Kinetic chemistry

1. Pharmaceutical Analytical chemistry (1)
Prepared by
Staff members of Analytical Chemistry Department

2. Gas diffusion and effusion
 Gaseous atoms are in constant random motion.
 Gaseous particles tend to diffuse as they have kinetic energy.
 Gas motion tends to be higher at higher temperature as the kinetic energy tends to be high at
elevated temperature.
 Effusion refers to motion of gaseous particles at small holes.
 Diffusion refers to diffusion of particles from higher concentration to lower concentration.
 Graham’s law: Diffusion or effusion of gaseous particles is inversely proportional to square root of
its molecular weight.

3. What’s the difference?
• Diffusion
• Gradual mixing of two gases.
• Effusion
• Process where gas particles inside a closed container escape through a
small opening.

4. The passage of a gas under pressure through a tiny opening.

5. Figure . Diffusion of particles from high concentration to lower
concentration.

6. The image on the left shows effusion, where the image on the right shows
diffusion. Effusion occurs through an orifice smaller than the mean free path
of the particles in motion, whereas diffusion occurs through an opening in
which multiple particles can flow through simultaneously.

7. Graham’s Law of Effusion
• Rates of effusion of gases (at the same T and P) are inversely proportional
to the square root of their molar masses.
 Graham's law states that the rate of diffusion or of effusion of a gas is inversely
(indirectly) proportional to the square root of its molecular weight.
 Thus, if the molecular weight of one gas is four times that of another, it would
diffuse through a porous plug or escape through a small pinhole in a vessel at
half the rate of the other (heavier gases diffuse more slowly than lighter ones).
RateA
Rate B
=
Molar MassB
Molar MassA


8. Rates
• Depends on the speed of a gas molecule.
• The speed of a gas molecule varies with the mass
• Lighter molecules move faster that heavier molecules (at the same temperature)
Q1:Compare the rates of effusion of H2 and O2 at the same temperature
and pressure.
Q2: A sample of H2 effuses through a porous container about 9 times
faster than an unknown gas. Estimate the molar mass of the unknown
gas.

9. Q3: At the same temperature, which molecule travels faster, O2 or N2? How much faster?
Q4: What is the molar mass of a gas if it diffuses 0.907 times the speed of argon gas?
N2 (lower molar mass)
(32/28) = 1.07 times faster
1/0.907 = (x/39.9)
(1/0.907)2 = ((x/39.9))2
1.216 = x/39.9
x = 48.5 g/mol

10. 6. How many milliliters of 5.5 M NaOH are needed to prepare 300 ml of 1.2 M
NaOH?
Answer: M1V1= M2V2
5.5× V1= 300×1.2 = 0.065 L V1= 65 ml
9. How many grams of SnCl2.2H2O are needed to prepare 75 ml of 0.25 M solution?
[Atomic weights: Sn= 23, O= 16, H=1, Cl=35.5
Answer:
SnCl2. 2 H2O = 118.7 + 2 (35.5) + 2 (2+16) = 225.7 it is molecular weight
Molarity (M) = weight (g)/M.wt× volume (L)
Weight= molarity× M.wt× volume (L) =0.25× 225.7×75/1000=4.3 g

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