would this reaction be endo or exothermic: Pb2+ + 2I- <----> PbI2 (s)?

Question

how would you know?

Answer 1

Pb2+ + 2I- <----> PbI2 (s)?
Most often this reaction is shown as a double replacement reaction as follows:

Pb(NO3)2 + 2 KI ==> PbI2¯ + 2 KNO3
Since both solutions have to be heated before mixing, you might think it is an endothermic reaction requiring heat. However, the Lead Iodide precipitate will not form till the mixture is cooled. This would imply the reaction is actually endothermic since cooling or removing heat energy from the solution results in the precipitate.

Answer 2

alba has got the concept completely the wrong way around.

shapsjo has got the correct observation but the conclusion made is also wrong. the mixture precipitates when cooled because the reaction prefers a low temperature environment where it can release energy (yes it is exothermic). it makes no sense to call the reaction endothermic because it obviously cant absorb any heat from a cold environment.

this reaction is exothermic, as are almost all precipitation reactions. you also need not consider whether this is a single or double displacement reaction because this reaction occurs in an aqueous medium. all you need is the net ionic equation (which is given here) as the rest are spectator ions and will neither release nor absorb heat.

even if you didnt know that precipitation reactions were exothermic, you can see from the net ionic eqn that only bonds are formed, none are broken, so the process is one that only releases energy.

if you had lessons in thermochemistry you would also note that the energy released is called enthalpy of lattice energy (when aqueous ions form a solid) and this is always a negative value (exothermic).

Answer 3

Exothermic. Pb-I bonds formed releases energy.

Answer 4

It has been a long standing dispute as to whether a reaction is exo or endo depending on one's reference frame to the material. If you are "outside" looking in then exo appears to be giving off heat but if you are "inside" looking out then to you exo means you are getting colder.

Answer 5

off the top of my head it has got to be endothermic. bond formation pb-I requires energy. While splitting up bonds (think of the hydrogen bomb where u split up the hydrogen bond) releases energy.

Answer 6

endothermic