Conservation of Mass

The Fundamental Law

The Law of Conservation of Mass states that matter is neither created nor destroyed in a chemical reaction. This principle is foundational in chemistry and ensures that the total mass of reactants equals the total mass of products in a closed system.

• Closed System: No matter can enter or leave; mass is strictly conserved.

• Open System: Mass may appear to change due to escape of gases, but the law still holds when all products are accounted for.

• Atomic Level: Atoms are rearranged, not destroyed or created. Every atom present at the start must be present at the end.

Example: If a reaction starts with 10g of methane and oxygen, the resulting carbon dioxide and water vapor will weigh exactly 10g in a closed system.

Balancing Chemical Equations

Quantitative Equality

Balancing chemical equations is essential to reflect the conservation of mass. The number of atoms for each element must be equal on both sides of the equation.

• Subscripts: Small numbers within a chemical formula (e.g., the 2 in H2O) indicate the ratio of elements in a molecule. Changing subscripts changes the chemical identity.

• Coefficients: Large numbers in front of a formula (e.g., the 2 in 2H2O) indicate the number of molecules. Only coefficients are used to balance equations.

Golden Rule: Never change subscripts when balancing equations. Altering subscripts changes the substance itself.

Example: Water vs. Hydrogen Peroxide

• H2O: Water, safe to drink.

• H2O2: Hydrogen peroxide, a bleach/poison.

The Balancing Strategy: Inventory Method

Balancing equations requires a systematic approach to ensure all atoms are accounted for:

1. List Elements: Identify every element present on both sides of the equation.

2. Initial Count: Count the number of atoms for each element.

3. Add Coefficients: Start with the most complex molecule or the most unbalanced atom.

4. Recount & Repeat: Update the inventory after each change.

5. Final Check: Ensure all ratios are simplified to the lowest possible integers.

Worked Example: Methane Combustion

Expert Tips for Balancing Equations

• Save H & O for Last: Balance oxygen and hydrogen last, as they often appear most frequently and may self-balance after other elements are set.

• Polyatomic Ions: If a polyatomic ion (e.g., SO4) appears on both sides, treat it as a single unit for faster balancing.

• Simplification: Always check if coefficients can be divided to achieve the lowest ratio (e.g., 2:2:4 becomes 1:1:2).

Summary Table: Subscripts vs. Coefficients

Additional info: The notes are directly relevant to "Chemical Reactions" and "Quantities in Chemical Reactions" chapters, covering conservation of mass, balancing equations, and the use of subscripts and coefficients.