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Write equations for the reactions of each of: (a) nitric acid (b) sulfuric acid *(c) hydroiodic acid, with each of: (i) sodium hydroxide (ii) zinc oxide (iii) ammonia.

So, the reaction of nitric acid with sodium hydroxide and zinc oxide is simple because I know that acid + base = salt + water. Therefore:

$$ \begin{align} \ce{HNO3(aq) + NaOH (aq) &-> NaNO3 (aq) + H2O (l)}\\ \ce{2HNO3(aq) + ZnO (s) &-> Zn(NO3)2 (aq) + H2O (l)} \end{align} $$

It makes sense. In the second part, for example, $\ce{Zn}$ is 2+ and $\ce{H}$ is -1. So using the criss-cross method, you get $\ce{Zn(NO3)2}$ and water. I understand this.

However, with ammonia:

$$\ce{HNO3(aq) + NH3 -> NH4NO3}$$

How am I supposed to know this equation? It's not metal + acid = salt + water. Is there word equations for everything?

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In the second part, for example, $\ce{Zn}$ is 2+ and $\ce{H}$ is -1. So using the criss-cross method, you get $\ce{Zn(NO3)2}$ and water. I understand this.

I don't agree on the -1 for hydrogen, let's look again at that example:

$$\ce{2HNO3(aq) + ZnO(s) -> Zn(NO3)2(aq) + H2O}$$

Metal oxides, such as zinc oxide, are basic anhydrides.

Imagine that

  1. $\ce{ZnO}$ was made by heating $\ce{Zn(OH)2}$ until it loses water:

$$\ce{Zn(OH)2 ->[\Delta] ZnO(s) + H2O}$$

  1. $\ce{ZnO}$ is a salt of $\ce{Zn^2+}$ and $\ce{O^2-}$

The reaction between nitric acid and zinc oxide and just a neutralization reaction between two protons and $\ce{O^2-}$ (which, as such, isn't present in solution).

Now for the reaction with ammonia: For this you may want to remember that there are different acid-base theories: The one that helps here is the Bronsted-Lowry theory:

  • Acids are proton donors

$\ce{HNO3}$ definitely is that.

  • Bases are proton acceptors

How can something else other than the $\ce{OH-}$ that you knew and the $\ce{O^2-}$ from $\ce{ZnO}$ be a proton acceptor = form a bond to a (proton)?

Is that possible if this acceptor doesn't even have a negative charge?

(Yes, otherwise I wouldn't buld up the tension like in a suspense story :D)

If the proton can't provide the electrons for the bond, the acceptor has to deliver them! And that can happen in the form of an electron pair, a double filled, non-bonding orbital.

Therefore a reaction, such as

$$\ce{H+ + NH3 <=> NH4+}$$

is perfectly valid. The rest of the example is just arranging the counter ions.

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How am I supposed to know this equation?

You have to memorise it. In aqueous solution, $\ce{NH3}$ deprotonates a small fraction of the water to give ammonium and hydroxide

$$ \ce{NH3 + H2O <=> NH4+ + OH−} \, , $$

and while it is usually impossible to isolate $\ce{NH4OH}$ itself, the above mentioned ions are there in the solution. And thus, an aqueous solution of $\ce{NH3}$ is a weak base and reacts with acids in the similar way as other weak bases.

How can I access all the word equations in chemistry so I am able to write every single equation given to me like these.

Again, learning general principles plus memorising all the special cases & exceptions is the only way.$%edit$

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  • $\begingroup$ Why then do I not have to memorise other reactions, such as acid + base = salt + water? Why did my textbook simply say "write out this equation". What is the point of making it an exercise if it simply requires memorisation? How am I supposed to know if I should be able to deduce this or memorise this? $\endgroup$
    – ROYGBIV
    Commented Mar 24, 2015 at 11:33

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