Chapter 15, Problem 79A

a.

Interpretation Introduction

Interpretation:

The sign of enthalpy change of the system $C_2{H}_{5}OH(s)\to C_2{H}_{5}OH(l)$ needs to be written.

Concept introduction:

The sign of enthalpy change of the system depends on the type of reaction.

Answer to Problem 79A

The sign of enthalpy change for the system:

$C_2{H}_{5}OH(s)\to C_2{H}_{5}OH(l)$

is positive.

Explanation of Solution

The state of the system is changing from solid to liquid which means that heat is being added to the system. Hence the enthalpy change is positive (work done on the system).

b.

Interpretation Introduction

Interpretation:

The sign of enthalpy change of the system $H_{2}O(g)\to H_{2}O(l)$ needs to be written.

Concept introduction:

The sign of enthalpy change of the system depends on the type of reaction.

Answer to Problem 79A

The sign of enthalpy change for the system:

$H_{2}O(g)\to H_{2}O(l)$

is negative.

Explanation of Solution

The state of the system is changing from gas to liquid which means that heat is escaping from the system. Hence the enthalpy change is negative(work done by the system).

c.

Interpretation Introduction

Interpretation:

The sign of enthalpy change of the system $C{H}_{3}OH(l)\to C{H}_{3}OH(g)$ needs to be written.

Concept introduction:

The sign of enthalpy change of the system depends on the type of reaction.

Answer to Problem 79A

The sign of enthalpy change for the system:

$C{H}_{3}OH(l)\to C{H}_{3}OH(g)$

is positive.

Explanation of Solution

The state of the system is changing from liquid to gas which means that heat is added to the system. Hence the enthalpy change is positive (work done on the system).

a.

Interpretation Introduction

Interpretation:

The sign of enthalpy change of the system $N{H}_3(l)\to N{H}_3(s)$ needs to be written.

Concept introduction:

The sign of enthalpy change of the system depends on the type of reaction.

Answer to Problem 79A

The sign of enthalpy change for the system:

$N{H}_3(l)\to N{H}_3(s)$

is negative.

Explanation of Solution

The state of the system is changing from solid to liquid which means that heat is released by the system. Hence the enthalpy change is negative (work done by the system).