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MCAT Chemistry Question — Alkali Metals & Water

The alkali metals (M) are a highly reactive group of elements that react with water according to the following reaction:

2M(s) + 2H2O(l) → 2MOH(aq) + H2(g)

Which of the following alkali metals will react the most violently with water?


  1. Sodium
  2. Magnesium
  3. Zinc
  4. Cesium
Click for Explanation

This question is testing your knowledge of reactivates among elements belonging to the alkali metals family. From outside knowledge, you should know that the alkali metals correspond to the first column of the periodic table. Additionally, the reaction given in the question stem shows the metal M is oxidized from its elemental state to an oxidation state of +1, which is the preferred oxidation state for elements in the first column. Thus, answer choices B and C can be eliminated.

To choose between sodium and cesium, you must apply the periodic table trends for ionization energy. Since the metal M loses an electron in the reaction with water, the metal that is the most easily oxidized will be the most reactive. The trend for ionization energy increases going up and to the right in the periodic table, which makes cesium the more easily oxidized element. Thus, D is the correct answer. If this does not make sense, think of it this way – the lower the ionization energy, the easier it is to remove an electron from the species, and the more reactive the species will be. Cesium, which is nearest the bottom left of the periodic table, has the lowest ionization energy of the given choices.

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