The substance CS₂ has a melting point of -110.8 °C and a boiling point of 46.3 °C. Its density at 20 °C is 1.26 g/cm³. It is highly flammable. (a) What is the name of this compound? (b) List the intermolecular forces that CS2 molecules exert on one another. (c) Write a balanced equation for the combustion of this compound in air. (You will have to decide on the most likely oxidation products.) (d) The critical temperature and pressure for CS2 are 552 K and 78 atm, respectively. Compare these values with those for CO₂ in Table 11.5 and discuss the possible origins of the differences.
The Correct Answer and Explanation is :
(a) Name of the compound:
The substance CS₂ is carbon disulfide. It consists of one carbon atom bonded to two sulfur atoms.
(b) Intermolecular forces:
Carbon disulfide molecules exert London dispersion forces (also known as Van der Waals forces) on one another. These forces are the result of temporary dipoles that form due to fluctuations in the electron cloud distribution. CS₂ is a non-polar molecule because the linear arrangement of the molecule causes the dipoles of the two C-S bonds to cancel each other out. Since CS₂ does not have dipole-dipole interactions or hydrogen bonding, the primary intermolecular force is London dispersion forces.
(c) Balanced equation for combustion:
The combustion of carbon disulfide in air (which consists of oxygen, O₂) typically leads to the formation of carbon dioxide (CO₂) and sulfur dioxide (SO₂) as oxidation products. The balanced equation for this reaction is:
[
\text{CS}_2 (l) + 3\text{O}_2 (g) \rightarrow \text{CO}_2 (g) + 2\text{SO}_2 (g)
]
This equation is balanced by ensuring the number of atoms of each element on both sides is equal. For each molecule of CS₂, one molecule of carbon dioxide and two molecules of sulfur dioxide are produced, using up three molecules of oxygen gas.
(d) Comparison with CO₂:
- Critical temperature (T_c): CS₂ has a critical temperature of 552 K, while CO₂ has a critical temperature of 304.2 K. The critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied. CS₂ has a higher critical temperature because it has stronger London dispersion forces compared to CO₂, as CS₂ is larger and has more electrons, which allows for greater temporary dipoles.
- Critical pressure (P_c): CS₂’s critical pressure is 78 atm, while CO₂’s critical pressure is 73.8 atm. Although these values are close, CS₂’s slightly higher critical pressure reflects the stronger intermolecular interactions (London dispersion forces), which require more pressure to achieve liquefaction.
The differences in critical temperature and pressure between CS₂ and CO₂ arise mainly from the molecular size and the nature of the intermolecular forces. CS₂ is a larger, non-polar molecule with stronger dispersion forces, while CO₂ is smaller and has weaker dispersion forces, resulting in lower critical values.