化学专业英语马永祥翻译36课

36

沸点和蒸馏Boiling

Points

of

Pure

Liquids

沸点的纯液体

Any

given

liquid,

when

admitted

into

a

closed

evacuated

space,

evaporates

until

the

vapor

attainsa

certain

definite

pressure,

which

depends

only

upon

the

temperature.

This

pressure,

which

is

the

pressure

exerted

by

the

vapor

in

equilibrium

with

the

liquid,

is

the

vapor

pressure

of

the

liquid

at

that

temperature.

As

the

temperature

increases

.

the

vapor

pressure

of

a

typical

liquid

x

increases

regularly

as

shown

by

the

generalized

vapor

pressure-temperature

curve

BC,

in

Fig.

5.

任何给定的液体,当进入一个封闭的疏散空间,蒸发到蒸汽形成某种明确的压力,仅仅依赖于温度。这种压力,这是由蒸汽压力与液体平衡的,是液体的蒸气压在温度。随着温度的升高。这个压力，是通过在与液体平衡的蒸汽施加的压力，是液体在该温度下的蒸汽压，其随着温度的增加而增加。一个典型液体x的蒸汽压的增加经常如图五所示，广义蒸汽压-温度曲线BC。At

the

temperature,

Tp,

where

the

vapor

pressure

reaches

101.3kPa.

x

begins

to

boil

and

Tp

is

called

the

NORMAL

BOILING

POINT

of

x.

Every

liquid

which

does

not

decompose

before

its

vapor

pressure

reaches

101.3kPa.

has

its

own

characteristic

boiling

point.

In

general,

the

boiling

point

of

a

substance

depends

upon

the

mass

of

its

molecules

and

the

strength

of

the

attractive

forces

between

them.

For

a

given

homologous

series,

the

boiling

points

of

the

member

compounds

rise

fairly

regularly

with

increasing

molecular

weight.

在温度、Tp,那里的蒸汽压力达到101.3

kPa。开始煮和x

Tp被称为正常沸点液体的x。在这个温度，当蒸汽压达到101.3

kPa时，液体x开始沸腾，这个蒸汽压力被叫做x的正常沸点。任何一种液体在它的蒸汽压达到101.3

kPa之前都不分解，每种液体的沸点都有其自身的特点。一般来说,一种物质的沸点取决于它的分子质量和它们之间的引力的强度。对于一个给定的同系物，其成员化合物的沸点随分子量的增加而增加。

Polar

liquids

tend

to

boil

higher

than

nonpolar

liquids

of

the

same

molecular

weight,

and

associated

polar

liquids

usually

boil

considerably

higher

than

nonassociated

polar

compounds.

极性液体的沸点往往高于相同分子量的非极性液体，而相关的极性液体的沸点远远高于一般的极性化合物。The

boiling

point

is

a

characteristic

constant

that

is

widely

used

in

the

identification

of

liquids.

Because

of

its

marked

dependence

upon

pressure

and

its

rather

erratic

response

to

impurities,

however,

it

is

generally

less

reliable

and

useful

in

characterization

and

as

a

criterion

of

purity

than

is

the

melting

point

for

solids1.

沸点是一个常数,广泛用于识别液体。然而，由于对压力的显著依赖性和对杂质的相当无规律的反应，沸点用于鉴别和作为纯度的判据方面，较之固体的熔点一般不太可信和较少使用。Boiling

Points

of

Solutions

沸点的解决方案

The

normal

boiling

point

of

any

solution

is

the

temperature

at

which

the

total

vapor

pressure

of

the

solution

is

equal

to

101.3kPa.

The

effect

of

any

solute,

Y,

on

the

boiling

point

of

X

will

depend,

then,

upon

the

nature

of

Y.

If

Y

is

less

volatile

than

X.

then

the

total

vapor

pressure

of

the

solution

is

lower,

at

any

given

temperature,

than

the

vapor

pressure

of

pure

X.

正常沸点的任何解决方案都是在其溶液总蒸汽压力等于101.3

kPa时的温度。任何溶质，Y在X上的沸点的效果将取决于Y的性质。如果Y比X难挥发。那么其总蒸汽压都比在任何给定的温度下纯X的蒸汽压低。

Such

a

case

is

represented

by

curve

B'C

,

in

which

the

experimentally

determined

values

tor

the

vapor

pressures

of

a

solution

are

plotted

against

temperature.

The

vapor

pressure

of

the

solution

does

not

reach

101.3kPa.

until

a

temperature

Tp'

is

attained.

In

other

words,

the

presence

of

the

less

volatile

solute

raises

the

boiling

point

of

X

from

Tp

to

Tp'.

A

solution

of

sugar

or

salt

in

water

is

a

familiar

example

of

this

type

of

solution.

根据溶质所确定实验值来绘制温度-蒸汽压力图，如曲线B'C所示。直到温度达到Tp'时，其溶液的蒸汽压才达到101.3

kPa。换句话说,较低挥发性溶质的存在使X的沸点从Tp提高到Tp’。这种类型的一个广为人知的例子就是糖或盐溶解在水中。Figure

5.

Generalized

vapor

pressure

diagrams

for

a

pure

liquid

(BO

,

for

a

solution

in

which

the

solute

is

less

volatile

than

the

solvent

(B'C),

and

for

a

solution

in

which

the

solute

is

more

volatile

than

the

solvent(B"C").

图5。广义水汽压图表示纯液体(BC,溶液中溶质比溶剂(B

'C)难挥发，以及另一溶液中含有更不稳定的溶剂(B“C”)。

On

the

other

hand,

if

Y

is

more

volatile

than

X.

then

the

total

vapor

pressure

of

the

solution

is

higher

than

that

of

pure

X,

as

shown

by

curve

B"C".

The

vapor

pressure

of

such

a

solution

reaches

101.3kPa.

at

temprature

Tp";

hence

the

effect

of

the

more

volatile

solute

is

to

lower

the

boiling

point

of

X

from

Tp

to

Tp".

A

solution

of

acetone

in

water

is

an

example

of

this

type.

另一方面,如果Y比X更易挥发。那么其溶液的总蒸汽压比纯X的高。,如图所示曲线B’’C’’所示。当一溶液的蒸汽压力达到101.3

kPa、温度达到Tp

",因此更多的挥发性溶质的作用是使X 的沸点从TP降低到TP’’。丙酮在水中的溶液就是这种类型的一个例子。

In

any

solution

of

two

liquids

X

and

Y,

the

molecules

of

X

are

diluted

by

molecules

of

Y,

and,

conversely,

the

molecules

of

Y

are

diluted

by

molecules

of

X.

You

would

therefore

expect

the

vapor

pressure

due

to

X

to

be

less

than

that

of

pure

X;

in

fact,

you

might

predict

that

the

PARTIAL

PRESSURE

due

to

X

would

be

proportional

to

the

molecular

concentration

of

X.

在任何溶液中的两种液体X和Y，X的分子被Y的分子稀释，相反,Y的分子被X的分子稀释。你因此会认为由此X的蒸汽压就小于纯的X。事实上,你可以预测,X的局部压力,与X的组分的浓度成正比。

Similarly,

the

partial

pressure

of

Y

might

be

expected

to

be

proportional

to

the

molecular

concentration

of

Y.

This

is,

in

fact,

the

relationship

which

holds

for

so

-called

ideal

solutions.

It

is

expressed

in

Raoult's

Law-the

partial

pressure

of

a

component

in

a

solution

at

a

given

temperature

is

equal

to

the

vapor

pressure

of

the

pure

substance

multiplied

by

its

mole

fraction

in

solution.

In

symbols,

for

a

solution

of

components

X

and

Y.

同样地,局部压力的Y也许被希望是与Y的分子浓度成正比。

.这是,事实上,这种关系可以使用与理想溶液。这由拉乌尔定律表述为：溶液中成分的局部压力等于在给定温度下的纯物质的蒸汽压乘以该物质在溶液中的摩尔分数。用符号X和Y表示溶液的组分。

Px=Px0Nx

where

Px

=

the

partial

pressure

of

X

in

solution,

Px=X在溶液中的分压Px0=

the

vapor

pressure

of

pure

X

at

that

temperature,

Px0

=纯X在该温度下的蒸汽压

Nx

=

the

mole

fraction

of

X

in

the

solution.Nx

=X在溶液中的摩尔分数Similarly.

ented

by

L1B.

Finally,

at

the

end

of

the

distillation,

the

liquid

phase

is

pure

toluene

boiling

at

110.6℃.

Similarly,

the

vapor

becomes

progressively

richer

in

toluene

also,

following

V1B.

Always,

however,

it

is

richer

in

benzene

than

is

the

liquid

with

which

it

is

in

equilibrium,

as

shown

by

the

points

of

intersection

of

any

horizontal

line

with

the

vapor

and

liquid

curves2.

在精馏过程中，随着L1B所表示的值，甲苯在液相中的浓度和沸点逐步增加。最后，在精馏结束时，液相是纯的甲苯，沸点为110.6℃。同样地，根据V1B曲线，甲苯蒸汽也随之变浓。然而，蒸汽较之其处于平衡的液体总是富含苯的，正如任一水平线与蒸汽和液体曲线的交点所示。Obviously,

a

single

simple

distillation

could

never

separates

20:80

molar

mixture

of

benzene

and

toluene

into

the

pure

components.

But

now

consider

what

would

be

accomplished

if

the

first

trace

of

vapor

formed

by

distillation

of

the

mixture

were

co