IB MYP 4-5 Chemistry -Law of conservation of mass- Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Law of conservation of mass- Study Notes
Key Concepts
- Law of Conservation of Mass
Law of Conservation of Mass
Law of Conservation of Mass
The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. During any chemical change, the total mass of the reactants equals the total mass of the products.
\( \mathrm{Total\ mass_{reactants} = Total\ mass_{products}} \)
In other words: Atoms are only rearranged to form new substances, but the number and type of atoms remain constant before and after the reaction.
Historical Background
This law was first proposed by Antoine Lavoisier in 1789, known as the “Father of Modern Chemistry.” He showed through careful experiments that when substances react in a closed container, the total mass remains unchanged — even if the substances appear very different afterward.
Explanation at the Particle Level
- In a chemical reaction, atoms are neither created nor destroyed.
- The bonds between atoms are broken and new ones are formed.
- Since the number of atoms of each element remains the same, the total mass remains constant.
Example:
\( \mathrm{2H_2 + O_2 \rightarrow 2H_2O} \)
Before and after the reaction, there are 4 hydrogen atoms and 2 oxygen atoms — just rearranged into a different compound (water).
Application in Balancing Chemical Equations
The law of conservation of mass is the foundation for balancing chemical equations. Every balanced chemical equation reflects that no atoms are lost or gained during the reaction.
Example:
Unbalanced: \( \mathrm{H_2 + O_2 \rightarrow H_2O} \)
Balanced: \( \mathrm{2H_2 + O_2 \rightarrow 2H_2O} \)
The balanced equation satisfies the conservation of mass — 4 H atoms and 2 O atoms on both sides.
Verification through Experiment
| Experiment | Observation | Inference |
|---|---|---|
| React sodium carbonate solution with dilute hydrochloric acid in a closed flask. | Effervescence due to carbon dioxide formation; total mass before and after reaction remains the same. | Mass is conserved even though new substances form. |
| Heating copper carbonate in a closed test tube. | Forms copper oxide and releases CO₂; total mass remains constant if gas is not lost. | Proves conservation of mass if system is closed. |
Important Notes
- The law applies only to closed systems (no substances enter or leave).
- It is valid for both physical and chemical changes.
- Although mass is conserved, energy changes (heat, light) may occur.
Applications of the Law
- Used to balance chemical equations.
- Helps calculate the amount of reactants and products (stoichiometry).
- Ensures accurate chemical manufacturing and industrial reactions.
- Forms the basis for quantitative chemical analysis.
Example
Hydrogen reacts with oxygen to form water. Show that mass is conserved.
▶️ Answer / Explanation
Step 1: Write the balanced equation: \( \mathrm{2H_2 + O_2 \rightarrow 2H_2O} \)
Step 2: Count atoms: 4 H and 2 O on both sides.
Step 3: Molar masses → \( \mathrm{(2×2) + 32 = 36\ g} \) reactants, \( \mathrm{2×18 = 36\ g} \) products.
Final Answer: Total mass remains the same; mass is conserved.
Example
In a closed vessel, 10 g of calcium carbonate is heated to produce 5.6 g of calcium oxide. Verify the law of conservation of mass.
▶️ Answer / Explanation
Equation: \( \mathrm{CaCO_3 \rightarrow CaO + CO_2} \)
Step 1: Mass of CaO = 5.6 g, so CO₂ = 10 – 5.6 = 4.4 g.
Step 2: Total mass before = 10 g, after = \( \mathrm{5.6 + 4.4 = 10\ g} \).
Final Answer: Total mass before = total mass after → law verified.
Example
A student burns 5 g of magnesium ribbon in 3 g of oxygen in a sealed container. The reaction forms magnesium oxide. Predict the total mass of products and explain.
▶️ Answer / Explanation
Equation: \( \mathrm{2Mg + O_2 \rightarrow 2MgO} \)
Step 1: Total reactant mass = \( \mathrm{5 + 3 = 8\ g} \).
Step 2: All oxygen combines with magnesium to form solid MgO.
Step 3: Total product mass = 8 g (no mass lost or gained).
Step 4: Reaction only rearranges atoms — no atoms destroyed.
Final Answer: Mass of magnesium oxide formed = 8 g → verifies the law of conservation of mass.