1st Term Week 9 Lesson Note: Chemical Combination For SS 1

 

Class: Senior Secondary School 1 (SSS 1)

Subject: Chemistry
Topic: Chemical Combination
Sub-Topic: The Laws of Chemical Combination
Duration: 40 Minutes

1. Introduction to Chemical Combination

When different elements react with each other to form chemical compounds, they do not combine at random. They do so in an orderly manner according to specific chemical rules.

These rules are known as the Laws of Chemical Combination. These fundamental laws govern how matter reacts and transforms quantitatively, laying the foundation for modern chemical equations and calculations (stoichiometry).

2. The Four Fundamental Laws of Chemical Combination

A. The Law of Conservation of Mass

This law was formulated by the French chemist Antoine Lavoisier in 1789.

• Statement: The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction, but can only be transformed from one form to another.

• Implication: In any closed chemical system, the total mass of the reactants before the reaction must equal the total mass of the products after the reaction.

• Mathematical Expression:

  Total Mass of Reactants = Total Mass of Products Experimental Verification: Heating a known mass of silver nitrate solution with sodium chloride solution inside a sealed flask produces a white precipitate of silver chloride, but the total mass measured on a balance remains completely unchanged before and after mixing.

B. The Law of Definite Proportions (Constant Composition)

This law was proposed by the French chemist Joseph Proust in 1799.

• Statement: The Law of Definite Proportions states that all pure samples of a particular chemical compound always contain the same elements combined together in the same fixed proportion by mass.

• Implication: The source or method of preparation of a pure chemical compound does not matter. It will always have the exact same chemical makeup by weight.

• Example: Pure water (H2O) can be obtained from rain, a borehole in Ughelli, or synthesized in a laboratory. Regardless of its origin, it will always contain Hydrogen and Oxygen combined in a strict mass ratio of 1:8 (since 2g of Hydrogen combines with 16g of Oxygen).

C. The Law of Multiple Proportions

This law was formulated by the English chemist John Dalton in 1803.

• Statement: The Law of Multiple Proportions states that if two elements 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.

• Implication: Elements can combine in different fixed patterns, but those patterns always change in distinct, whole steps.

• Example: Carbon and Oxygen can combine to form two different gases:

  • Carbon Monoxide (CO): 12g of Carbon combines with 16 of Oxygen.

  • Carbon Dioxide (CO2): 12g of Carbon combines with 32g of Oxygen.

  • Comparing the masses of Oxygen (16g and 32 g) that combine with a fixed mass of Carbon (12g), the ratio is 16:32 =1:2, which is a simple whole-number ratio.

D. The Law of Reciprocal Proportions

This law was proposed by Jeremias Richter in 1792.

• Statement: The Law of Reciprocal Proportions states that the weights of two elements which combine separately with a fixed weight of a third element are either the same or are in a simple multiple ratio of the weights in which they combine with each other.

• Example: Consider three elements: Carbon (C), Hydrogen (H), and Oxygen (O).

  • Carbon combines with Hydrogen to form Methane (CH4).

  • Oxygen combines with Hydrogen to form Water (H2O).

  • The weights of Carbon and Oxygen that react with a fixed weight of Hydrogen can be used to mathematically predict the exact whole-number weight ratio when Carbon and Oxygen directly combine to form Carbon Dioxide (CO2).

3. Summary Table of the Laws

Law	Primary Scientist	Key Focus	Key Concept
Conservation of Mass	Antoine Lavoisier	Weight before & after	Mass is unchanged
Definite Proportions	Joseph Proust	Elements inside a single compound	Fixed ratio by mass
Multiple Proportions	John Dalton	Two elements forming multiple compounds	Simple whole-number ratios
Reciprocal Proportions	Jeremias Richter	Three elements interacting together	Interlocking weight ratios

Evaluation Exercises (For Classwork/Assignment)

1. State the Law of Conservation of Mass and explain its significance in balancing chemical equations.

2. Two samples of copper oxide were analyzed. Sample A contained 2.0 g} of copper combined with 0.5{ g} of oxygen. Sample B contained 4.0 g} of copper combined with 1.0 g of oxygen. Show whether these results illustrate the Law of Definite Proportions.

3. Differentiate clearly between the Law of Definite Proportions and the Law of Multiple Proportions.

4. Why is John Dalton's atomic theory closely linked to the validation of these laws?