Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.
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 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.
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)
Here calcium oxide reacts with water to form calcium hydroxide. You can recognize calcium hydroxide as a base because of the hydroxide ions it can give away.
This is the exact reaction that takes place when water is added to dry cement. Calcium oxide, or quicklime, is an ingredient in cement. For this reason, it is really important to protect your skin when you are working with cement or grout for tiling.
Continued contact of your hands with these products can lead to irritation. This is because calcium hydroxide further reacts with the water in your skin, breaking down into calcium and hydroxide ions. The hydroxide ions are very caustic.
In this lesson, you have learned that the term anhydride means 'without water.' Acid anhydrides are non-metal oxides that can react with water to form acidic solutions. Important acid anhydride reactions include the reaction of water vapor with sulfur trioxide and carbon dioxide. Base anhydrides are Group I and II metal oxides that can react with water to form basic solutions. An important use for base anhydrides is in the production of cement.
Once you are finished, you should be able to:
- Recall what an anhydride is
- Identify and describe the two main types of anhydrides
- Discuss the role of acid anhydrides in acid rain
- State the origin for the oxides that form base anhydrides
- State which oxides form acids and which oxides form bases
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