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Acid & Base Anhydrides: Definition & Examples

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  • 0:00 Definition of Anhydrides
  • 1:12 Acid Anhydrides
  • 4:04 Base Anhydrides
  • 5:35 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

In this video lesson, you will learn the definition and the types of anhydrides that form acids and bases in water. You will also discover where you can commonly find anhydrides and why they are important. A short quiz will test your knowledge.

Definition of Anhydrides

What do a bag of cement, acid rain and the synthesis of aspirin have in common?

I know, tricky question so early in the lesson. Well, the answer is they all involve important chemical molecules called anhydrides. The term anhydride means 'without water.' Anhydrides are able to react with water to produce either an acid or a base.

Before we look at anhydrides in more detail, I want to quickly remind you of what we mean by an acid and a base. The simplest definition of an acid is a substance that releases hydrogen ions, H+, in water. And the simplest definition of a base is a substance that releases hydroxide ions, OH-, in water.

These simple definitions work well for us here. We can recognize an acid molecule because it has hydrogen ions to give away. We can recognize a base because it has hydroxide ions to give away. Anhydrides can thus be divided into two main types, and we will look at each one in turn.

Acid Anhydrides

Acid anhydrides are molecules that form acidic solutions in water. And acid anhydrides are the oxides of non-metals that can react with water. In solution, they form molecular acids. It is important to realize that not all non-metal oxides are acid anhydrides, only those that can react with water. For example, carbon monoxide is an oxide of carbon but does not react with water. Therefore, it is not an acid anhydride.

Here we have two common and important reactions.

The first is the formation of sulfuric acid:

SO3(g) + H2O --> H2SO4(aq)

Here sulfur trioxide gas reacts with water to form sulfuric acid. You can recognize sulfuric acid because it has two hydrogens to give away.

This reaction is important because it is a producer of acid rain. Acid rain is formed from pollution. In particular, acid rain is usually formed from pollution from coal plants that produce sulfur dioxide, SO2.

Once in the air, sulfur dioxide reacts with oxygen to form sulfur trioxide. This is the acid anhydride in our reaction. Once it is formed, it quickly reacts with water vapor in the atmosphere and falls as acid rain. Acid rain has devastating effects on the environment.

For this reason, regulations have been introduced to reduce the amount of sulfur dioxide in the atmosphere from industry. These regulations have successfully decreased the amount of acid rain.

Another important acid anhydride reaction is the formation of carbonic acid:

CO2(g) + H2O --> H2CO3(aq)

This time, carbon dioxide is reacting with water to form carbonic acid. Again, you can recognize it as an acid because it has hydrogen to give away.

Carbonic acid is also involved in the formation of acid rain. But it is even more important in its role in changing the pH levels of our oceans, rivers, and streams.

From our reaction, I am sure you can see that the more carbon dioxide there is in the atmosphere, the more carbonic acid will be produced. This is of real concern to scientists as the oceans become more acidic because of increased levels of atmospheric carbon dioxide.

Acid anhydrides are also used extensively in organic chemistry. They are used in the manufacture of all sorts of things, including industrial chemicals, pharmaceuticals, perfumes and explosives. If you have a bad headache, you may take aspirin. So, an important reaction for many people is the use of acid anhydride in the synthesis of aspirin.

Base Anhydrides

Let us now move onto base anhydrides. These are molecules that form basic solutions in water. Base anhydrides are the oxides of Group I and II metals. They react with water to form the hydroxide ion as one of the products.

A typical base anhydride reaction is the reaction of calcium oxide:

CaO(s) + H2O --> Ca(OH)2(aq)

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