How Do Metals React With Acid?
Most of the metals found in nature react with acids, but not all of them react in the same way. Metals react with acids to form a corresponding salt solution and hydrogen gas. The salt solution formed will be based on the reacting metal and the acid.
The name of the salt solution thus formed is given using the first part being the name of the metal used and the secound part from the acid.
What Happens When Metals React with Acids?
The reaction of an acid with metal is an exothermic reaction. When a piece of metal is dipped in an acid solution, the size of the metal decreases. This is because, when metal reacts with acid solutions, a salt solution is formed and hydrogen gas is liberated. The hydrogen gas is liberated in the form of bubbles. That is, when a metal is made to react with an acid, the metal usually displaces hydrogen from the acid.
The evolving hydrogen gas can be tested by introducing a burnt splint into the evolving gas. Since hydrogen is flammable, it will produce a small explosion with a popping sound. A general reaction of acid with metal can be given as:
Metal + Acid $\rightarrow$ Salt Solution + Hydrogen Gas
The reactions of metals with acids are actually redox reaction. That is, the metal gets oxidised and the acid gets reduced during the reaction. The oxidation and reduction happen simultaneously, so it is considered as a redox reaction.
Consider the reaction of zinc metal with acid.
$\mathrm{Zn}_{\text {(s) }}+2 \mathrm{H}_{\text {(aq) }}^{+} \rightarrow \mathrm{Zn}^{2+}{ }_{(\mathrm{aq})}+\mathrm{H}_{2(\mathrm{~g})}$
The half reactions are :
$\mathrm{Zn}_{(\mathrm{s})} \rightarrow \mathrm{Zn}_{(\text {(aq) }}^{2+}+2 \mathrm{e}^{-} \text {(oxidation) }$
$2 \mathrm{H}_{(\mathrm{aq})}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2(\mathrm{~g})} \text { (reduction) }$
Here, Zn loses electrons. That is, Zn gets oxidised. Hydrogen ions acquire electrons and get reduced.
Reactivity Series
The reactivity series is the series of metals arranged in the decreasing order of their reactivity. It is also known as an activity series. In a reactivity series, the most reactive metals are on top and the least reactive metals are on the bottom. The reactivity series of metals is shown below.
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Reaction of Acid with Metal
Most of the metals react with dilute mineral acids, namely dilute hydrochloric acid, dilute sulphuric acid, etc. to produce a salt solution and hydrogen gas.
Reaction of Metal With Dilute Hydrochloric Acid
Metals react with dilute hydrochloric acid to give their corresponding chloride salt and hydrogen.
For example: When a reaction of zinc metal with acid dil.HCl takes place, it will give ZnCl2 and H2 gas as the product.
$\mathrm{Zn}+\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2}+\mathrm{H}_{2}$
Similar is the situation when aluminium and magnesium reacts with dil.HCl. Aluminium reacts with dil.HCl to form Aluminium Chloride, and Mg reacts to form Magnesium Chloride. The reaction is as follows:
$\mathrm{Al}+\mathrm{HCl} \rightarrow \mathrm{AlCl}_{3}+\mathrm{H}_{2} \\ $ $\mathrm{Mg}+\mathrm{HCl} \rightarrow \mathrm{MgCl}_{2}+\mathrm{H}_{2} $
The least reactive metals below the hydrogen atom in the reactivity series, like Cu, Ag and Au do not react with dilute hydrochloric acid and do not produce hydrogen gas. The most reactive metals like sodium (Na) and potassium (K) are never made to react with dil.HCl. Since these metals are very reactive, they react very violently with the dilute acids and the reaction will be very exothermic and a huge amount of heat is liberated causing explosion.
Reaction of Metal With Dilute Sulphuric Acid
When a metal reacts with the acid H2SO4, it forms their corresponding sulphates and hydrogen gas.
For example: Metals such as Zn, Al, Mg and Fe react with dilute H2SO4 to form hydrogen gas and ZnSO4, Al2(SO4)3, MgSO4 and FeSO4 respectively.
$\mathrm{Zn}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{ZnSO}_{4}+\mathrm{H}_{2} \\$ $\mathrm{Fe}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{FeSO}_{4}+\mathrm{H}_{2}$
The least reactive metals like Cu also do not react with dilute sulphuric acid to produce hydrogen. However, they react with hot concentrated sulphuric acid but do not produce hydrogen.
Reaction of Metal With Nitric Acid
The reaction of metal with nitric acid will produce metal nitrates. Hydrogen gas is not evolved in this reaction. This is because nitric acid is a very strong oxidising agent and any hydrogen evolved is oxidised to water.
NO (nitrogen monoxide) gas is evolved in the reaction of metals with dilute nitric acid, whereas NO2 (nitrogen dioxide) gas is evolved in the reaction with concentrated nitric acid.
The general reaction of metals with dilute nitric acid is given as:
$\mathrm{HNO}_{3}+\text { Metal } \rightarrow \text { Metal Nitrate }+\mathrm{NO}+\mathrm{H}_{2} \mathrm{O}$
Example : $\mathrm{Zn}+8 \mathrm{HNO}_{3}(\text { dil }) \rightarrow 3 \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}+2 \mathrm{NO}+4 \mathrm{H}_{2} \mathrm{O}$
The general reaction of metals with concentrated nitric acid is as given below:
$\mathrm{HNO}_{3}+\text { Metal } \rightarrow \text { Metal nitrate }+\mathrm{NO}_{2}+\mathrm{H}_{2} \mathrm{O}$
Example : $\mathrm{Zn}+4 \mathrm{HNO}_{3} \rightarrow \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}+\mathrm{NO}_{2}+2 \mathrm{H}_{2} \mathrm{O}$
Conclusion
Reaction of an acid with metal form salt and hydrogen gas. All metals do not react in a similar way with the acids. The reaction depends upon the position of the metal in the reactivity series. Highly reactive metals are never made to react with dilute mineral acids. The least reactive metals do not react with other metals and they do not produce hydrogen gas.
Metals like Mg, Zn, and Al react with dilute mineral acids such as dil.HCl and dil.H2SO4 to form their corresponding chlorides and sulphates respectively. Metal nitrates are produced in the reaction of metals with nitric acid, and no hydrogen gas is evolved.
FAQs on Reaction of Metals With Acids for JEE
1. How does concentrated acid react with the least reactive metals?
The least reactive metals are gold, silver, platinum, i.e., noble metals. These metals do not react with any mineral acids like hydrochloric acid, sulphuric acid or even nitric acid. It means, they can’t dissolve in any single component acids.
But they can react with aqua regia. Aqua regia is an acidic solution formed by mixing concentrated hydrochloric acid and concentrated nitric acid in a 3:1 molar ratio. When gold reacts with aqua regia, the nitric acid acts as an oxidiser and oxidises Au to Au3+. Then hydrochloric acid reacts with these Au3+ ions to form tetrachloroaurate (III) ions.
2. Why are metals reacted with dilute acids and not usually reacted with concentrated acids?
Most of the metals are made to react with dilute acids, and they can form hydrogen gas and corresponding salts. Many of the acids in concentrated form are very powerful oxidising agents and may produce undesirable by-products on reacting with metals. Also, the reaction will be very exothermic in nature.
For example, the reaction of zinc with concentrated sulphuric acid will result in the formation of SO2 as a by-product. The reaction is as follows:
$\mathrm{Zn}+\text { conc. } \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{ZnSO}_{4}+\mathrm{SO}_{2}+\mathrm{H}_{2} \mathrm{O}$
3. Explain some physical properties of metal.
The main physical characteristics of metals.Thin sheets of metal can be crushed out. It indicates that they have the quality of malleability. Metals can bend easily. They are able to be drawn into wires. Metals are efficient heat and electrical conductors. Metals have a glossy appearance because they are lustrous. The tensile strength of metals is high. It implies that they can support large weights.Metals have a resonant sound. It indicates that they produce a ringing sound when we strike them. Metals are brittle. It implies that they are difficult to cut.
