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?

Short answer

Two compounds with the same empirical formula can have markedly different properties because their molecular structures can differ. The empirical formula represents the simplest whole number ratio of elements in a compound, while the molecular formula shows the actual number of atoms of each element in a single molecule. For example, formaldehyde (H2CO) and ethylene glycol (HOCH2CH2OH) both have the empirical formula CH2O but differ in molecular structure, leading to differences in properties like physical states and viscosity.

Step-by-step solution

Define empirical formula

An empirical formula represents the simplest whole number ratio of elements in a compound. It does not necessarily represent the exact number of atoms of each element, but rather the ratio of those atoms.

Define molecular formula

A molecular formula shows the actual number of atoms of each element in a single molecule of the compound. While the empirical formula represents the simplest ratio of elements, the molecular formula represents the real composition of the compound.

Explain the difference between empirical and molecular formulas

The same empirical formula can correspond to different molecular formulas. This means that two compounds can have the same ratio of elements but have different actual numbers of atoms within their molecules. For example, the empirical formula CH can correspond to the molecular formulas CH (methylene) or C2H2 (acetylene).

Discuss the importance of molecular structure

The properties of a compound are not solely determined by its composition; the molecular structure of the compound also plays a significant role. Two compounds with the same empirical formula can have different molecular structures, which can lead to different properties, such as physical states and viscosity.

Provide an example

Let us consider two compounds with the empirical formula CH2O: formaldehyde (H2CO) and ethylene glycol (HOCH2CH2OH). Formaldehyde is a gas at room temperature, while ethylene glycol is a viscous liquid. The difference in their properties can be attributed to their molecular structures. Formaldehyde has a planar structure with a double bond between carbon and oxygen, whereas ethylene glycol has a more complex structure with two hydroxyl groups that can participate in hydrogen bonding. This hydrogen bonding enables ethylene glycol to form a viscous liquid, while formaldehyde does not have this characteristic, and hence exists as a gas. In conclusion, two compounds with the same empirical formula can have markedly different properties due to differences in their molecular structures, which affect the properties such as physical states and viscosity.

Key concepts

Chemical Composition

Understanding the chemical composition of a compound involves analyzing the variety and ratio of the elements that it's made up of. The empirical formula presents a simplified representation, showing the simplest whole number ratio of elements in a compound. This can be thought of as the 'recipe' for the basic building blocks of a substance. For instance, glucose, fructose, and ribose all have the empirical formula C₆H₁₂O₆, indicating a consistent ratio among carbon, hydrogen, and oxygen atoms. However, these substances are distinctly different in their natures. This is because the empirical formula does not specify the exact quantity or arrangement of atoms.

Improving upon the textbook exercise solution, we should note that while the empirical formula is useful for identifying substances in a broad sense, it doesn't reveal the complexities of molecular interactions or how atoms are bonded within the molecule. This information is essential for predicting reactivity and understanding the behavior of a compound under various conditions.

Molecular Structure

Diving deeper into the concept of molecular structure, it's clear that the physical and chemical properties of a compound are intrinsically linked to the way its atoms are connected. The molecular structure encompasses not only the arrangement of atoms but also the types of chemical bonds between them, including single, double, or triple bonds, and the three-dimensional shape of the molecule itself.

For a more thorough grasp of this concept, let's look at isomers—molecules that share the same molecular formula but have different arrangements of atoms. Structural (or constitutional) isomers have different covalent arrangements of their atoms. For example, propanol and isopropanol both have the formula C₃H₈O, but propanol has a straight-chain structure, whereas isopropanol has a branched-chain structure. This slight variance greatly influences their respective boiling points and toxicities.

Moreover, another form of isomers, stereoisomers, share the same bond structure but differ in the three-dimensional orientations of the atoms. This leads to properties as distinct as the smells of oranges versus lemons, both of which can contain the molecular formula C₁₀H₁₆O but differ in molecular arrangement.

Physical State of Compounds

The physical state of a compound at a given temperature and pressure—whether it be solid, liquid, or gas—is a result of the balance between kinetic energy and intermolecular forces. Molecular structure heavily impacts these intermolecular attractions, which include hydrogen bonding, van der Waals forces, and dipole-dipole interactions. Molecules with strong intermolecular bonds, like hydrogen bonds found in water, typically exist as liquids or solids at room temperature.

To enhance the exercise solution, consider the example of water (H₂O) and carbon dioxide (CO₂). Despite both having 'O' for oxygen, their differences in molecular structure bring about very different states: water is a liquid whereas carbon dioxide is a gas under standard conditions. This disparity is due to water's ability to form hydrogen bonds because of the polar covalent bonds between hydrogen and oxygen atoms, as opposed to carbon dioxide, which is a linear, non-polar molecule and cannot form such strong intermolecular forces.

In practice, understanding the molecular structure becomes invaluable for predicting the physical state of a compound. It helps explain why certain medicines are absorbed better as tablets (solid) rather than as inhalants (gas) and why oils (liquid) can lubricate machinery parts whereas gases cannot.