Problem 26 - General Chemistry

Question

a. Is it possible to add heat to a pure substance and not observe a temperature change? If so, provide examples. b. Describe, on a molecular level, what happens to the heat being added to a substance just before and during melting. Do any of these molecular changes cause a change in temperature? Part 2: Consider two pure substances with equal molar masses: substance A, having very strong intermolecular attractions, and substance \(\mathrm{B}\), having relatively weak intermolecular attractions. Draw two separate heating curves for \(0.25\) -mol samples of substance \(A\) and substance \(B\) in going from the solid to the vapor state. You decide on the freezing point and boiling point for each substance, keeping in mind the information provided in this problem. Here is some additional information for constructing the curves. In both cases, the rate at which you add heat is the same. Prior to heating, both substances are at \(-50^{\circ} \mathrm{C}\), which is below their freezing points. The heat capacities of \(\mathrm{A}\) and \(\mathrm{B}\) are very similar in all states. a. As you were heating substances \(\mathrm{A}\) and \(\mathrm{B}\), did they melt after equal quantities of heat were added to each substance? Explain how your heating curves support your answer. b. What were the boiling points you assigned to the substances? Are the boiling points the same? If not, explain how you decided to display them on your curves. C. According to your heating curves, which substance reached the boiling point first? Justify your answer. d. Is the quantity of heat added to melt substance A at its melting point the same as the quantity of heat required to convert all of substance \(\mathrm{A}\) to a gas at its boiling point? Should these quantities be equal? Explain.

Step-by-Step Solution

Answer

Heat can be added without temperature change during phase transitions. Substance B, with weaker forces, reaches its boiling point first and requires less heat. Heat required for melting A is less than for vaporization.

Step 1: Understanding Heat Addition Without Temperature Change

Yes, it is possible to add heat to a pure substance without an increase in temperature. This typically occurs during phase changes, such as melting and boiling. When a solid melts, the added heat is used to overcome the intermolecular forces between particles rather than increasing their kinetic energy, which accounts for temperature rise.

Step 2: Molecular Level Changes During Melting

Just before and during melting, the heat added to a substance is used to break the orderly structure of the solid state. The energy goes into overcoming the cohesive forces holding the molecules together in a fixed lattice, transitioning them into a more disordered liquid state. Although the molecular arrangement changes, the kinetic energy (and thus temperature) of molecules initially remains unchanged.

Step 3: Comparing Intermolecular Forces

Substance A, with strong intermolecular attractions, will require more heat to overcome these forces during phase transitions than substance B, which has weaker attractions. This difference affects the amount of heat required during melting and boiling, reflected in their heating curves.

Step 4: Heating Curves for Substances A and B

Draw two heating curves starting at -50°C for both substances. Substance A will have a higher melting and boiling point due to stronger intermolecular forces. In both curves, plot phases: solid, liquid, and vapor with respective plateaus at phase changes but longer for A due to higher transition enthalpies.

Step 5: Analyzing Equal Heat Addition During Melting

Due to stronger intermolecular forces in substance A, more heat is needed to reach its melting point compared to substance B. Consequently, substance B melts after less heat is added. The heating curves will show a lower melting point for B, supporting this conclusion.

Step 6: Determining and Justifying Boiling Points

Assign higher boiling points for substance A, reflecting its stronger molecular attractions. Substance B will have a lower boiling point due to weaker forces. Substance A shows a later and higher temperature phase transition on the curve, highlighting different energetics of phase changes.

Step 7: Identifying Which Substance Boiled First

Substance B, with weaker intermolecular forces, will reach its boiling point first, as shown by its lower boiling point on the heating curve. This reflects the smaller amount of energy required to vaporize compared to substance A.

Step 8: Comparing Heat Required for Melting and Vaporization of A

The heat needed to melt substance A at its melting point is usually less than that required to vaporize it at its boiling point. This discrepancy is due to the larger energy gap required to completely separate molecules into the vapor phase compared to merely moving them apart into a liquid.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces that exist between the molecules of a substance. These forces play a crucial role in determining various properties of the substance, such as melting and boiling points.
Strong intermolecular forces mean that the molecules are more tightly held together. Consequently, more heat energy is required to overcome these forces and allow a phase change from solid to liquid, or liquid to gas.

• For example, in substance A, which has strong intermolecular attractions, it takes more heat to melt and boil compared to substance B.
• This is because breaking strong intermolecular forces requires more energy.

Substance B, on the other hand, with weaker intermolecular forces, reaches these state changes with less heat input. Understanding these forces helps to predict physical properties like heat required for phase changes.

Temperature Change

During phase changes, when a substance changes its state from solid to liquid or liquid to gas, heat is added to the substance. However, the temperature doesn't change despite this addition of heat.
Instead, this heat is used to overcome the intermolecular forces, allowing the molecules more freedom to move rather than increasing their speed. This is why there is a plateau in temperatures observed during phase transitions.

• This plateau indicates that temperature remains constant during melting and boiling, even as heat is continually being added.
• The energy absorbed is utilized entirely for the transition process.

Heating Curve

A heating curve is a graphical representation of the temperature of a substance as heat is added to it over time. It shows the different phases of the substance and the points at which phase changes occur.
Significantly, it highlights the phases where temperature remains constant during phase transitions, despite continuous heat addition.

• The curve typically begins with a solid phase where temperature rises linearly as heat is added.
• As the melting point is reached, a plateau appears, indicating a phase transition from solid to liquid with no change in temperature.
• After melting, the liquid phase again shows a linear rise until the boiling point, where another plateau represents the transition from liquid to gas.

In substances with strong intermolecular forces, the plateaus are longer because more energy is required to affect the change.

Molecular Level Changes

On the molecular level, phase changes involve significant structural changes. During melting, molecules absorb heat, allowing them to break free from their fixed positions in the solid-state lattice.

• As the orderly structure of the solid disrupts, the substance turns into a liquid, where molecules can move more freely.
• During boiling, molecules separate completely into a gaseous state.
• The heat energy required here is used in breaking these intermolecular bonds, rather than increasing the kinetic energy of the molecules.

It's important to note that although molecular structure changes, the kinetic energy—and thus temperature—does not increase during the actual phase transition.