General Chemistry Q Help Needed

[RiPSTONE]

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After a hand pump has been operating, the shaft is warm and the tip of the needle is cool. This is because:

A. airiscompressedintheshaftofthepump,andit expands as it enters the needle tip.

B. air is compressed in the shaft of the pump, and it expands as it leaves the needle tip.

C. air expands in the shaft of the pump, and it is compressed as it enters the needle tip.

D. air expands in the shaft of the pump, and it is compressed as it leaves the needle tip.

Answer is B and I thought it was D because exothermic feels warmer and endothermic feel cooler.

 

[el_duderino]

Compressing gas makes it warmer. Gas leaves the shaft because the pump compresses it, and it decompresses at the tip as it leaves.

 

[RiPSTONE]

Thank you. Does this make sense?

When air is compressed the molecules become closer together and "form bonds" = exothermic = feels warmer

and

When air expands the molecules get farther apart and "bonds are broken" = endothermic = feels cooler

 

[el_duderino]

I wouldn't put it that way. You're not really forming bonds simply compressing gas.

 

[RiPSTONE]

Okay, but why does the gas get warmer?
I'm trying to think of PV = nRT now and compressed means smaller V, so greater P while n and R are held constant, so how would we know the effect on T?

 

[el_duderino]

Because you're doing work on the gas. The energy you put into compressing it turns into heat. When the gas decompresses, it does work on its surroundings therefore losing energy.

 

[RiPSTONE]

Thank you for the clarification!