Acids and Bases part 2

For part one, click here.

In Part 1, I talked about basic properties of acids and bases, the Arrhenius and Bronsted-Lowry theories, strong and weak acids and bases and some standard reactions. Today I'm going to finish off the standard reactions, and then go off onto some slightly new stuff. (I say "slightly new" because nothing you learn at this stage is really completely new but has its roots in something you already know. Remember that when you're learning "new" stuff, okay?)

Okay, on to some new standard reactions-  amphoteric metal/metal oxide/metal hydroxide reactions with bases. I hate these reactions. I know that expressing my hatred for these reactions isn't going to help you learn them, but whatever. Adding to my hatred for amphoteric metal reactions is that I can't explain them to save my life, which is also why I can't remember them. It doesn't help that complex ions are annoying either (in my humble opinion anyway).

First up is the good ol' amphoteric metal + base reaction:

Amphoteric metal + base -> complex ion + hydrogen gas

2Al + 2NaOH + 6H₂O à 2NaAl(OH)₄ + 3H₂

Then there's the amphoteric metal oxide and amphoteric metal hydroxide with base reactions:

Amphoteric metal oxide + base -> complex ion

Al₂O₃ + 2NaOH + 3H₂O à 2NaAl(OH)₄

Amphoteric metal hydroxide + base -> complex ion

Zn(OH)₂ + 2NaOH à Na₂Zn(OH)₄

Please note that the amphoteric metal and amphoteric metal oxide reactions occur with water as a reactant.

I can't really explain what a complex ion is (okay, well I can tell you my inference that a complex ion has both a metal and a non-metal component, but you probably already inferred that), but I can give a few examples from my revision book:

Tetrahydroxoaluminate (aluminate): Al(OH)₄^-

Tetrahydroxozincate (zincate): Zn(OH)₄^2-

Tetrahydroxochromate (chromate): Cr(OH)₄^-

Don't get too scared by those super-long names, by the way. Just remember, "tetra" means 4 and "hydroxo" is a reference to the hydroxide ion. Anyway, you don't really need to know the names of these ions.

The charge on the ion is just the positive charge on the metal part subtract 4 (since each hydroxide ion contributes a -1 charge and there are four hydroxide ions in a tetrahydroxo- complex ion).

Since I don't really understand amphoteric metal equations, the way I memorise these is just by thinking about what's similar and what's different between the three equations. Remembering which equation corresponds to which difference is the problem though...

The main similarity is that all equations produce complex ions. There are a couple more obvious differences though.

Differences:

• Only the amphoteric metal reaction produces hydrogen gas. Maybe this is due to the lack of oxygen atoms attached to the original amphoteric metal.
• Only the amphoteric metal hydroxide reaction doesn't involve water. I'm guessing that this is due to the fact that a hydroxide already includes hydrogen atoms so that a reaction with water isn't required.

Now that I've written that, it seems to make slightly more sense to me...

Now onto the slightly new stuff. First I'm going to talk about monoprotic and polyprotic acids. Monoprotic acids only have one hydrogen atom per molecule, while polyprotic acids have multiple hydrogen atoms per molecule and therefore can ionise multiple times. Remember, mono- means one, and poly- means multiple. You can also call acids with two hydrogen atoms per molecule "diprotic" acids, and I'm assuming that you can call those with three hydrogen atoms "triprotic" acids.

I'm going to provide an example on how polyprotic acids undergo successive ionisation. The example that I'm going to provide here is a strong diprotic acid, sulfuric acid.

H₂SO₄ + H₂O -> H₃O⁺ + HSO₄^-

HSO4^- + H₂O <--> H₃O⁺ + SO₄^2-

In the first reaction, sulfuric acid completely ionises to form the hydrogen ion (which doesn't exist very long in water and quickly forms the hydronium ion) as well as the hydrogen sulfate ion. The hydrogen sulfate ion then reacts to produce the hydrogen-then-hydronium ion, as well as the sulfate ion. Since hydrogen sulfate is a weak acid, it only partially ionises in water. Some hydrogen sulfate will stay as hydrogen sulfate. The final solution (by this I mean what you end up with as a result of these reactions, not the Nazi Party's Final Solution of course- sorry for the really lame not-very-funny joke) will therefore have no sulfuric acid, lots of hydronium ions, quite a few hydrogen sulfate ions and a small amount of sulfate ions.

All this talk of hydronium ions reminds me that I need to tell y'all about pH. pH, which runs on a scale between 1 (or is it 0?) and 14, is a measure of how acidic or basic a solution is. Acids have a low pH while bases have a high pH. Neutral substances have a pH of 7. You can measure the pH of a substance by using an indicator such as litmus paper or universal indicator.

Mathematically, though, what is pH? Well, pH runs on a logarithmic scale, which is why I wrote about logs before I wrote this post, in a post aptly titled "Logarithms," though I must say I didn't explain logs all too well there. Here's the equation:

pH = -log [H+]

[H+] is hydrogen ion concentration (in moles/litre).

Don't worry, mathophobes, it's easy enough to plug this equation into a calculator to get the pH from the hydrogen ion concentration. (You don't even have to remember the equation because it's in your data sheet.) The hard part is getting the hydrogen ion concentration in the first place.

With a strong acid, it's easy. (Well, if it doesn't require more effort than I think it requires, then it's easy.) Since a strong acid completely ionises in water, the number of moles of hydrogen in the strong acid is the number of moles of hydrogen ions in the resulting solution. Weak acids are a bit different, though, because you have to know what percentage of the acid actually ionises. I've never actually gotten a weak acid question, but I assume that what you do is you work out the number of moles as normal, but then you multiply this by the percentage required.

With bases, where you're normally given something random like hydroxide ions to work with, it's slightly harder. What we have to think about here is the water constant, which is 1 * 10^(-14) (which is really just 10^(-14)). The product of the hydrogen ion and hydroxide ion concentrations is equal to this water constant. (You don't have to remember this number either because it's also on your data sheet.) We now have a nifty little formula for you to rearrange and use and abuse at your will in order to work out hydrogen ion concentrations.

[H+][OH-] = 1 * 10^(-14)

As long as you can figure out either the hydrogen ion or hydroxide ion concentrations, you can work out the concentration of the other kind of ion. You can take the hydrogen ion concentration to work out the pH.

Maths-savvy people would know that you can rearrange the pH equation to get another useful equation (full working is provided to make up for my terrible explanation on logs in the other post):

pH = -log [H+]
-pH = log [H+]
10^(-pH) = [H+]

Now you can whack in the pH to find the hydrogen ion concentration! Yay!

All this stuff about pH gives us one more way in which we can differentiate acids and bases. Acids have a low pH while bases have a high pH. Therefore, most indicators will show different colours depending on whether it's an acid or a base that you're looking at. (However, some indicators have a narrow range and are better used for distinguishing weak acids from strong acids or something along those lines, so be careful.)

Last thing I need to talk about is properties and reactions of non-metal and metal oxides including their reaction with water. Now, this is a topic that I'm shaky with and put a big asterisk (my symbol for "need to revise") next to. One thing I can tell you, though, is that most metal oxides are basic, just like metal hydroxides, which is why they neutralise acids (see Acids and Bases part 1). Most non-metal oxides are the opposite- they're acidic. (Mind you, this is a general rule only- there are amphoteric metal oxides and all that jazz.) Now, as for their reactions with water... I'll have to look it up in my textbook.

Acidic non-metal oxides with water- The oxygen from the water joins with the non-metal oxide to make an ion while the hydrogen from the water completes the package by turning that ion into an acid. Here's some examples:

CO₂ + H₂O -> H₂CO₃

SO₂ + H₂O -> H₂SO₃

_{Basic metal oxides with water- If the metal oxide is soluble, it will react to form hydroxide ions. The presence of all of these hydroxide ions means that the resulting solution is basic.}

_{Aaaand that's pretty much it for acids and bases. Next up will be either redox or organic chemistry- I'm thinking probably organic chem because I hate redox and want to give myself some more breathing room before I have to deal with it. If I'm in a really good mood tomorrow (or whenever I write my next post), though, I might do redox. We'll see.}