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Say we have a given redox reaction: $$\ce{MnO4- + Fe^2+ -> Fe^3+ + Products}$$ This is an unbalanced redox reaction, just depicting how $\ce{Fe^2+}$ is oxidized to $\ce{Fe^3+}$ by $\ce{KMnO4}$. Now, depending on the medium, the following reactions may take place: In an acidic medium, $\ce{MnO4-}$ is reduced to $\ce{Mn^2+}$ by accepting 5 electrons $$\ce{... 2 In acidic medium the equation must mention somewhere that some \ce{H+} ions appear somewhere in the equation. In basic medium, the equation must mention somewhere that some \ce{OH-} ions appear somewhere in the equation, For example, you may say that permanganate ion reacts in acidic conditions to produce \ce{Mn^2+}. In basic conditions, it could not ... 0 I understand the procedure but not the questions. So there are two reactions:$$\ce{2HCl(aq) + CaCO3(s) -> H2CO3(aq) + 2 Cl-(aq) + Ca2+(aq)}\ce{HCl(aq) + NaOH(aq) -> H2O(l) + Cl-(aq) + Na+(aq)} The first reaction has reactants in non-stoichiometric amounts. The second reaction, a titration, is done with stoichiometric amounts, yielding a ...
When constructing chemical equations, you cannot work out the subscript of each product. The formula cannot be guessed. In your example, you must know the formula $\ce{Ca, N2}$ and $\ce{Ca3N2}$. This being given, your job is to find the good coefficients 3, 1 and 1.