Question

Rationalize why chalk (calcium carbonate) has a higher melting point than motor oil (large compound made from carbon and hydrogen), which has a higher melting point than water and engages in relatively strong hydrogen bonding interactions.

Short answer

Chalk (calcium carbonate) has the highest melting point among the three substances due to its strong ionic bonding, requiring a significant amount of energy to overcome. Motor oil, consisting of large non-polar carbon and hydrogen-based molecules, has a higher melting point than water because it mainly exhibits London dispersion forces, which increase with molecular size. Water has a melting point between the other two substances due to its relatively strong hydrogen bonding interactions, which are stronger than London dispersion forces in small molecules but weaker than ionic bonding.

Step-by-step solution

Identifying the Intermolecular Forces in Each Substance

In order to rationalize the differences in melting points, we need to first identify the types of intermolecular forces present in each substance: 1. Chalk (calcium carbonate, CaCO3) is an ionic compound made of calcium ions (Ca2+) and carbonate ions (CO3 2-). Therefore, its main intermolecular force is ionic bonding. 2. Motor oil consists of large carbon and hydrogen-based molecules, which makes it a non-polar substance. Its main intermolecular force is London dispersion forces (Van der Waals forces). 3. Water (H2O) is a polar substance, which means that it has a partial positive charge and a partial negative charge on its molecules. Its main intermolecular force is hydrogen bonding.

Discuss the Strength of Each Intermolecular Force

Now, we will discuss the different strengths of the intermolecular forces found in each substance: 1. Ionic bonding is the strongest type of intermolecular force. It involves the electrostatic attraction between oppositely charged ions, in this case, Ca2+ and CO3 2-. This strong attraction requires a significant amount of energy to overcome. 2. London dispersion forces are the weakest type of intermolecular force. They are temporary attractions caused by random fluctuations in the electron cloud surrounding a non-polar molecule. These forces are generally weaker in smaller molecules but can become stronger in larger molecules, like those found in the motor oil, due to the increase in surface area for interactions. 3. Hydrogen bonding is a stronger force than London dispersion forces but weaker than ionic bonding. It occurs between molecules when hydrogen is bonded to a highly electronegative atom, such as oxygen, creating a strong dipole-dipole attraction between the partial positive charge on the hydrogen and the partial negative charge on the electronegative atom.

Relating the Strength of Intermolecular Forces to Melting Points

The melting point of a substance is directly related to the strength of its intermolecular forces. A substance with stronger intermolecular forces will require more energy to overcome these forces and, therefore, will have a higher melting point. In our exercise: 1. Chalk has ionic bonding, the strongest intermolecular force, which requires a significant amount of energy to overcome. Thus, chalk has the highest melting point among three substances. 2. Motor oil has London dispersion forces due to its large carbon and hydrogen-based molecules. These forces are stronger than those in small non-polar molecules, so motor oil has a higher melting point than water, which has hydrogen bonding interactions. 3. Water can engage in relatively strong hydrogen bonding interactions compared to London dispersion forces in small molecules. As a result, water has a higher melting point than other substances that rely solely on London dispersion forces, but these forces are not as strong as ionic bonding in chalk, making water's melting point lower than that of chalk. To summarize, chalk has the highest melting point due to strong ionic bonding, motor oil has a higher melting point than water due to its large non-polar molecules and London dispersion forces, and water's melting point is in between because of its relatively strong hydrogen bonding interactions.

Key concepts

Ionic Bonding

Ionic bonding is a type of chemical bond where atoms with opposite electric charges (ions) attract each other to form a compound. This occurs typically between metals, which lose electrons to become positively charged cations, and non-metals, which gain electrons to become negatively charged anions.

For example, in chalk, which is calcium carbonate (CaCO₃), ionic bonding happens between calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻). The ionic bond is exceptionally strong because of the coulombic force, which is the force of attraction between charged particles. As a result, compounds with ionic bonding, like chalk, require a lot of energy to change states, contributing to their high melting points.

London Dispersion Forces

London dispersion forces, also referred to as Van der Waals forces, are intermolecular forces that occur between non-polar molecules. These forces are the result of temporary fluctuations in the distribution of electrons within molecules, which create instantaneous dipoles that induce dipoles in neighboring molecules. This leads to a weak attraction.

Though London dispersion forces are the weakest type of intermolecular interaction, their strength increases with the size of the molecule and the accompanying increase in the surface area. This factor is significant in molecules like those in motor oil, which is made up of long carbon and hydrogen chains. The large molecular size of motor oil gives rise to stronger London dispersion forces, placing its melting point above substances with smaller non-polar molecules.

Hydrogen Bonding

Hydrogen bonding is a special type of polar bonding that significantly affects the physical properties of compounds. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, and is attracted to another electronegative atom in a different molecule. For instance, water molecules (H₂O) exhibit hydrogen bonding where the hydrogen is attracted to the oxygen of a neighboring water molecule.

This bonding is stronger than London dispersion forces but weaker than ionic bonding. The strong intermolecular attraction in hydrogen bonds gives substances like water relatively high melting and boiling points compared to those substances that only exhibit London dispersion forces.

Melting Point Comparison

A melting point comparison among substances can be understood by looking at the type and strength of their intermolecular forces. Generally, the stronger the intermolecular forces within a substance, the higher the temperature needed to separtate molecules from each other, hence, a higher melting point.

In the exercise, chalk, with ionic bonds, has the highest melting point due to the significant energy required to break the strong electrostatic forces between ions. Motor oil's considerable molecular size leads to stronger London dispersion forces and a melting point higher than that of water. Water, with hydrogen bonds, has a melting point lower than chalk but higher than compounds with only London dispersion forces due to the strength of its hydrogen bonds.

Intermolecular Force Strength

The strength of intermolecular forces is a critical factor that determines the physical properties of substances, including melting point. These forces range from weak Van der Waals (London dispersion) forces in non-polar molecules to the strong electrostatic forces observed in ionic bonding.

The hierarchy of intermolecular force strength, from weakest to strongest, typically is: London dispersion forces, dipole-dipole interactions, hydrogen bonding, and ionic bonding. The substance's melting point is correspondingly affected; the stronger the intermolecular forces, the higher the melting point. By understanding the relationship between force strength and melting point, one can predict relative melting points in various substances.