Action of Alkalis on Metal Oxides and Metal Hydroxides

Action of Alkalis on Metal Oxides and Metal Hydroxides: Alkalis are bases that are soluble in water. An alkali is a basic and ionic salt of an alkali metal or an alkaline earth metal. Sodium hydroxide or potassium hydroxide are some alkalis. These alkalis react differently with metal oxides and hydroxides. Among all the metals, only zinc, aluminium and lead are some of the metals that react with alkalis to give the corresponding metal salt and liberate hydrogen gas. Since these metals are amphoteric in nature, i.e., they act as both acids and bases, they react with alkalis.

Now, when we talk about metal oxides, they are basic in nature, and we can think that they cannot react with alkalis that are also base. But, in some special cases, this happens! The oxides of the amphoteric metals like zinc oxide, aluminium oxide, and lead oxide act as both acids as well as bases. So, these acidic oxides can react with alkalis to give a neutralisation reaction. These metal oxides dissolve in water to form metal hydroxides. In this article, we will learn about the action of alkalis on metal oxides and hydroxides.

What are Alkali Metals?

The term ‘alkali’ refers to the basic or alkaline nature of their respective metal hydroxides. These compounds are known as alkali metals because when they react with water, they usually form alkalis that are strong water-soluble bases that can easily neutralise acids. Alkali metals are present in the first column of the periodic table.
Alkali elements include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr).

What are Metal Oxides and Metal Hydroxides?

The oxygen atom when bound with metal, it gives metal oxides. When metals bind with hydroxide group \((−\text {OH})\), they form metal hydroxides. Metal oxides are also called basic oxides because they react with water to form hydroxides, also known as bases.

Type of Metal oxides and Metal Hydroxides That React With Alkali

We know that metal oxides and hydroxides are basic in nature. Hence, it is not possible for a basic metal oxide to react with an alkali or base. Thus, only a few metal oxides and hydroxides that are amphoteric in nature react with alkalis. The metal oxides and hydroxides that show dual behaviour, i.e., they show both acidic as well as basic properties are known as amphoteric oxides. Some amphoteric oxides include zinc oxide, aluminium oxide, and lead oxide. They show both acidic as well as basic character. Let’s learn about the action of alkalis on some metal oxides and hydroxides one by one.

1. Action of Alkalis on Metal Oxide

• Action of Alkalis on Zinc Oxide

Zinc Oxide \((\mathrm{ZnO})\) reacts with an alkali sodium hydroxide \((\mathrm{NaOH})\) at a given temperature to form a salt sodium zincate \(\left(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\right)\) and water. In this condition, zinc oxide acts as an acid that reacts with a base to give neutralisation reaction.

\(\mathrm{ZnO}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

If instead of sodium hydroxide, potassium hydroxide \((\text {KOH})\) is added, then the salt formed is potassium zincate \(\left(\mathrm{K}_{2} \mathrm{ZnO}_{2}\right)\). The chemical reaction involved is given below:

\(\mathrm{ZnO}+2 \mathrm{KOH} \rightarrow \mathrm{K}_{2} \mathrm{ZnO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

• Action of Alkalis on Aluminium Oxide

Aluminium Oxide \(\left(\mathrm{Al}_{2} \mathrm{O}_{3}\right)\) reacts with an alkali sodium hydroxide \((\mathrm{NaOH})\) at a given temperature to form a salt sodium aluminate \(\left(\mathrm{NaAlO}_{2}\right)\) and water. In this condition, aluminium Oxide acts as an acid that reacts with a base to give neutralisation reaction. The chemical reaction is given below:

\(\mathrm{Al}_{2} \mathrm{O}_{3}+2 \mathrm{NaOH} \rightarrow 2 \mathrm{NaAlO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

In place of sodium hydroxide, potassium hydroxide \((\mathrm{KOH})\) is added, then the salt formed is potassium aluminate \(\left(\mathrm{KAlO}_{2}\right)\). The chemical reaction involved is given below:

\(\mathrm{Al}_{2} \mathrm{O}_{3}+2 \mathrm{KOH} \rightarrow 2 \mathrm{KAlO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

• Action of Alkalis on Lead Oxide

Lead Oxide \((\text {PbO})\), which is yellow in colour reacts with an alkali sodium hydroxide \((\mathrm{NaOH})\) at a given temperature to form a colourless and soluble salt sodium plumbite \(\left(\mathrm{Na}_{2} \mathrm{PbO}_{2}\right)\) and water. In this condition, lead oxide acts as an acid that reacts with a base to give neutralisation reaction. The chemical reaction is given below:

\(\mathrm{PbO}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{PbO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

In place of sodium hydroxide, potassium hydroxide \((\mathrm{KOH})\) is added, then the salt formed is potassium plumbite \(\left(\mathrm{K}_{2} \mathrm{PbO}_{2}\right)\). The chemical reaction involved is given below:

\(\mathrm{PbO}+2 \mathrm{KOH} \rightarrow \mathrm{K}_{2} \mathrm{PbO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

2. Action of Alkalis on Metal Hydroxide

• Action of Alkalis on Zinc Hydroxide

Zinc Hydroxide \(\left(\mathrm{Zn}(\mathrm{OH})_{2}\right)\) reacts with an alkali sodium hydroxide \((\mathrm{NaOH})\) at a given temperature to form a salt sodium zincate \(\left(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\right)\) and two molecules of water. In this condition, zinc hydroxide acts as an acid that reacts with a base to give neutralisation reaction.

\(\mathrm{Zn}(\mathrm{OH})_{2}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

If instead of sodium hydroxide, potassium hydroxide \((\text {KOH})\) is added, then the salt formed is potassium zincate \(\left(\mathrm{K}_{2} \mathrm{ZnO}_{2}\right)\). The chemical reaction involved is given below:

\(\mathrm{Zn}(\mathrm{OH})_{2}+2 \mathrm{KOH} \rightarrow \mathrm{K}_{2} \mathrm{ZnO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

• Action of Alkalis on Aluminium Hydroxide

Aluminium hydroxide \(\left(\mathrm{Al}(\mathrm{OH})_{3}\right)\) reacts with an alkali sodium hydroxide \((\mathrm{NaOH})\) at a given temperature to form a salt sodium aluminate \(\left(\mathrm{NaAlO}_{2}\right)\) and water. In this condition, aluminium hydroxide acts as an acid that react with a base to give neutralisation reaction. The chemical reaction is given below:

\(\mathrm{Al}(\mathrm{OH})_{3}+\mathrm{NaOH} \rightarrow \mathrm{NaAlO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

In place of sodium hydroxide, potassium hydroxide \((\text {KOH})\) is added, then the salt formed is potassium aluminate \(\left(\mathrm{KAlO}_{2}\right)\). The chemical reaction involved is given below:

\(\mathrm{Al}(\mathrm{OH})_{3}+\mathrm{KOH} \rightarrow \mathrm{KAlO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

• Action of Alkalis on Lead Hydroxide

Lead hydroxide \(\left(\mathrm{Pb}(\mathrm{OH})_{2}\right)\), reacts with an alkali sodium hydroxide \((\mathrm{NaOH})\) at a given temperature to form a colourless and soluble salt sodium plumbite \(\left(\mathrm{Na}_{2} \mathrm{PbO}_{2}\right)\) and water. In this condition, lead hydroxide acts as an acid that reacts with a base to give neutralisation reaction. The chemical reaction is given below:

\(\mathrm{Pb}(\mathrm{OH})_{2}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{PbO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

If, instead of sodium hydroxide, potassium hydroxide \((\text {KOH})\) is added, then the salt formed is potassium plumbite \(\left(\mathrm{K}_{2} \mathrm{PbO}_{2}\right)\). The chemical reaction involved is given below:

\(\mathrm{Pb}(\mathrm{OH})_{2}+2 \mathrm{KOH} \rightarrow \mathrm{K}_{2} \mathrm{PbO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

Learn About Acids, Bases and Salts

Summary

In short, Alkalis are water-soluble bases. Sodium hydroxide or potassium hydroxide are some alkalis. We know very well that metal oxides and hydroxides are basic in nature. Hence, it is not possible for a basic metal oxide to react with an alkali or base. Due to this reason, some metal oxides and hydroxides that are amphoteric in nature only react with alkalis. The amphoteric metal oxides and hydroxides show dual behaviour, i.e., they show both acidic as well as basic properties. Some amphoteric metal oxides include zinc oxide, aluminium oxide, and lead oxide. They show both acidic as well as basic character. So, these acidic oxides can react with alkalis to give a neutralisation reaction. These metal oxides dissolve in water to form metal hydroxides. Metal oxides and hydroxides react with alkalis like sodium hydroxide, potassium hydroxide, etc., to form respective salt and water. The only difference between the reaction of alkalis with metal oxides and hydroxides is that in the case of metal oxides, one water molecule of water is formed and in the case of metal hydroxides, two water molecules of water are formed.

FAQs

Q.1. How are alkali metal oxides and hydroxides prepared?
Ans: The oxygen atom, when bound with metal, gives metal oxides. When metals bind with hydroxide group \((−\text {OH})\), they form metal hydroxides. Metal oxides are also called basic oxides because they react with water to form hydroxides, also known as bases. Metal oxides in water form metal hydroxides.

Q.2. Are alkalis, oxide and hydroxides of metals?
Ans: All alkali metals form different oxides, hydroxides, carbonates, and nitrates. They are among the most reactive elements as they have the weakest nuclear charge in the respective period. They have the tendency to lose one valence electron in the outermost shell and form strong ionic bonds with their anions. Hence, alkalis are mostly oxide and hydroxides of alkali metals.

Q.3. Which metal oxides form alkalis?
Ans: The oxides of alkali metals react to form a simple or alkaline metal hydroxide with water they are alkalis or bases that are soluble in water. For example, alkali metals like Sodium, potassium, lithium, etc., form alkalis like sodium hydroxide, potassium hydroxide, and lithium hydroxide.

Q.4. What happens when an acid reacts with metal oxides and metal hydroxides explain with proper reactions?
Ans: When acids react with metal oxides, respective salt along with water are formed. Since metal oxides are bases, they neutralise acids.
\(\mathrm{CuO}+2 \mathrm{HCl} \rightarrow \mathrm{CuCl}_{2}+\mathrm{H}_{2} \mathrm{O}\)
Similarly, salt and water are produced when acids react with metal hydroxides. Metal hydroxides are also bases as they neutralise acids. For example, sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.
\(\mathrm{NaOH}+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}\)

Q.5. How do alkalis react with metal oxides and metal hydroxides? Explain with examples.
Ans: Metal oxides and hydroxides are basic in nature. Hence, it is not possible for a basic metal oxide to react with an alkali or base. Thus, only a few metal oxides and hydroxides that are amphoteric in nature react with alkalis. The metal oxides and hydroxides that show dual behaviour, i.e., they show both acidic as well as basic properties, are known as amphoteric oxides. Some amphoteric oxides include zinc oxide, aluminium oxide, and lead oxide. They show both acidic as well as basic character. For example, zinc oxide and zinc hydroxide react with sodium hydroxide to form sodium zincate and water.
\(\mathrm{ZnO}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2}+\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{Zn}(\mathrm{OH})_{2}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\).

Q.6. Are metal oxides acidic or basic?
Ans: Metal oxides are basic in nature. The oxygen atom, when bound with metal, it gives metal oxides. When metals bind with hydroxide group \((−\text {OH})\), they form metal hydroxides. Metal oxides are also called basic oxides because they react with water to form hydroxides, also known as bases or alkalis.

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