Diffusion

Question 1

What is diffusion?

Answer 1

spread of particles of a substance due to thermal motion

Question 2

What is flux?

Give the formula.

Unit?

Answer 2

flux = J_V = how much volume of a fluid passes through a cross section at a given time)

• J_V = flow density per time = Δv / (Δt * ΔA)
• in [mol/(m²*s)]

Question 3

Knowing what flux is explain what Fick’s first law states.

Why does it happen?

Give the formula.

Answer 3

happens to equilibrate 2 different concentrations

Fick’s first law:

flux is proportional to the concentration drop (hence 2 media with different concentrations)

J_V = -D * Δc/Δx

• D = diffusion coefficient
• Δc/Δx = concentration drop = concentration change at a unit distance

Question 4

What is the diffusion coefficient D?

Explain on what it depends giving its formula.

Answer 4

gives the amount of material that diffuses through a unit area at a unit time at a unit concentration drop

• in [m²/s]

D = kT / (6πηr)

depends on:

• size (πr) + shape of diffusing particle
• viscosity (η) + temperature (T) of medium

Question 5

What does Fick’s second law state?

Answer 5

predicts how diffusion causes the concentration to change with time

D * Δt * Δ(Δc/Δx) / Δx + c(t) = c(t + Δt)

if the spatial distribution of the concentration is known at a given time it gives the distribution at a later time t + Δt

Question 6

With which method can the amount of diffused material in our measurement be calculated?

Why?

Answer 6

because K⁺ and Cl^- ions diffusing form the gel cause the distilled water to become an electrolyte thus conductivity can be measured

⇒ conducitivity directly proportional to concentration of the solution

Question 7

Explain the relation of conductivity to resistance using a formula.

What is its unit?

Answer 7

reciprocal of resistance R

1/R = 1/ρ * A/l = σ * A/l

• ρ = resistivity
• σ = conductivity
• in [Sm^-1]

⇒ conductivity = conductance in a solution btw 2 electrodes of 1m2 at 1m distance

Question 8

What is a cell constant?

How can it be determined?

Answer 8

calibration constant of every measuring cell

can be determined with calibration solution of known conductivity

• A/l = C/σ
• depends on length and cross section of the measuring cell