Question

The melting point of benzene is \(5.5^{\circ} \mathrm{C}\) and its boiling point is \(80.1{ }^{\circ} \mathrm{C}\). Sketch a heating curve for benzene from \(0{ }^{\circ} \mathrm{C}\) to \(100^{\circ} \mathrm{C} .\) a. What is the state of benzene at \(15^{\circ} \mathrm{C} ?\) b. What happens on the curve at \(5.5^{\circ} \mathrm{C}\) ? c. What is the state of benzene at \(63^{\circ} \mathrm{C}\) ? d. What is the state of benzene at \(98^{\circ} \mathrm{C}\) ? e. At what temperature will both liquid and gas be present?

Short answer

a: Liquid at 15°C. b: Phase change at 5.5°C. c: Liquid at 63°C. d: Gas at 98°C. e: 80.1°C for liquid and gas.

Step-by-step solution

Understand the Temperature Range and Phases

Identify the key temperature points: 0°C, 5.5°C, 80.1°C, and 100°C. These temperatures are important for determining the state transitions of benzene.

Sketch the Heating Curve

Draw a graph with temperature on the x-axis (from 0°C to 100°C) and energy on the y-axis. Mark three distinct regions: solid (0°C to 5.5°C), liquid (5.5°C to 80.1°C), and gas (80.1°C to 100°C). Indicate the phase change points at 5.5°C (melting) and 80.1°C (boiling).

Determine the State of Benzene at 15°C

Since 15°C is between the melting point (5.5°C) and boiling point (80.1°C), benzene is in the liquid state at 15°C.

Describe the Curve at 5.5°C

At 5.5°C, benzene transitions from a solid to a liquid. The curve will show a horizontal line as it absorbs heat without changing temperature.

Determine the State of Benzene at 63°C

Since 63°C is also between the melting point (5.5°C) and boiling point (80.1°C), benzene is in the liquid state at 63°C.

Determine the State of Benzene at 98°C

Since 98°C is above the boiling point (80.1°C), benzene is in the gaseous state at 98°C.

Identify Temperatures with Both Liquid and Gas

During the phase change from liquid to gas, both liquid and gas states will be present. This occurs at the boiling point of 80.1°C.

Key concepts

Melting Point

The melting point of a substance is the temperature at which it transitions from solid to liquid. For benzene, this temperature is 5.5°C. At the melting point, the substance absorbs energy, causing the molecules to overcome their rigid structure. During this transition, the temperature remains constant until all the solid turns into liquid. For benzene, this is depicted as a horizontal line on the heating curve.

Boiling Point

The boiling point is the temperature at which a liquid turns into a gas. Benzene boils at 80.1°C. At this point, the liquid molecules absorb enough energy to break free from the liquid's surface, resulting in a phase change to gas. Again, the temperature stays constant during this transition until all the liquid has evaporated. On the heating curve, this phase change is shown as another horizontal line.

Phase Changes

Phase changes occur when a substance transitions between different states of matter (solid, liquid, gas). Key phase changes for benzene include melting (solid to liquid at 5.5°C) and boiling (liquid to gas at 80.1°C). Each phase change involves energy absorption without a change in temperature. This is because the energy goes into changing the molecular structure rather than raising the temperature.

States of Matter

Benzene, like all substances, can exist in three states of matter: solid, liquid, and gas. Below 5.5°C, benzene is solid. Between 5.5°C and 80.1°C, benzene is liquid. Above 80.1°C, benzene becomes gaseous. The state of matter is determined by temperature and pressure, with specified transition points illustrating phase changes on the heating curve.

Temperature and Energy Graph

A temperature and energy graph, or heating curve, illustrates how a substance's temperature changes as energy is added. For benzene, the heating curve will show different slopes and horizontal lines:

• From 0°C to 5.5°C: Solid benzene's temperature increases.
• At 5.5°C: Benzene melts; temperature is constant.
• From 5.5°C to 80.1°C: Liquid benzene's temperature increases.
• At 80.1°C: Benzene boils; temperature is constant.
• From 80.1°C to 100°C: Gaseous benzene's temperature increases.

The horizontal lines at melting and boiling points indicate energy used for phase changes without temperature increase.