Chemistry: The Molecular Science
Chemistry: The Molecular Science
5th Edition
ISBN: 9781285199047
Author: John W. Moore, Conrad L. Stanitski
Publisher: Cengage Learning
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Chapter 9, Problem 37QRT
Interpretation Introduction

Interpretation:

The amount of mass of CCl2F2 must evaporate to freeze 2 mole of water initially at 20C has to be calculated.

Concept Introduction:

Calculation of heat energy:

When both the phases are same means a transition is taking place within the same phase like between liquid to liquid.  Then, ΔH0=m×s×Δθ.

Where, m= mass of substance, s= Specific heat of substance in a particular phase and Δθ= Temperature difference.

When two phases are different means a transition is taking place between two different phases like between solid to liquid.  Then, ΔH0=n×ΔfusorvapH0.

Where, n= No. of moles of substance , ΔfusH0= Fusion enthalpy of a substance in its solid phase and ΔvapH0= enthalpy of vaporization of a substance in its liquid phase.

Expert Solution & Answer
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Answer to Problem 37QRT

The amount of mass of CCl2F2 must evaporate to freeze 2 mole of water initially at 20C is 51.9g.

Explanation of Solution

Given data:

The specific heat capacity of liquid water is 4.184Jg1C1.  The fusion of enthalpy for solid ice is 6.02kJ/mol. The vaporization enthalpy of CCl2F2 is 289J/g.

Calculation of mass of water:

  No. of moles (n)=mMm=n×M=2mol×18.0152g/mol=36.0304g36g.

  1. 1. Cool the water from 20oC to 0oC

The heat energy evolved during this process can be calculated as given below.

  ΔH0=m×s×Δθ=(36g)(4.184Jg1C1)[(0C)(20C)]=3012.48J=3012.48J×1kJ1000J=3.01248kJ3.0kJ.

Where, m= mass of water, s= Specific heat of liquid water and Δθ= Temperature difference.

  1. 2. Freeze the ice at 0oC

The heat energy evolved during this process can be calculated as given below.

  ΔcrystH=ΔfreezeH=ΔfusH=6.02kJ/mol

  ΔH0=(n)×(ΔfusH0)=(2mol)×(6.020kJmol)=12.040kJ12.0kJ.

Where, n= No. of moles of water and ΔfusH0= Fusion enthalpy of solid ice.

Now, the total heat energy evolved during freezing of 2 mole of water from 20C is the sum total of all the heat energies required for each transition:

  ΔH=3.0kJ12kJ=-15kJ.

The heat energy required to evaporate one gram of CCl2F2 is 289J.  So the amount of CCl2F2 will be evaporated by 15kJ energy can be calculated as given below.

  15kJ×1000J1kJ×1g289J=51.9g.

Therefore, the amount of mass of CCl2F2 must evaporate to freeze 2 mole of water initially at 20C is 51.9g.

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