9. Which of these process is always exothermic?

Question

1. melting
2. condensing
3. boiling
4. Evaporation

Answer 1

Exothermic means that heat is removed or released from the system in question.  An easy way to look at this is to ask yourself the question, what do I have to do to something in order for this to happen?  Do I have to add heat to it or should I cool it down (remove heat)?

 

1. melting - if you want to melt something, you heat it up!  This means heat must be added to the system for this change to occur.  How do you melt ice quickly?  Put it on the stove or run hot water over it.  Melting is endothermic.

2. condensing - Condensing is the opposite of vaporization. Where does condensing occur?  On the outside of your cold drink! Why?  Because the molecules of water vapor touching the cold glass will lose their heat, slow down, and become liquid droplets.  Therefore, condensation is exothermic, because heat must be removed from the gaseous molecules in order for them to condense. 

 

If you have trouble seeing this, look at the opposite process - vaporization, which takes liquid to a gas.  Doesn't that take energy input to accomplish, and therefore is Endothermic?  If so, then the opposite of vaporization (condensation) will require the opposite energy flow (Exothermic)

3. boiling - How do you boil water?  Do you put heat into it or do you remove heat from it?  You put it on a stove and put heat into the water to make it boil.  Therefore, boiling is endothermic.

4. Evaporation - Another word for this is vaporization - this is the opposite of condensation. If you want liquid to evaporate more quickly, what do you do?  You heat it up.  This is why we use hairdryers to blow hot air on our heads to dry our hair!  Evaporation requires heat for the molecules to go from liquid to gas, therefore it is endothermic.

Answer 2

A general rule of thumb is "Exothermic reactions create a decrease in disorder". Let's use this:

1. This goes from a solid to a liquid, liquids have more disorder than solids. This is endothermic.
2. This goes from a gas to a liquid, gases have more disorder than solids. This is exothermic.
3  This goes from a liquid to a gas. This is endothermic.

4. This goes from a liquid to a gas. This is endothermic.

The rule of thumb works because everything must cause an increase in entropy(disorder). thus if it seems like the opposite, the reaction must be releasing the lost energy to the surroundings.

Sorry for any grammar mistakes, currently have a kitten attacking me.