Problem 80 - Chemistry Matter and Change

Question

The molar enthalpy of fusion of methanol is 3.22 \(\mathrm{k} / \mathrm{mol}\) What does this mean?

Step-by-Step Solution

Answer

The molar enthalpy of fusion is the energy needed to melt one mole of methanol, which is 3.22 kJ.

Step 1: Understanding Molar Enthalpy of Fusion

The molar enthalpy of fusion is the amount of energy required to change one mole of a substance from a solid to a liquid state without a change in temperature. It is a measure of the energy needed to overcome the forces holding the molecules together in the solid state.

Step 2: Analyzing the Given Value

In this exercise, we are given that the molar enthalpy of fusion of methanol is 3.22 kJ/mol. This means that 3.22 kilojoules of energy is required to melt one mole of methanol.

Step 3: Relating Molar Enthalpy to Physical Process

For methanol, this value indicates the energy necessary for phase transition. When one mole of solid methanol is heated at its melting point, exactly 3.22 kJ of energy is needed to convert it into the liquid form, without any increase in temperature.

Key Concepts

Phase Transition

A phase transition refers to the process where a substance changes from one state of matter to another. Common phase transitions include moving from solid to liquid (melting), liquid to gas (vaporization), and others like solid to gas (sublimation). Understanding the phase transition involves recognizing that it occurs without altering the temperature at the melting or boiling point of a substance.
For methanol, when it reaches its melting point, turning from solid to liquid is a classic case of a phase transition known as fusion. During this process, the particles in the solid gain enough energy to overcome the forces that hold them in a fixed position. The energy needed for this change is termed as molar enthalpy of fusion, representing the energy per mole of substance undergoing the phase change.

Energy Requirement

Energy plays a vital role in phase transitions, such as melting. The specific energy required to melt a mole of a substance at its melting point without changing its temperature is called its molar enthalpy of fusion. For the case of methanol, this value is 3.22 kJ/mol. This means that exactly 3.22 kilojoules of energy must be provided to overcome the molecular forces in one mole of solid methanol so that it can transform into the liquid state.
This energy requirement, also known as latent heat, does not affect the temperature of the substance. Instead, it provides the necessary energy to weaken the attractions between the molecules in the solid, causing them to slide past one another freely, forming a liquid.

Mole Concept

The mole concept provides an easy way to measure and compare amounts of substances in chemistry. A mole is defined as Avogadro's number of particles, which is approximately 6.022 x 10²³ of atoms, molecules, or ions. This unit connects the microscopic world of atoms and molecules with the macroscopic grams and liters we measure in labs.
When we say that the molar enthalpy of fusion of methanol is 3.22 kJ/mol, we are describing the energy needed to melt one mole of methanol, containing 6.022 x 10²³ methanol molecules. This standardized unit makes it easier for chemists and students to understand and calculate the energy changes involved during phase transitions across different substances.