Strong and weak acids and bases

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Go to a discussion of the pH of strong acids and bases.

I. Historical Introduction

I'm not going to write this yet. I spent two hours going over my materials and I couldn't get started. While I think the events are understandable to high schoolers, I think it's going to just get too long. Maybe, I'll attack it another day. Besides, your teacher ain't gonna test you on the history!!

All acids, bases, and salts are electrolytes. From history, this meant that, in an electrochemical cell, the current flowed. Non-electrolytes, such as sugar, do not allow current to flow in an electrochemical cell.

Svante Arrhenius, in 1884-1887, showed that electrolytes dissolve to give ions in solution.

There was a problem, starting in the mid-1880s. Certain electrolytes (called weak) behaved in solution according to what was called the Ostwald dilution law. Other electrolytes (called strong) did not follow this law and there was no explanation why.

II. The Modern Meaning of Strong

The explanation for why strong electrolytes behaved the way they did was first suggested in 1904, but was not proven until 1923. Strong electrolytes are 100% dissociated into ions in solution. The original strong electrolyte molecule, as such, does not exist in solution. The only thing present are ions. Sodium chloride is an example of a strong electrolyte. Only Na+ and Cl¯ exist in solution. NaOH is another. Only Na+ and OH¯ exist in the solution.

Now, having said that, if the solution is sufficiently concentrated, you form what are called ion pairs. Not molecules, mind you. The Na+ and the Cl¯ join up briefly to form NaCl and so the effective dissociation is slightly less than 100%. Having said that, we will act like strong electrolytes always dissociate 100%.

Certain acids are considered to be strong, which means they are dissociated 100% in solution.

HCl hydrochloric acid HBr hydrobromic acid
HI hydroiodic acid HNO3 nitric acid
H2SO4 sulfuric acid H2SeO4 selenic acid
HClO3 chloric acid HClO4 perchloric acid
HBrO3 bromic acid HBrO4 perbromic acid
HIO3 iodic acid HIO4 periodic acid

You ought to memorize this list, because almost every other acid is weak. The most common strong acid example used by teachers is HCl. Watch out for a teacher who tries to trip you up by using another strong acid on the test while having used HCl all the time in class.

Five of the acids above do not often appear on lists of strong acids:

H2SeO4, HBrO3, HBrO3, HIO3, HIO4

Five points about the above list:

  1. The 100% dissociation idea begins to break down as solutions become more concentrated. Usually if the acid is 100% dissociated in solutions of 1.0-molar or less, it is called strong.
  2. Sulfuric acid and selenic acid are considered strong only in their first dissociation step (this is, when they lose their first H+).
  3. Here's a possible problem for you, the student: you are given a problem which treats the sulfuric acid (or selenic acid) as 100% ionzed in BOTH dissociation steps. What to do? How to solve the problem when you KNOW it's incorrect? Advice: do the problem in the manner asked for by the problem. You may choose if you want to discuss it with your instructor.
  4. I once saw HSCN on a list for strong acids. Only once. There are a few others which are "almost" strong and sometimes a textbook author will include one in his or her own list. Picric acid is one example of an "almost strong" acid.
  5. Chromic acid (H2CrO4) is sometimes listed as a strong acid.

Certain bases are considered to be strong.

 LiOH   lithium hydroxide
 NaOH   sodium hydroxide
 KOH   potassium hydroxide
 RbOH   rubidium hydroxide
 CsOH   cesium hydroxide
 *Ca(OH)2   calcium hydroxide
 *Sr(OH)2   strontium hydroxide
 *Ba(OH)2   barium hydroxide

* Completely dissociated in solutions of 0.01 M or less. These are insoluble bases which ionize 100%. The other five in the list can easily make solutions of 1.0 M and are 100% dissociated at that concentration.

I once saw a mnemonic device to help remember which bases are the strong bases. Look to your periodic table to see this arrangement:


You will notice that the pattern of the elements traces out the outline of a lower case letter "b," as in base. (Ignore the line.) Please note that I included Fr and Ra. They are also strong bases but, due to radioactivity issues, they are usually ignored. That being said, watch out for the teacher that will never mention Fr or Ra in the context of strong bases and then include them on a test.

The ones most often used in teaching examples are NaOH and KOH. In fact, the others sorta look funny in the list because the ChemTeam thinks he has never, ever used anything other than NaOH or KOH as an example when discussing strong bases.

There are other strong bases. However, these substances are seldom discussed in an introductory class, so you probably won't see them on a test. I mention them here so you can, at least, be aware of them. There are two categories to be aware of:

  1. The various metallic oxides, such as Na2O or CaO, will make a strong base in solution. However, it is not the oxide that ionizes (for example, Na2O ionizing into Na+ and O2¯ ions. The oxide reacts with the water to make hydroxide ion. Here's an example:
    Na2O + H2O ---> 2NaOH
  2. Another category of stong bases are the amides, such as KNH2 (potassium amide) or Ca(NH2)2 (calcium amide). Once again, there is a chemical reaction which produces hydroxide. With the amides, the NH2¯ pulls a hydrogen ion off a water molecule to make the hydroxide ion.

Memorize the above list, since almost everything else is weak. Same warning as to teacher behavior as above.

All salts, of which there are many, are considered to be strong electrolytes. The three most common examples are:

NaCl - sodium chloride
KCl - potassium chloride
Na2SO4 - sodium sulfate

Three more, randomly selected:

MgSO4 - magnesium sulfate
KBr - potassium bromide
FeCl2 - iron(II) chloride

III. The Modern Meaning of Weak

Weak electrolytes will dissociate in solution, but they do so less than 100%. The usual examples students study dissociate only 1% to 5%.

In fact, this partial dissociation was known to be happening in the mid-1880s, when the Ostwald dilution law was announced. It was just that it took time for the correct explanation (that being 100% dissociation) of strong electrolyte behavior to occur to someone.

The classic weak acid is acetic acid. In fact, it has its own abbreviation of HAc, where H means hydrogen and Ac means acetate. The ChemTeam will try to use several different weak acids in the examples to follow.

The classic weak base is ammonia (NH3).

Also, we run into a bit of a technicality in the language. Here is the Brønsted-Lowry equation for ammona dissolving in water:

NH3 + H2O ⇌ NH4+ + OH¯

The ammonia does not dissociate in the same sense that HAc dissociates in water. Generally speaking, the word ionization is used here rather than dissociation.

Go to a discussion of the pH of strong acids and bases.

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