Ch.7 - Thermochemistry

Problem 42

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Chapter 7, Problem 42

A system absorbs 196 kJ of heat and the surroundings do 117 kJ of work on the system. What is the change in internal energy of the system?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. In a closed system, the change in internal energy is equal to the heat added to the system minus the work done by the system. This principle is fundamental for understanding energy conservation in thermodynamic processes.

Internal Energy

Internal energy is the total energy contained within a system, encompassing both kinetic and potential energy at the molecular level. It is a state function, meaning it depends only on the current state of the system, not on how it reached that state. Changes in internal energy are crucial for analyzing thermodynamic processes.

Work and Heat Transfer

Work and heat are two primary ways energy is transferred into or out of a system. Work is done on the system when external forces act on it, while heat is energy transferred due to a temperature difference. Understanding how to quantify these transfers is essential for calculating changes in internal energy using the First Law of Thermodynamics.

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Related Practice

Identify each energy exchange as primarily heat or work and determine whether the sign of ΔE is positive or negative for the system. a. Sweat evaporates from skin, cooling the skin. (The evaporating sweat is the system.) b. A balloon expands against an external pressure. (The contents of the balloon is the system.) c. An aqueous chemical reaction mixture is warmed with an external flame. (The reaction mixture is the system.) Identify energy exchanges as primarily heat or work. Determine whether the sign of E is positive or negative for the system.

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Textbook Question

Identify each energy exchange as primarily heat or work and determine whether the sign of ΔE is positive or negative for the system. Identify each energy exchange as primarily heat or work. a. A rolling billiard ball collides with another billiard ball. The first billiard ball (defined as the system) stops rolling after the collision. b. A book falls to the floor. (The book is the system). c. A father pushes his daughter on a swing. (The daughter and the swing are the system). Identify each energy exchange as primarily heat or work.

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Textbook Question

A system releases 622 kJ of heat and does 105 kJ of work on the surroundings. What is the change in internal energy of the system?

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Textbook Question

The air in an inflated balloon (defined as the system) warms over a toaster and absorbs 115 J of heat. As it expands, it does 77 kJ of work. What is the change in internal energy for the system?

We pack two identical coolers for a picnic, placing 24 12-ounce soft drinks and five pounds of ice in each. However, the drinks that we put into cooler A were refrigerated for several hours before they were packed in the cooler, while the drinks that we put into cooler B were at room temperature. When we open the two coolers three hours later, most of the ice in cooler A is still present, while nearly all of the ice in cooler B has melted. Explain this difference.

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Textbook Question

A kilogram of aluminum metal and a kilogram of water are each warmed to 75 °C and placed in two identical insulated containers. One hour later, the two containers are opened and the temperature of each substance is measured. The aluminum has cooled to 35 °C, while the water has cooled only to 66 °C. Explain this difference.

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