Metals and Non-Metals - Part III

Metals react with oxygen, water, dilute acids, and other salts. In the ensuing discussions we will study these reactions. These reactions will give us some idea about chemical reactivity of metals. 

1. Metals are electropositive : Atoms of metal elements have 1 to 3 electrons in the outermost shells. The atoms can therefore loose electrons while forming compounds. The metal atoms become electropositive in a reaction. For example, Na atom easily gives off its one electron in the M-shell to become Na⁺.  

2. Reaction with oxygen : All metals react with oxygen to form metal oxides. A single metal can form various oxides depending on the valence state of the metal atom. For example Fe can form oxides FeO, Fe₂O₃, Fe₃O₄, etc. Metal oxides are basic in nature; when dissolved in water they form alkaline solution. Metal oxide bonds are ionic bonds. Metal atoms loose electrons to oxygen atoms while forming the metal oxide. Not all metals react with oxygen at room temperature. Some metals need to be held at high temperatures before they start reacting with oxygen. 

i) Sodium reacts with oxygen or air at ordinary room temperature. The resultant is sodium oxide. Sodium is a highly reactive metal and catches fire easily when exposed to air. 

Na₂O when dissolved in water becomes NaOH or sodium hydroxide. Potassium (K) metal also shows similar behavior. 

ii) Magnesium does not react with oxygen at room temperature. For the reaction to start, Mg has to be heated in air so that magnesium oxide or MgO is formed. 

Since heat is required for starting the above reaction, it means that Mg is less reactive than Na. Calcium shows similar reaction as Mg. 

iii) Zinc metal reacts with oxygen in the air only after heating strongly. Zinc forms ZnO. 

The reaction shows that Zn is less reactive than Mg. 

iv) Fe reacts with oxygen more slowly than Zn. A typical reaction of Fe with oxygen is shown below. 

v) Copper metal reacts with oxygen in the air very slowly. Prolonged strong heating will result into copper oxide. The reaction is shown below. 

Thus in terms of reactivity of metals with oxygen, we can say that Na is the most reactive metal and Cu is the least reactive of the examples that we have seen above. 

To express the order of reactivity of the metals with oxygen, we can say that 

                    Na    >    Mg    >    Zn    >    Fe    >   Cu
                        
             Most                     Decreasing                    Least 
            reactive                  reactivity                        reactive

3. Metal reaction with water : All metals react with water to form metal oxides and metal hydroxides. Hydrogen gas is released in this reaction. Metal oxides are basic in nature; when dissolved in water they form alkaline solution. Not all metals react with water at room temperature. Some metals react with cold water, some metals react with hot water, and other metals react with steam. There are some metals that do not react with water at all. These reactions will become more clear in the following discussion.

i) Sodium reacts with water at ordinary room temperature, or even cold water. Sodium, as discussed earlier, is a highly reactive metal. Sodium forms NaOH when reacted with water. Hydrogen gas is released. 

ii) Magnesium reacts slowly with water at room temperature or with cold water. But Mg reacts with hot water easily to form MgO. Hydrogen gas is released.

Since heat is required for starting the above reaction, it means that Mg is less reactive than Na. 

iii) Zinc metal reacts with steam. Zinc forms ZnO (zinc oxide) and hydrogen are formed.

The reaction shows than Zn is less reactive than Mg. 

iv) Fe reacts with water more slowly than Zn. Iron has to be heated to a red-hot condition and steam has to be introduced for Fe to react with water (steam). Iron oxide (with Fe²⁺ and Fe³⁺) and hydrogen are formed. 

v) Copper metal does not react with water or steam. This shows that Cu is less reactive than Fe. 

Thus in terms of reactivity of metals with water, we can say that Na is the most reactive metal and Cu is the least reactive of the examples that we have seen above. 

To express the order of reactivity of the metals with water, we can say that  

                    Na    >    Mg    >    Zn    >    Fe    >   Cu
                        
             Most                     Decreasing                     Least 
            reactive                  reactivity                         reactive

4. Metal reaction with dilute acids : Most metals react with dilute acids by replacing hydrogen and forming a salt. Hydrogen gas is released. Metals like Cu, Ag, Au, Pt, Hg, do not react with dilute acids. The rate of reaction depends on the reactivity of the metal. Na reacts vigorously with dilute acids, whereas Zn or Fe react very slowly with dilute acids. 

With dilute hydrochloric acids, metals react to give metal chloride and hydrogen. Some examples are given below.

With dilute sulphuric acids, metals react to give metal sulphate and hydrogen. Some examples are given below. 

Reactions with dilute nitric acids and metals are not as straight forward as the reactions shown above. Nitric acid is an oxidizing acid and releases O from its NO₃^--radical, hence its reactions with metals does not lead to metal-salt and hydrogen. 

From the dilute acid reactions, we can conclude that Na is more reacts most rapidly and Cu does not react at all. So the order of reactivity of these metals with dilute acids can be written as 

To express the order of reactivity of the metals with dilute acids, we can say that  

                    Na    >    Mg    >    Zn    >    Fe    >   Cu
                        
             Most                     Decreasing                     Least 
            reactive                  reactivity                         reactive

5. Metal reactions with salt solutions : Reactivity of metals determine which salt is formed when a metal is added to a salt solution. More reactive metal displaces the less reactive metal in the metal salt. These are called displacements reactions. Examples below will show some typical examples of metal displacement reactions.

6. Metal reactions with metal oxides : Reactivity of metals determine which metal displaces another metal from the metal-oxide. As discussed earlier, more reactive metal displaces the less reactive metal in the metal oxide. 

When CuO is heated with Mg, MgO is formed and Cu is released. This is because Mg is more reactive than Cu. This reaction is irreversible, as Cu will never be able to displace Mg from MgO. 

7. Metal reactions with chloride : Metals have excess electrons and chlorine (Cl) needs one electron to complete its orbit. Hence metals react with Cl easily. Na, Ca react readily with Cl to form NaCl and CaCl₂. Mg reacts with Cl on heating to form MgCl₂. Zn forms a chloride easily. Fe and Cu need to be heated to form their chlorides. All chlorides are electrovalent or ionic compounds. The reactions are shown below. 

8. Metal reactions with hydrogen : Na, K, Mg and Ca react with H to form metal-hydrides. Other metals do not react with H easily. To form metal-hydrides, the metals have to be heated and hydrogen gas has to be passed over the heated metal fillings. Metals have to readily give up their electron to the hydrogen to form the metal-hydrogen bond. NaH, KH, CaH₂ and MgH₂ are common metal-hydrides. The reactions are shown below. 

Copper (Cu), iron (Fe) and zinc (Zn) are less reactive metals are they do not react with hydrogen and their metal-hydrides are not formed, even when heated. 

9. Metals as reducing agents : Metals have excess unbound electrons and are able to give these electrons easily. In chemistry this property of a substance to donate electrons is known as reducing property and the substance is called a reducing agent. For example Na gives off an electron to any other substance which is forming a bond with itself. Thus Na is a reducing agent. Na atoms becomes ionized, hence Na itself is getting oxidized. The reaction is shown below. 

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