# thermodynamics question

#### sendwich

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-delta H -->exothermic rxn but you NEED energy to break bonds?? how does this make sense?

how is condensation (going from air-->liquid) an exothermic reaction? your'e going to LESS entropy, right? isn't that saying water-->is exothermic? (which isn't, it's endothermic) but you're going to a smaller entropy, correct?

i just dont understnad the break bonds=need energy....isn't that a +H to the reaction?

...confused.
thanks

#### TicAL

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gh said:
-delta H -->exothermic rxn but you NEED energy to break bonds?? how does this make sense?
In order to break bonds, energy does need to be put into the system. When the new bonds in the products form, however, they are at a more stable lower energy state than the original reactants. Since they are at a lower energy state, the excess energy is given off to the system (hence it being exothermic).

how is condensation (going from air-->liquid) an exothermic reaction? your'e going to LESS entropy, right? isn't that saying water-->is exothermic? (which isn't, it's endothermic) but you're going to a smaller entropy, correct?

i just dont understnad the break bonds=need energy....isn't that a +H to the reaction?

...confused.
thanks
Examine the opposite reaction. Going from liquid to air would require heat to be put into to break the liquid bonds to make the transition to gaseous state, this would be endothermic.

You can also think of going from gas to liquid as forming a less energy state (air molecules move with a higher kinetic energy than liquid). Since you're reducing the overall energy state, the left over energy is given off to the system.

#### cfdavid

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gh said:
-delta H -->exothermic rxn but you NEED energy to break bonds?? how does this make sense?

how is condensation (going from air-->liquid) an exothermic reaction? your'e going to LESS entropy, right? isn't that saying water-->is exothermic? (which isn't, it's endothermic) but you're going to a smaller entropy, correct?

i just dont understnad the break bonds=need energy....isn't that a +H to the reaction?

...confused.
thanks
Yep, entropy DOES decrease. But remember that exothermic is relative to enthalpy and not entropy. You can have a decrease in entropy of a system, while having a negative (exothermic) enthalpy.

To condense water vapor to form water, temperature is released. "Cold" doesn't "cool" the vapor. Rather, heat is released from vapor, and this forms the liquid. Since heat is released, it's exothermic.