Chapter 7. Microscale Gas Chemistry:

Ethyne Information

A. Appearance

    Ethyne is commonly referred to as acetylene, the gas used by welders in oxy-acetylene torches.  Ethyne is a colorless, highly flammable gas with a characteristic odor.  The odor is pure ethyne is not unpleasant, however, traces of phosphine (PH3) are frequently present giving ethyne the disagreeable odor that most people attribute to ethyne.  The phosphines arises from trace levels of calcium phosphide, Ca3P2, present in calcium carbide, the compound used to generate ethyne.  Pure ethyne (100%) does not burn.  When mixed with air, even as little as 3%, ethyne forms explosive mixtures and forms explosive mixtures with air over a wide range of compositions.
 

B. Physical Properties of C2H2

 
Ethyne, C2H2
Atomic mass: 26.02 g/mol
melting point -80.8 oC
boiling point -84.0 oC


     Ethyne is a linear molecule with a carbon-carbon triple bond.  Interesting comparisons are noted between ethyne and the other two-carbon hydrocarbons, ethane (C2H6) and ethene (C2H4).

    The following table summarizes the differences in bond length and strength between these two-carbon hydrocarbons.  The carbon-carbon bond length becomes progressively shorter through the series ethane, ethene and ethyne.  The carbon-carbon bond strength also becomes stronger through the series with the triple bond having a strength of 839 kJ/mol.

 
C-C Bond
C-C Bond Length (pm)
C-C Bond Strength (kJ/mol)
Ethane, C2H6
single
153
347
Ethene, C2H4
double
134
614
Ethyne, C2H2
triple
120
839

C. History
     Ethyne was first prepared by John Davy in 1836.
 

D. Natural Abundance
     Ethyne occurs in small amounts in nature.  It is found in swamp gases, and in coal and petroleum deposits.
 

E. Industrial Production
     Ethyne is manufactured by the reaction of calcium carbide with water, the same reaction that we will employ in this chapter.  The reaction is:

CaC2(s) + 2 H2O(l)  ---> C2H2(g) + Ca(OH)2(aq, s)

F. Industrial Uses
     Ethyne, commonly referred to as acetylene, is the gas used by welders in oxy-acetylene torches.  Ethyne burns in air with the production of considerable amounts of soot.  The reaction between ethyne and air is complex and depends on the amount of oxygen available.  All three of these reactions take place:

2 C2H2(g) + O2(g) ---> 4 C(s) + 2 H2O(g)

2 C2H2(g) + 3 O2(g) ---> 4 CO(g) + 2 H2O(g)

2 C2H2(g) + 5 O2(g) ---> 4 CO2(g) + 2 H2O(g)

When oxygen is limited, soot, C(s), is produced as given in the first reaction.  Oxy-acetylene torches use pure oxygen.  Under these conditions, an extremely hot and brilliantly white flame is produced and the products are almost exclusively CO2 and H2O as per the last equation.
 

G. Gas Density of C2H2
      The density of ethyne is 1.064 g/L at 25 oC which is 10% less than that of air.

H. Gas Solubility of C2H2
     Ethyne is slightly soluble in water; 1 mL water will dissolve 1 mL ethyne.  The gas also dissolves in a wide variety of organic solvents.  For example, acetone (propanone) dissolves 25 volumes of ethyne!
 
 

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