science question?

Question

how are endothermic and exothermic reactions related to loss or gain of heat

Answer 1

"Exothermic" comes from the Greek and means "heat goes outside" i.e. gives off heat. (The Ex in Exothermic derives from the Greek word for "stranger" or "outsider".)

"Endothermic" again comes from Greek. It means "heat goes inside" i.e. absorbs heat.

Exothermic reactions occur when two (or more) molecules react in a way that produces something more stable than its components taken individually. In general, such reactions involve sharing electrons that weren't previously shared. The result is usually a very stable molecule.

Endothermic reactions occur when the reactants combine in a way that is less stable. These reactions often involve actually moving electrons from one atom to another permanently, sometimes by breaking bonds that shared those electrons.

There are exceptions to every rule, of course. Salt formation can be endothermic OR exothermic depending on the specifics of the reaction.

Here's an example of a sequence of related reactions that is both endothermic AND exothermic.

Take gasoline and light it with a match. The match supplies enough heat to form oxygen radicals - endothermic because you have to break the bond between the two parts of the O2 molecule. But then the oxygen radicals start combustion of the gasoline to form CO2 and H2O - and that is exothermic.

The heat expelled by the combustion continues the cycle of forming oxygen radicals, which affects the gasoline, which continues the cycle, ... until you are out of either O2 or gasoline.

It is hard to decide ahead of time which reactions will be endo and which will be exo, but if you can end up making very stable compounds, odds are it will be exothermic.

Answer 2

exothermic is the loss of heat by a system

endothermic is the gain of heat by a system

lighting a match is exothermic

melting ice is endothermic