Diffusion -Research of gas diffusion

Date ： May.26th, 2011 Speaker ： Chen,Yang,

Deng

Purpose

1. Improve the diffusion experiment in high school chemistry course, to prevent the excessive inhalation of irritant gases

2. Measure the gas diffusion rate to calculate the molecular weights of HCl and NH3 gas, and compare them with the theoretical values

3. Discuss the influent

factors of gas diffusion

rate

Principle of gas diffusion experiments (1) ＊ Gas diffusion rate (R) : the volume or mole of

the diffusion gas per second 

R = volume of diffusion gas / time

or R = mole of diffusion gas / time

Principle of gas diffusion experiments(2)

＊ Graham’s law ： Under the condition of the same temperature and pressure, the gas diffusion rate is proportional to the reciprocal of the square root of the gas molecular weight.

R1/ R2= /

R1 ： The diffusion rate of gas 1 R2 ： The diffusion rate of gas 2 M1 ： The molecular weight of the gas 1 M2 ： The molecular weight of the gas 2

1M 2M

Principle of gas diffusion experiments(3)

＊ When the temperature and pressure of gas are different, the diffusion rate  can be derived as ＊ Two different gases (assumed to be ideal

gas) are compared： or = constant

P1 ： Partial pressure of gas 1 P2 ： Partial pressure of gas 1

t1 ： Diffusion time of gas 1 t2 ： Diffusion time of gas 1

2

22

1

11

M

tp

M

tP

M

P

1

2

2

1

t

t

R

R

TM

P R

Original experiment of gas diffusion

NH3(g) + HCl(g) → NH4Cl(s)

In this experiment, the gas diffuses in the glasstube, so the diffusion rate can be expressed as

R = diffusion distance(r) of gas / time(t)The diffusion times of the two different gases are the

same.

NH4OH(aq) HCl(aq)

)

NH4Cl(s)

Gas diffusion rate

According to the Graham’s law ： R1/ R2= /

and

R = diffusion distance(r) of gas / time(t)

t is the sameM1/2 r = constant

M ： Molecular Weight

1M 2M

Improved experiment

Experimental diagram

Glass tube (30cm)↑

volumetric flask

10cmuniversal

indicator paper

Result

gas  diffusion

050

100150200250

300350

10 15 20 25

temperature(℃)

diff

usio

n tim

e(s)

Ammonia

Hydrochloricacid

t(s)vaper

PM √ M Pt/√M

16 580 174.1231

062250.7

31

12 580 174.1231

061688.0

48

10 580 174.1231

061406.7

07

18 580 174.1231

062532.0

72

Ammonia  Result at 15 ℃

t(s)vaper

PM √ M Pt/√M

168 100 36.56.0415

232780.7

56

106 100 36.56.0415

231754.5

24

85 100 36.56.0415

231406.9

3

162 100 36.56.0415

232681.4

43

Hydrochloric acid Result at 15 ℃

t(s)vaper

PM √ M Pt/√M

5 580 174.1231

06703.35

33

10 580 174.1231

061406.7

07

14 580 174.1231

061969.3

89

15 580 174.1231

062110.0

6

Ammonia  Result at 15.5 ℃

Hydrochloric acid Result at 15.5 ℃

t(s)vaper

PM √ M Pt/√M

54 100 36.56.0415

23893.81

44

70 100 36.56.0415

231158.6

48

74 100 36.56.0415

231224.8

57

95 100 36.56.0415

231572.4

51

Ammonia  Result at 17 ℃

t(s)vaper

PM √ M Pt/√M

14 580 174.1231

061969.3

89

12 580 174.1231

061688.0

48

11 580 174.1231

061547.3

77

9 580 174.1231

061266.0

36

Hydrochloric acid Result at 17 ℃

t(s)vaper

PM √ M Pt/√M

80 100 36.56.0415

231324.1

69

95 100 36.56.0415

231572.4

51

73 100 36.56.0415

231208.3

05

74 100 36.56.0415

231224.8

57

0

500

1000

1500

2000

2500

15 15.5 17

temperature(℃)

Pt/√

M Ammonia

Hydrochloricacid

Comparison

Conclusion

1. We try to improve the diffusion experiment, but the experimental results although are slightly improved, but still not as good as expect, maybe some factors are not taken into account.

2. It takes about 158 seconds for HCl(g) to pass 10cm of the tube  at 17 , but only ℃14 seconds for NH3(g). The experimental data do not match the Graham’s law.

Conclusion

3. The saturated vapor pressures of concentrated HCl(aq) and NH3(aq) are much different, so they will affect the diffusion rate in the glass tube in the experiments.

So, adding the factor of saturated vapor pressure to the diffusion law, the experimental data are more consistent with the theoretical data.

Thank for your listening!