BORING EDUCATION — LIVE EXPERIMENTProduct is live. Experiments are running. Feedback is being collected. Improvements are underway.Welcome to Boring Education. We're glad you're here while we're figuring out how to make learning better.
BORINGEDUCATION

1.4bFree

Hydrides: why water boils at 100 °C when it “should” not

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

The problem

Look down group VIA: H2Te boils at −1.8 °C, H2Se at −41.3 °C, H2S at −59.6 °C. The molecules get lighter and the boiling point keeps falling. By that pattern, water should boil somewhere below −60 °C. It would be a gas, and there would be no rivers, no chai and no you.

Instead water boils at 100 °C. Something extra holds water molecules together that H2S does not have. That something is the hydrogen bond, and this topic explains where it comes from.

Hydrides are binary compounds of hydrogen with other elements. By their bonding they are ionic, intermediate or covalent, and covalent character increases from left to right across the periodic table.

Step 1 / 7

Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

Look down group VIA: H2Te boils at −1.8 °C, H2Se at −41.3 °C, H2S at −59.6 °C. The molecules get lighter and the boiling point keeps falling. By that pattern, water should boil somewhere below −60 °C. It would be a gas, and there would be no rivers, no chai and no you.

Instead water boils at 100 °C. Something extra holds water molecules together that H2S does not have. That something is the hydrogen bond, and this topic explains where it comes from.

Hydrides are binary compounds of hydrogen with other elements. By their bonding they are ionic, intermediate or covalent, and covalent character increases from left to right across the periodic table.

Key terms

Ionic hydrides
Group IA and the heavier members of group IIA form ionic hydrides containing the hydride ion, H−. They are crystalline solids with high melting and boiling points, and they conduct electricity when molten.
Intermediate hydrides
The hydrides of beryllium and magnesium sit between the two classes. They have polymeric structures and a covalent nature.
Covalent hydrides
Formed by elements with electronegativity above 1.8. They are gases or volatile liquids that do not conduct. Their stability rises across a period and falls down a group: HF is the most stable.
Hydrogen bonding
Across a period the other element grows more electronegative, so its bond to hydrogen becomes more polar. H2O and HF are so polar that their molecules form hydrogen bonds with each other.

Short questions with model answers

  1. Q1. Give a brief reason: alkali metals give ionic hydrides.

    • Alkali metals are strongly electropositive.
    • Na → Na+ + e− · H + e− → H−

    They transfer an electron to hydrogen, giving ionic solids of M+ and H−.

  2. Q2. Use Table 1.5: how much higher is the boiling point of water than that of hydrogen sulphide, and why?

    • 100 − (−59.6) = 159.6 °C
    • O–H is far more polar than S–H.

    159.6 °C higher, because hydrogen bonds hold water molecules together.

  3. Q3. Down group IVA the boiling point rises from CH4 to SnH4. By how much, and why is there no anomaly at the top?

    • −52 − (−164) = 112 °C
    • Bigger molecules, stronger forces between them.
    • CH4 has no hydrogen bonding.

    By 112 °C; CH4 is not hydrogen-bonded, so it simply follows the trend.

Common mistakes

  • ✗ Writing that ionic hydrides such as NaH contain H+ ions.

    ✓ They contain H−, the hydride ion: hydrogen gains the electron that the metal loses.

  • ✗ Saying covalent hydrides become more stable down a group.

    ✓ Stability decreases down a group and increases across a period. Fluorine forms the most stable hydride; thallium, lead and bismuth the least stable.

  • ✗ Explaining water's high boiling point by saying H2O is the heaviest molecule in its group.

    ✓ It is the lightest. Its boiling point is high only because of hydrogen bonding, which H2S, H2Se and H2Te lack.

MCQs

  1. 1. Ionic hydrides contain the ion:

    1. (a) H+
    2. (b) H−
    3. (c) OH−
    4. (d) H3O+
    Show answer

    (b) H−, the hydride ion, formed when hydrogen accepts the metal's electron.

  2. 2. The hydrides of beryllium and magnesium are:

    1. (a) ionic
    2. (b) intermediate
    3. (c) gases
    4. (d) hydrogen-bonded
    Show answer

    (b) Intermediate: polymeric structures with a covalent nature, between ionic and covalent hydrides.

  3. 3. Which element forms the most stable hydride?

    1. (a) bismuth
    2. (b) carbon
    3. (c) fluorine
    4. (d) lead
    Show answer

    (c) Fluorine (HF). Stability rises across a period and falls down a group; Tl, Pb and Bi hydrides are least stable.

  4. 4. According to Table 1.5, the boiling point of H2S is:

    1. (a) 100 °C
    2. (b) −59.6 °C
    3. (c) −41.3 °C
    4. (d) −1.8 °C
    Show answer

    (b) −59.6 °C. −41.3 °C is H2Se and −1.8 °C is H2Te.

  5. 5. Covalent hydrides are formed by elements with electronegativity greater than:

    1. (a) 0.9
    2. (b) 1.2
    3. (c) 1.8
    4. (d) 3.0
    Show answer

    (c) Above 1.8 on the Pauling scale; hydrogen itself is 2.1, so most of these bonds are polar.

Quick revision

  • Hydrides are ionic (IA and heavier IIA, with H−), intermediate (Be, Mg) or covalent (electronegativity above 1.8).
  • Covalent hydride stability rises across a period and falls down a group; HF is the most stable.
  • Boiling points rise down a group, except H2O, HF and NH3, which hydrogen bonding lifts far above the trend (water: 100 °C).