Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Define matter and its properties.
• Explain the laws of chemical combination.
• Calculate empirical and molecular formulae.
• Write and balance chemical equations.
• Determine the percentage composition of elements.

Lesson summary

Welcome, future scientists and engineers! Today, we delve into one of the most powerful tools in chemistry: Stoichiometry. Think of it as the 'recipe' for chemical reactions. Just like a good cook needs to know the exact amount of flour, sugar, and eggs to bake a perfect cake, a chemist must know the exact amounts of reactants needed to produce a desired amount of product. In Ghana, this is not just theory. From the manufacturing of vital medicines at pharmaceutical companies in Accra, to producing fertilizers for our cocoa farms, to the food processing industries making our favourite drinks and snacks, stoichiometry is the principle that ensures efficiency, safety, and profitability.

Lesson notes

This lesson builds upon your knowledge of the mole, molar mass, and writing chemical formulae. We will now apply these concepts to understand the quantitative nature of chemical reactions. A. The Foundation: Laws of Chemical Combination

Three fundamental laws govern how substances combine chemically: Law of Conservation of Mass: In any ordinary chemical reaction, matter is neither created nor destroyed. The total mass of the reactants must equal the total mass of the products. This is why we must balance chemical equations. Law of Constant Proportion (or Definite Proportions): A chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or method of preparation. For example, water (H₂O) is *always* made of 2 parts hydrogen and 16 parts oxygen by mass (a ratio of 1:8). Law of Multiple Proportions: If two elements 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. E.g., in water (H₂O) and hydrogen peroxide (H₂O₂), the mass of oxygen that combines with 2g of hydrogen is 16g and 32g respectively, a simple 1:2 ratio. B. Balancing Chemical Equations

A balanced equation has the same number of atoms of each element on both the reactant (left) side and the product (right) side. This upholds the Law of Conservation of Mass.

Rules/Steps to Balance Equations: Write the correct formulae for all reactants and products. Count the atoms of each element on both sides of the arrow. Use coefficients (numbers placed *in front* of the chemical formulae) to balance the atoms. Start with the most complex compound or elements that appear in only one reactant and one product. Balance polyatomic ions (like SO₄²⁻, PO₄³⁻) as a single unit if they appear unchanged on both sides. Balance hydrogen and oxygen atoms last, as they often appear in multiple compounds. Final Check: Do a final count to ensure all elements are balanced and that the coefficients are in their simplest whole-number ratio.

Evaluation guide

• Perform calculations involving stoichiometric relationships .
• Experiential Learning :
• Work in small mixed ability groups to perform experiments on some common reactions i n the
• Problem -Based Learning: In a class activity,