Chapter+13,+Section+1

**S11.A.1.1.2** Analyze and explain the accuracy of scientific facts, principles, theories, and laws.
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 * This page contains information that is specific to Chapter thirteen material.** **Click here** **to see the lab that we will be performing for this chapter.**
 * THEORIES OF ACIDS AND BASES**

 Neutralisation happens because hydrogen ions and hydroxide ions react to produce water. Hydrochloric acid is neutralised by both sodium hydroxide solution and ammonia solution. In both cases, you get a colourless solution which you can crystallise to get a white salt - either sodium chloride or ammonium chloride. These are clearly very similar reactions. The full equations are: In the sodium hydroxide case, hydrogen ions from the acid are reacting with hydroxide ions from the sodium hydroxide - in line with the Arrhenius theory. However, in the ammonia case, there don't appear to be any hydroxide ions! You can get around this by saying that the ammonia reacts with the water it is dissolved in to produce ammonium ions and hydroxide ions: This is a reversible reaction, and in a typical dilute ammonia solution, about 99% of the ammonia remains as ammonia molecules. Nevertheless, there are hydroxide ions there, and we can squeeze this into the Arrhenius theory. However, this same reaction also happens between ammonia gas and hydrogen chloride gas. In this case, there aren't any hydrogen ions or hydroxide ions in solution - because there isn't any solution. The Arrhenius theory wouldn't count this as an acid-base reaction, despite the fact that it is producing the same product as when the two substances were in solution. That's silly!
 * The Arrhenius Theory of acids and bases**
 * The theory**
 * Acids are substances which produce hydrogen ions in solution.
 * Bases are substances which produce hydroxide ions in solution.
 * Limitations of the theory**

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 * The Bronsted-Lowry Theory of acids and bases**

The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it. Hydroxide ions are still bases because they accept hydrogen ions from acids and form water. An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to them. When hydrogen chloride gas dissolves in water to produce hydrochloric acid, the hydrogen chloride molecule gives a proton (a hydrogen ion) to a water molecule. A co-ordinate (dative covalent) bond is formed between one of the lone pairs on the oxygen and the hydrogen from the HCl. Hydroxonium ions, H3O+, are produced.
 * The theory**
 * An acid is a proton (hydrogen ion) donor.
 * A base is a proton (hydrogen ion) acceptor.
 * The relationship between the Bronsted-Lowry theory and the Arrhenius theory**

When hydrogen chloride dissolves in water, almost 100% of it reacts with the water to produce hydroxonium ions and chloride ions. Hydrogen chloride is a strong acid, and we tend to write this as a one-way reaction: || You will find more about [|strong and weak acids] on another page in this section. || In fact, the reaction between HCl and water is reversible, but only to a very minor extent. In order to generalise, consider an acid HA, and think of the reaction as being reversible. Thinking about the //forward reaction:// But there is also a //back reaction// between the hydroxonium ion and the A- ion: The reversible reaction contains //two// acids and //two// bases. We think of them in pairs, called **//conjugate pairs//**. When the acid, HA, loses a proton it forms a base, A-. When the base, A-, accepts a proton back again, it obviously refoms the acid, HA. These two are a conjugate pair. //Members of a conjugate pair differ from each other by the presence or absence of the transferable hydrogen ion.// If you are thinking about HA as the acid, then A- is its conjugate base. If you are thinking about A- as the base, then HA is its conjugate acid. The water and the hydroxonium ion are also a conjugate pair. Thinking of the water as a base, the hydroxonium ion is its conjugate acid because it has the extra hydrogen ion which it can give away again. Thinking about the hydroxonium ion as an acid, then water is its conjugate base. The water can accept a hydrogen ion back again to reform the hydroxonium ion. This is the reaction between ammonia and water that we looked at earlier: Think first about the forward reaction. Ammonia is a base because it is accepting hydrogen ions from the water. The ammonium ion is its conjugate acid - it can release that hydrogen ion again to reform the ammonia. The water is acting as an acid, and its conjugate base is the hydroxide ion. The hydroxide ion can accept a hydrogen ion to reform the water. Looking at it from the other side, the ammonium ion is an acid, and ammonia is its conjugate base. The hydroxide ion is a base and water is its conjugate acid. You may possibly have noticed (although probably not!) that in one of the last two examples, water was acting as a base, whereas in the other one it was acting as an acid. A substance which can act as either an acid or a base is described as being **//amphoteric//**. || An //amphiprotic// substance is one which can both donate hydrogen ions (protons) and also accept them. Water is a good example of such a compound. The water acts as both an acid (donating hydrogen ions) and as a base (by accepting them). The "protic" part of the word refers to the hydrogen ions (protons) either being donated or accepted. Other examples of amphiprotic compounds are amino acids, and ions like HSO4- (which can lose a hydrogen ion to form sulphate ions or accept one to form sulphuric acid). But as well as being amphiprotic, these compounds are also //amphoteric//. Amphoteric means that they have reactions as both acids and bases. So what is the difference between the two terms? All amphiprotic substances are also amphoteric - but the reverse isn't true. There are amphoteric substances which don't either donate or accept hydrogen ions when they act as acids or bases. There is a whole new definition of acid-base behaviour that you are just about to meet (the Lewis theory) which doesn't necessarily involve hydrogen ions at all. A Lewis acid is an electron pair acceptor; a Lewis base is an electron pair donor (see below). Some metal oxides (like aluminium oxide) are amphoteric - they react both as acids and bases. For example, they react as bases because the oxide ions accept hydrogen ions to make water. That's not a problem as far as the definition of amphiprotic is concerned - but the reaction as an acid is. The aluminium oxide doesn't contain any hydrogen ions to donate! But aluminium oxide reacts with bases like sodium hydroxide solution to form complex aluminate ions. You can think of lone pairs on hydroxide ions as forming dative covalent (coordinate) bonds with empty orbitals in the aluminium ions. The aluminium ions are accepting lone pairs (acting as a Lewis acid). So aluminium oxide can act as both an acid and a base - and so is amphoteric. But it //isn't// amphiprotic because //both// of the acid reaction and the base reaction don't involve hydrogen ions. I have gone through 40-odd years of teaching (in the lab, and via books and the internet) without once using the term amphiprotic! I simply don't see the point of it. The term amphoteric takes in all the cases of substances functioning as both acids and bases without exception. The term amphiprotic can only be used where both of these functions involve transference of hydrogen ions - in other words, it can only be used if you are limited to talking about the Bronsted-Lowry theory. Personally, I would stick to the older, more useful, term "amphoteric" unless your syllabus demands that you use the word "amphiprotic". ||
 * Conjugate pairs**
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 * Note:** I am deliberately missing state symbols off this and the next equation in order to concentrate on the bits that matter.
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 * The HA is an acid because it is donating a proton (hydrogen ion) to the water.
 * The water is a base because it is accepting a proton from the HA.
 * The H3O+ is an acid because it is donating a proton (hydrogen ion) to the A- ion.
 * The A- ion is a base because it is accepting a proton from the H3O+.
 * //A second example of conjugate pairs//**
 * Amphoteric substances**
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 * Note:** You might also come across the term **//amphiprotic//** in this context. The two words are related and easily confused.