Problem 46 - Chemistry: The Central Science

Question

Two compounds have the same empirical formula. One substance is a gas, whereas the other is a viscous liquid. How is it possible for two substances with the same empirical formula to have markedly different properties?

Step-by-Step Solution

Answer

Two substances with the same empirical formula can have markedly different properties due to differences in their molecular formulas and structures. The empirical formula only indicates the simplest ratio of elements, while the molecular formula shows the actual number of atoms of each element in a compound. Different molecular structures lead to variations in intermolecular forces, molecular size, and shape, all of which affect properties such as boiling point, melting point, viscosity, and volatility. For instance, ethene (C2H4) and benzene (C6H6) share the empirical formula CH2 but have different molecular structures, resulting in ethene being a gas and benzene being a liquid at room temperature.

Step 1: Define empirical formula and molecular formula

An empirical formula is a chemical formula that shows the simplest ratio of elements in a compound, without indicating the actual number of atoms. On the other hand, a molecular formula is a chemical formula that displays the actual number of atoms of each element in a compound.

Step 2: Explain how two compounds may have the same empirical formula but different molecular formulas

Two compounds can have the same empirical formula if they have the same ratio of elements but different molecular formulas if they contain different numbers of atoms of each element. For example, hydrogen peroxide (H2O2) and water (H2O) have different molecular formulas but share the same empirical formula (HO).

Step 3: Discuss the role of molecular structure in determining properties

The molecular structure of a compound plays a significant role in determining its properties such as boiling point, melting point, viscosity, and volatility. Factors that influence these properties include strength of intermolecular forces (e.g. hydrogen bonding, dipole-dipole, and London dispersion forces), molecular size, and shape. As a result, two compounds with the same empirical formula but different molecular structures can have distinct properties.

Step 4: Provide an example of two substances with the same empirical formula but markedly different properties

One example of two substances with the same empirical formula but different properties is ethene (C2H4) and benzene (C6H6). Both compounds have the empirical formula CH2, but ethene is a gas at room temperature, while benzene is a liquid. The different properties are due to the varying molecular structures and intermolecular forces – ethene has a simple linear structure with weak London dispersion forces, whereas benzene has a cyclic structure and exhibits stronger pi-pi stacking interactions resulting in its liquid state. In conclusion, two substances with the same empirical formula can have markedly different properties due to differences in their molecular formulas and structures, which influence the strength of intermolecular forces and other factors that dictate their properties.