Question

The alkaline earth metal that reacts vigorously with water at room temperature is:
A) Ca
B) Be
C) Mg
D) Ba

Answer 1

Hint: Group 2 elements are called alkaline earth metals. Recall the elements that constitute group 2. There is a trend in the reactivity of alkaline earth metals with water. As we go down the group, group 2 elements become more reactive towards water.

Complete answer:
Group 2 elements comprise beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) and these elements are known as alkaline earth metals. Group 2 elements react with cold water, except beryllium and magnesium, to give metal hydroxide and water.
$M+2H_{2}O\to M(OH{)}_2+H_2$
Magnesium and beryllium react with steam to give metal oxide and hydrogen gas. Alkaline earth metals become more reactive towards water as we go down the group.
Explanation of the above trend:
When alkaline earth metals react with water to form metal oxides or hydroxides, metal ions are formed. The formation of these ions from the original metal requires input of energy. First and second ionisation energies are required to convert the metal atoms into ions with a +2 charge.
As we go down the group, ionisation energy decreases because it gets easier to form the ions due to less attraction of electrons towards the nucleus in heavier atoms. Thus, the reaction becomes easier as the energy needed to form positive ions falls. Consequently, this leads to lower activation energy and therefore, faster reactions. Barium (Ba) is the second last alkaline earth metal and is the heaviest metal in its group. It requires the least ionisation energy and hence activation energy to form ions. So, it reacts vigorously with water at room temperature.

Thus, option D is correct.

Note:
When metal reacts with cold water, we get metal hydroxide and when metal reacts with steam, the metal oxide is formed. This is because metal hydroxides thermally decompose to give the metal oxide and water.