Question

unction of temperature, ponential functions Figure I shows measured vapor pressures of several volatile liquids as a function and can be considered as a small part of the overall phase diagram. Figure 1 The nonlinear plots are exponential which can be linearized to create Fimine 900- plotting the natural logarithm of vapor por In P. vs. 1/T after converting temperature, 7607 - 3457-----78.5/100.0/- Kelvin. garithm of vapor pressure, Figure 2 700-Diethyl ether Water Diethyl ether Vapor pressure (tor) Ethanol In P Water 100 0 100 20 40 60 80 Temperature (°C) The second type of plot is the basis for the Clausius-Clapeyron equation (Equation 1): (Equation 1) In P=(- (Equation 1) In P AC where: R is the universal gas constant of 8.314J/K.mol AH is in J/mol T is in Kelvin C is a constant This form of Equation 1 is very similar to others you will encounter later in your studies of chemistry The nature of the constant C will be explained more fully in discussions of changes in free energy and equilibrium constants. Since this is a linear equation, it has the general form y=mx+b, so it can be shown that any two points of related temperature and pressure for a given liquid can be described quantitatively by Equation 2: 272 Chapter 11: Phases of Matter

Answer 1

1. The normal boiling points of

(a) Diethyl ether = 34.5$\degree C$

(b) Ethanol = 78.5$\degree C$

(c) Water = 100$\degree C$

It is known from the graph of Figure 1. This is because the boiling occurs at atmospheric pressure which is 760 torr. Hence, temperature corresponding to 760 torr gives the boiling point for the respective liquids.

2. From the graph in Figure 1, a temperature constant line is dawn at 20$\degree C$. It can be obtained from there that ETHANOL has the highest vapour pressure at 20$\degree C$

3. In equation 1, the slope is :  $-\frac{\Delta H_{vap}}{R}$

4. Based on the second plot and my response to question3, it is clearly visible from the second plot that water has the greater slope than ethanol. Thus water has greater ${\Delta H_{vap}}$ than ethanol.