Answer :
To explain why the molar enthalpies of vaporization increase in the order CH4 < C2H6 < C3H8, we need to understand the role of intermolecular forces (IMFs), specifically dispersion forces, and how they vary with the molecular properties of these substances.
1. Definition of Enthalpy of Vaporization:
The molar enthalpy of vaporization is the amount of energy required to convert one mole of a liquid substance into its gaseous form at constant temperature and pressure.
2. Type of Intermolecular Forces:
For CH4 (methane), C2H6 (ethane), and C3H8 (propane), the primary intermolecular force present is dispersion (London) forces. These are temporary attractive forces that result when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.
3. Effect of Molecular Size and Molar Mass:
The strength of dispersion forces is directly influenced by molecular size and molar mass:
- CH4 (Methane): Methane has the smallest molecular size and the lowest molar mass among the three substances.
- C2H6 (Ethane): Ethane has a larger molecular size and a higher molar mass than methane.
- C3H8 (Propane): Propane has the largest molecular size and the highest molar mass among these three substances.
4. Polarizability:
Polarizability refers to how easily the electron cloud around a molecule can be distorted to form a temporary dipole. Larger molecules with more electrons have greater polarizability. As a molecule's size and molar mass increase, its polarizability also increases.
5. Strength of Dispersion Forces:
As the size, molar mass, and polarizability of the molecules increase, the dispersion forces become stronger. Stronger dispersion forces require more energy to overcome during the phase transition from liquid to gas.
6. Order of Increasing Molar Enthalpy of Vaporization:
- CH4 (Methane): Due to its small size and low molar mass, methane has the weakest dispersion forces and consequently the lowest molar enthalpy of vaporization.
- C2H6 (Ethane): Ethane, being larger and heavier than methane, has stronger dispersion forces and a higher molar enthalpy of vaporization.
- C3H8 (Propane): Propane, with the largest size and highest molar mass among the three, has the strongest dispersion forces and therefore the highest molar enthalpy of vaporization.
Conclusion:
The molar enthalpies of vaporization increase in the order CH4 < C2H6 < C3H8 because the polarizability of the molecules, and therefore the strength of dispersion forces, increase with the size and molar mass of the hydrocarbons. This results in a greater amount of energy needed to vaporize larger and heavier molecules.
1. Definition of Enthalpy of Vaporization:
The molar enthalpy of vaporization is the amount of energy required to convert one mole of a liquid substance into its gaseous form at constant temperature and pressure.
2. Type of Intermolecular Forces:
For CH4 (methane), C2H6 (ethane), and C3H8 (propane), the primary intermolecular force present is dispersion (London) forces. These are temporary attractive forces that result when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.
3. Effect of Molecular Size and Molar Mass:
The strength of dispersion forces is directly influenced by molecular size and molar mass:
- CH4 (Methane): Methane has the smallest molecular size and the lowest molar mass among the three substances.
- C2H6 (Ethane): Ethane has a larger molecular size and a higher molar mass than methane.
- C3H8 (Propane): Propane has the largest molecular size and the highest molar mass among these three substances.
4. Polarizability:
Polarizability refers to how easily the electron cloud around a molecule can be distorted to form a temporary dipole. Larger molecules with more electrons have greater polarizability. As a molecule's size and molar mass increase, its polarizability also increases.
5. Strength of Dispersion Forces:
As the size, molar mass, and polarizability of the molecules increase, the dispersion forces become stronger. Stronger dispersion forces require more energy to overcome during the phase transition from liquid to gas.
6. Order of Increasing Molar Enthalpy of Vaporization:
- CH4 (Methane): Due to its small size and low molar mass, methane has the weakest dispersion forces and consequently the lowest molar enthalpy of vaporization.
- C2H6 (Ethane): Ethane, being larger and heavier than methane, has stronger dispersion forces and a higher molar enthalpy of vaporization.
- C3H8 (Propane): Propane, with the largest size and highest molar mass among the three, has the strongest dispersion forces and therefore the highest molar enthalpy of vaporization.
Conclusion:
The molar enthalpies of vaporization increase in the order CH4 < C2H6 < C3H8 because the polarizability of the molecules, and therefore the strength of dispersion forces, increase with the size and molar mass of the hydrocarbons. This results in a greater amount of energy needed to vaporize larger and heavier molecules.
