Chemistry Unit Notes on Chemical Reactions and the Conservation of Mass

Writing Balanced Chemical Equations

  • Importance of Balancing:

    • Combines ideas to create new understandings.

    • Accounts for every atom involved in a reaction.

  • Atomic Mass: Atoms consist of neutrons, protons, and electrons; these cannot be created or destroyed during a reaction.

Fundamental Concepts

Law of Conservation of Mass

  • During a chemical reaction, matter cannot be created or destroyed.

  • Balancing equations reflects the rearrangement of atoms.

Practical Applications

Quantitative Chemistry

  • Uses the law of conservation to illustrate that the mass of products equals the mass of reactants.

  • Chemical equations are balanced in terms of the number of atoms of each element on both sides.

  • Examples:

    • Unbalanced Reaction: H2 + Cl2 → HCl

    • Balanced Reaction: H2 + Cl2 → 2HCl

    • Indicates 1 molecule of H2 and 1 molecule of Cl2 reacting to form 2 molecules of HCl.

Observations from Reactions

  • Some reactions might show a perceived change in mass due to gases escaping.

  • Example: Magnesium reaction with oxygen to produce magnesium oxide.

    • Chemical Equation: 2Mg + O2 → 2MgO

    • Mass results:

    • Initial crucible mass: 0.23 g

    • Final crucible mass: 0.41 g

Thermal Decomposition

  • Reaction example: Calcium carbonate (CaCO3) decomposing into calcium oxide (CaO) and carbon dioxide (CO2).

    • Chemical Equation: CaCO3 → CaO + CO2

    • Important for understanding mass loss.

    • Mass results:

    • Initial mass: 0.54 g

    • Final mass: 0.36 g

Activity: Balancing Chemical Equations

  • Skills Developed:

    • Communicative skills in understanding and using chemical symbols.

Practice Questions

  1. What is the law of conservation of mass?

  2. Why might some reactions appear to show a change in mass?

  3. Provide two examples of reactions where a change in mass may seem apparent.

  4. Balance the following equations:

    • a) H2 + O2 → H2O

    • b) Ca + HCl → CaCl2 + H2

    • c) Li + H2O → LiOH + H2

    • d) NH3 + O2 → NO + H2O

    • e) K + O2 → K2O

  5. Count the atoms and elements in NaAl(OH)4.

  6. Interpret the following formulae:

    • a) 2HCl

    • b) Cl2

  7. Decomposition of hydrogen peroxide (H2O2) to water and oxygen:

    • a) Write a balanced symbol equation, including state symbols.

    • b) Explain why the produced water has a lower mass than the original hydrogen peroxide.