Problem 33 - Chemistry

Question

Rationalize why chalk (calcium carbonate) has a higher melting point than motor oil (composed of large compounds containing only carbon and hydrogen), which has a higher melting point than water (a compound that exhibits hydrogen bonding).

Step-by-Step Solution

Answer

In conclusion, chalk (calcium carbonate) has the highest melting point due to its strong ionic bonds. Motor oil has a higher melting point than water because its large hydrocarbon molecules create stronger London dispersion forces compared to those in water. Water has a lower melting point compared to chalk and motor oil because it has hydrogen bonds, which are stronger than the London dispersion forces in motor oil but weaker than the ionic bonds in chalk.

Step 1: Understand Intermolecular Forces

Intermolecular forces are the forces between the molecules of a substance. These forces can be categorized into three main types: ionic bonds, covalent bonds, and hydrogen bonds. 1. Ionic bonds occur between ions of opposite charges, like those found in salts. 2. Covalent bonds occur between non-metal atoms sharing electrons, forming molecules. 3. Hydrogen bonds are a type of dipole-dipole interaction occurring between molecules containing a hydrogen atom bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. We need to understand the intermolecular forces present in each substance (chalk, motor oil, and water) to rationalize their melting points.

Step 2: Identify the Intermolecular Forces in Each Substance

1. Chalk (Calcium Carbonate): Chalk is an ionic compound with the chemical formula CaCO3. Calcium carbonate is made up of Ca^(2+) cations and CO3^(2-) anions, held together by strong ionic bonds. 2. Motor oil: Motor oil is composed of large carbon and hydrogen-based compounds known as hydrocarbons. In these compounds, the carbon and hydrogen atoms are covalently bonded. The intermolecular forces present in motor oil are London dispersion forces (also known as van der Waals forces), which are relatively weak interactions between nonpolar molecules. 3. Water: Water (H2O) is a covalent compound consisting of hydrogen and oxygen atoms. The intermolecular force present in water is hydrogen bonding which occurs between the hydrogen atom of one water molecule and the oxygen atom of another water molecule.

Step 3: Compare the Strength of Intermolecular Forces and Melting Points

1. Chalk (Calcium Carbonate): The ionic bonds in calcium carbonate are strong, requiring a significant amount of energy to break. Therefore, chalk has a high melting point. 2. Motor oil: The London dispersion forces in motor oil are relatively weak compared to ionic bonds and hydrogen bonds. However, the large size of hydrocarbon molecules can create more substantial London dispersion forces between molecules, leading to a higher melting point than water but lower than chalk. 3. Water: Hydrogen bonds are stronger than London dispersion forces but weaker than ionic bonds. Thus, water has a higher melting point than motor oil due to its hydrogen bonding, but a lower melting point compared to chalk. In conclusion, chalk has the highest melting point due to its ionic bonds, followed by motor oil with stronger London dispersion forces because of its large hydrocarbon molecules, and finally, water with a lower melting point resulting from its hydrogen bonding.

Key Concepts

Ionic Bonds

Ionic bonds are one of the strongest types of chemical bonds. They occur when atoms transfer electrons to one another, resulting in the formation of charged ions.
Typically, this happens between a metal and a non-metal. The metal atom becomes a positively charged cation, while the non-metal atom becomes a negatively charged anion. These oppositely charged ions are attracted to each other, creating a strong bond.Characteristics of Ionic Bonds:

• Usually form between metals and non-metals.
• Create charged ions that attract each other.
• Form crystalline structures, such as salts.
• Exhibit high melting and boiling points because of strong attractions.

Calcium carbonate, commonly known as chalk, is a classic example of an ionic compound. It consists of calcium ions \(\text{Ca}^{2+}\) and carbonate ions \(\text{CO}_3^{2-}\). The strong attraction between these ions requires a lot of energy to break, leading to its high melting point compared to other substances without ionic bonds.

Covalent Bonds

Covalent bonds are formed when two atoms share one or more pairs of electrons. This type of bond typically occurs between non-metal atoms. Instead of transferring electrons, the atoms involved in covalent bonding share them, allowing each atom to attain a stable electron configuration. Characteristics of Covalent Bonds:

• Form between non-metal atoms.
• Involve sharing of electron pairs.
• Create molecules with varied bonding strengths.
• Can exist as single, double, or triple bonds, affecting bond strength.

Motor oil is composed of large molecules with carbon and hydrogen atoms bonded covalently. These molecules are held together by London dispersion forces, which are weak compared to ionic or hydrogen bonds. However, the size and structure of these molecules allow for increased interactions, giving motor oil a higher melting point than substances with smaller, less complex covalent molecules, such as water.

Hydrogen Bonds

Hydrogen bonds are a specific type of dipole-dipole interaction. They occur when a hydrogen atom, covalently bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine), interacts with another electronegative atom in a different molecule. These bonds are stronger than typical dipole-dipole interactions, but weaker than ionic bonds. Characteristics of Hydrogen Bonds:

• Occur between molecules containing H bonded to O, N, or F.
• Create significant but moderate attractive forces.
• Responsible for many of water's unique properties.
• Lead to higher boiling and melting points than simple covalent interactions.

Water, made of hydrogen and oxygen, exhibits hydrogen bonding. This intermolecular force arises between the hydrogen atom of one water molecule and the oxygen atom of another. Although hydrogen bonds are not as strong as ionic bonds, they are significant enough to give water properties essential for life, such as a relatively high melting point compared to substances held together only by weak forces.