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which process represents a decrease in entropy

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The general idea here is that entropy increases as disorder and randomness increase.

Similarly, entropy decreases as disorder and randomness decrease.

Now, randomness and disorder increase as a substance goes from solid to liquid, and finally to gas. On the other hand, randomness and disorder decrease as a substance goes from gas to liquid, and finally to solid.

So right from the start, you know that any reaction that has a gas as a reactant and a liquid as a product, for example, will result in a decrease in entropy.

In your case, you have


A liquid is being converted to two gases, so entropy will increase.


A solid is being dissolved to produce solvated ions, so in general, you can say that entropy will increase. This is not a very good example because there are solids that can be dissolved in water to a decrease in entropy.

I'm not going to go into why that is the case, just keep in mind that it is possible.

Now, the first two reactions involve gaseous reactants and gaseous products. In such cases, look at the total number of moles of gas present on the two sides of the chemical equation.

When it comes to reactions that involve gases, you will have

  • more moles of gas on the reactants' side → decrease in entropy
  • more moles of gas on the products' side → increase in entropy

Notice that this reaction


has a total of 3 moles of gas on the reactants' side and only 2 moles of gas on the products' side.

This means that this reaction will show a decrease in entropy.

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