Chemical Combination, Valency and Simple Calculations for SS 1

Class: SS 1
Term: Second Term
Week: 7
Age: 15 years
Duration: 60 minutes
Subject: Chemistry
Curriculum Theme: The Chemical World
Focal competence: Demonstrating the knowledge of how the chemical laws regulate natural processes, interactions and the formation of natural compounds
Key competencies/values: Research and Problem Solving; Problem Solving
Skills:

• Handling (i.e. setting up) of apparatus to demonstrate the laws of chemical combination

Previous Lesson: Electronic Configuration of the First 20 Elements and Basic Atomic Terms
Topic: Chemical Combination
Subject Matter: Meaning of Chemical Combination, Valency of elements and their role in chemical combination. Solve simple word, problems involving chemical laws

Specific Objectives

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

Cognitive Domain

• Define Chemical Combination.
• Explain the laws of conservation of matter, constant composition, reciprocal proportion, and multiple proportions.
• Determine valency of elements from their atomic configuration.
• Explain the relationship between the valency and reactivity of elements to form new substances.
• Solve simple problems involving the laws of conservation of matter, constant composition, reciprocal proportion, and multiple proportions.

Psychomotor Domain

• Perform simple experiments to demonstrate chemical combination laws.

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
• The HeadTeacher Scheme of work For The New Revised Senior Secondary Education Curriculum (SSEC)
• School Chemistry textbook for Senior Secondary Schools
• Chemistry texts and workbooks

Instructional Materials

The teacher will teach this lesson with the aid of:

• Periodic Table chart
• Chart showing electronic configuration of elements
• Laboratory equipment for demonstrating the law of conservation of matter (e.g., conical flask, stopper, balance, dilute acid, marble chips)
• Whiteboard/Chalkboard and markers/chalk

Rationale for the Lesson

This lesson is foundational in chemistry, providing students with an understanding of how atoms interact to form molecules and compounds. It introduces the quantitative laws that govern chemical reactions, which are essential for predicting reaction outcomes and understanding the composition of substances. Grasping these concepts will enable students to comprehend more complex chemical processes in subsequent topics.

Prerequisite/Previous Knowledge

Students should have prior knowledge of atoms, elements, compounds, mixtures, and basic atomic structure including protons, neutrons, electrons, and electronic configuration.

Lesson Content/Board Summary

Chemical Combination

Meaning of Chemical Combination

Chemical combination is the process by which two or more elements or compounds react chemically to form a new substance with different properties. This process involves the rearrangement of atoms and the formation of new chemical bonds.

Laws of Chemical Combination

Chemical reactions are governed by fundamental laws that describe the quantitative relationships between reactants and products.

Law of Conservation of Matter (Mass)

This law states that in any closed system, the total mass of the reactants before a chemical reaction must be equal to the total mass of the products after the reaction. Matter cannot be created or destroyed, only rearranged.

Example: When 10g of calcium carbonate ((CaCO_3)) is heated, it decomposes to form 5.6g of calcium oxide ((CaO)) and 4.4g of carbon dioxide ((CO_2)).

(CaCO_3(s) rightarrow CaO(s) + CO_2(g))

Mass of reactant = 10g

Mass of products = 5.6g + 4.4g = 10g

Thus, mass is conserved.

Example 1

Question: 12g of magnesium reacts completely with oxygen to form 20g of magnesium oxide. How much oxygen reacted?

Solution:

Step 1: Write the balanced chemical equation (conceptual, for understanding).

(Magnesium + Oxygen rightarrow Magnesium text{ } oxide)

Step 2: Apply the Law of Conservation of Mass.

Mass of reactants = Mass of products

(12g text{ } (Mg) + Mass text{ } of text{ } Oxygen = 20g text{ } (MgO))

Step 3: Solve for the unknown mass.

(Mass text{ } of text{ } Oxygen = 20g – 12g = 8g)

Answer: (8g) of oxygen reacted.

Law of Constant Composition (Definite Proportions)

This law states that a given chemical compound always contains its component elements in fixed ratio by mass, regardless of its source or method of preparation.

Example: Water ((H_2O)) always contains hydrogen and oxygen in a mass ratio of 1:8. This means for every 1g of hydrogen, there are 8g of oxygen in water.

Example 2

Question: A sample of carbon dioxide ((CO_2)) weighing 22g contains 6g of carbon and 16g of oxygen. Another sample of carbon dioxide weighing 44g contains 12g of carbon and 32g of oxygen. Show that these data illustrate the Law of Constant Composition.

Solution:

Step 1: Calculate the mass ratio of Carbon to Oxygen for the first sample.

(Mass text{ } of text{ } Carbon : Mass text{ } of text{ } Oxygen = 6g : 16g = 3 : 8)

Step 2: Calculate the mass ratio of Carbon to Oxygen for the second sample.

(Mass text{ } of text{ } Carbon : Mass text{ } of text{ } Oxygen = 12g : 32g = 3 : 8)

Step 3: Compare the ratios.

Since the ratio of carbon to oxygen by mass is constant (3:8) in both samples of carbon dioxide, the data illustrates the Law of Constant Composition.

Answer: The mass ratio of carbon to oxygen is 3:8 in both samples, confirming the law.

Law of Multiple Proportions

This law states that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers.

Example: Carbon and oxygen form two compounds: carbon monoxide (CO) and carbon dioxide ((CO_2)).

1. In CO, 12g of carbon combines with 16g of oxygen.
2. In (CO_2), 12g of carbon combines with 32g of oxygen.

Fixing the mass of carbon at 12g, the masses of oxygen that combine with it are 16g and 32g. The ratio of these masses of oxygen is (16:32 = 1:2), which is a simple whole number ratio.

Example 3

Question: Nitrogen and oxygen form two oxides. In the first oxide, 28g of nitrogen combines with 16g of oxygen. In the second oxide, 28g of nitrogen combines with 32g of oxygen. Show how these data illustrate the Law of Multiple Proportions.

Solution:

Step 1: Identify the fixed mass of one element.

The mass of nitrogen is fixed at 28g in both oxides.

Step 2: Identify the masses of the other element that combine with the fixed mass.

Mass of oxygen in first oxide = 16g

Mass of oxygen in second oxide = 32g

Step 3: Find the ratio of these masses of oxygen.

(Ratio = 16g : 32g = 1 : 2)

Answer: The ratio of the masses of oxygen that combine with a fixed mass of nitrogen is 1:2, which is a simple whole number ratio, thus illustrating the Law of Multiple Proportions.

Law of Reciprocal Proportions

This law states that if two different elements combine separately with a fixed mass of a third element, the ratio of the masses in which they do so is either the same as or a simple multiple of the ratio of the masses in which they combine with each other.

Example: Consider hydrogen (H), oxygen (O), and sulphur (S).

1. Hydrogen combines with oxygen to form water ((H_2O)). 2g of H combines with 16g of O (ratio 1:8).
2. Hydrogen combines with sulphur to form hydrogen sulphide ((H_2S)). 2g of H combines with 32g of S (ratio 1:16).
3. Oxygen combines with sulphur to form sulphur dioxide ((SO_2)). 32g of S combines with 32g of O (ratio 1:1).

Fixing the mass of hydrogen at 2g:

Mass of O combining with 2g H = 16g

Mass of S combining with 2g H = 32g

Ratio of masses of O:S = (16:32 = 1:2)

Now, consider the ratio of masses of O and S when they combine with each other in (SO_2):

Mass of O : Mass of S = (32:32 = 1:1)

The ratio of (1:2) (from H) and (1:1) (from direct combination) are related by a simple multiple (2/1 = 2). This demonstrates the law.

Valency of Elements

Definition of Valency

Valency is the combining power of an element. It represents the number of hydrogen atoms that an atom of an element can combine with or displace. Alternatively, it is the number of electrons an atom needs to gain, lose, or share to achieve a stable electron configuration (usually an octet or duplet).

Determining Valency from Atomic Configuration

The valency of an element is primarily determined by the number of electrons in its outermost shell (valence electrons) and its tendency to achieve a stable electron configuration (like that of a noble gas).

1. Elements with 1, 2, or 3 valence electrons: Tend to lose these electrons to form positive ions. Their valency is 1, 2, or 3 respectively.
   1. Sodium (Na): Electronic configuration 2, 8, 1. Loses 1 electron, valency = 1.
   2. Magnesium (Mg): Electronic configuration 2, 8, 2. Loses 2 electrons, valency = 2.
   3. Aluminium (Al): Electronic configuration 2, 8, 3. Loses 3 electrons, valency = 3.
2. Elements with 5, 6, or 7 valence electrons: Tend to gain electrons to complete their octet. Their valency is 3, 2, or 1 respectively (8 – number of valence electrons).
   1. Nitrogen (N): Electronic configuration 2, 5. Gains 3 electrons, valency = 3.
   2. Oxygen (O): Electronic configuration 2, 6. Gains 2 electrons, valency = 2.
   3. Chlorine (Cl): Electronic configuration 2, 8, 7. Gains 1 electron, valency = 1.
3. Elements with 4 valence electrons: Tend to share 4 electrons to complete their octet. Their valency is 4.
   1. Carbon (C): Electronic configuration 2, 4. Shares 4 electrons, valency = 4.
4. Elements with 8 valence electrons (Noble Gases, except Helium with 2): Have a stable octet and do not readily combine. Their valency is 0.

Role of Valency in Chemical Combination

Valency dictates the ratio in which atoms combine to form stable chemical compounds. Atoms combine in such a way that their valencies are satisfied, leading to a neutral compound. The “criss-cross” method is often used to determine the chemical formula:

1. Write the symbols of the elements side by side.
2. Write their valencies as superscripts.
3. Criss-cross the valencies (swap them) as subscripts.
4. Simplify the subscripts to the lowest whole number ratio.

Example: Formation of Magnesium Oxide

1. Symbols: Mg O
2. Valencies: (Mg^{2+} O^{2-}) (Magnesium valency 2, Oxygen valency 2)
3. Criss-cross: (Mg_2O_2)
4. Simplify: (MgO)

Example: Formation of Aluminium Oxide

1. Symbols: Al O
2. Valencies: (Al^{3+} O^{2-}) (Aluminium valency 3, Oxygen valency 2)
3. Criss-cross: (Al_2O_3)
4. Simplify: (Al_2O_3) (already in simplest ratio)

Teaching Methods/Instructional Techniques

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

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Questioning/Recall

Teacher’s Activity: The teacher greets the students and asks questions to review previous knowledge on elements, compounds, and atomic structure. For example: “What is an element?” “What is the outermost shell of an atom called?”

Pupils’ Activity: Pupils respond to the questions, recalling concepts of elements and atomic shells.

Learning Point: Activating prior knowledge

Step 2: Meaning of Chemical Combination

Time: 10 minutes

Teaching Skill: Explanation/Discussion

Teacher’s Activity: The teacher explains the meaning of chemical combination, giving simple examples like the formation of water from hydrogen and oxygen. The teacher guides students to discuss why elements combine.

Pupils’ Activity: Pupils listen, participate in the discussion, and contribute examples of chemical combinations.

Learning Point: Definition of chemical combination

Step 3: Laws of Chemical Combination (Conservation of Matter and Constant Composition)

Time: 15 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher introduces the Laws of Chemical Combination. The teacher explains and demonstrates the Law of Conservation of Matter using a simple experiment (e.g., reacting marble chips with dilute acid in a stoppered conical flask on a balance). The teacher then explains the Law of Constant Composition with examples like water and carbon dioxide.

Pupils’ Activity: Pupils observe the demonstration, listen to explanations, and ask questions for clarification. They discuss in groups to state the laws.

Learning Point: Understanding conservation and constant composition

Step 4: Laws of Chemical Combination (Multiple and Reciprocal Proportions)

Time: 10 minutes

Teaching Skill: Explanation/Examples

Teacher’s Activity: The teacher explains the Law of Multiple Proportions using examples like carbon monoxide and carbon dioxide. The teacher then explains the Law of Reciprocal Proportions with suitable examples involving three elements.

Pupils’ Activity: Pupils listen, take notes, and discuss the examples provided by the teacher.

Learning Point: Understanding multiple and reciprocal proportions

Step 5: Valency of Elements

Time: 10 minutes

Teaching Skill: Explanation/Guided Practice

Teacher’s Activity: The teacher defines valency and explains how to determine it from the electronic configuration of elements using the Periodic Table. The teacher provides examples (Na, O, Cl, C) and guides students to determine the valency of other elements.

Pupils’ Activity: Pupils define valency, determine valencies of elements, and explain its role in compound formation.

Learning Point: Valency and its determination

Step 6: Solving Problems

Time: 5 minutes

Teaching Skill: Problem Solving/Application

Teacher’s Activity: The teacher works through simple problems involving the laws of chemical combination, demonstrating step-by-step solutions on the board. The teacher encourages students to attempt similar problems.

Pupils’ Activity: Pupils observe the problem-solving process and attempt to solve simple problems.

Learning Point: Applying chemical laws to problems

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 chemical combination.
2. State the Law of Conservation of Matter.
3. How is valency determined from electronic configuration?
4. If 10g of hydrogen reacts with 80g of oxygen to form water, what is the mass of water formed?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Assessment of understanding

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 chemical combination, laws, and valency 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 importance of chemical combination laws and valency in understanding chemical reactions.

Pupils’ Activity: Pupils listen and ask any final questions.

Learning Point: Consolidating key concepts

Continuous Assessment/Further Study

Type: Homework/Practice Exercise

Instruction: Answer the following questions in your notebook.

1. Explain the Law of Constant Composition with an example.
2. Determine the valency of the following elements given their atomic numbers:
   1. Potassium (K, Z=19)
   2. Sulphur (S, Z=16)
   3. Phosphorus (P, Z=15)
3. Nitrogen and oxygen form two compounds. In the first compound, 7g of nitrogen combines with 4g of oxygen. In the second compound, 7g of nitrogen combines with 8g of oxygen. Show how these data illustrate the Law of Multiple Proportions.
4. Using the criss-cross method, write the chemical formula for the compound formed between:
   1. Calcium (valency 2) and Chlorine (valency 1)
   2. Aluminium (valency 3) and Sulphur (valency 2)

Lesson Keywords

• Chemical Combination – The process where elements or compounds react to form new substances.
• Valency – The combining power of an element, determined by its outermost electrons.
• Law of Conservation of Matter – States that mass is neither created nor destroyed in a chemical reaction.
• Law of Constant Composition – States that a compound always has elements in fixed mass ratios.
• Law of Multiple Proportions – States that if two elements form more than one compound, the masses of one element combining with a fixed mass of the other are in simple whole number ratios.
• Law of Reciprocal Proportions – Relates the ratios of masses of two elements combining separately with a third element to their own combining ratio.

Differentiation

For students who grasp concepts quickly, provide additional complex problems involving the chemical laws or challenge them to research historical context of these laws. For students needing more support, offer simplified examples, provide a chart of common valencies, and guide them through step-by-step problem-solving with more direct assistance.

Suggested Lesson Videos

For further understanding, search on YouTube for:

• chemical combination laws SS1 chemistry
• valency and electronic configuration chemistry