Reactions with Water: Group 2 Elements
Meet the Alkaline Earth Metals
Group 2 is a family of elements in the periodic table that includes Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). They are all shiny, silvery-white metals. The name "alkaline earth" comes from the fact that their oxides are found in the earth's crust and they form alkaline solutions (hydroxides) when reacted with water. A key property is that they all have two electrons in their outer shell, which they readily lose to form $2^+$ ions, making them highly reactive.
The General Reaction and Its Equation
The general chemical reaction between a Group 2 metal (represented by 'M') and water is a classic example of a single displacement reaction. The metal pushes out the hydrogen from the water. The general word equation and chemical formula are:
The balanced chemical equation is: $M_{(s)} + 2H_2O_{(l)} \rightarrow M(OH)_{2(aq)} + H_{2(g)}$
This reaction is also exothermic, meaning it releases heat energy. The metal hydroxide produced dissolves in water to form an alkaline solution, which turns universal indicator or phenolphthalein purple or pink, indicating a base.
Reactivity Trend: A Journey Down the Group
One of the most fascinating aspects of Group 2 chemistry is how the reactivity changes as you move from the top to the bottom of the group. The reactivity increases down the group. This trend can be explained by atomic structure. As you go down the group, the number of electron shells increases. The two outer electrons are therefore further from the attractive force of the nucleus and are more easily lost. This makes it easier for the metal to form a $2^+$ ion and react.
| Element | Reaction with Cold Water | Observations | Chemical Equation |
|---|---|---|---|
| Beryllium (Be) | No visible reaction. | A very thin, protective oxide layer forms on its surface, preventing reaction. | No reaction with cold water. |
| Magnesium (Mg) | Very slow with cold water, reacts faster with steam. | With cold water, tiny bubbles of hydrogen gas appear very slowly. The solution turns weakly alkaline. | $Mg_{(s)} + 2H_2O_{(l)} \rightarrow Mg(OH)_{2(aq)} + H_{2(g)}$ (very slow) |
| Calcium (Ca) | Steady reaction. | Fizzing, bubbles of hydrogen gas are produced. The water turns cloudy white as calcium hydroxide is less soluble. The solution is strongly alkaline. | $Ca_{(s)} + 2H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} + H_{2(g)}$ |
| Strontium (Sr) | Vigorous reaction. | Rapid fizzing and a lot of heat is produced. The metal moves around on the water surface. | $Sr_{(s)} + 2H_2O_{(l)} \rightarrow Sr(OH)_{2(aq)} + H_{2(g)}$ |
| Barium (Ba) | Very vigorous, potentially explosive. | Extremely rapid fizzing, the metal may spark or ignite the hydrogen gas. The reaction must be conducted with extreme caution. | $Ba_{(s)} + 2H_2O_{(l)} \rightarrow Ba(OH)_{2(aq)} + H_{2(g)}$ |
A Closer Look: Magnesium and Calcium in Action
Let's examine two of the most commonly studied Group 2 elements in more detail.
Magnesium has a interesting dual behavior. In cold water, the reaction is barely noticeable because the magnesium hydroxide produced is insoluble and forms a protective layer on the metal, stopping further reaction. However, if you use magnesium in the form of a powder or ribbon and pass steam over it when it's hot, the reaction is much more vigorous, producing magnesium oxide and hydrogen gas: $Mg_{(s)} + H_2O_{(g)} \rightarrow MgO_{(s)} + H_{2(g)}$.
Calcium, on the other hand, reacts readily with cold water. If you drop a small piece of calcium metal into a beaker of water, you will see immediate fizzing as hydrogen gas is produced. The water will also become warm to the touch (exothermic) and turn cloudy. The cloudiness is due to the formation of a suspension of sparingly soluble calcium hydroxide, $Ca(OH)_2$, which is also known as slaked lime.
From the Lab to Real Life: Practical Applications
The products of these reactions are not just laboratory curiosities; they have important uses in our daily lives and industries.
Calcium Hydroxide, $Ca(OH)_2$: This is one of the most useful products. It is used in agriculture to neutralize acidic soils (a process called liming). It is also a key ingredient in the mortar used in construction, and it is used in water treatment to adjust the pH of drinking water.
Magnesium Hydroxide, $Mg(OH)_2$: This compound is sparingly soluble in water and is the active ingredient in milk of magnesia, a common antacid used to relieve indigestion. It works by neutralizing excess stomach acid.
Hydrogen Gas, $H_2$: While not typically produced industrially from metal-water reactions due to cost, the hydrogen gas produced in these reactions is the same gas that can be used as a clean fuel, burning with a pale blue flame to produce water.
Important Questions
Why does reactivity increase as you go down Group 2?
The reactivity increases down the group due to atomic size. As you move down, each element gains an extra electron shell. This means the two outer electrons are further away from the positive nucleus. The attraction between the nucleus and these outer electrons becomes weaker, making it easier for the atom to lose them and form a positive ion ($M^{2+}$). Since chemical reactions involve losing electrons, the easier it is to lose electrons, the more reactive the metal is.
Why is the reaction of magnesium with steam different from cold water?
In cold water, the magnesium hydroxide $Mg(OH)_2$ that forms is insoluble and sticks to the metal, creating a protective layer that prevents further reaction. With steam, the energy provided by the heat breaks this layer, allowing a continuous and faster reaction to occur. The high temperature also provides the activation energy needed for a more favorable reaction pathway, leading to the formation of magnesium oxide ($MgO$) instead of the hydroxide.
How can you test that the gas produced is hydrogen?
The classic test for hydrogen gas is the "squeaky pop" test. You collect the gas in a small test tube and hold a burning splint near the mouth of the tube. If the gas is hydrogen, it will ignite with a characteristic squeaky pop sound as it reacts rapidly with oxygen in the air to form water vapor.
Conclusion
The reactions of Group 2 elements with water provide a clear and dramatic demonstration of periodic trends in chemistry. From the inertness of beryllium to the explosive nature of barium, the increase in reactivity is a direct result of the underlying atomic structure. These reactions are not just theoretical; they produce compounds like calcium hydroxide and magnesium hydroxide that are vital to agriculture, medicine, and industry. Understanding these reactions helps us see the beautiful pattern and practical power of chemistry in the world around us.
Footnote
1 Alkaline: A substance with a pH greater than 7 that can neutralize an acid. Alkaline solutions feel soapy and turn red litmus paper blue.
2 Exothermic Reaction: A chemical reaction that releases energy, usually in the form of heat, into its surroundings.
3 Single Displacement Reaction: A chemical reaction where one element replaces another element in a compound.
4 Reactivity Series: A list of metals arranged in order of their decreasing reactivity, with the most reactive metal at the top.