Water-reactive substances

Water-reactive substances[1] are those that spontaneously undergo a chemical reaction with water, as they are highly reducing in nature.[2] Notable examples include alkali metals, sodium through caesium, and alkaline earth metals, magnesium through barium.

Some water-reactive substances are also pyrophoric, like organometallics and sulphuric acid, and should be kept away from moisture. The use of acid-resistant gloves and face shield are required and should be handled in fumehoods.[3]

Water-reactive substances are classified as R2 under the UN classification system and as Hazard 4.3 by the United States Department of Transportation.

All chemicals that react vigorously with water or liberate toxic gas when in contact with water are recognised for their hazardous nature in the 'Approved Supply List,'[4] or the list of substances covered by the international legislation on major hazards[5] many of which are commonly used in manufacturing processeses.

Alkali metals
Group 1: Alkali metals

The alkali metals (Li, Na, K, Rb, Cs, and Fr) are the most reactive metals in the periodic table - they all react vigorously or even explosively with cold water, resulting in the displacement of hydrogen.

The Group 1 metal (M) is oxidised to its metal ions, and water is reduced to hydrogen gas (H2) and hydroxide ion (OH−), giving a general equation of:

2 M(s) + 2 H2O(l) ⟶ 2 M+(aq) + 2 OH−(aq) + H2(g) [6]

The Group 1 metals or alkali metals become more reactive in higher periods of the periodic table.

Alkaline earth metals
Group 2: Alkaline earth metals

The alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra) are the second most reactive metals in the periodic table, and, like the Group 1 metals, have increasing reactivity in the higher periods. Beryllium (Be) is the only alkaline earth metal that does not react with water or steam, even if metal is heated to red heat.[7] Additionally, beryllium has a resistant outer oxide layer that lowers its reactivity at lower temperatures.

Magnesium shows insignificant reaction with water, but burns vigorously with steam or water vapor to produce white magnesium oxide and hydrogen gas:

Mg(s) + 2 H2O(l) ⟶ Mg(OH)2(s) + H2(g)

A metal reacting with cold water will produce metal hydroxide. However, if a metal reacts with steam, like magnesium, metal oxide is produced as a result of metal hydroxides splitting upon heating.[8]

The hydroxides of calcium, strontium and barium are only slightly water-soluble but produce sufficient hydroxide ions to make the environment basic, giving a general equation of:

M(s) + 2 H2O(l) ⟶ M(OH)2(aq) + H2(g) [9]

Reactivity series of metals
Order of reactivityMetalReactions with water or steam
most reactivepotassium (K)very vigorous reaction with cold water
↑sodium (Na)vigorous reaction with cold water
↓calcium (Ca)less vigorous reaction with cold water
least reactivemagnesium (Mg)slow reaction with cold water, vigorous with hot water
  • If metals react with cold water, hydroxides are produced.
  • Metals that react with steam form oxides.
  • Hydrogen is always produced when a metal reacts with cold water or steam.[10]
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References
  1. "The MSDS HyperGlossary: Water Reactive". Interactive Learning Paradigms Incorporated. Retrieved 2007-05-10.
  2. Raymond, Chang (2010). Chemistry (PDF) (tenth ed.). Americas, New York: McGraw-Hill. pp.897-898. ISBN 0077274318. Retrieved 27 February 2018.
  3. The University of Iowa. "Reactive Chemicals". Environmental Health & Safety. Retrieved 2 March 2018.
  4. Quinn, D. J.; Davies, P. A. (2003). "MODELLING RELEASES OF WATER REACTIVE CHEMICALS" (PDF). Symposium Series. 149. Retrieved 25 February 2018.
  5. Kapias, T; Griffiths, RF (2001). REACTPOOL: A new model for accidental releases of water-reactive chemicals (PDF). Crown. ISBN 0-7176-1995-8. Retrieved 25 February 2018.
  6. Landas, Trevor. "Reactions of Main Group Elements with Water". Chemistry LibreTexts. Retrieved 9 February 2017.
  7. Pilgaard, Michael. "Beryllium: Chemical Reactions". Michael Pilgaard's Table of the Elements. Retrieved 16 February 2018.
  8. Clark, Jim. "Reactions of the Group 2 Elements with Water". ChemGuide. Retrieved 16 February 2018.
  9. Landas, Trevor. "Reactions of Main Group Elements with Water". Chemistry LibreTexts. Retrieved 16 February 2018.
  10. Gallagher, RoseMarie; Ingram, Paul (2009). Chemistry IGCSE Revision Guide. Great Clarendon Street, Oxford OX2 6DP: Oxford University Press. pp. 114–115.CS1 maint: location (link)
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