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Endothermic or exothermic?

Cu(s) + 4HNO3 --> Cu(NO3)2(aq) + 2NO2(g) + 2H2O

How do you know if this is endothermic or exothermic?

Thanks in advance

kingchemist Sat, 11/07/2009 - 05:08

It is exothermic from experience.
However, it is not easy to tell just from the equation.

You may know that bond breaking is an endothermic process while bond making is an exothermic process. You are making two molecular products, NO2 and H2O so energy will be released when these are made. However, copper metal has to be broken apart so this requires energy. HNO3 is already ionised (H+ and NO3-) so nothing to be done with this. Also Cu(NO3)2 is also ionised (Cu2+ and 2NO3-) so no energy released here. However, when Cu dissolves it forms Cu2+ ions which form bonds with H2O to make Cu2+(aq), more energy will be released.

So it's whether the energy needed to break metallic bonds in Cu is more than or less than the energy released when NO2 and H2O and Cu2+(aq) are made