Tin (Sn) is a metallic element found in Group 14 of the periodic table. It has been used for millennia, notably as a component in bronze. The common form is silvery-white beta-tin. Tin is considered a low-reactivity metal, making it useful in protective coatings and alloys. Its chemistry is characterized by its amphoteric nature, meaning it reacts with both acidic and alkaline solutions.
How Tin Reacts with Air and Water
Tin exhibits high resistance to corrosion when exposed to air at ambient temperatures. This stability is due to passivation, where the surface spontaneously forms a layer of tin(IV) oxide (\(\text{SnO}_2\)). This protective oxide film shields the underlying metal from further reaction with oxygen.
Heating tin significantly increases its reactivity with air. When the metal is heated above \(150^\circ\text{C}\), it readily reacts with oxygen, forming tin(IV) oxide.
Tin does not react with cold or liquid water, maintaining stability suitable for food storage containers. However, if tin is subjected to steam at very high temperatures, a slow reaction occurs. This high-temperature interaction yields tin(IV) oxide and releases hydrogen gas (\(\text{Sn} + 2\text{H}_2\text{O} \rightarrow \text{SnO}_2 + 2\text{H}_2\)).
Reactions with Acids
Tin reacts differently depending on whether the acid is non-oxidizing or oxidizing, influencing the final product and the oxidation state. With non-oxidizing acids, such as dilute hydrochloric acid (\(\text{HCl}\)), the reaction is slow, forming a tin(II) salt and evolving hydrogen gas. For example, in hydrochloric acid, the products are tin(II) chloride and hydrogen (\(\text{Sn} + 2\text{HCl} \rightarrow \text{SnCl}_2 + \text{H}_2\)). This reaction oxidizes the tin metal from 0 to \(+2\). Concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)) also reacts slowly with tin, but the rate increases when heated.
Oxidizing acids like concentrated nitric acid (\(\text{HNO}_3\)) react vigorously with tin, converting the metal to its highest oxidation state of \(+4\). The strong oxidizing capability means the primary product is hydrated tin(IV) oxide, sometimes called metastannic acid (\(\text{H}_2\text{SnO}_3 \cdot \text{nH}_2\text{O}\)). This white, amorphous product is insoluble and precipitates out of the solution.
The reaction with concentrated nitric acid does not produce hydrogen gas, as the acid is reduced to nitrogen oxides. The outcome depends on the acid’s concentration; dilute nitric acid may instead produce a soluble tin(II) nitrate salt.
Reactions with Strong Bases
Tin is notable for its amphoteric character, meaning it is readily attacked by both acids and strong bases. While many metals are inert to alkaline solutions, tin reacts with hot, concentrated solutions of strong bases, such as sodium hydroxide (\(\text{NaOH}\)).
The reaction with a strong alkali produces a soluble hydroxo-complex and releases hydrogen gas. When tin reacts with hot concentrated sodium hydroxide, it forms the hydroxostannate(II) ion, \([\text{Sn}(\text{OH})_4]^{2-}\). The general equation for this reaction is \(\text{Sn} + 2\text{OH}^- + 2\text{H}_2\text{O} \rightarrow [\text{Sn}(\text{OH})_4]^{2-} + \text{H}_2\). The formation of this soluble complex allows the tin to dissolve completely in the alkaline solution.
Reactions with Halogens and Other Non-metals
Tin reacts vigorously with halogens, often under mild conditions. Reactions with chlorine (\(\text{Cl}_2\)) and bromine (\(\text{Br}_2\)) are particularly vigorous and often occur spontaneously. In these reactions, tin is oxidized to its \(+4\) state, readily forming tin(IV) halides, such as tin(IV) chloride (\(\text{SnCl}_4\)). The general form of this highly exothermic reaction is \(\text{Sn} + 2\text{X}_2 \rightarrow \text{SnX}_4\), where \(\text{X}\) represents the halogen.
Tin also reacts with other non-metals when heated, such as sulfur (\(\text{S}\)). When heated with sulfur, it forms tin sulfides, which can be either tin(II) sulfide (\(\text{SnS}\)) or tin(IV) sulfide (\(\text{SnS}_2\)), depending on the conditions.