I don't understand well the acid-base reactions, what happens with the ions? are they stable?
If we change HCl for NaOH, does the same happen?
The purpose of adding acid to an oxidizing agent is to help it function more effectively in aqueous solution. Such agents can function well enough in the solid state, and often with much more rapid release of energy. Just try mixing some potassium permanganate with magnesium powder and igniting the mixture, although I would of course recommend extreme caution when doing so. Don't confine it, otherwise it can explode. Wear safety glasses, and use a fuse on the end of a long pole for ignition. This combination is actually used in military flash-bang grenades.
With K*2Cr2O7* and HCl:
The H+ ions from the acid combine with the oxygens from the oxidizing agent to make water molecules. When said agent gets reduced, K+ and Cr3+ ions are left over. These combine with the Cl- ions from the HCl to leave KCl and CrCl*3* in solution.
You should remember that when potassium dichromate is dissolved in water, it dissociates like this:
K*2Cr2O7* + H*2O —–> 2 CrO4*2- + 2 K+ + 2 H+
The overall reaction of the CrO*4*2- ions looks like this:
CrO*42- + 8 H+ + 3 e– —–> Cr3+ + 4 H2*O
This can of course be written assuming that you are starting from dichromate ions:
Cr*2O72- + 14 H+ + 6 e– —–> 2 Cr3+ + 7 H2*O
The reaction in alkaline solution is similar, except that water molecules are involved on the left hand side instead of H+ ions from acid. The reaction equation looks like this:
CrO*42- + 4 H2O + 3 e– —–> Cr(OH)3* + 5 OH-
Notice that the Cr3+ isn't left in solution – it combines with OH- ions to precipitate Cr(OH)3. This can also be written assuming that you are starting with dichromate ions:
Cr*2O72- + 14 H2O + 6 e– —–> 2 Cr(OH)3* + 8 OH-
In this case, the oxidizing agent is reduced by the same amount (Chromium goes from +6 to +3) whether in acid or alkaline solution. But not all oxidizing agents behave this way. Another common oxidizing agent, potassium permanganate, reacts quite differently in alkaline solution than it does in acid solution.
The reaction of MnO*4*- ions in acid solution is:
MnO*4– + 8 H+ + 5 e– —–> Mn2+ + 4 H2*O
The reaction in neutral or alkaline solution is:
MnO*4– + 2 H2O + 3 e– —–> MnO2* + 4 OH-
In acid solution, the manganese goes from +7 down to +2. But in neutral or alkaline solution it is less powerful, going from +7 down to +4.