Lesson Explainer: Reactions of Oxides Chemistry

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

An oxide is a common type of chemical compound that consists of at least one atom of oxygen combined with another element.

Definition: Oxide

An oxide is a chemical compound that contains at least one oxygen atom chemically bonded to an atom of another element.

There are a vast number of oxides of different elements, including both metals and nonmetals. Oxides have many uses in our society. Calcium oxide (CaO) is used in the production of concrete, and nitrous oxide (NO2) is used as an anesthetic for dental procedures.

Example 1: Defining the Constituent Parts of an Oxide

Which element does an oxide—a compound containing two elements—contain at least one atom of?

Answer

An oxide is a chemical compound that contains the element oxygen. The oxygen in a compound will be bonded to one another element, creating a binary compound. For example, when copper metal reacts with oxygen gas, copper(II) oxide (CuO) is formed. An oxide, therefore, contains at least one atom of oxygen.

Common metal oxides include magnesium oxide, sodium oxide, and copper(II) oxide. The ease with which metal oxides form relates to the reactivity of the metal.

The group 1 alkali metals are well known for being very reactive and are consequently stored under oil to prevent reactions with oxygen and moisture in the air. If a piece of a group 1 metal such as lithium is freshly cut, the shiny surface soon becomes dull as the lithium reacts with oxygen in the air, forming a layer of lithium oxide (LiO2): 4Li()+O()2LiO()sgs22

Less reactive metals, such as copper, will not react as readily with oxygen. Orange-brown copper metal must be heated strongly in a Bunsen burner flame for the reaction with oxygen to proceed. This results in black copper(II) oxide forming on the surface of the copper metal: 2Cu()+O()2CuO()sgs2

One of the most common nonmetal oxides is carbon dioxide. As one of the products of respiration in the human body, carbon dioxide leaves our body each time we exhale. It is also produced through the combustion of carbon: C()+O()CO()sgg22

Burning sulfur in pure oxygen in a gas cylinder causes the sulfur to burn with a brilliant blue flame, producing the gas sulfur dioxide: S()+O()SO()sgg22

Sulphur burning in a glass

Notice that the photograph shows a piece of litmus paper at the back of the gas cylinder that has turned red and started to bleach. This bleaching is due to the acidic sulfur dioxide gas being produced.

The final example of nonmetal oxide formation is the reaction of phosphorus with oxygen. In its pure state, phosphorus is a very reactive element and will spontaneously burst into flames in the presence of oxygen: P()+5O()PO()42410sgs

Oxides can be classified in terms of their acidic or basic nature. There are four classifications: acidic, basic, amphoteric, and neutral.

As we move across each period of the periodic table, the atomic number increases, as does the acidic nature of the oxides of these elements. This results in metal oxides generally being considered basic and nonmetal oxides being considered acidic.

An oxide’s acidity or basicity greatly affects their reactions with acids and alkalis. Basic metal oxides will react with mineral acids to form the relevant salt and water. For example, copper oxide will react with dilute hydrochloric acid: CuO()+2HCl()CuCl()+HO()saqaql22

This type of reaction is known as a neutralization reaction, which has the following general equation.

Equation: Neutralization

Acid+basesalt+water

It is possible to use this general equation to help us to determine unknown substances. For example, if the salt produced from a neutralization reaction contained chloride ions, then the acid used must have been hydrochloric acid. This can be seen in the reaction between sodium oxide and hydrochloric acid: NaO()+2HCl()2NaCl()+HO()22saqaql

If a neutralization reaction produced a salt containing sulfate ions, then sulfuric acid must have been used: CaO()+HSO()CaSO()+HO()saqaql2442

Finally, if a neutralization reaction produced a salt that contained nitrate ions, nitric acid must have been used to neutralize the base: MgO()+2HNO()Mg(NO)()+HO()saqaql3322

Example 2: Determining the Metal Oxide and Acid Necessary to Form Zinc Chloride

A metal oxide and an acid reacted together to form zinc chloride and water. What were the metal oxide and the acid?

  1. Zinc oxide and hydrochloric acid
  2. Zinc oxide and sulfuric acid
  3. Zinc metal and nitric acid
  4. Zinc metal and hydrochloric acid
  5. Zinc chloride and zinc oxide

Answer

The reaction between a metal oxide and acid is an example of a neutralization reaction. In a neutralization reaction, an acid reacts with a base to form a salt and water. In this case, the salt that has formed is zinc chloride.

An acid consists of hydrogen ions and one of many anions. These anions will be the same anions that appear in the salt product. The cation or positive metal ion in the salt will come from the metal oxide. Finally, the oxygen atom from the oxide will bond with the hydrogen from the acid to form water.

In this particular example, the salt produced is zinc chloride. As such, the zinc must have come from zinc oxide and the chloride must come from hydrochloric acid. Therefore, the correct answer is A.

While the basic nature of metal oxides can be observed through their reaction with dilute acids, some metal oxides are partially soluble in water and will form alkaline solutions: MgO()+HO()Mg(OH)()slaq22

As we move across the periodic table and examine the nonmetal oxides, the change in acidic nature of these compounds becomes apparent.

For example, carbon dioxide is slightly soluble in water and will form an acidic solution of carbonic acid: CO()+HO()HCO()2223glaq

We can confirm the carbonic acid solution is acidic with universal indicator. Addition of a few drops of universal indicator to the solution will produce a red color, indicating the solution to be acidic. Acidic solutions are formed when nonmetal oxides dissolve in water.

Another nonmetal acidic oxide is phosphorus pentoxide, which will react with sodium hydroxide solution to produce sodium phosphate and water: PO()+12NaOH()4NaPO()+6HO()410342saqaql

Example 3: Identifying Which of a Selection of Oxides Is Acidic

Which of the following oxides is most likely to be an acidic oxide?

  1. CuO
  2. NaO2
  3. MgO
  4. CaO
  5. CO2

Answer

In this question, we are given five different oxides and need to identify which one is likely to be an acidic oxide.

At first glance, it appears that the selection of oxides in this question all seem similar. However, upon closer inspection, we can see that answer E has one crucial difference.

Carbon dioxide is a nonmetal oxide as opposed to copper oxide, sodium oxide, magnesium oxide, and calcium oxide, which are all metal oxides. Metal oxides are basic, whereas nonmetal oxides are acidic, and so the oxide that is most likely to be acidic from this selection is answer E, CO2.

Not all oxides can be considered purely acidic or basic oxides. Metals such as zinc and aluminum have oxides that are both acidic and basic in nature and are referred to as being amphoteric.

An amphoteric oxide can react with both acids and bases. Aluminum oxide will react with dilute hydrochloric acid to form aluminum chloride and water: AlO()+6HCl()2AlCl()+3HO()2332saqaql

However, aluminum oxide will also react with sodium hydroxide solution, forming a salt known as sodium aluminate and water: AlO()+6NaOH()2NaAlO()+3HO()23332saqaql

In the first reaction, aluminum oxide is behaving as a base, and in the second, it is behaving as an acid.

Another oxide that displays acidic and basic characters and can be considered amphoteric is zinc oxide, which reacts with dilute mineral acids and alkalis in the following ways: ZnO()+HSO()ZnSO()+HO()ZnO()+2NaOH()NaZnO()+HO()saqaqlsaqaql2442222

There is one final type of oxide to consider. Neutral oxides are neither acidic nor basic in nature and so do not react with acids or bases. Examples of neutral oxides include carbon monoxide (CO) and nitrous oxide (NO2).

Example 4: Predicting the Color a Universal Indicator Will Change into in Different Aqueous Solutions of Oxides

To determine the pH of various oxides, an experiment was set up. Three beakers were filled with 0.5 L of deionized water, and a few drops of universal indicator were added. A spatula of the following oxides was then added to each beaker.

What color will each solution change to following the addition of the oxide?

  1. A: green, B: red, C: blue
  2. A: red, B: green, C: blue
  3. A: blue, B: green, C: red
  4. A: red, B: blue, C: green
  5. A: blue, B: red, C: green

Answer

In this question, we have three different oxides of phosphorus, magnesium, and aluminum. Magnesium is a metal in group 2 of the periodic table and forms basic oxides that dissolve in water to form alkaline solutions. Phosphorus is in group 15 of the periodic table and, as a nonmetal, dissolves in water to form an acidic solution. Aluminum oxide forms an amphoteric oxide that is both acidic and basic in nature.

Universal indicator is a common indicator used to determine if a solution is acidic or alkaline. In the presence of an acid, universal indicator will turn a solution yellow to red. In the presence of an alkaline solution, universal indicator will turn a solution blue to purple. The universal indicator will be green in the presence of a neutral solution.

If we combine this knowledge, we should expect beaker A containing the acidic phosphorus oxide to be red in color. Beaker B contains the basic magnesium oxide and so we would expect this solution to be blue in color, and beaker C containing the amphoteric aluminum oxide to be green in color. This corresponds to answer D, the correct answer.

General trends exist in the oxides of the elements in certain groups in the periodic table. In groups on the left-hand side of the periodic table, such as group 1 and group 2, the basic properties of the oxides increase as we descend the group.

If we consider the reactions we have looked at as a whole, we can see that as we move across the periodic table, the atomic number increases and the oxides of the elements become increasingly acidic in nature.

In order to explain some of these concepts, we can consider acids and bases as hydroxy compounds with a general formula of MOH, where M is the atom of the element we are examining.

The hydroxy compound can ionize in two different ways: MOHM+OHMOHMO+H()()Base+Acid+

The manner in which the hydroxy compounds ionize is determined by the attraction and repulsion that exists between the three constituent ions in the substance. The following illustration shows a triangle with attractive forces existing between oppositely charged ions and repulsion between ions with like charges.

We can use this model to explain the trends within the groups that we previously discussed. If the attractive force between H+ and O is greater than that between M+ and O, the substance will ionize to form a base. For example, as we move down group 1, more basic oxides are formed. The increasing atomic numbers, and consequent increased size of the ion, reduce charge density, therefore reducing the attraction between M+ and O, more readily forming hydroxide ions.

If the attractive force between M+ and O is greater than that between H+ and O, the substance will ionize to form an acid. This is what we see happening as we move across the periods of the periodic table from group 1 to group 17.

When nonmetal oxides are ionized as acids, the strength of the oxygenated acid formed depends on the number of oxygen atoms in the anion, which are not linked to hydrogen atoms.

We can analyze some examples of this altering strength by using the general formula MO(OH), which builds upon the one we have previously used.

The higher the number of non-hydrogen-bonded oxygen atoms (O), the greater the strength of the acid, as can be seen in the examples in the following table.

There are other less common forms of oxides, such as peroxides and superoxides. A peroxide is a specific type of compound where two oxygen atoms are joined by a single covalent bond, such as in the compound hydrogen peroxide, shown below.

OOHH

Superoxide compounds contain a negative superoxide ion, O2, such as KO2. Superoxide compounds are particularly important to our biology as our immune system produces superoxide compounds to kill invading microorganisms.

Example 5: Describing the Oxide of Aluminum

AlO23 is an oxide that can react with acids as well as bases, as shown in the following equations: AlO+3HSOAl(SO)+3HOAlO+2NaOH2NaAlO+HO232424322322

Which of the following describes that oxide?

  1. It is a neutral metal oxide.
  2. It is an amphoteric nonmetal oxide.
  3. It is a peroxide.
  4. It is an amphoteric metal oxide.
  5. It is a neutral nonmetal oxide.

Answer

In this question, we are asked to describe aluminum oxide from the two given reactions. The first reaction shows aluminum oxide reacting with an acid, sulfuric acid. The second equation shows aluminum oxide reacting with a base, sodium hydroxide. From these equations, we can see that aluminum oxide reacts with both acids and bases.

Answer A describes aluminum as a neutral metal oxide. However, neutral oxides do not react with dilute mineral acids or alkaline solutions and so this answer must be incorrect.

Answer B describes aluminum oxide as an amphoteric nonmetal oxide, but its location in group 13 of the periodic table means the aluminum is a metal and so this answer is incorrect as well.

A peroxide is a specific type of compound that has two oxygen atoms bound together by a single covalent bond. Aluminum oxide has three oxygen atoms and cannot be considered a peroxide and so answer C is incorrect.

Answer D describes aluminum oxide is an amphoteric metal oxide. Aluminum is indeed a metal due to its position in group 13 of the periodic table and can be considered amphoteric as it reacts with both acids and alkalis, as can be seen in the two chemical equations given above.

As answer E consists of choices we have already eliminated, the correct answer is D.

Let’s summarize the points discussed in this explainer.

Key Points

  • An oxide is a compound that contains at least one oxygen atom and an atom of another element.
  • The ease with which an oxide can be formed from an element relates to the reactivity of that particular metal or nonmetal.
  • Metals generally form basic oxides.
  • Nonmetals generally form acidic oxides.
  • Some elements in the center of the periodic table form amphoteric oxides, which will react with acids and bases.
  • Basic oxides react with acids to form salts and water.
  • Acidic oxides react with alkalis to form salts and often water.
  • pH tests can be used to determine if an oxide is acidic or basic.
  • Some elements form neutral oxides, which do not react with acids or bases.
  • The acidic character of an oxide increases across the periodic table.
  • The higher the number of non-hydrogen-bonded oxygen atoms (O), the greater the strength of the acid.

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