Question

The critical temperatures \((\mathrm{K})\) and pressures \((\mathrm{atm})\) of a series of halogenated methanes are as follows: $$ \begin{array}{lcccc} \text { Compound } & \mathbf{C C l}_{3} \mathbf{F} & \mathbf{C C l}_{2} \mathbf{F}_{2} & \mathbf{C C I F}_{3} & \mathbf{C F}_{4} \\ \hline \text { Critical temperature } & 471 & 385 & 302 & 227 \\ \text { Critical pressure } & 43.5 & 40.6 & 38.2 & 37.0 \end{array} $$ (a) List the intermolecular forces that occur for each compound. (b) Predict the order of increasing intermolecular attraction, from least to most, for this series of compounds. (c) Predict the critical temperature and pressure for \(\mathrm{CCl}_{4}\) based on the trends in this table. Look up the experimentally determined critical temperatures and pressures for \(\mathrm{CCl}_{4}\), using a source such as the CRC Handbook of Chemistry and Physics, and suggest a reason for any discrepancies.

Short answer

The intermolecular forces for all the compounds are dipole-dipole interactions and London dispersion forces. The order of increasing intermolecular attraction is: CF4 < CClF3 < CCl2F2 < CCl3F. The predicted critical temperature and pressure for CCl4 are around 500 K and 45 atm, while the experimentally determined values are 556.4 K and 45.6 atm. The discrepancy between the predicted and experimental values could be due to factors such as the oversimplification of our trend analysis, differences in molecular geometries, atomic interactions, and other factors affecting critical temperature and pressure.

Step-by-step solution

List intermolecular forces for each compound.

All the given compounds are halogenated methanes. The general molecular formula of these compounds is: \(\mathrm{C}_{1}\mathrm{H}_{4-n}\mathrm{X}_{n}\); where n is the number of halogenating molecules, X represents the halogen atoms(Cl or F), and (4-n) represents the number of Hydrogen molecules. Since the given molecules contain polar bonds (C-F and C-Cl), they exhibit dipole-dipole interactions. In addition to this, all of these molecules contain elements with lone electron pairs leading to London dispersion forces (also called Van der Waals forces). In summary: - CCl3F has dipole-dipole interactions and London dispersion forces. - CCl2F2 has dipole-dipole interactions and London dispersion forces. - CClF3 has dipole-dipole interactions and London dispersion forces. - CF4 has dipole-dipole interactions and London dispersion forces. #b: Order of increasing intermolecular attraction#

Predict the order of increasing intermolecular attraction.

The order of increasing intermolecular attraction will depend mostly on the strength of the dipole-dipole interactions, as London dispersion forces will be similar for these compounds. The strength of dipole-dipole interactions increases with an increasing number of polar C-Cl bonds. So, the order of increasing intermolecular attraction would be: CF4 < CClF3 < CCl2F2 < CCl3F. #c: Predicting critical temperature and pressure for CCl4#

Predict critical temperature and pressure for CCl4.

To predict the critical temperature and pressure for CCl4, we will identify trends in the given data for other halogenated methanes: Compound Critical Temp (K) Critical Press (atm) CCl3F (One Cl replaced with F) 471 43.5 CCl2F2 (Two Cls replaced with Fs) 385 40.6 CClF3 (Three Cls replaced with Fs) 302 38.2 CF4 (All Cls replaced with Fs) 227 37.0 From the given data, we observe that: 1. As the number of F atoms increases and the number of Cl atoms decreases, the critical temperature decreases. 2. As the number of F atoms increases and the number of Cl atoms decreases, the critical pressure decreases slightly. 3. For CCl4, since all the Cl atoms are still present, we can expect its critical temperature and pressure to be higher than CCl3F. Hence, we can predict the critical temperature for CCl4 to be around 500K and critical pressure to be around 45 atm. To compare our prediction with experimental values, we can check a reliable source like the CRC Handbook of Chemistry and Physics. The experimentally determined values for CCl4 are: - Critical temperature: 556.4 K - Critical pressure: 45.6 atm

Compare predicted values with experimental values and suggest reasons for discrepancies.

Our predicted values for critical temperature and pressure are close to the experimental values. However, there is some discrepancy, which could be attributed to various factors such as the oversimplification of our trend analysis, differences in molecular geometries, atomic interactions, and other factors that affect critical temperature and pressure, which were not taken into consideration.

Key concepts

Halogenated Methanes

Halogenated methanes are compounds derived from methane \((\text{CH}_4)\) by replacing one or more hydrogen atoms with halogen atoms such as chlorine \((\text{Cl})\) or fluorine \((\text{F})\). These substitutions lead to variations like \(\text{CCl}_3\text{F}\), \(\text{CCl}_2\text{F}_2\), \(\text{CClF}_3\), and \(\text{CF}_4\). The presence of halogens significantly alters the chemical and physical properties.

These changes impact properties such as

• Dipole moment - due to polar C-Cl and C-F bonds
• Boiling and melting points
• Reactivity

Moreover, all the halogenated methanes discussed in the exercise feature polar bonds, which contributes to their intermolecular interactions like dipole-dipole interactions. Additionally, the size and number of halogen atoms influence the London dispersion forces, thereby affecting the compound's overall stability and reactivity in gaseous states.

Critical Temperature

The critical temperature of a compound is defined as the highest temperature at which it can exist as a liquid, regardless of the pressure applied. It's a crucial concept in understanding phase transitions.

For halogenated methanes:

• \(\text{CCl}_3\text{F}\) has a critical temperature of 471K
• \(\text{CCl}_2\text{F}_2\) is 385K
• \(\text{CClF}_3\) is 302K
• \(\text{CF}_4\) is 227K

This data shows a clear trend: as you replace chlorine atoms with fluorine atoms, the critical temperature decreases. Chlorine is heavier and larger than fluorine, contributing to stronger intermolecular forces, resulting in higher critical temperatures.

Predicting the critical temperature of \(\text{CCl}_4\) was based on this trend, resulting in a rough estimate of 500K. However, detailed calculations and experimental results showed it to be 556.4K, hinting that additional structural factors aside from just halogen substitutions can affect critical temperatures.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where positive and negative charges attract each other. In halogenated methanes, these attractions are significant due to the C-Cl and C-F bonds. The more polar a molecule, the stronger these interactions are, which impacts the compound's physical properties like boiling point and solubility.

Consider the order of intermolecular attraction strength: \(\text{CF}_4 < \text{CClF}_3 < \text{CCl}_2\text{F}_2 < \text{CCl}_3\text{F}\). This sequence is primarily determined by the dipole-dipole interactions arising from the number and nature of C-Cl bonds.

These interactions lead to various effects:

• Higher boiling points as dipole interactions increase
• Changes in solubility in polar solvents
• Variations in melting points

Thus, understanding dipole-dipole interactions helps explain why certain halogenated methanes have higher critical temperatures and pressures than others.

London Dispersion Forces

London dispersion forces, a type of van der Waals forces, are weak intermolecular interactions arising from temporary, induced dipoles in atoms or molecules. They are universal, affecting both polar and non-polar entities alike.

In halogenated methanes, London dispersion forces complement dipole-dipole interactions. These forces are generally weaker than dipole-dipole forces but become more significant as the number of electrons increases, which occurs with larger halogen atoms or multiple substitutions.

Key characteristics of London dispersion forces include:

• They increase with molecular size and mass
• Contribute to the boiling and melting point elevations
• Important for structural stability in gases

While dispersal forces are similar across these halogenated methanes, their collective effect along with other forces defines features such as the critical temperature and intermolecular interaction strength. Understanding these helps in predicting behaviors like phase changes under various conditions.