When do solids and liquids drop out?

[wired202808]

Advertisement - Members don't see this ad

For entropy related problems i.e. Which of the following shows an increase in entropy? Do you count all solids states?

 

[mh0000]

The question is a little vague, and so will be the answer. It is a little difficult to predict a change in entropy of a reaction unless it involves a change in the number of moles of gas. If it doesn't, then one must use tabulated entropy values to calculate the entropy change of the reaction. Heating usually increases the entropy, as well as dissolving most salts (the 3+ ions usually show a decrease in entropy due to their high charge density), molecular rotation and higher molecular masses also tend to have a higher entropy for the same phase. Heavier elements also tend to have higher entropy.

Hope this helps a little

 

[wired202808]

The question is a little vague, and so will be the answer. It is a little difficult to predict a change in entropy of a reaction unless it involves a change in the number of moles of gas. If it doesn't, then one must use tabulated entropy values to calculate the entropy change of the reaction. Heating usually increases the entropy, as well as dissolving most salts (the 3+ ions usually show a decrease in entropy due to their high charge density), molecular rotation and higher molecular masses also tend to have a higher entropy for the same phase. Heavier elements also tend to have higher entropy.

Hope this helps a little

ok so heres what i meant:

Question: Which of the following shows an increase in entropy:
answer: 1M of solid + 1M of Liquid -> 1 M aqueous + 1 mole of gas

So is this the answer because it went from solid to liquid to gas or because you did not include solid and liquid as Moles and therefore went from O moles to 2 moles?

 

[mh0000]

ok so heres what i meant:

Question: Which of the following shows an increase in entropy:
answer: 1M of solid + 1M of Liquid -> 1 M aqueous + 1 mole of gas

So is this the answer because it went from solid to liquid to gas or because you did not include solid and liquid as Moles and therefore went from O moles to 2 moles?

This will show an increase in entropy largely in part of the increase in the number of moles of gas. It also looks like a salt (the solid reactant) was dissolved, which also tends to increase entropy (except highly charged ions as mentioned in my previous post).

 

[wired202808]

This will show an increase in entropy largely in part of the increase in the number of moles of gas. It also looks like a salt (the solid reactant) was dissolved, which also tends to increase entropy (except highly charged ions as mentioned in my previous post).

right so are you then not counting any moles on the reactant side because its a solid and liquid there n then +1 on the products because of the gas? or are you looking at it simply as an increase in entropy because it went from a solid reactant to a gaseous product?

 

[mh0000]

right so are you then not counting any moles on the reactant side because its a solid and liquid there n then +1 on the products because of the gas? or are you looking at it simply as an increase in entropy because it went from a solid reactant to a gaseous product?

Yes, a change in entropy requires to look at how the reactants changed into products. Like I said before, it's not always so simple to look at a reaction adn predict the direction of entropy change without calculating it. However, there are a few instances in which you can just look at the reaction and do so. A change in the number of moles of gas is usually a good indicator of how the entropy will change (assuming there are even gas reactants or products, or even if there is a change in # of moles). Phase changes, dissolving, heating\, breaking open a ring into a linear chain can also indicate a change in entropy.