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Reactions of Group 2 Elements with Water - Chemwiki
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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Descriptive Chemistry > s-Block Elements > Group 2 Elements: The
Alkaline Earth Metals > Chemistry of the Group 2 Elements > Reactions of Group 2 Elements with Water
Reactions of Group 2 Elements with Water
This page discusses the reactions of the Group 2 elements (beryllium, magnesium, calcium, strontium and barium) with water, using these reactions to describe the trend in
reactivity in Group 2.
Beryllium
Beryllium has no reaction with water or steam, even at high heat.
Magnesium
Magnesium burns in steam to produce white magnesium oxide and hydrogen gas.
Very clean magnesium ribbon has a mild reaction with cold water, given below. After several minutes, hydrogen gas bubbles form on its surface, and the coil of magnesium ribbon
usually floats to the surface. However, the reaction is short-lived because the magnesium hydroxide formed is almost insoluble in water and forms a barrier on the magnesium
preventing further reaction.
Note: As a general rule, if a metal reacts with cold water, the metal hydroxide is produced. If it reacts with steam, the metal oxide is formed. This is because the metal hydroxides
thermally decompose to the oxide and water.
Calcium, strontium and barium
These metals react with cold water with increasing vigor to give the metal hydroxide and hydrogen. Strontium and barium have reactivities similar to that of lithium. Calcium, for
example, reacts fairly vigorously and exothermically with cold water. Bubbles of hydrogen gas are given off, and a white precipitate (of calcium hydroxide) is formed, together with an
alkaline solution (also of calcium hydroxide, which is slightly water-soluble). The equation for the reactions of any of these metals would is as follows:
The hydroxide solubilities increase down the group. Calcium hydroxide is mainly formed as a white precipitate (although some does dissolve). Less precipitate is formed down the
group with increasing solubility.
Summary of the trend in reactivity
The Group 2 metals become more reactive with water down the group.
Beryllium as a special case
An additional reason for the lack of reactivity of beryllium compared with the rest of the group is that beryllium has a strong resistant layer of oxide on its surface which lowers its
reactivity (a similar phenomenon occurs with aluminum). This, along with beryllium's position in the group, indicates that beryllium is quite unreactive.
The enthalpy changes for the reactions
The enthalpy change of a reaction is a measure of the amount of heat absorbed or evolved when the reaction takes place. An enthalpy change is negative if heat is evolved, and
positive if it is absorbed. Calculate the enthalpy change for the possible reactions between beryllium or magnesium and steam gives the following values:
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2/18/2015
Reactions of Group 2 Elements with Water - Chemwiki
Notice that both possible reactions are strongly exothermic, giving out almost identical amounts of heat. However, only the magnesium reaction actually happens. The explanation
for the different reactivities must lie somewhere else. Similarly, calculating the enthalpy changes for the reactions between calcium, strontium or barium and cold water reveals that
the amount of heat evolved in each case is almost exactly the same—about -430 kJ mol-1. The reason for the increase in reactivity must again lie elsewhere.
The activation energies for the reactions
The activation energy for a reaction is the minimum amount of energy which is needed in order for the reaction to take place. It doesn't matter how exothermic the reaction would be
once it got started - if there is a high activation energy barrier, the reaction will take place very slowly, if at all.
When Group 2 metals react to form oxides or hydroxides, metal ions are formed. The formation of the ions from the original metal involves various stages all of which require the
input of energy - contributing to the activation energy of the reaction. These stages involve the input of:
the atomization energy of the metal. This is the energy needed to break the bonds holding the atoms together in the metallic lattice.
the first + second ionization energies. These are necessary to convert the metal atoms into ions with a 2+ charge.
After this, there will be a number of steps which give out heat again - leading to the formation of the products, and overall exothermic reactions.
The graph shows the effect of these important energy-absorbing stages as you go down Group 2.
Notice that the ionization energies dominate this - particularly the second ionization energies. Ionization energies fall down the group. Because it gets easier to form the ions, the
reactions will happen more quickly.
Summarizing the increase in reactivity The reactions of the Group 2 elements proceed more readily as the energy needed to form positive ions falls. This is mainly due to a decrease in ionization energy down the group.
This leads to lower activation energies, and therefore faster reactions.
Contributors
Jim Clark (Chemguide.co.uk)
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