The following data present the temperatures at which certain vapor pressures are achieved for dichloromethane (CH2Cl2) and methyl iodide (CH3I): (c) The order of volatility of these two substances changes as the temperature is increased. What quantity must be different for the two substances for this phenom- enon to occur?

Verified step by step guidance

1. Step 1: Understand the concept of volatility. Volatility is the ability of a substance to vaporize. It is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with lower vapor pressure.
2. Step 2: Recall that the vapor pressure of a substance depends on the temperature and the heat of vaporization (the energy required to convert a given quantity of a substance from a liquid into a gas at a given pressure). This relationship is given by the Clausius-Clapeyron equation: ln(P2/P1) = -ΔHvap/R (1/T2 - 1/T1), where P1 and P2 are the vapor pressures at temperatures T1 and T2 respectively, ΔHvap is the heat of vaporization, and R is the gas constant.
3. Step 3: Understand that if the order of volatility of two substances changes as the temperature is increased, it means that the temperature dependence of their vapor pressures is different. This can only happen if their heats of vaporization (ΔHvap) are different.
4. Step 4: Therefore, for the order of volatility of dichloromethane (CH2Cl2) and methyl iodide (CH3I) to change as the temperature is increased, their heats of vaporization must be different.
5. Step 5: In conclusion, the quantity that must be different for the two substances for this phenomenon to occur is the heat of vaporization.

Key Concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of molecules to escape from the liquid phase into the gas phase. As temperature increases, the kinetic energy of the molecules also increases, leading to higher vapor pressures. Understanding vapor pressure is crucial for analyzing the volatility of substances.

Volatility

Volatility refers to the tendency of a substance to vaporize. A more volatile substance has a higher vapor pressure at a given temperature, meaning it evaporates more readily. The order of volatility between substances can change with temperature due to differences in intermolecular forces, such as hydrogen bonding or van der Waals forces, which affect how easily molecules can escape into the vapor phase.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. These forces, including hydrogen bonds, dipole-dipole interactions, and London dispersion forces, significantly influence the physical properties of substances, including boiling points and vapor pressures. For the order of volatility to change with temperature, the strength of these intermolecular forces must differ between the two substances, affecting their vapor pressures at elevated temperatures.