s used to keep food cold, it must absorb heat from the food.

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Thus, the reaction is endothermic.

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And finally, is our reaction product, a gas, more or less disordered than our reactant,

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a solid?

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Gas molecules are further apart and more dispersed. [Gas bellowing]

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Thus, they are more disordered than molecules in a solid.

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We don’t blame them.

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Everybody gets a little crazy when they’re far away from their pals. [Gas on a doctor's bed and doctor appears]

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Now, looking at our handy chart, we see that ?G is negative, ?H is

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positive, and ?S is positive.

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So let’s mosey on down back to our potential answers…

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It looks like choice (C) is the one we’re looking for. [Answer C circled and C dances in a club]

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It’s okay, choice C, we can’t all be positive all of the time.

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2 out of 3 times ain't half bad.

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