PP Presentation
GAS FORMING REACTIONS - alternative classification
A. CaCO3 (s) + 3 HCl (aq) ® CaCl2 (aq) + H2O (l) + CO2 (g)
There is no precipitate and no change in O.N. (Ca stays as +2, Cl as -1)
It is an ACID-BASE REACTION involving an intermediate H2CO3
called CARBONIC ACID, which immediately dissociates to H2O and CO2
2 H+ + CO32- ® “H2CO3 “ ® H2O + CO2
B. Zn (s) + 2 HCl (aq) ® ZnCl2 (aq) + H2 (g)
There is no precipitate. However there is an obvious change in O.N.
It is also an example of an OXIDATION-REDUCTION REACTION
Zn goes from O.N. = 0 in the metal to O.N. = +2 in the chloride salt
H goes from O.N. = +1 in HCl (aq) to O.N. = 0 in the elemental gas