3.1 | The conservation of mass, chemical equations and stioichmetry

Conservation of Mass

• Law of Conservation of Mass:

  • States that mass is neither created nor destroyed in chemical reactions.

  • Atoms are rearranged during reactions, but the total number of each type of atom remains constant.

  • The same collection of atoms is present before and after a reaction.

Chemical Reactions

• Chemical Equations:

  • Represent chemical reactions symbolically.

  • Consist of reactants (starting substances) on the left side and products (substances produced) on the right side.

  • Example: The combustion of hydrogen with oxygen:

    • Unbalanced: H2 + O2 -> H2O

    • Balanced: 2 H2 + O2 -> 2 H2O

• Coefficients:

  • Numbers placed in front of chemical formulas indicating relative amounts of substances.

  • Balance equations by ensuring equal numbers of atoms for each element on both sides.

Balancing Chemical Equations

• To balance equations:

  1. Write the unbalanced equation.

  2. Identify reactants and products.

  3. Determine coefficients.

  4. Check atom count on both sides for balance.

• Example: Balancing the combustion of methane (CH4)

  • Unbalanced equation: CH4 + O2 -> CO2 + H2O

  • Steps to balance:

    • Carbon (C): 1 C in CH4 and 1 C in CO2, add coefficient 1 to both.

    • Hydrogen (H): 4 H in CH4 and 2 in H2O, add coefficient 2 before H2O.

    • Oxygen (O): 2 O from O2 needed for 2 CO2 and H2O (total 4 O), add coefficient 2 before O2.

  • Final balanced equation: CH4 + 2 O2 -> CO2 + 2 H2O

Stoichiometry

• Definition:

  • The study of quantities involved in chemical reactions and balanced equations.

  • Determines how much product is formed or how much reactant is necessary using mole ratios from balanced equations.

• Applications:

  • Used extensively in chemistry and industry to control and predict amounts in reactions.

Chemical States and Conditions

• Physical States in Equations:

  • Indicated by symbols: (g) for gas, (l) for liquid, (s) for solid, (aq) for aqueous.

  • Example with states: CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(g)

• Condition Symbols:

  • Symbols like Greek letter delta (Δ) indicate conditions such as heat addition in reactions.