[Free] Short Notes on Carbides for CSIR NET Chemical Science!! (Download PDF)

By Neetesh Tiwari|Updated : September 15th, 2021

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at Byjus Exam Prep come up with short notes on Carbideswhich comes under the Inorganic Chemistry section of the Chemical Science syllabus

Our experienced Exam experts have meticulously designed this set of short notes on the Carbides to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at Byjus Exam Prep come up with short notes on Carbides, which comes under the Inorganic Chemistry section of the Chemical Science syllabus

Our experienced Exam experts have meticulously designed this set of short notes on the Carbides to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.

Study Notes on Carbides

The formation of various binary compounds of carbon with metals and metalloids is known as the carbides. These can be classified as follows:

Saline carbides: These are largely ionic solids and are formed by the elements of Groups 1 and 2 and by aluminium.

Metallic carbides: These have a metallic conductivity and lustre and are formed by the d-block elements.

Metalloid carbides: These are hard covalent solids that are formed by boron and silicon.

Let's discuss two types of carbides in detail.

(a) Saline carbides: These are the metal-carbon compounds in which metals have a high electropositive character. Non-metal carbides are generally hard and are semiconductors. For groups 1 and 2, these carbides can be divided into three categories:

Graphite intercalation compounds are formed by group 1 metals. Their formation takes place by the reaction between graphite and alkali metal vapour or between graphite and metal-ammonia solution. Example: KC8, dicarbides (or ‘acetylides’), which contain the C22– anion, and methides, which formally contain the C4– anion. A series of alkali metal-graphite intercalation compounds can be prepared with different metal: carbon ratios, including KC8 and KC16.

The formation of dicarbides takes place by a broad range of electropositive metals, including those from Groups 1 and 2 and the lanthanoids. The C22– ion has a very short CC distance in some dicarbides which is consistent with it being a triply bonded [C≡C]2 ion isoelectronic with [C≡N] and N≡N. The structures of some dicarbides are related to rock salt, but replacement of the spherical Cl ion by the elongated [C≡C]2– ion leads to an elongation of the crystal along with one of the axes, and a resulting tetragonal symmetry. In lanthanoid dicarbides, the CC bond is significantly longer which suggests that for them the simple triply bonded structure is not a good approximation. Carbides like Be2C and AI4C3 are borderline between saline and metalloid, and the isolated C ion is only formally C4–. The presence of directional bonding in carbides is indicated by the crystal structures of methides, which are not those expected for the simple packing of spherical ions.

The principal synthetic routes to the saline carbides and acetylides of Groups 1 and 2 are very straightforward. These can be formed by the direct reaction of the elements at high temperatures.

The formation of graphite intercalation compounds is another example of a direct reaction but is carried out at much lower temperatures. 

The reaction of metal oxide and carbon at a high temperature is given below.

Crude calcium carbide is prepared in electric arc furnaces by this method. The carbon acts both as a reducing agent to remove the oxygen and as a source of carbon to form the carbide.

Reaction of ethyne (acetylene) with a metal-ammonia solution:

2 Na(am) | C2H2(g) → Na2C2(s) | H2(g)

The above reaction takes place under mild conditions and leaves the carbon-carbon bonds of the starting material intact. As the ethyne molecule acts as a very weak Bronsted acid (pK2 = 25), the reaction can be regarded as a redox reaction between a highly active metal and a weak acid to yield H2 (with H+ the oxidizing agent) and the metal dicarbide.

The saline carbides have a high electron density on the C atom, so they can be readily oxidized and protonated. For example, calcium carbide reacts with weak acid water which results in the formation of ethyne.

This reaction is readily understood as the transfer of a proton from a Bronsted acid (H2O) to the conjugate base (C22–) of a weaker acid (HC≡CH). Similarly, the controlled hydrolysis or oxidation of the graphite intercalation compound KC8 restores the graphite and produces a hydroxide or oxide of the metal:

In KC8, which is a graphite intercalation compound, the potassium atoms lie in a symmetrical array between the sheets. 

The above structure is of the calcium carbide structure. Because C22– is not spherical, the cell is elongated along one axis. This crystal is therefore tetragonal rather than cubic.

(b) Metallic carbides: Generally, d-Metal carbides are hard materials with the carbon atom octahedrally surrounded by metal atoms. The d metals provide the largest class of carbides. Examples are Co6Mo6C and Fe3Mo3C. They are also referred to as interstitial carbides because it was long thought that the structures were the same as those of the metals and that they were formed by the insertion of C atoms in octahedral holes. In fact, the structure of the metal and the metal carbide often differ. The presence of the hardness and other properties of metallic carbides indicates the strong metal-carbon bonding is present in them. Some of these carbides are economically and technologically useful materials. Tungsten carbide (WC), for example, is used to cut tools and high-pressure apparatus such as that used to produce diamonds. Cementite, Fe3C, is a major constituent of steel and cast iron.

Metallic carbides with composition MC have an FCC or hcp arrangement of metal atoms with the C atoms present in the octahedral holes. The FCC arrangement results in a rock-salt structure. The C atoms in carbides of composition M2C occupy only half the octahedral holes between the close-packed metal atoms. A-C atom in an octahedral hole is formally hyper coordinate (untypically high coordination number) because it is surrounded by six metal atoms. 

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Neetesh TiwariNeetesh TiwariMember since Aug 2020
Working Experience of more than 2 years in Education Industry. Currently associated to CSIR-NET at Gradeup. Guiding students to crack the exam and fulfil their dreams.
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