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Tips for Acid-Base Net Ionic Equations - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are some tips, and tricks for writing acid-base net ionic equations. Now we all know that if you add an acid and a base, you always get 2 products; salt, and water.

So in a strong acid, and strong base for example, you have HCl plus NaOH. You end up with NaCl plus H2O. But that's not the net ionic equation. What happens is the NaCl cancels out, because the NaCl is aqueous. So that ionizes, and you're left with the water.

So what you end up with is you always end up H+ plus OH- yields H2O, because HCl, and NaOH, since they're strong, they fully dissociate. So the Na+, and the Cl - are spectator ions. So with a strong acid and strong base, the net ion equation is always H+ plus OH- yields H2O. Super easy.

So in number two, say if we have HCl and Ammonia, NH3. Ammonia is a weak base. So when you add them together, you end up with NH4Cl, then the water is in the solution that you have there. Well, what happens is, so the HCl fully ionizes or dissociates. So you end with H+. But the Cl- is actually a spectator ion here. The NH3 only partially dissociates. So the NH3 keep it together, so keep weak together. Then you end up with NH4+.

So basically the shortcut is, when you have H+, strong acid, you always put the H+ there plus the NH3, so you keep that together, because it's weak. All you do is you add together what you have, to make your product. So that should make that easy.

We'll use the same philosophy with number 3. Weak acid, strong base. So say if I have a weak acid HF, Hydrofluoric acid, and I add it to a strong base. Let's just use Sodium Hydroxide again. Then I make my products. I get NaF plus H2O.

So then if you take a look here. So HF, that was a weak acid, so weak keep together. So weak keep together, because it only partially dissociates plus NaOH, the Na would be a spectator ion. So I have to take care of that. So I'm left with OH-, because the OH- becomes fully dissociated from a strong base yields. Since we took the Na+, then we keep the F- that we have there, and then plus the H2O.

So if you take a look here, so the shortcut is, keep the weak acid together. So that's why I have HF plus when I say strong base, you just use Hydroxide. Then yields, then basically what you want to do is you want to make water, so you basically take one away. Taking H plus OH from the acid, and then that's how you get the water. Then you get your F- which is 0, you get up your HF.

Then the last one; weak acid, and weak base. So let's do HF plus NH3. So what ends up happening is since they're already weak, keep together. So both of them are weak, so keep them together. The acid is going to become so you have H+. Then you are left with F- plus NH4+. That's all you've got to do.

So keep them together if its a weak acid, and a weak base. Then all you have is to donate the proton, or the H+ from the acid to the weak base, then you get your products.

So hopefully this mini-tutorial gives you some ideas of the patterns that you need for writing acid-base in their ionic equations. Have a good one.