hydrates 2

Covalent Network Solids and Hydrates

Definitions and Properties

  • Covalent Network Solids: These are solid materials where atoms are bonded by covalent bonds in a continuous network. They are characterized by having high melting points and hardness due to the strong bonding.

  • Diamonds: A covalent network solid made of carbon.

    • Properties of Diamonds:

    • Hardness: Very hard, considered the hardest known natural material.

    • Melting Points: Very high.

  • Graphite: Another allotrope of carbon.

    • Properties of Graphite:

    • Softness: Relatively softer than diamonds.

    • Melting Points: Lower than those of diamonds.

Examples of Covalent Network Solids

  1. Diamond: Formed solely from carbon atoms, exhibiting strong covalent bonding.

  2. Graphite: Mentioned previously as a carbon allotrope, characterized by layers that easily slide over each other.

  3. Quartz: A silicon dioxide network that forms a three-dimensional (3D) structure.

Hydrates

  • Hydrates: Compounds that include water in their crystalline structure.

    • Example Problem: Calculate the percent water of a hydrate.

    • Initial mass of hydrate: 5.60 g before heating.

    • Mass of residue after heating (anhydrous substance): 3.92 g.

    • Mass of water lost:
      extMassofwater=5.60extg−3.92extg=1.68extgext{Mass of water} = 5.60 ext{ g} - 3.92 ext{ g} = 1.68 ext{ g}

    • Percent water in the hydrate:
      ext{Percent water} = rac{1.68 ext{ g}}{5.60 ext{ g}} imes 100 ext{ 5} = 30.0 ext{ 5}

  • Methods of Removing Water from a Hydrate:

    1. Heating:

    • The water in hydrates is typically loosely bound and vaporizes upon heating, although some hydrates may decompose.

    1. Desiccation:

    • Placing the hydrate in a desiccator or near a desiccant to absorb water vapor.

Determining the Formula of a Hydrate

To ascertain the water content of an ionic hydrate:

  1. Measure the mass of the hydrate before and after heating.

    • Mass of sample before heating (hydrate): e.g., 5.60 g.

    • Mass of anhydride (after heating): e.g., 3.92 g.

  2. Calculate the mass of water:
    extMassofwater=extMassofhydrate−extMassofanhydrideext{Mass of water} = ext{Mass of hydrate} - ext{Mass of anhydride}

  3. Convert mass to moles:

    • extMolesofanhydride=racextMassofanhydrideextMolarmassofanhydrideext{Moles of anhydride} = rac{ ext{Mass of anhydride}}{ ext{Molar mass of anhydride}}

    • extMolesofwater=racextMassofwaterextMolarmassofwaterext{Moles of water} = rac{ ext{Mass of water}}{ ext{Molar mass of water}}

  4. Divide by the smallest number of moles to establish the ratio of water molecules to anhydride.

  5. Write the empirical formula as:
    extAnhydrouscompoundext•extnumberofwatermoleculesext{Anhydrous compound} ext{•} ext{number of water molecules}

Example Calculations

  1. Barium Hydroxide Hydrate:

    • Initial mass = 6.00 g, Final mass of anhydrous compound = 3.26 g.

    • Calculation:

      • extMassofwater=6.00extg−3.26extg=2.74extgext{Mass of water} = 6.00 ext{ g} - 3.26 ext{ g} = 2.74 ext{ g}

      • Molar mass of anhydrous Ba(OH)₂ = 171.35 g/mol.

      • Moles of Ba(OH)₂ = rac{3.26}{171.35} ≈ 0.019 mol.

      • Moles of water = rac{2.74}{18.015} ≈ 0.152 mol.

      • Ratio: $ rac{0.152 ext{ mol H}2O}{0.019 ext{ mol Ba(OH)}2} ext{ gives approximately} 8$, leading to
        – extFormula:Ba(OH)2•8H2Oext{Formula: Ba(OH)₂ • 8H₂O}

  2. Magnesium Iodide Hydrate:

    • Initial mass = 1.628 g, Final mass = 1.072 g.

    • Calculation:

      • extMassofwater=1.628extg−1.072extg=0.556extgext{Mass of water} = 1.628 ext{ g} - 1.072 ext{ g} = 0.556 ext{ g}

      • Molar mass of MgI₂ = 278.11 g/mol.

      • Moles of anhydride = $ rac{1.072}{278.11} ≈ 0.00385 ext{ mol}$

      • Moles of water = $ rac{0.556}{18.015} ≈ 0.03087 ext{ mol}$

      • Ratio: rac0.030870.00385extgivesapproximately8rac{0.03087}{0.00385} ext{ gives approximately} 8

      • Final formula: extMgI<em>2•8H</em>2Oext{MgI}<em>2 • 8H</em>2O

  3. Chromium (III) Nitrate Hydrate:

    • Given data: 40.50% water by mass.

    • Molar mass of Cr(NO₃)₃ is needed to calculate the molecular formula based on the % water content.

    • To find the mass ratio of water to the whole hydrate, assume a 100 g sample: 40.50 g water, leaving 59.50 g of anhydrous compound, then use the molar masses to determine the formula.