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?


2 Answers 2


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.


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.$%edit$

  • $\begingroup$ 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? $\endgroup$
    Mar 24, 2015 at 11:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.