Problem 78 - General Chemistry: Principles and Modern Applications

Question

An important test for the purity of an organic compound is to measure its melting point. Usually, if the compound is not pure, it begins to melt at a lower temperature than the pure compound.(a) Why is this the case, rather than the melting point being higher in some cases and lower in others?(b) Are there any conditions under which the melting point of the impure compound is higher than that of the pure compound? Explain.

Step-by-Step Solution

Answer

Impure organic compounds typically melt at a lower temperature because impurities disrupt regular intermolecular forces. Rarely, if impurities interact with the compound in a way that strengthens these forces, the melting point might be higher.

Step 1: Answering Question A

Impurities in an organic compound disrupt the uniformity of the forces between the molecules of that compound. A pure organic compound melts at a specific temperature because that's when the intermolecular forces of the compound are overcome and the solid starts to turn into liquid. When there are impurities, these may disrupt the forces between the molecules, causing the compound to start melting at a lower temperature because the intermolecular forces between the molecules are not as strong as they would be in a pure compound.

Step 2: Answering Question B

In general, impurities lower the melting point of a compound as they disrupt the regular structure of the compound. An exception to this general rule might occur if the impurity interacts with the compound in a way that actually strengthens the intermolecular forces. However, such situations are rare with organic compounds. In fact, it is more common for the opposite to occur with the melting point being lowered due to impurity.

Key Concepts

Organic Compound Purity

Purity in organic compounds is crucial because it directly impacts the physical properties, like melting point. Chemists often use melting points to verify the purity of a substance. For a pure organic compound, its melting point range is usually quite narrow—within 1-2°C. This narrow range happens because all molecules are identical, and they require a specific amount of energy to overcome the consistent intermolecular forces and transition from solid to liquid.

When an organic compound has impurities, it is no longer uniform. These foreign molecules disrupt the orderly structure, causing irregularities in the crystal lattice. As a result, fewer molecules require the same amount of energy to melt, broadening and lowering the melting point range. This is why a clear, concise melting point is a good indicator of a compound's purity. In synthesis and analysis, ensuring purity allows chemists to accurately predict and control reactions.

Intermolecular Forces

Intermolecular forces are the invisible attractions that hold molecules together in the solid state. These forces are critical because they determine many physical properties, including melting and boiling points. In a pure organic compound, the molecules are held together by consistent and specific intermolecular forces. To transition from solid to liquid, these forces need to be overcome, which happens at the compound's melting point.

Types of intermolecular forces include:

• Van der Waals forces: These are weak forces present in all molecules, especially significant in non-polar organic compounds.
• Dipole-dipole interactions: Occur in polar molecules, where positive and negative charges attract each other.
• Hydrogen bonds: A strong type of dipole interaction where hydrogen is bound to a highly electronegative element like oxygen or nitrogen.

These forces collectively ensure that the compound remains in its solid state until sufficient energy is supplied to break them.

Impurities Effect on Melting Point

The presence of impurities affects the melting point of a compound through disruption of intermolecular forces. When an impure substance is heated, the impurities interfere with the regular molecular arrangement. This means less energy is required to weaken these unevenly distributed forces, causing the compound to melt at a lower temperature than its pure counterpart.

Although rare, there are instances where impurities could potentially raise a melting point. This occurs if the impurity forms stronger forces with the compound compared to the existing ones. Yet, in most organic compounds, impurities lead to melting point depression. Understanding this concept is crucial for chemists as they aim to identify, quantify, and remove impurities to obtain substances with desired characteristics for various applications.