Reactions of Group 2 Elements with Acids
- Page ID
This page discusses the reactions of the Group 2 elements (beryllium, magnesium, calcium, strontium and barium) with common acids.
Reactions with dilute hydrochloric acid
Each metal reacts with dilute hydrochloric acid, producing bubbles of hydrogen gas and a colorless solution of the metal chloride:
\[ X + 2HCl \rightarrow XCl_2 + H_2\]
These reactions become more vigorous down the group.
Reactions with dilute sulfuric acid
These are more complicated, because of the formation of insoluble sulfates.
Beryllium and magnesium
These metals react with with dilute sulfuric acid just as they did with dilute hydrochloric acid; the reaction between magnesium and dilute sulfuric is familiar to many beginning chemists. Hydrogen gas is formed, along with colorless solutions of beryllium or magnesium sulfate. For example:
\[ Mg + H_2SO_4 \rightarrow MgSO_4 + H_2\]
Calcium, strontium and barium
Calcium sulfate is slightly soluble, and strontium and barium sulfates are essentially insoluble. When exposed to sulfuric acid, a layer of insoluble sulfate is formed on each of these metals, slowing or stopping the reaction entirely. In the calcium case, some hydrogen is produced, along with a white precipitate of calcium sulfate.
Reactions with nitric acid
These reactions are more complicated. When a metal reacts with an acid, the metal usually reduces hydrogen ions to hydrogen gas. The elemental metal is oxidized to metal cations in the process.
However, nitrate ions are easily reduced to nitrogen monoxide and nitrogen dioxide. Metals reacting with nitric acid, therefore, tend to produce oxides of nitrogen rather than hydrogen gas. If the acid is relatively dilute, the reaction produces nitrogen monoxide, although this immediately reacts with atmospheric oxygen, forming nitrogen dioxide. If concentrated nitric acid is used, nitrogen dioxide is formed directly.
Various sources disagree on whether beryllium reacts with nitric acid. Beryllium forms a strong oxide layer (similar to that of aluminum) which slows reactions down until it has been removed.
Some sources say that beryllium does not react with nitric acid. However, procedures for making beryllium nitrate by reacting beryllium powder with nitric acid are readily available. One source uses semi-concentrated nitric acid, claiming that the gas evolved is nitrogen monoxide. This is a reasonable conclusion.
The reactivity of beryllium seems to depend on its source, and how it was manufactured. It is possible that small amounts of impurities in the metal can affect its reactivity.
The other Group 2 metals
The rest of the Group 2 metals produce hydrogen gas from very dilute nitric acid, but this gas is contaminated with nitrogen oxides. Colorless solutions of the metal nitrates are also formed. Taking magnesium as an example, if the solution is very dilute:
\[ Mg + 2HNO_3 \rightarrow Mg(NO_3)_2 + H_2\]
At moderate concentrations (even with very dilute acid, this occurs to some extent):
\[ 3Mg + 8HNO_3 \rightarrow 3Mg(NO_3)_2 + 2NO + 4H_2O\]
And with concentrated acid:
\[ Mg + 4HNO_3 \rightarrow Mg(NO_3)_2 + 2NO_2 + 2H_2O\]
Contributors and Attributions
Jim Clark (Chemguide.co.uk)