7.3 Conservation of Mass and Balancing Chemical Equations

Conservation of Mass

• Balancing Chemical Equations
    - The concept that atoms rearrange is more accepted now than it was 200 years ago.
    - An early misunderstanding was that when a candle burned, the wax simply vanished, suggesting that matter could disappear.

7.3.1 Conservation of Mass

• Antoine-Laurent Lavoisier:
    - Lavoisier demonstrated that the mass after burning a candle is equal to the mass before burning.
    - The apparent loss of mass results from gases released into the atmosphere.
    - This foundational observation led to the Law of Conservation of Mass:
      - Matter cannot be created or destroyed during a chemical reaction.

7.3.2 Law of Constant Proportion

• Law of Constant Proportion:
    - States that a compound always contains the same relative amounts of each element, regardless of how it is formed.
    - Example: Carbon Dioxide (CO2):
      - Consistently composed of about 27% mass of carbon.
      - Carbon dioxide contains one carbon atom and two oxygen atoms, which is fixed regardless of its formation process (e.g., from a sherbet reaction or combustion in an engine).
    - Lavoisier posthumously recognized as the Father of Modern Chemistry after his execution during the French Revolution.

7.3.3 Open and Closed Systems

• Definitions:
    - Chemistry System: The specific reaction under investigation.
    - Surroundings: Everything outside the system.
    - Closed System:
      - Does not allow matter transfer to or from its surroundings.
    - Open System:
      - Allows matter transfer to or from surroundings.

• Examples of Systems:
    - Open System:
      - Evaporation: Molecules escape from the liquid phase to vapor, which can result in an apparent loss of mass.
    - Closed System:
      - A sealed flask retains molecules, hence mass is conserved as they cannot escape.

7.3.4 Balancing Chemical Equations

• Combustion of Methane:
    - Reaction breakdown:
      - Methane (CH4) reacts with oxygen (O2) to produce water (H2O) and carbon dioxide (CO2).
      - Molecular Composition:
        - CH4: 1 Carbon, 4 Hydrogen.
        - O2: 2 Oxygen (diatomic).
        - H2O: 2 Hydrogen, 1 Oxygen.
        - CO2: 1 Carbon, 2 Oxygen.

• Mass Conservation in Equations:
    - The equation must balance, with the number of atoms of each element on both sides being equal.
    - Example: The balanced equation for methane combustion:
      - $ext{CH}_4 + 2 ext{O}_2 <br>ightarrow ext{CO}_2 + 2 ext{H}_2 ext{O}$.

Steps to Balance a Chemical Equation

1. Identify Reactants and Products:
      - Create a word equation based on reaction observations or reliable resources.
        - Example: Reaction between hydrogen and oxygen produces water vapor:
        - Word Equation: Hydrogen + Oxygen → Water.

2. Determine Chemical Formulae:
      - Identify the correct chemical formulae for each reactant and product.
        - Hydrogen: H2, Oxygen: O2, Water: H2O.

3. Write the Equation:
      - Write reactants on the left and products on the right based on the word equation.

4. Balance Atoms:
      - List elements and count atoms on both sides of the equation:
        | Element | Reactants | Products |
        |---------|-----------|----------|
        | H | 2 | 2 |
        | O | 2 | 1 |
     - Adjust coefficients to balance the atoms on both sides by modifying the number in front of formulas.

• Example of Balancing Hydrogen & Oxygen:
    - Original Equation: $ext{H}_2 + ext{O}_2 <br>ightarrow ext{H}_2 ext{O}$.
    - New Equation (after balancing): $2 ext{H}_2 + ext{O}_2 <br>ightarrow 2 ext{H}_2 ext{O}$.

1. Include States of Matter:
      - Use symbols to indicate states:
        - Solid (s), Liquid (l), Aqueous (aq), Gas (g).
      - Example of complete equation with states:
        $2 ext{H}_2(g) + ext{O}_2(g) <br>ightarrow 2 ext{H}_2 ext{O}(g)$.

Case Study: Balancing the Combustion of Methane

1. Determine Reactants and Products:
      - Reactants: Methane (CH4), Oxygen (O2).
      - Products: Carbon Dioxide (CO2), Water (H2O).

2. Chemical Formulae:
      - Methane: CH4, Oxygen: O2, Carbon Dioxide: CO2, Water: H2O.

3. Write the Equation:
      - Replace words with formulae:
      - $ext{CH}_4 + ext{O}_2 <br>ightarrow ext{CO}_2 + ext{H}_2 ext{O}$.

4. Balance the Number of Atoms:
      - Count atoms on both sides:
        | Element | Reactants | Products |
        |---------|-----------|----------|
        | C | 1 | 1 |
        | H | 4 | 2 |
        | O | 2 | 3 |
      - Adjust coefficients to balance hydrogen (put 2 before H2O) and oxygen (put 2 before O2):
        - Final balanced equation: $ext{CH}_4 + 2 ext{O}_2 <br>ightarrow ext{CO}_2 + 2 ext{H}_2 ext{O}$.
      - Note: Include states of matter:
        $ext{CH}_4(g) + 2 ext{O}_2(g) <br>ightarrow ext{CO}_2(g) + 2 ext{H}_2 ext{O}(g)$.

Activities

• Modeling Equations:
    1. Use marshmallows to create models of different compounds.
    2. Write out reactions to produce each compound. Example: $ext{Pi} + 2 ext{Wh} <br>ightarrow ext{Pi} ext{Wh}_2$.

Quick Quiz

• Questions or activities relating to the content covered in the material.