Calcium Carbide - Definition, Structure, Formula, Uses with FAQs

Structure Of Calcium Carbide

Calcium Carbide formula

The calcium carbide formula is $\mathrm{CaC}_2$.

Also read -

• NCERT Solutions for Class 11 Chemistry
• NCERT Solutions for Class 12 Chemistry
• NCERT Solutions for All Subjects

Carbide

Carbide is one of the most important terms which is used in both organic and inorganic chemistry. It is a chemical compound which consists of carbon and metallic or semi-metallic elements. It is the ionic or covalent bond that connects the carbide group to metallic or semi-metallic elements.

The formula of carbide ion is (C^4-) and that of dicarbide ion is (C^-2₂). It exists in ionic form and denotes that dicarbide ions are composed of two ‘C’ atoms. The formula of dicarbide (C^-2₂) shows that it exists in dianionic form. Among various carbides, carbides of silicon, carbides of tungsten have a significant role because of their physical properties like strength, hardness and the ability to resist to chemical attacks at high temperatures.

Aluminium carbide (Al₄C₃), Beryllium carbide (Be₂C), Calcium carbide ($\mathrm{CaC}_2$) etc. are some examples of carbide. Iron carbide or also called (Cementite) is an important component of steel and cast iron.

General Properties of Carbides

• Carbides generally possess a very high melting point.

• Carbides conduct heat and electricity.

• Carbides are compounds with metallic lustre properties.

• Alkali metal carbides are generally soft and transparent in a pure state. They are decomposed easily by water or acids and result in the formation of aliphatic hydrocarbons. They will not conduct electric current in solid-state.

• All the coinage metal carbides are found to be coloured and explosive powders. Examples are Cu₂C₂ (dark brown), Ag₂C₂ (white), Au₂C₂ (yellow).

• The carbides of alkaline earth metals are decomposed by dilute acids or water to form acetylene gas and they react with N₂.

$\begin{aligned} & \mathrm{CaC}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2+\mathrm{C}_2 \mathrm{H}_2 \\ & \mathrm{CaC}_2+\mathrm{N}_2 \rightarrow \mathrm{CaCN}_2+\mathrm{C}\end{aligned}$

Magnesium oxide is reduced to magnesium by such carbides. They also react with NH₃ and Br₂.

$3 \mathrm{MgO}+\mathrm{CaC}_2 \rightarrow 3 \mathrm{Mg}+\mathrm{CaO}+2 \mathrm{CO}$

$\mathrm{CaC}_2+4 \mathrm{NH}_3 \rightarrow \mathrm{CaCN}_2+\mathrm{NH}_4 \mathrm{CN}+4 \mathrm{H}_2$

$\mathrm{CaC}_2+4 \mathrm{Br}_2 \rightarrow \mathrm{CaBr}_2+\mathrm{C}_2 \mathrm{Br}_6$

• Be₂C has brick red colour and can scratch glass and quartz. Al₄C₃ is pale yellow in colour and it can be decomposed by water to produce methane. It can react with N₂ to form Al₅C₃N and form alumina on reaction with oxygen.

$\mathrm{Al}_4 \mathrm{C}_3+6 \mathrm{O}_2 \rightarrow 2 \mathrm{Al}_2 \mathrm{O}_3+3 \mathrm{CO}_2$

Preparation of Carbides

• Carbides can be synthesised by heating metal oxide with carbon.

$\mathrm{CaO}+3 \mathrm{C} \rightarrow \mathrm{CaC}_2+\mathrm{CO}$

• It can also be manufactured from acetylene.

$\mathrm{C}_2 \mathrm{H}_2+2 \mathrm{Li} \rightarrow \mathrm{LiC}_2+\mathrm{H}_2$

Different Types of Carbide

On the basis of the bond present between the carbide ion and the metal element, carbides are generally of the following types.

• Ionic carbides- The combination of alkali or alkaline earth metals with carbide ions results in the formation of ionic carbides. The electrostatic attractive force binds these ions. There exists a huge electronegativity difference in this type of carbides. CaC₂ is an example.

• Covalent carbides- Low electropositive elements such as ‘Si’ and ‘B’ combines with carbide ions to form covalent carbides. In this type of carbides, there exists a low electronegativity difference. ‘SiC’ (Carborundum), B₄C are examples.

• Interstitial carbides- The combination of transition metals and carbide molecules forms interstitial carbides. In this type of carbide, carbide ions occupy the interstitial site of the metal lattice. TiC is an example.

• Intermediate transition metal carbide – These types of carbides are also composed of transition metal and carbide ion and their size is found to be similar. Carbide of iron or known as cementite (Fe₃C) is an example.

Uses of Carbides

• Most of the carbides are found to be very hard. So they are used as abrasive tools, drilling and cutting tools.

• They are used as reducing agents in metallurgy.

• Al₄C₃ is used to synthesize methane whereas CaC₂ is used for the manufacture of acetylene.

• Carbides are also employed as furnace lining.

• Boron carbide is generally used to cut diamond, to make filaments of lamps and also to drill holes in rocks.

• The carbide of tungsten is nowadays used in making jewellery and also in the manufacture of various surgical tools.

Calcium Carbide

An important chemical compound with formula CaC₂ is known as calcium carbide or calcium acetylide. One of its industrial use is that it is used for the synthesize of acetylene and calcium cyanamide.

In the pure state, it is found to be colourless, but anyway, the pieces of technical grade of CaC₂ are grey or brown in colour and contain $80-85 \%$ of CaC₂. The rest of it is composed of CaO, Ca₃P₂, CaS, Ca₃N₂, SiC etc.

CaC₂ is mainly employed in the manufacture of acetylene gas and also to produce acetylene in carbide lamps. It is also used to synthesize various chemicals and fertilizers and is also involved in steel making.

Preparation

Calcium carbide is synthesized in an industrial scale by using an electric arc furnace by combining the mixture of coke and lime at $2200^{\circ} \mathrm{C}$. This is an endothermic reaction that requires 110 kilocalories per mole and very high temperature to expel CO.

$\mathrm{CaO}+3 \mathrm{C} \rightarrow \mathrm{CaC}_2+\mathrm{CO}$

The high temperature condition which is required for the manufacture of CaC₂ cannot be gained through traditional combustion. Hence this reaction should be carried out in an electric furnace using graphite electrodes.

Also, students can refer,

• NCERT solutions for Class 12 Chemistry Chapter 10 The S-Block Elements
• NCERT Exemplar Class 12 Chemistry Solutions Chapter 10 The S-Block Elements
• NCERT notes Class 12 Chemistry Chapter 10 The S-Block Elements

Applications of Calcium Carbide

• Production of Acetylene: CaC₂ reacts with water to form acetylene and calcium hydroxide.

$\mathrm{CaC}_{2(\mathrm{~S})}+2 \mathrm{H}_2 \mathrm{O}_{(\mathrm{aq})} \rightarrow \mathrm{C}_2 \mathrm{H}_{2(\mathrm{~g})}+\mathrm{Ca}(\mathrm{OH})_{2(\mathrm{aq})}$

• Production of calcium cyanamide: The reaction of CaC₂ with N₂ at a very high-temperature forms calcium cyanamide. It is generally known as nitrolime and is used as a fertilizer. It undergoes hydrolysis to form cyanamide H₂NCN.

$\mathrm{CaC}_2+\mathrm{N}_2 \rightarrow \mathrm{CaCN}_2+\mathrm{C}$

• Steel making: CaC₂ is used in the desulfurization of iron and also as a fuel in steelmaking to extend the scrap ratio to liquid iron.

• Deoxidizer: In ladle treatment it acts as a strong deoxidizer.

• As a ripening agent: CaC₂ is used as a source of acetylene gas which functions as an alternative ripening agent like ethylene.

• CaC₂ is also used to find out the moisture content of the soil.

• CaC₂ is nowadays sold as a mole repellent.

Structure of Calcium Carbide

The common crystalline form of CaC₂ at room temperature is found to be a distorted rock salt structure in which $\mathrm{C}^{-2}{ }_2$ units are lying parallel. It can be shown as:

Also check-

Some Solved Examples

Question: In curing cement plaster, water is sprinkled from time to time. This helps in:

1) keeping it cool

2) (correct) developing interlocking needle­-like crystals of hydrated silicates

3) Hydrating sand and gravel mixed with cement

4) converting sand into silicic acid.

Solution:

Water helps in the hydration of calcium aluminates and calcium silicates, which change into their colloidal gels by developing interlocking needle-like crystals of hydrated silicates.

The hydrolysis leads to the formation of calcium hydroxide which is useful in binding the particles of calcium silicate together.

Hence, the answer is option (2).

Question 2: When calcium carbide reacts with water, the gas evolved is:

A) Ethylene
B) Acetylene
C) Methane
D) Ethane

Solution:

$\mathrm{CaC}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{C}_2 \mathrm{H}_2+\mathrm{Ca}(\mathrm{OH})_2$

The gas produced is acetylene (C₂H₂).

Hence, the answer is option (2).

Question 3: Which of the following are true for $\mathrm{CaC}_2$?
1. Produced by heating lime and coke in an electric arc furnace.
2. Hydrolysis gives $\mathrm{C}_2 \mathrm{H}_2$.
3. $\mathrm{C}_2{ }^{2-}$ ion has a triple bond.
4. It is used to make calcium cyanamide.

A) 1, 2, 3
B) 2, 3, 4
C) 1, 2, 4
D) 1, 2, 3, 4

Solution:

All statements are correct. CaC₂ is a versatile industrial raw material.

Hence, the answer is option (4).

Question 4: When 6.4 g of CaC₂ reacts completely with water, the volume of acetylene obtained at STP (22.4 L/mol) is __ L.

1) 2.24L

2) 1.24L

3) 2.27L

4) 1.22L

Solution:

Molar mass $\mathrm{CaC}_2=64 \mathrm{~g} / \mathrm{mol}$.
Moles of $\mathrm{CaC}_2=6.4 / 64=0.1 \mathrm{~mol}$.
$1 \mathrm{~mol} \mathrm{CaC} \mathrm{C}_2 \rightarrow 1 \mathrm{~mol} \mathrm{C}_2 \mathrm{H}_2$.
So, $0.1 \mathrm{~mol} \rightarrow 0.1 \times 22.4=2.24 \mathrm{~L}$.

Hence, the answer is option (1).