s Block Elements
9.0 Compounds of Calcium
9.0 Compounds of Calcium
1. Calcium oxide or Quick lime ($CaO$):
- Preparation: By heating limestone ($CaCO_3$) in a rotary kiln at $1070-1270\ K$. $$CaC{O_3}\mathop \to \limits^\Delta CaO + C{O_2}$$ The carbon dioxide is removed as soon as it is produced to enable the reaction to proceed to completion.
- Properties:
a. It is also known as Burnt lime and is white amorphous substance.
b. When react with $SiO_2$ and $P_2O_5$ at high temperature gives calcium silicate ($CaSiO_3$) and calcium phosphate $\left[ {C{a_3}{{(P{O_4})}_2}} \right]$ resp.
$CaO$ is used as flux in blast furnace for removal of impurities in the form of slag.
c. When water is added to lime a hissing sound is produced along with clouds of steam. The lime forms slaked lime $[Ca(OH)_2]$.
d. Calcium oxide reacts with moist chlorine to form bleaching powder. $$CaO + C{l_2} \to CaOC{l_2}\left[ {Mixture{\text{ }}of{\text{ }}CaC{l_2} + {\text{ }}Ca{{\left( {OCl} \right)}_2}} \right]$$
e. Calcium oxide on reaction with moist $SO_2$ forms $CaSO_3$ and with moist $CO_2$ forms $CaCO_3$.
f. Reaction with moist $HCl$ gas forms $CaCl_2$.
g. Reaction with Carbon at ${2000^ \circ }C$ forms calcium carbide ($CaC_2$).
- Uses:
a. It is an important primary material for manufacturing cement and is the cheapest form of alkali.
b. It is used in the manufacture of sodium carbonate from caustic soda.
c. It is employed in the purification of sugar and in the manufacture of dye stuffs.
2. Calcium carbonate ($CaCO_3$):
- Preparation:
a. Carbon dioxide when passed through lime water gives calcium carbonate. $$Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O$$
b. Calcium chloride on reaction with sodium carbonate gives calcium carbonate. $$CaC{l_2} + N{a_2}C{O_3} \to CaC{O_3} + 2NaCl$$
- Properties:
a. It is a white powder insoluble in water.
b. Dissolves in water in presence of $CO_2$ due to the formation of calcium bicarbonate. $$CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}$$
- Uses: It is used as a building material in the form of marble and in the manufacture of quick lime. Calcium carbonate along with magnesium carbonate is used as a flux in the extraction of metals such as iron. It is also used as an antacid, mild abrasive in tooth paste, a constituent of chewing gum, and a filler in cosmetics.
3. Calcium chloride ($CaCl_2$):
- Preparation: Calcium oxide, calcium hydroxide or calcium carbonate when treated with $HCl$ gives calcium chloride. $$\begin{equation} \begin{aligned} CaO + 2HCl \to CaC{l_2} + {H_2}O \\ Ca{(OH)_2} + 2HCl \to CaC{l_2} + 2{H_2}O \\ CaC{O_3} + 2HCl \to CaC{l_2} + {H_2}O + C{O_2} \\ CaC{l_2}\left( {solution} \right)\mathop \to \limits^{crystallzation} CaCl{_2}.6{H_2}O\left( {Crystal} \right) \\\end{aligned} \end{equation} $$
- Properties:
a. It is a colourless deliquescent crystalline substance which is soluble in water as well as in alcohol.
b. Crystals of calcium chloride when strongly heated gives off water of crystallisation.
4. Plaster of Paris ($CaS{O_4}.\frac{1}{2}{H_2}O$):
- Preparation: When gypsum is heated at about ${120^ \circ }C - {130^ \circ }C$, plaster of paris is formed. $$CaS{O_4}.2{H_2}O\mathop \to \limits^\Delta \left( {CaS{O_4}} \right).\frac{1}{2}{H_2}O + \frac{3}{2}{H_2}O\;\left( {{\text{about }}{{120}^ \circ }C - {{130}^ \circ }C} \right)$$$$CaS{O_4}.2{H_2}O\mathop \to \limits^\Delta CaS{O_4}\left( {{\text{deadburnt}}} \right)\left( {{\text{above }}{{200}^ \circ }C} \right)$$$$CaS{O_4}.2{H_2}O\mathop \to \limits^\Delta 2CaO + 2S{O_2} + {O_2}\left( {{\text{above }}{{1200}^ \circ }C} \right)$$
- Properties:
a. It is a white crystalline solid. It is sparingly soluble in water.
b. It becomes anhydrous at about ${200^ \circ }C$. Anhydrous form is known as dead burnt plaster. $$CaS{O_4}.2{H_2}O\mathop \to \limits^{120^\circ C} CaS{O_4}.\frac{1}{2}{H_2}O\mathop \to \limits^{200^\circ C} CaS{O_4}$$
5. Calcium hydroxide ($Ca(OH)_2$):
- Preparation:
a. From quick lime: $$\begin{equation} \begin{aligned} CaO + {H_2}O \to Ca{(OH)_2} \to {\text{suspended in water}}\mathop \to \limits^{filter} Ca{\left( {OH} \right)_2}\left[ {{\text{Clear Solution (lime water)}}} \right] \\ {\text{suspended in water}} \to Ca{\left( {OH} \right)_2}\left[ {{\text{Suspension Milk of lime}}} \right] \\\end{aligned} \end{equation} $$
b. From $CaCl_2$: $$CaC{l_2} + 2NaOH \to Ca{(OH)_2} + 2NaCl$$
- Properties:
a. It gives $CaCO_3$ and $Ca(HCO_3)_2$ with $CO_2$. $$Ca{(OH)_2}{\text{(lime water)}} + C{O_2} \to CaC{O_3} \downarrow {\text{(milky)}} + {H_2}O$$
On prolong treatment with $CO_2$ milkiness disappear due to formation of $Ca(HCO_3)_2$. $$CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}{\text{(soluble)}}$$
6. Portland Cement:
- Mixture of Ca silicates and calcium aluminates with small amount of gypsum. $CaO = 50 - 60\%$ , $SiO_2 = 20-25\%$, $Al_2O_3 = 5-10\%$, $MgO = 2-3\%$, $Fe_2O_3 = 1-2\%$, $SO_3 = 1-2\%$.
- If lime in excess, cement cracks during settings. If lime is less, cement with weak strength.
