01.08 Diffusion in Fluids

1. Introduction to Diffusion

Diffusion is a fundamental process in chemistry where particles move from an area of higher concentration to an area of lower concentration until they are evenly distributed. This movement occurs due to the random motion of particles.

Key Concepts:

• Movement Direction: Particles move from regions of higher concentration to lower concentration.
• Equilibrium: Eventually, particles are evenly spread, and concentration is uniform throughout the medium.
• Rate of Diffusion:
  • Gases: Diffuse quickly due to high particle speed and greater spacing.
  • Liquids: Diffuse slower than gases because particles are closer together.
  • Solids: Diffusion does not occur as particles are fixed in place.

Example:

When a drop of ink is placed in water, it spreads out until the color is uniform throughout the water.

2. Dissolving and Diffusion

Dissolving is a process where a solute (solid, liquid, or gas) disperses uniformly in a solvent (usually a liquid), forming a solution.

Example: Dissolving Potassium Manganate(VII) in Water

1. Initial Stage:
   • A crystal of potassium manganate(VII) is placed at the bottom of a dish of water.
   • The surrounding water becomes purple as the solid begins to dissolve.
2. Diffusion Process:
   • Particles move from the solid’s surface into the water.
   • Over time, the purple color spreads throughout the dish, indicating even distribution.
3. Complete Solution:
   • The crystal fully dissolves, and the solution is uniformly purple.

Significance:

• The spread of solute particles in the solvent is an example of diffusion.
• In biological systems, diffusion is crucial for processes like oxygen transport from lungs to blood and carbon dioxide removal.

3. Diffusion of Gases

Gaseous particles move rapidly and spread out to occupy the entire available space. This behavior is governed by the kinetic theory of matter.

Key Points:

• High-Speed Movement: Gas molecules travel at approximately 1,800 km/hour.
• Frequent Collisions: Gas particles collide millions of times per second, resulting in random paths.
• Pressure: Gas pressure arises from these collisions with container walls.

Example: Bromine Gas in a Jar

1. Setup:
   • A few drops of liquid bromine are placed in a gas jar and sealed.
2. Process:
   • Bromine easily vaporizes, spreading as brown gas throughout the jar.
3. Observation:
   • After some time, the entire jar contains brown bromine gas, illustrating gas diffusion.

Importance:

• Sensing Smells: Diffusion allows odor molecules to reach our noses.
• Breathing: Facilitates oxygen and carbon dioxide exchange in the lungs.

4. Factors Affecting the Rate of Diffusion

a. Mass of Particles:

• Heavier Molecules: Move more slowly than lighter ones at the same temperature.
• Example: In an experiment, ammonia (Mₐ = 17) diffuses faster than hydrochloric acid (Mₐ = 36.5), resulting in the smoke ring forming closer to the hydrochloric acid source.

b. Temperature:

• Higher Temperature: Increases particle speed, enhancing diffusion rate.
• Lower Temperature: Slows down particles, reducing diffusion rate.

c. Medium of Diffusion:

• Gases: Diffuse faster due to greater particle speed and lower density.
• Liquids: Diffuse slower as particles are closer together.
• Solids: No diffusion as particles are fixed in place.

d. Size of Molecules:

• Smaller Molecules: Diffuse more quickly than larger ones.
• Example: Hydrogen gas (smallest molecule) diffuses faster than nitrogen or oxygen.

5. Experimental Investigation of Diffusion in Liquids

Objective:

To demonstrate diffusion in liquids through the formation of an insoluble precipitate when ions meet in a solution.

Materials:

• Petri dish
• Tweezers
• White tile or paper
• Silver nitrate crystal
• Potassium iodide crystal
• Distilled or deionized water
• Test tubes
• Silver nitrate solution
• Potassium iodide solution
• Dropping pipettes
• Eye protection

Safety Precautions:

• Wear eye protection.
• Handle crystals with tweezers.
• Do not ingest chemicals.
• Wash hands after handling.
• Dispose of silver nitrate solution properly (do not pour down the drain).

Procedure:

1. Preparation:
   • Place a Petri dish on a white tile or paper and fill it nearly to the top with deionized water.
2. Adding Crystals:
   • Using tweezers, place a crystal of silver nitrate on one side of the dish and a crystal of potassium iodide on the opposite side.
3. Observation:
   • Watch as the crystals dissolve and a new compound forms, indicated by a precipitate.
4. Result:
   • The precipitate (ammonium chloride) forms where the ions meet, demonstrating diffusion.

Questions to Consider:

1. What is the precipitate formed in this reaction?
   • Answer: Silver iodide (AgI) is formed as a precipitate.
2. Write a word equation for the reaction.
   • Answer: Silver nitrate solution + Potassium iodide solution → Silver iodide (precipitate) + Potassium nitrate solution.
3. What factors control where the solid is formed in the Petri dish?
   • Answer: The concentration gradients and the rate of diffusion of each ion determine where the precipitate forms.
4. Why does the solid not form exactly in the middle of the dish?
   • Answer: Because silver nitrate and potassium iodide diffuse at different rates due to differences in their molecular masses.

6. Kinetic Particle Theory and States of Matter

The kinetic particle theory explains the behavior of particles in different states of matter based on their movement and arrangement.

States of Matter:

1. Solid:
   • Particles are tightly packed in a fixed structure.
   • Particles vibrate but do not move from place to place.
   • Properties: Definite shape and volume, incompressible, no diffusion.
2. Liquid:
   • Particles are close together but can move past each other.
   • Properties: Definite volume, takes the shape of the container, slight compressibility, limited diffusion.
3. Gas:
   • Particles are far apart and move freely at high speeds.
   • Properties: No definite shape or volume, highly compressible, rapid diffusion.

Key Points:

• Particle Movement: In solids, minimal; in liquids, moderate; in gases, rapid.
• Energy: Higher in gases than in liquids and solids.
• Diffusion Rates: Gases > Liquids > Solids.

Example Activity:

• Modeling Particles: Use marbles to represent particles in different states. Arrange them in a tight grid for solids, loosely for liquids, and spread out for gases. Observe how they move when energy (heat) is added.

7. Practical Applications of Diffusion

a. Breathing:

• Oxygen Diffusion: From lungs to blood.
• Carbon Dioxide Diffusion: From blood to lungs.

b. Sensing Smells:

• Odor molecules diffuse through the air to reach our noses.

c. Industrial Processes:

• Gas mixing in chemical reactors.
• Perfume spreading in a room.

8. Sample Questions and Answers

Question 8:

A small amount of liquid bromine is placed in a gas jar, which is then sealed with a lid. Evaporation of the liquid bromine takes place.

Bri(l) → Br₂(g)

Explain why, after about an hour, the gaseous bromine molecules have spread to evenly occupy the whole container using the kinetic theory.

Answer: According to the kinetic theory, bromine molecules move rapidly and collide frequently with each other and the container walls. These high-speed, random motions cause the bromine gas to spread out uniformly, occupying the entire available space in the container.

Question 9:

A teacher conducted a diffusion demonstration using methylamine (Mₐ = 31) and hydrochloric acid (Mₐ = 36.5), replacing ammonia with methylamine.

a. Where would you predict the position of the smoke ring to be in this experiment? Explain your answer.

Answer: The smoke ring would form closer to the hydrochloric acid source. Since methylamine (Mₐ = 31) is lighter than hydrochloric acid (Mₐ = 36.5), methylamine diffuses faster, allowing hydrochloric acid to catch up and react, forming the precipitate nearer to the acid source.

b. Suggest other gases similar to hydrochloric acid that could replace it in this demonstration.

Answer: Hydrogen chloride (HCl) gas is similar to hydrochloric acid and could replace it. Other examples include sulfur dioxide (SO₂) or nitrogen dioxide (NO₂), which are also acidic gases.

Question 10:

Experiments comparing the rate of diffusion of different gases can be done using a porous pot apparatus.

Explain why the levels of liquid change when hydrogen is placed outside the porous pot cylinder containing air.

Answer: When hydrogen gas is placed outside the porous pot, it begins to diffuse into the pot, replacing some of the air inside. Since hydrogen is lighter than air, it diffuses rapidly, causing a decrease in air pressure inside the pot. This pressure difference results in the liquid levels in the side tube changing as hydrogen enters the pot.

9. Key Vocabulary

• Diffusion: Movement of particles from high to low concentration.
• Solute: Substance dissolved in a solvent.
• Solvent: Substance that dissolves the solute.
• Precipitate: Insoluble solid formed from a reaction in a solution.
• Kinetic Particle Theory: Theory that explains the behavior of particles in different states of matter based on their motion and energy.
• Porous Pot: A container with small pores allowing gas molecules to pass through.
• Volatile: Easily vaporized at normal temperatures.
• Equilibrium: State where concentrations are uniform, and no net movement of particles occurs.

10. Activity: Modeling States of Matter

Objective: Create a model to represent particles in solids, liquids, and gases, demonstrating their movement and arrangement.

Materials:

• Balls or marbles (to represent particles)
• Tray or dish
• Transparent cover (optional)

Procedure:

1. Solids:
   • Arrange marbles in a tight, fixed grid on the tray.
   • Represent minimal movement with small vibrations.
2. Liquids:
   • Scatter marbles loosely, allowing them to move past each other.
   • Demonstrate fluid movement by gently shifting the tray.
3. Gases:
   • Spread marbles widely across the tray.
   • Move the tray rapidly to show high-speed, random motion.

Discussion Points:

• Why do solids have definite shapes?
• Why can liquids flow but solids cannot?
• Why do gases expand to fill their containers?

11. Summary

• Diffusion is the movement of particles from higher to lower concentration, essential in various natural and industrial processes.
• States of Matter (solid, liquid, gas) differ in particle arrangement and movement, affecting diffusion rates.
• Kinetic Particle Theory explains the behavior of particles based on their energy and motion.
• Experimental Investigations help visualize and understand diffusion processes.
• Practical Applications of diffusion include biological processes like breathing and everyday phenomena like scent distribution.