Unit 3 - Worksheet 1 Energy Bar Charts | Lecture notes Chemistry ... - Free Printable

Problem Analysis:

The worksheet asks us to use energy bar charts to represent the changes in energy storage and flow for three different scenarios. We need to consider thermal energy (\(E_{th}\)), potential energy (\(E_{ph}\)), and chemical energy (\(E_{ch}\)) in each case. Additionally, we must describe how the arrangement and motion of molecules change from the initial to the final state.

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Solution:

#### 1. A cup of hot coffee cools as it sits on the table.

- Initial State:
- The coffee is hot, so it has a high amount of thermal energy (\(E_{th}\)).
- There is no significant change in potential energy (\(E_{ph}\)) or chemical energy (\(E_{ch}\)) since the coffee's composition remains unchanged.
- Energy Bar Chart:
- \(E_{th}\): High
- \(E_{ph}\): Low (unchanged)
- \(E_{ch}\): Low (unchanged)

- Final State:
- As the coffee cools, its thermal energy decreases because heat is transferred to the surroundings.
- Potential energy and chemical energy remain unchanged.
- Energy Bar Chart:
- \(E_{th}\): Low
- \(E_{ph}\): Low (unchanged)
- \(E_{ch}\): Low (unchanged)

- Energy Flow:
- Heat flows out of the coffee into the surroundings.

- Molecular Changes:
- Initial: Molecules in the coffee are moving rapidly due to high thermal energy.
- Final: Molecules slow down as thermal energy decreases.

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#### 2. A can of cold soda warms as it is left on the counter.

- Initial State:
- The soda is cold, so it has low thermal energy (\(E_{th}\)).
- There is no significant change in potential energy (\(E_{ph}\)) or chemical energy (\(E_{ch}\)).
- Energy Bar Chart:
- \(E_{th}\): Low
- \(E_{ph}\): Low (unchanged)
- \(E_{ch}\): Low (unchanged)

- Final State:
- As the soda warms, its thermal energy increases because heat is absorbed from the surroundings.
- Potential energy and chemical energy remain unchanged.
- Energy Bar Chart:
- \(E_{th}\): High
- \(E_{ph}\): Low (unchanged)
- \(E_{ch}\): Low (unchanged)

- Energy Flow:
- Heat flows into the soda from the surroundings.

- Molecular Changes:
- Initial: Molecules in the soda are moving slowly due to low thermal energy.
- Final: Molecules speed up as thermal energy increases.

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#### 3. A tray of water (20°C) is placed in the freezer and turns into ice cubes (-8°C).

- Initial State:
- The water is liquid at 20°C, so it has moderate thermal energy (\(E_{th}\)).
- There is no significant change in potential energy (\(E_{ph}\)) or chemical energy (\(E_{ch}\)).
- Energy Bar Chart:
- \(E_{th}\): Moderate
- \(E_{ph}\): Low (unchanged)
- \(E_{ch}\): Low (unchanged)

- Final State:
- The water freezes into ice at -8°C. During freezing, thermal energy decreases, and some of this energy is converted into potential energy due to the ordered arrangement of water molecules in the solid state.
- Energy Bar Chart:
- \(E_{th}\): Low
- \(E_{ph}\): High (due to increased order in the solid state)
- \(E_{ch}\): Low (unchanged)

- Energy Flow:
- Heat flows out of the water into the freezer.

- Molecular Changes:
- Initial: Water molecules are in a liquid state, moving freely with moderate thermal energy.
- Final: Water molecules form a rigid, ordered structure in the solid state (ice), with reduced thermal energy and increased potential energy.

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#### 4. Where does the energy that leaves the system in #3 go? How does this energy transfer affect the room temperature in the kitchen? Do you have any experience that supports your answer?

- Where does the energy go?
- The energy that leaves the system in scenario #3 (freezing water) goes into the freezer and eventually into the surrounding environment (the kitchen).

- Effect on Room Temperature:
- When the freezer removes heat from the water to freeze it, the heat is expelled into the kitchen through the freezer's cooling mechanism (e.g., condenser coils).
- This causes the room temperature in the kitchen to increase slightly.

- Experience:
- If you've ever felt the warm air coming from the back or sides of a refrigerator or freezer, you've experienced this phenomenon. The heat removed from the inside is released into the surrounding area, warming it up.

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Final Answers:

1. Coffee Cooling:
- Initial: \(E_{th}\) (High), \(E_{ph}\) (Low), \(E_{ch}\) (Low)
- Final: \(E_{th}\) (Low), \(E_{ph}\) (Low), \(E_{ch}\) (Low)
- Energy Flow: Heat flows out of the coffee.
- Molecular Changes: Molecules slow down.

2. Soda Warming:
- Initial: \(E_{th}\) (Low), \(E_{ph}\) (Low), \(E_{ch}\) (Low)
- Final: \(E_{th}\) (High), \(E_{ph}\) (Low), \(E_{ch}\) (Low)
- Energy Flow: Heat flows into the soda.
- Molecular Changes: Molecules speed up.

3. Water Freezing:
- Initial: \(E_{th}\) (Moderate), \(E_{ph}\) (Low), \(E_{ch}\) (Low)
- Final: \(E_{th}\) (Low), \(E_{ph}\) (High), \(E_{ch}\) (Low)
- Energy Flow: Heat flows out of the water into the freezer.
- Molecular Changes: Molecules transition from a liquid to a solid state, reducing thermal energy and increasing potential energy.

4. Energy Transfer and Room Temperature:
- The energy that leaves the system goes into the freezer and then into the kitchen.
- This causes the room temperature in the kitchen to increase slightly.
- Personal experience supports this, as refrigerators and freezers release heat into the surrounding area.

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Final Answer Box:
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