Gas Laws, Charles’ Law, Celsius–Kelvin Conversion and Avogadro’s Law for SS 1

Class: SS 1
Term: Third Term
Week: 1
Age: 15 years
Duration: 60 minutes
Subject: Chemistry
Curriculum Theme: The chemical world
Focal competence: Illustrating the gas laws with appropriate equations and solving problems related to the laws
Key competencies/values: Critical Thinking
Skills:

• Deriving the gas equation from statements of gas laws

Previous Lesson: Petroleum Or Crude Oil: Petrochemicals, Octane Number and LNG
Topic: Gas Laws
Subject Matter: Meaning of Gas Laws, Charles law, Conversion between Celsius scale and kelvin scale, Kinetic theory and the behaviour of gases, Graham Law of Diffusion GayLussac’s law, Avogadro’s law

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define Gas Laws.
• State the various gas laws (Charles’, Gay-Lussac’s, Graham’s, Avogadro’s).
• Explain the Kelvin Scale of temperature and its relationship to the Celsius Scale.
• Relate the behaviour of gases to the Kinetic theory.

Affective Domain

• Appreciate the importance of gas laws in industrial processes.
• Demonstrate curiosity in understanding the principles governing gas behaviour.

Psychomotor Domain

• Express the gas laws as equations relating temperature, pressure, and volume of gases.
• Solve simple mathematical problems involving the gas laws.
• Illustrate Graham’s Law of diffusion using experimental materials.

Social Domain

• Collaborate effectively during group activities and discussions.

Reference Materials

The following resources were used in planning this lesson:

• 2025 New Revised Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• A suitable Chemistry textbook for SS 1
• The HeadTeacher Scheme of work For The New Revised Senior Secondary Education Curriculum (SSEC)

Instructional Materials

The teacher will teach this lesson with the aid of:

• Pictures/Charts on the applications of gas laws in industrial processes.
• Cotton wool
• Ammonia solution
• Concentrated HCl solution
• Thermometer
• Balloons
• Measuring cylinder
• Stopwatch

Rationale for the Lesson

Understanding gas laws is fundamental to comprehending the physical properties and behaviour of gases, which are ubiquitous in our environment and industrial applications. This lesson provides the foundational knowledge for further studies in chemistry and physics, enabling students to predict and explain phenomena involving gases.

Prerequisite/Previous Knowledge

Students should have a basic understanding of matter, states of matter, temperature, pressure, and volume from their Junior Secondary School science lessons.

Lesson Content/Board Summary

Gas Laws

Meaning of Gas Laws

Gas laws are a set of scientific laws that describe the relationships between the macroscopic properties of gases, such as pressure, volume, temperature, and amount (number of moles). These laws were developed over centuries of observations and experiments to explain the behaviour of ideal gases.

Charles’ Law

Charles’ Law states that for a fixed mass of gas at constant pressure, the volume of the gas is directly proportional to its absolute temperature (temperature in Kelvin).

Mathematically, this can be expressed as:

(V propto T)

Or

(frac{V}{T} = k) (where k is a constant)

For two different states of the same gas:

(frac{V_1}{T_1} = frac{V_2}{T_2})

Where:

1. (V_1) = initial volume
2. (T_1) = initial absolute temperature
3. (V_2) = final volume
4. (T_2) = final absolute temperature

This means that as the temperature of a gas increases, its volume also increases, assuming the pressure remains constant.

Conversion Between Celsius Scale and Kelvin Scale

The Kelvin scale (absolute temperature scale) is the standard temperature scale used in gas laws because it starts at absolute zero (0 K), which is the lowest possible temperature where molecular motion theoretically stops. The Celsius scale is a relative scale where 0 °C is the freezing point of water.

Relationship:

To convert temperature from Celsius (°C) to Kelvin (K), add 273.15 (often rounded to 273) to the Celsius temperature.

Formula:

(T(K) = T(°C) + 273)

Where:

1. (T(K)) = Temperature in Kelvin
2. (T(°C)) = Temperature in Celsius

Example 1

Question: Convert 25 °C to Kelvin.

Solution:

Step 1: Write the formula.

(T(K) = T(°C) + 273)

Step 2: Substitute the values.

(T(K) = 25 + 273)

Step 3: Simplify and write the answer.

(T(K) = 298 K)

Answer: (298 K)

Example 2

Question: Convert 373 K to Celsius.

Solution:

Step 1: Rearrange the formula to solve for Celsius.

(T(°C) = T(K) – 273)

Step 2: Substitute the values.

(T(°C) = 373 – 273)

Step 3: Simplify and write the answer.

(T(°C) = 100 °C)

Answer: (100 °C)

Kinetic Theory of Gases and the Behaviour of Gases

The Kinetic Theory of Gases provides a microscopic explanation for the macroscopic behaviour of gases. Its main postulates are:

1. Gases consist of a large number of tiny particles (atoms or molecules) that are in continuous, random motion.
2. The volume occupied by the gas particles themselves is negligible compared to the total volume of the container.
3. There are no significant attractive or repulsive forces between gas particles.
4. Collisions between gas particles and with the walls of the container are perfectly elastic (no energy loss).
5. The average kinetic energy of the gas particles is directly proportional to the absolute temperature of the gas.

How it explains gas behaviour:

1. Pressure: Gas particles constantly collide with the walls of the container. The force exerted by these collisions per unit area is the pressure of the gas.
2. Temperature: Temperature is a measure of the average kinetic energy of the gas particles. Higher temperature means faster-moving particles and thus more frequent and forceful collisions, leading to higher pressure or volume.
3. Volume: Gases expand to fill their containers because their particles are in constant random motion and there are large spaces between them. Increasing temperature increases kinetic energy, causing particles to move faster and further apart, leading to expansion (e.g., a balloon expanding when heated).

Graham’s Law of Diffusion

Graham’s Law of Diffusion states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its molar mass (or density), at constant temperature and pressure.

Mathematically, this can be expressed as:

(Rate propto frac{1}{sqrt{M}})

Or

(frac{Rate_1}{Rate_2} = sqrt{frac{M_2}{M_1}})

Where:

1. (Rate_1) = rate of diffusion of gas 1
2. (Rate_2) = rate of diffusion of gas 2
3. (M_1) = molar mass of gas 1
4. (M_2) = molar mass of gas 2

This means lighter gases diffuse faster than heavier gases. For example, ammonia (NH₃, M = 17 g/mol) diffuses faster than hydrogen chloride (HCl, M = 36.5 g/mol).

Gay-Lussac’s Law (Pressure Law)

Gay-Lussac’s Law states that for a fixed mass of gas at constant volume, the pressure of the gas is directly proportional to its absolute temperature (temperature in Kelvin).

Mathematically, this can be expressed as:

(P propto T)

Or

(frac{P}{T} = k) (where k is a constant)

For two different states of the same gas:

(frac{P_1}{T_1} = frac{P_2}{T_2})

Where:

1. (P_1) = initial pressure
2. (T_1) = initial absolute temperature
3. (P_2) = final pressure
4. (T_2) = final absolute temperature

This means that as the temperature of a gas increases, its pressure also increases, assuming the volume remains constant.

Avogadro’s Law

Avogadro’s Law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules (or moles).

Mathematically, this can be expressed as:

(V propto n)

Or

(frac{V}{n} = k) (where k is a constant)

For two different states of the same gas:

(frac{V_1}{n_1} = frac{V_2}{n_2})

Where:

1. (V_1) = initial volume
2. (n_1) = initial number of moles
3. (V_2) = final volume
4. (n_2) = final number of moles

This law implies that at standard temperature and pressure (STP: 0 °C and 1 atm), one mole of any gas occupies a volume of 22.4 dm³ (molar volume).

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Observation, Problem Solving, Group Work.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Questioning/Recalling

Teacher’s Activity: The teacher greets the students and reviews previous knowledge by asking questions about the states of matter and properties of gases. The teacher then introduces the topic of Gas Laws, explaining that they describe how gases behave under different conditions.

Pupils’ Activity: Pupils respond to questions about states of matter and listen attentively to the introduction of the new topic.

Learning Point: Introduction to Gas Laws

Step 2: Meaning of Gas Laws and Charles’ Law

Time: 10 minutes

Teaching Skill: Explanation/Definition

Teacher’s Activity: The teacher explains the general meaning of Gas Laws and then introduces Charles’ Law, stating its definition and mathematical expression. The teacher provides simple examples of its application.

Pupils’ Activity: Pupils listen, define Gas Laws, state Charles’ Law, and copy the definition and equation into their notebooks.

Learning Point: Charles’ Law principles

Step 3: Conversion Between Celsius Scale and Kelvin Scale

Time: 10 minutes

Teaching Skill: Explanation/Problem Solving

Teacher’s Activity: The teacher explains the importance of the Kelvin scale in gas laws and demonstrates how to convert temperatures between Celsius and Kelvin using the formula. The teacher works through the provided examples on the board.

Pupils’ Activity: Pupils pay attention, copy the conversion formula, and practice solving the example problems.

Learning Point: Celsius-Kelvin conversion

Step 4: Kinetic Theory and the Behaviour of Gases

Time: 10 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher explains the postulates of the Kinetic Theory of Gases and relates them to the observable behaviour of gases (pressure, temperature, volume). The teacher guides students to blow up a balloon and discusses how the expansion relates to the kinetic theory (Activity 2).

Pupils’ Activity: Pupils listen, blow up balloons, observe, and discuss the relationship between balloon expansion and kinetic theory.

Learning Point: Kinetic Theory postulates

Step 5: Graham’s Law of Diffusion

Time: 10 minutes

Teaching Skill: Demonstration/Observation

Teacher’s Activity: The teacher introduces Graham’s Law, stating its definition and mathematical expression. The teacher then demonstrates Graham’s Law using cotton wool soaked in ammonia and concentrated HCl solutions, observing the formation of ammonium chloride closer to the HCl end (Activity 1).

Pupils’ Activity: Pupils listen, state Graham’s Law, observe the demonstration carefully, and discuss the implications regarding diffusion rates of gases.

Learning Point: Graham’s Law application

Step 6: Gay-Lussac’s Law and Avogadro’s Law

Time: 5 minutes

Teaching Skill: Explanation/Comparison

Teacher’s Activity: The teacher introduces Gay-Lussac’s Law and Avogadro’s Law, stating their definitions and mathematical expressions. The teacher highlights the conditions under which each law applies.

Pupils’ Activity: Pupils listen, state Gay-Lussac’s Law and Avogadro’s Law, and note their respective equations.

Learning Point: Other Gas Laws

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define Gas Laws.
2. State Charles’ Law.
3. Convert 50 °C to Kelvin.
4. Mention two postulates of the Kinetic Theory of Gases.
5. State Graham’s Law of Diffusion.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding of Gas Laws

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on Gas Laws, Charles’ Law, Celsius-Kelvin conversion, Kinetic Theory, Graham’s Law, Gay-Lussac’s Law, and Avogadro’s Law into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson content

Step 9: Conclusion

Time: 5 minutes

Teaching Skill: Summarisation

Teacher’s Activity: The teacher briefly summarises the main points of the lesson, reinforcing the definitions and relationships between the gas laws and the Kinetic Theory. The teacher encourages students to review the concepts learned.

Pupils’ Activity: Pupils listen attentively and ask any final questions for clarification.

Learning Point: Consolidation of concepts

Continuous Assessment/Further Study

Type: Homework/Practice Exercise

Instruction: Answer the following questions in your notebook:

1. A gas occupies a volume of 200 cm³ at 27 °C. What will be its volume at 77 °C if the pressure remains constant?
2. State Avogadro’s Law and write its mathematical expression.
3. Explain how the Kinetic Theory of Gases accounts for the pressure exerted by a gas.
4. If gas A diffuses at a rate of 10 cm/s and has a molar mass of 4 g/mol, what is the molar mass of gas B if it diffuses at 5 cm/s under the same conditions?
5. Convert the following temperatures: a) 0 °C to Kelvin, b) 300 K to Celsius.

Lesson Keywords

• Gas Laws – Scientific laws describing relationships between gas properties.
• Charles’ Law – Volume proportional to absolute temperature at constant pressure.
• Kelvin Scale – Absolute temperature scale, 0 K is absolute zero.
• Kinetic Theory of Gases – Explains gas behaviour based on particle motion.
• Graham’s Law of Diffusion – Rate of diffusion inversely proportional to square root of molar mass.
• Gay-Lussac’s Law – Pressure proportional to absolute temperature at constant volume.
• Avogadro’s Law – Equal volumes of gases contain equal moles at same temperature and pressure.

Differentiation

Support: For students who struggle with mathematical problems, provide additional step-by-step guidance for temperature conversions and simple gas law calculations. Use visual aids and analogies to explain the Kinetic Theory more concretely.

Extension: Challenge advanced students to research the Ideal Gas Law and its derivation from the individual gas laws, or to investigate real-world applications of gas laws in engineering or medicine.

Suggested Lesson Videos

For further understanding, students can search on YouTube for:

• “Gas Laws for SS1 Chemistry”
• “Charles Law explanation and examples”
• “Celsius to Kelvin conversion”
• “Kinetic Theory of Gases simplified”
• “Graham’s Law of Diffusion experiment”