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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145당신이 사용하는 게 아니라요?교과서 변경
모든 교과서McMurry 8th EditionCh.9 - Thermochemistry: Chemical Energy문제 35a
9장, 문제 35a

Imagine a reaction that results in a change in both volume and temperature: (a) Has any work been done? If so, is its sign positive or negative?

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1. In the context of chemistry, work is done when a system expands or contracts against an external pressure. If the volume of the system changes, it means that work has been done.
2. The sign of the work done depends on the direction of the volume change. If the system expands (volume increases), it does work on the surroundings, and the work is considered negative. This is because the system is losing energy to do the work. In contrast, if the system contracts (volume decreases), work is done on the system by the surroundings, and the work is considered positive because the system is gaining energy.
3. The change in temperature does not directly indicate whether work has been done or not. However, it can be related to the change in internal energy of the system. If the temperature increases, the internal energy of the system increases and vice versa. But this change in internal energy can be due to work done on or by the system, heat transferred to or from the system, or both.
4. To determine the exact amount of work done, you would need more information such as the external pressure and the exact change in volume. The work done (w) can be calculated using the formula w = -PΔV, where P is the external pressure and ΔV is the change in volume.
5. Remember, these signs are based on the convention used in chemistry. In physics, the signs may be reversed. Always pay attention to the context and the convention being used.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Work in Thermodynamics

In thermodynamics, work is defined as the energy transferred when a force is applied over a distance. For a gas, work can be calculated using the formula W = -PΔV, where P is pressure and ΔV is the change in volume. If the volume increases (expansion), work is done by the system and is considered negative, while if the volume decreases (compression), work is done on the system and is positive.
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First Law of Thermodynamics

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle implies that the change in internal energy of a system (ΔU) is equal to the heat added to the system (Q) minus the work done by the system (W): ΔU = Q - W. Understanding this relationship is crucial for analyzing energy changes in reactions involving volume and temperature changes.
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가이드 코스
01:18
First Law of Thermodynamics

Sign Convention for Work

The sign convention for work in thermodynamics is essential for determining the direction of energy transfer. When a system does work on its surroundings (expansion), the work is considered negative, indicating energy leaving the system. Conversely, when work is done on the system (compression), it is positive, reflecting energy entering the system. This convention helps clarify the energy dynamics during reactions that involve changes in volume and temperature.
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가이드 코스
01:52
Work Function Calculation Example
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교과서 질문

Imagine a reaction that results in a change in both volume and temperature:

(b) Has there been an enthalpy change? If so, what is the sign of ∆H? Is the reaction exothermic or endothermic?

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교과서 질문

Redraw the following diagram to represent the situation (b) when work has been lost by the system.

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교과서 질문

Redraw the following diagram to represent the situation (a) when work has been gained by the system and

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교과서 질문
A piece of dry ice (solid CO2) is placed inside a balloon, and the balloon is tied shut. Over time, the carbon dioxide sub- limes, causing the balloon to increase in volume. Give the sign of the enthalpy change and the sign of work for the sublima- tion of CO2.
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