Heat Transfer

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Physical Chemistry › Heat Transfer

Questions 1 - 3

A chemical reaction is run in which $545 \textup{ J}$ of work is done by the system and the internal energy changes by $-811\textup{ J}$ . What is the total amount of heat transferred?

$-266\textup{ J}$

$266\textup{ J}$

$-837 \textup{ J}$

$837\textup{ J}$

$-26\textup{ J}$

Explanation

The First Law of thermodynamics states that for a system that only exchanges energy by heat or work:

$\Delta E = w + q$

Work is done by the system, therefore

$\Delta E = -545\textup{ J}$

$q = \Delta E - w$

$q=-266\textup{ J}$

A chemical reaction is run in which $466\textup{ J}$ of heat is absorbed and the internal energy changes by $212\textup{ J}$ . What is the amount of work done?

$-254\textup{ J}$

$254\textup{ J}$

$-678\textup{ J}$

$678\textup{ J}$

$-166\textup{ J}$

Explanation

The First Law of thermodynamics states that for a system that only exchanges energy by heat or work:

$\Delta E = w + q$

Heat is absorbed by the system, therefore

$q = 466 \textup{ J}$

So $w = \Delta E - q$

$w=212\textup{ J}-466\textup{ J} = -254\textup{ J}$ done by the system.

An automobile engine provides $522\textup{ J}$ of work to push the pistons and generates $2239\textup{ J}$ of heat that must be carried away by the cooling system. What is the change in the internal energy of the engine?

$-2761\textup{ J}$

$2761\textup{ J}$

$1717\textup{ J}$

$-1717\textup{ J}$

$-8341\textup{ J}$

Explanation

The First Law of thermodynamics states that for a system that only exchanges energy by heat or work:

$\Delta E = w + q$

The heat is given off by the system, so $q = -2239 \textup{ J}$ . Similarly, work is being done by the system, so $w=-522 \textup{ J}$
$\Delta E = -522\textup{ J} -2239\textup{ J}=-2761\textup{ J}$

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