Hydrogen • Topic 2 of 3

Hydrides & Water

When hydrogen combines with another element it forms a hydride. The character of the hydride depends on the partner element, and the most important hydride of all — water — shapes life and chemistry on Earth.

Types of hydrides

  • Ionic (saline) hydrides: formed by the most electropositive s-block metals (e.g. NaH, CaH2). They are crystalline, non-volatile solids containing the H− ion. On electrolysis of the molten hydride, hydrogen is liberated at the anode, proving the negative charge on hydrogen. They react violently with water: NaH + H2O → NaOH + H2.
  • Covalent (molecular) hydrides: formed by p-block non-metals (e.g. CH4, NH3, H2O, HF). They are volatile, with discrete molecules held by weak van der Waals forces; those with N, O or F show hydrogen bonding, giving abnormally high boiling points.
  • Metallic (interstitial) hydrides: formed by many d- and f-block metals, with hydrogen in the interstitial holes of the lattice (e.g. Pd, Ti). They are often non-stoichiometric (e.g. TiH1.7) and store hydrogen. The d-block metals of groups 7–9 generally do not form hydrides — the hydride gap.

Structure and properties of water

Water is a bent molecule with an H–O–H angle of about 104.5°; oxygen is sp3 hybridised with two lone pairs. Strong hydrogen bonding gives water its anomalies: high boiling point, high specific heat, high surface tension, and the fact that ice (an open H-bonded cage) is less dense than liquid water, so it floats. Water is an excellent solvent and is amphoteric.

Hard and soft water

Soft water lathers readily with soap. Hard water does not, because dissolved Ca2+ and Mg2+ salts react with soap to give an insoluble scum.

  • Temporary hardness is due to the bicarbonates Ca(HCO3)2 and Mg(HCO3)2. It is removed simply by boiling (which decomposes the bicarbonate to insoluble carbonate) or by Clark's method (adding a calculated amount of slaked lime): Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O.
  • Permanent hardness is due to the chlorides and sulphates of calcium and magnesium and is not removed by boiling. It is removed by washing soda (Na2CO3), by the permutit / zeolite process (Na2Z exchanges Na+ for Ca2+/Mg2+), or by modern synthetic ion-exchange resins that swap H+ and OH− for the dissolved ions, giving demineralised water.

Heavy water (D2O)

Heavy water is water in which both hydrogen atoms are deuterium. It is obtained by the prolonged electrolysis of ordinary water (the lighter H2O is electrolysed faster, enriching the residue in D2O) or by fractional distillation. It has a higher boiling point (101.4°C), higher density (1.106 g cm−3) and reacts more slowly than ordinary water. Its main use is as a moderator in nuclear reactors, where it slows fast neutrons without absorbing them.

Ion-exchange softening of hard water using a resin columnresin bed(Na-Z / H-resin)hard water (Ca2+, Mg2+)soft water (Na+)Ca/Mg ionsheld by resin
Solved Examples
1
Worked Example
Classify the following hydrides and give the type: NaH, CH4, TiH1.7.
Solution
  1. NaH is formed by an electropositive s-block metal and contains H−, so it is an ionic (saline) hydride.
  2. CH4 is formed by a p-block non-metal as discrete molecules, so it is a covalent (molecular) hydride.
  3. TiH1.7 is a non-stoichiometric d-block hydride with hydrogen in the lattice holes, so it is a metallic (interstitial) hydride.

Answer: NaH ionic, CH4 covalent, TiH1.7 metallic.

2
Worked Example
Why does hard water fail to give lather with soap?
Solution
  1. Hard water contains dissolved Ca2+ and Mg2+ ions.
  2. These ions react with the sodium stearate of soap to form insoluble calcium and magnesium stearates.
  3. The insoluble scum forms instead of lather, wasting soap.

Answer: Ca2+ and Mg2+ ions precipitate soap as an insoluble scum, so no lather forms.

3
Worked Example
Write the reaction by which Clark's method removes temporary hardness.
Solution
  1. Temporary hardness is due to the bicarbonate Ca(HCO3)2.
  2. A calculated amount of slaked lime, Ca(OH)2, is added.
  3. The reaction precipitates insoluble calcium carbonate: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O.

Answer: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O.

4
Worked Example
Explain how the zeolite (permutit) process removes permanent hardness, and how the exhausted zeolite is regenerated.
Solution
  1. Hard water is passed through sodium zeolite (Na2Z), which exchanges its Na+ for the Ca2+/Mg2+ in water: Na2Z + Ca2+ → CaZ + 2Na+.
  2. The water leaving the bed is soft because its hardness-causing ions are now held in the resin.
  3. When exhausted, the zeolite (now CaZ) is regenerated by flushing it with a concentrated brine (NaCl) solution: CaZ + 2NaCl → Na2Z + CaCl2.

Answer: Na2Z swaps Na+ for Ca2+/Mg2+ to soften water; regenerated with brine: CaZ + 2NaCl → Na2Z + CaCl2.

5
Worked Example
Ice floats on water. Explain this anomaly using the structure of water.
Solution
  1. In liquid water, hydrogen bonds form and break, allowing molecules to pack fairly closely.
  2. In ice, each water molecule is hydrogen-bonded to four others in an open, cage-like tetrahedral network.
  3. This open structure has more empty space, so ice is less dense than liquid water and therefore floats.

Answer: The open hydrogen-bonded cage of ice has a lower density than liquid water, so ice floats.

6
Worked Example
How is heavy water obtained and why is it used in nuclear reactors?
Solution
  1. Heavy water D2O is obtained by the prolonged electrolysis of ordinary water; the lighter H2O is electrolysed faster, so the residue is enriched in D2O.
  2. It can also be concentrated by fractional distillation.
  3. In a reactor it acts as a moderator: it slows down fast neutrons by collisions without absorbing them, helping sustain the chain reaction.

Answer: Made by prolonged electrolysis of water; used as a neutron moderator in nuclear reactors.

Key Points

  • Hydrides are ionic/saline (NaH, CaH2; contain H−), covalent/molecular (CH4, NH3, H2O) or metallic/interstitial (Pd, Ti hydrides; store hydrogen).
  • Water is a bent molecule (H–O–H ≈ 104.5°) whose hydrogen bonding gives high boiling point, high specific heat and floating ice.
  • Temporary hardness (bicarbonates) is removed by boiling or Clark's method: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O.
  • Permanent hardness (chlorides/sulphates) is removed by washing soda, the zeolite/permutit process, or synthetic ion-exchange resins.
  • Heavy water D2O, made by prolonged electrolysis, is denser and slower-reacting and is used as a moderator in nuclear reactors.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Which of these is an ionic (saline) hydride?
Explanation: NaH is formed by an electropositive s-block metal and contains the H− ion, making it an ionic hydride.
Q2.Temporary hardness of water is caused by:
Explanation: Temporary hardness is due to Ca(HCO3)2 and Mg(HCO3)2, removed by boiling or Clark's method.
Q3.Permanent hardness of water can be removed by:
Explanation: Permanent hardness (chlorides/sulphates) is not removed by boiling; the zeolite or ion-exchange process swaps the Ca2+/Mg2+ ions out.
Q4.Heavy water is mainly used as a:
Explanation: D2O slows fast neutrons without absorbing them, so it acts as a moderator in nuclear reactors.
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