Lesson Video: Reactions of Oxides Chemistry

In this video, we will learn how to describe and write equations for the reactions of oxides.

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Video Transcript

In this video, we will learn what an oxide is and which oxides are acidic, basic, amphoteric, and neutral. We’ll look at some chemical equations for how some oxides react with acids. First, let’s ask ourselves, what is an oxide? An oxide is a compound which contains the element oxygen bonded to another element. For example, in carbon dioxide, oxygen is bonded to carbon. Some commonly occurring oxides which you may have heard of are iron(III) oxide, which is the main component of rust, sulfur dioxide, which is sometimes used as a food preservative, dinitrogen monoxide, which is the IUPAC name and whose common name is nitrous oxide, this is laughing gas, water, silicon dioxide or silica, which is the main component of sand, aluminum oxide, and magnesium oxide.

Perhaps you’ve produced magnesium oxide in the lab. A pair of tongs holds magnesium ribbon in a Bunsen burner flame. A highly exothermic reaction occurs with much heat and light being given off as the magnesium reacts with oxygen in the air to produce magnesium oxide. All oxides can be categorized into one of four groups: acidic, basic, amphoteric, or neutral oxides. Let’s investigate this and start with acidic oxides. Acidic oxides are those which when they react with water produce acids. These oxides contain nonmetals from groups 14 to 17 of the periodic table. The general equation is nonmetal oxide plus water react to give an acid. Let’s look at some examples.

When sulfur dioxide gas reacts with water, sulfurous acid is produced, H2SO3. When the nonmetal oxide, carbon dioxide, reacts with water, carbonic acid or H2CO3 is produced. These two reactions can occur in the environment when there is much SO2 and CO2 in the atmosphere. Liquid water and rain drops can interact with carbon dioxide and sulfur dioxide in the atmosphere and produce the two acids carbonic and sulfurous acid. These two acids are components of acid rain. One last example of when a nonmetal oxide interacts with water to produce an acid is the reaction of nitrogen dioxide gas with water to produce nitric acid. If a few drops of universal indicator were added to solutions of these acids, it would turn red-orange, proving that the nonmetal oxides SO2, CO2, and NO2 when they react with water do indeed produce acids.

Now let’s have a look at basic oxides. A basic oxide is an oxide which when it reacts with water forms a base or an alkali. In general, basic oxides contain metals from groups one or two of the periodic table. The general equation is metal oxide plus water react to give a base or alkali. An alkali is a substance containing the hydroxide ion OH−. Here are two examples of equations where metal oxides produce hydroxides or alkali solutions. In the first, sodium oxide reacts with water to produce sodium hydroxide, and in the second, calcium oxide reacts with water to produce calcium hydroxide. Sodium hydroxide is highly soluble in water, while calcium hydroxide is only slightly soluble. Nevertheless, if a few drops of universal indicator were added to these solutions, it would turn blue-purple, confirming that the solutions are basic or alkaline and proving that these oxides do indeed react with water to produce bases or alkalis.

So far, we have seen that nonmetal oxides or acidic oxides react with water to produce acids and that metal oxides or basic oxides react with water to produce a base. Acidic oxides can also act like acids and react with a base to produce salt and water. And basic oxides can act like a base by reacting with an acid to produce salt and water. The general rule applies: an acid reacts with a base to produce salt and water. A bit later in this video, we will look specifically at how basic oxides react with acids to produce salt and water. For now, let’s move on to the third type of oxide, amphoteric oxides.

Amphoteric oxides, unlike acidic and basic oxides, usually don’t dissolve in or react with water. However, they do show both acidic and basic properties. They behave like an acid when they react with a base, and they behave like a base when they react with an acid. These oxides contain metals such as copper, zinc, lead, beryllium, aluminum, and tin. We saw a moment ago that when an acid and a base react, they produce salt and water. So because amphoteric oxides can behave like acids or bases, there are two general equations to look at. When these oxides behave like an acid, the equation is amphoteric oxide plus base, giving salt and water. And when they react like a base, the equation is amphoteric oxide plus acid, giving salt and water.

Let’s look at an example for each. Aluminum oxide is amphoteric. It does not dissolve in or react with water and can act like an acid or a base. When it reacts with a base such as sodium hydroxide, sodium aluminate, salt, and water are produced. Note that this formula is a simplification. Aluminum can form quite complex ions in solution. The formula here for sodium aluminate is actually the formula for the solid anhydrous product. But sodium aluminate in the presence of water is highly soluble, and so will react with water to form a hydrated compound with a complex formula, which we will not look at here. When this amphoteric oxide reacts with an acid, the salt aluminum chloride is produced. This dual nature of amphoteric oxides is indicated by their name. The word amphoteric comes from the Greek word amphoteroi, meaning both.

Let’s move on to the last type of oxide, neutral oxides. Neutral oxides do not show acidic or basic properties and do not react with acids or bases. There are only a few known neutral oxides, and these include carbon monoxide, nitrous oxide, and nitric oxide. Again, neutral oxides do not undergo reactions with acids or bases. Now, let’s have a look specifically of how basic oxides react with acids to produce salt and water, as well as some more examples of how amphoteric oxides can act as bases, react with acids, and produce salt and water. When sodium oxide reacts with hydrochloric acid, sodium chloride and water are the products. Notice that the anion in the acid and the cation in the basic oxide determine which salt is produced.

Can you guess what the acid is in this next equation? Magnesium oxide is largely insoluble in water. However, in dilute, warmed acidic solution, it can react to produce salt and water, in this case, magnesium nitrate and water. The magnesium cation in the salt product comes from the oxide, and the NO3 or the nitrate ion must have come from the acid. There are two nitrate ions, which means there must be two positive charges in the acid or two H+ ions. Combining these ions together, we get, we get two HNO3, which is the acid nitric acid. We saw an example earlier of how an amphoteric oxide can act as a base and react with an acid. Let’s look at one more example.

The reaction of zinc(II) oxide with sulfuric acid produces the salt zinc sulfate and water. Again, the cation in the salt comes from the oxide, and the anion in the salt comes from the acid. So far, we’ve looked at the types of oxides, how they react, and we’ve looked at many equations. Before we do a practice example, let’s do something a bit different. Let’s investigate how different elements react with oxygen to produce oxides and how this gives us a basic idea of a reactivity series for the elements.

Some elements react with oxygen more vigorously than others. Gold has low to no reactivity with oxygen. We say it’s inert and does not react. Silver and mercury are very slow and resistant to react with oxygen. More metals have been added to this list in a specific order, and this is based on increasing reactivity with oxygen, in other words, an increase in the ease with which these elements react when increasing the vigor. The elements at the far right of the series react easily and vigorously with oxygen, requiring little energy to undergo this reaction, with potassium metal being the most vigorous or we say the most reactive.

It’s important to know that all these metals can be induced to react with oxygen under the correct conditions, even gold. But here we are talking about their natural reactivity. The more reactive an element is, the more likely it is to be found in nature bonded to oxygen or other elements. This list here is called a reactivity series. It shows the general trend or order with which the elements react with oxygen. You’ll notice only metals written on this reactivity series. But nonmetals can also react with oxygen. The vigor with which hydrogen reacts fits in between iron and zinc. Let’s look in some detail at a specific reactivity series of four nonmetals. Bear in mind, though, that these nonmetals can be placed into the top reactivity series in amongst the metals, according to their relative reactivity with oxygen.

There are many known oxides of chlorine. However, chlorine does not react with oxygen in the air and in normal conditions. Energy input is needed to induce a reaction. Of these four nonmetals, chlorine is the least reactive. Carbon also does not generally react spontaneously with oxygen. Think about a piece of charcoal on a barbecue. It must first be heated till red hot. Then it will react and burn with oxygen in the air. Sulfur reacts a bit more vigorously. It will catch fire when heated over a Bunsen burner. Phosphorus, however, reacts quite vigorously and ignites spontaneously in oxygen in the air. Phosphorus is the most reactive towards oxygen from the elements in this series. So from their reaction with oxygen, we can conclude an increasing reactivity from chlorine to carbon to sulfur to phosphorus. Now it’s time to have a look at an example before we summarize everything we have learnt.

To determine the pH of various oxides, an experiment was set up. Three beakers were filled with 0.5 liters of deionized water and a few drops of universal indicator were added. A spatula of the following oxide was then added to each beaker. What color will each solution change to following the addition of the oxide? (A) A: blue, B: green, and C: red. (B) A: green, B: red, and C: blue. (C) A: blue, B: red, and C: green. (D) A: red, B: green, and C: blue. Or (E) A: red, B: blue, and C: green.

An oxide is a compound consisting of oxygen bonded to another element. P2O10, which was added to the first beaker, is a nonmetal oxide because it is composed of the nonmetal phosphorus bonded to oxygen. MgO and Al2O3 are both examples of metal oxides because Mg, magnesium, is a metal and Al, aluminum, is also a metal. And these metals are bonded to oxygen. In general, when a nonmetal oxide reacts with water, an acid is formed. This usually occurs when a nonmetal is from groups 14 to 17 of the periodic table. When the metal in a metal oxide is from groups one or two of the periodic table, for example, magnesium, and the oxide reacts with water, a base or alkali solution is usually formed.

Note, however, that there are always exceptions to the rule. For example, beryllium oxide is not soluble in water and does not react with water under normal conditions. Other metal oxides containing metals which are not in groups one or two of the periodic table, for example, copper, zinc, lead, aluminum, and tin, when they are placed in water, they do not usually react and are usually insoluble. Now, magnesium oxide is also not very soluble in water, but tiny amounts of it will dissolve and react with water to produce a base or alkaline solution.

We were told that universal indicator was added to each beaker to determine the pH. Universal indicator is red in the highly acidic region of the pH scale, then orange-yellow, then green around neutral point, then blue, and at the far end of the spectrum in the highly basic region, it is purple. We have seen that when a nonmetal oxide reacts with water, an acid forms in the case of beaker A. So the indicator will turn red in beaker A. In beaker B, a base or alkali will form as the metal oxide reacts with water, so the indicator will turn a purple-blue color. And in beaker C, when the metal oxide is added, no reaction occurs.

Aluminum oxide is an example of an amphoteric oxide. And again, these are not usually soluble, nor do they usually react with water, although they can react with acids and bases. Because there is no reaction with water, the pH of the water in this beaker will remain neutral, and the indicator will appear green. So the color change in each beaker because of the addition of an oxide will be A: red, B: blue, and C: green.

Let’s summarize what we have learnt. An oxide is a compound containing oxygen bonded to another element. When a nonmetal oxide reacts with water, an acid is produced. When a metal oxide reacts with water, a base is produced. When an amphoteric oxide is placed in water, it does not dissolve or react. And when a neutral oxide is placed in water, there is also no reaction. These are the general trends. When a nonmetal oxide reacts with a base, salt and water are produced. Salt and water are also produced when a metal oxide reacts with an acid. And for amphoteric oxides which can react with an acid or a base because they act as acids or bases, again salt and water are the products.

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