Law of Mass Conservation and Chemical Reaction Rates

The Law of Conservation of Mass

• Definition: Mass is neither created nor destroyed during a chemical reaction.

• Mass Relationship: The total mass of the reactants is exactly equal to the total mass of the products.

• Experimental Evidence: In a reaction between barium chloride and sodium sulfate ($BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + NaCl$), the mass before mixing and after the reaction remains constant at $150.2137\,g$.

• Atomic Basis: The same kinds and numbers of atoms are present before and after a reaction; because the atoms themselves do not change, their total mass must remain the same.

Calculating Reacting Masses

• Relative Atomic and Formula Masses: These values are used to determine actual reacting masses.

  • Example: $Ca (40) + Cl_2 (71) \rightarrow CaCl_2 (111)$.

• Calculation Rule: $\text{Reactant 1} + \text{Reactant 2} = \text{Product}$. To find a missing reactant: $\text{Missing Reactant} = \text{Product Mass} - \text{Known Reactant Mass}$.

• Specific Examples:

  • Lithium and sulfur: $14\,g\,Li + 64\,g\,S \rightarrow 78\,g\,Li_2S$.

  • Magnesium and oxygen: $48\,g\,Mg + 32\,g\,O_2 \rightarrow 80\,g\,MgO$.

  • Carbon and oxygen: $12\,g\,C + 32\,g\,O_2 \rightarrow 44\,g\,CO_2$.

  • Potassium and oxygen: $156\,g\,K + 32\,g\,O_2 \rightarrow 188\,g\,K_2O$.

Reaction Rates and Collision Theory

• Reaction Rate: A measure of how quickly reactants are converted into products.

• Collision Theory: Chemical reactions occur only when particles collide with enough energy and in the correct orientation.

• Activation Energy: The minimum energy required for a reaction to start.

• Effective Collisions: Successful collisions that result in product formation; more effective collisions lead to a faster reaction rate.

Factors Affecting Reaction Rates

• Temperature: Higher temperature increases particle movement and the frequency/energy of collisions (e.g., sugar dissolves faster in hot water).

• Concentration: Higher concentration (e.g., $2.0\,mol\,dm^{-3}\,HCl$ vs. $0.5\,mol\,dm^{-3}\,HCl$) provides more particles in the same space, leading to more collisions.

• Surface Area: Smaller particle sizes (crushed solids or powders) expose more particles for collision, speeding up the reaction.

• Catalysts: Substances that speed up reactions by providing an alternative pathway with lower activation energy without being consumed (e.g., digestive enzymes like Pepsin).

Industrial and Environmental Applications

• Food Preservation: Methods like refrigeration, drying, salting, sugaring, pickling, and smoking slow down chemical reactions and bacterial growth.

• Materials Production:

  • Haber Process: Catalysts produce ammonia from nitrogen and hydrogen for fertilizers.

  • Other uses: Production of plastics (polymers), medicines, and cement.

• Pollution Control: Catalytic Converters in cars use catalysts (platinum, palladium, rhodium) to convert harmful gases like Carbon monoxide ($CO$), Nitrogen oxides ($NO_x$), and Hydrocarbons ($HC$) into safer gases like $CO_2$, $N_2$, and $H_2O$.

• Corrosion: Rusting is a slow oxidation reaction accelerated by water, oxygen, and salt.

• Fire Control: Fire is a rapid combustion reaction requiring the Fire Triangle: Heat, Fuel, and Oxygen. Removing one component stops the reaction.

Energy Changes: Exothermic and Endothermic

• Exothermic Reactions: Release energy (heat) to the surroundings; the surroundings become warmer. Products have lower energy than reactants.

  • Examples: Burning wood, respiration, combustion, and neutralization.

• Endothermic Reactions: Absorb energy (heat) from the surroundings; the surroundings become cooler. Products have higher energy than reactants.

  • Examples: Photosynthesis, cooking eggs, baking bread, and instant cold packs.

Review Questions and Discussion

• Case Study: Refrigeration: Why should meat be refrigerated? Refrigeration provides a low temperature which results in fewer collisions and slower bacterial growth, extending shelf life.

• Case Study: Dissolving: Why does powdered medicine dissolve faster than tablets? Powders have a larger surface area, allowing more particles to be exposed for collision.

• Categorization Exercises:

  • Exothermic: Burning LPG, Fireworks, Burning candle.

  • Endothermic: Cooking rice, Photosynthesis, Melting candle wax, Melting ice, Instant cold pack.

• General Inquiry:

  • Why do industries use catalysts instead of just increasing temperature? To save time and money by speeding up reactions without the excessive energy costs of high heat.

  • Which factor explains crushed medicine working faster? Surface Area.

  • Which factor explains refrigeration? Temperature.