Calcium oxide

 

Calcium oxide

Chemical formula	CaO
OTP appearance	white solid
Molar Mass(g/mol)	56.0774
Enthalpy of Formation(kJ/mol)	-685
Density(g/cc)	3.34
Melting Point(°C)	2613
Boiling Point(°C)	2850
NFPA 704	0 3 2

Uses

Primary

• Feedstock for calcium hydroxide, lye, and all other hydroxides

Secondary

• Heat without fire:

  Std Enthalpy of Formation:

  CaO(s) + H₂O(l) → Ca(OH)₂(aq)ΔH=-63.7

  -635 - 285.83 = -987 - 67

Natural occurrence

• Does not occur naturally

Hazards

• Corrosive to skin
• Calcium oxide dust irritates the eyes and lungs
• Mixed with water produces substantial heat as well as calcium hydroxide which is caustic.

Production

From calcium carbonate

• Calcium oxide is produced by heating calcium carbonate above 550°C, often in a lime kiln.

  CaCO₃ → CaO + CO₂

1. Place 100 units by mass of calcium carbonate in a lidded ceramic container
2. Repeat
   1. Place the container in the lime kiln
   2. Heat to 900°C (orange heat) for 30 minutes
   3. Remove from fire and measure mass.
      N.B. You can use the weight loss to estimate total time to conversion: 100 → 90 is 10 units (out of 44) of mass removed. Estimate 2 hrs to convert all.
3. until mass is 56-57 units.

   NB: The above presumes pure calcium carbonate as feedstock. Impurities may increase or decrease the target weight.

4. The material is now mostly calcium oxide.

Storage

Calcium oxide decomposes when exposed to air along two routes:

• Absorb carbon dioxide from the air to become calcium carbonate

CaO + CO₂ → CaCO₃

• Absorb water from the air to become calcium hydroxide

CaO + H₂O → Ca(OH)₂

Thus, storage is a problem. For these reasons, calcium oxide is most often used as quickly as it is produced.