O Level Chemistry: Particulate Nature of Matter (Kinetic Particle Theory) Part 1

Last Sunday, i met the new group of students for the 2010 Sun Sec 3 Chemistry Class under Ultimate Leap Programme.

Since it was the 1st session, i decided to start with a very basic topic known as Particulate Nature of Matter, otherwise known as Kinetic Particle Theory to many. This should be something that most students would have learned and mastered in Sec 1/2 in their secondary schools.

To my surprise, many of the students were not conversant in this topic when i gave them the Pre-Test!

Today, i am going to share with you the overview checklist on what you need to know about this topic as well as discuss on one very important concept that are frequently tested in GCE ‘O’ Levels / IGCSE / IB / IP Chemistry Exams.

Overview Checklist on what you need to know:

1. Recognise & draw the particulate models for Solids, Liquids & Gases
2. Describe the arrangement & movement of the particles in the 3 States of Matter
3. Explain why Gases can be compressed, but not Solids & Liquids
4. Describe what happens to the particles during melting, boiling, condensation and freezing
5. Interpret Temp / Time Graphs for Changes of States
6. State the connection between relative molecular mass of a gas & its rate of diffusion
7. State some examples of diffusion of gases in everyday life

Of the above, the favourite question that examiners like to ask is “connection between relative molecular mass of a gas & its rate of diffusion“. Let’s take a look at what we need to know in this section.

In diffusion, the particles of one substance move between the particles of another substance. This happens in both liquids and gases. [Note: for GCE ‘O’ Levels – only Diffusion in Gases will be tested].

Example: When a perfume is sprayed and released into the air, the particles of the perfume spread through the air by moving between the particles of the air. The perfume is said to diffuse through the air.

Speed of diffusion depends alot on the temperature. The higher the temperature, the faster the molecules move and the faster the rate of diffusion. For Physics students, you normally hear the following: higher the temperature, the higher the kinetic energy!

However, molecules/atoms of different gases move at different speeds, even at the same temperature. They do NOT move at the same speed! The average rate of diffusion of the particular gas molecules/atoms is related to the mass of the particles involved.

Quick Tips:

Smaller the Mass -> Faster they move -> Faster diffusion rate

Larger the Mass -> Slower they move -> Slower diffusion rate

{Analogy: “Fatter i am , the slower i run“}

Masses of different gas molecules can be compared by their relative molecular masses.

Let’s check out one example.

Example:
Which gas will diffuse slower – Nitrogen Dioxide (NO2) or Sulfur Dioxide (SO2)?

Working:
Mr of NO2 = 46
Mr of SO2 = 64

Conclusion:
Nitrogen Dioxide (NO2) has a smaller relative molecular mass than oxygen, so on average its molecules will diffuse much faster than those of Sulfur Dioxide (SO2).

Alright, with that concept, you can try out the following exam-based question on your own to see how much you have learned.

Quick Check 1:
Which statement explains why carbon monoxide, CO, and ethene, C2H4, diffuse at the same rate?
A. They both contain the element, carbon
B. They both react with oxygen
C. They are both gases at room temperature
D. They have the same relative molecular mass

&

Quick Check 2:
Samples of gases, methane, CH4, and hydrogen, H2, are at the same temperature. Compare the speeds of the molecules in these two gases.

Go try it out!!! =)

PS: Do write down your answers & working in the Comments Section below. I will respond with my suggested answers shortly.

Related Articles:

• O Level Chemistry – Rate of Reaction Mini Series Part 2
• O Level Chemistry: Difference between Atom, Molecule & Particle
• O Level Chemistry – Rate of Reaction Mini Series Part 1
• O Level Chemistry: Mole Concepts / Chemical Calculations
• O Levels Chemistry – Selected Exam Questions Cont’d