Chemical Changes and the Conservation of Mass

Chemical Changes

  • Chemical changes involve the formation of new substances.

  • Example: Carbon monoxide + sugar + water → cold (a chemical reaction occurs, forming a new substance).

  • Energy is required to separate bonds and form new ones.

    • Endothermic: Absorbs energy/heat for the reaction to occur.

    • Exothermic: Releases energy/heat during the reaction.

  • Mass and atoms are conserved (mass remains the same).

  • The number of molecules can change; the structure of the compound changes.

  • Example: H₂ + Cl₂ → 2HCl. If heat is added, hydrogen and chlorine separate. Hydrogen bonds with hydrogen, and chlorine bonds with chlorine; molecules change.

Chemical Equations

  • Chemical changes result in the formation of a product from reactants, leading to a chemical equation.

  • Unbalanced equation example: H2O2 \rightarrow H2O + O2 (hydrogen peroxide decomposes into water and oxygen).

  • Balancing an equation is crucial.

    • Example: 2H2O2 \rightarrow 2H2O + O2

    • The number of atoms of each element must be the same on both sides of the equation.

States of Matter in Equations

  • Specify the state of matter using abbreviations:

    • (l) for liquid

    • (s) for solid

    • (g) for gas

    • (aq) for aqueous solution

  • Aqueous solutions (aq) are formed when a solid is dissolved in water. This is a physical change.

  • Example: Coffee solution or copper chloride solution.

  • Using the aqueous solution in a reaction makes it a chemical change.

Conservation of Atoms and Mass

  • Law of Conservation of Matter: In a balanced chemical equation, the total number of atoms of each element in the reactants equals the total number of atoms of each element in the products.

  • Example: Gas methane (CH₄) burns in oxygen (O₂), producing carbon dioxide (CO₂) and water (H₂O).

    • Unbalanced: CH4(g) + O2(g) \rightarrow CO2(g) + H2O(l)

Balancing Chemical Equations - Example

  • Reactants: Carbon (C), Hydrogen (H), Oxygen (O)

  • Products: Carbon (C), Hydrogen (H), Oxygen (O)

  • Unbalanced atoms:

    • Reactant side: 1 C, 4 H, 2 O

    • Product side: 1 C, 2 H, 3 O

  • Balanced equation: CH4(g) + 2O2(g) \rightarrow CO2(g) + 2H2O(l)

Applying Heat and States of Matter

  • Heat is applied (endothermic reaction), symbolized by a triangle (\$\Delta\$) above the arrow.

  • Specify states: (g) for gas, (l) for liquid.

  • The amount of atoms in the reactant is equal to the amount of atoms in the product; this is conservation of mass.

    • A balanced chemical equation means that the reactants' total amount of atoms will be equal to the atoms of the product side for each element.

Conservation of Mass - Calculations

  • To verify conservation of mass, calculate the mass of reactants and products using the periodic table.

  • Reactant side: CH₄ + 2O₂

    • Carbon: 1 atom * 12 u = 12 u

    • Hydrogen: 4 atoms * 1 u = 4 u

    • Oxygen: 4 atoms * 16 u = 64 u

    • Total reactant mass: 12 + 4 + 64 = 80 u

  • Product side: CO₂ + 2H₂O

    • Carbon: 1 atom * 12 u = 12 u

    • Hydrogen: 4 atoms * 1 u = 4 u

    • Oxygen: 4 atoms * 16 u = 64 u

    • Total product mass: 12 + 4 + 64 = 80 u

  • The mass of reactants equals the mass of products (80 u = 80 u), demonstrating the conservation of mass.

Practical Applications and Observations

  • The practical demonstrates the conservation of mass, showing that what you start with is what you end with.

  • Definition: In a balanced chemical equation, the total number of atoms of each element in the reactant is equal to the total number of atoms of each element present in that product.

  • Average calculation: Add all measurements together and divide by the number of measurements.

Important Concepts for Practical

  • Write balanced chemical equations from word descriptions.

  • Know formulas for sodium hydroxide and hydrochloric acid.

  • Distinguish between chemical and physical changes.

  • Write ratios of compounds and compare them.

  • Explain observations and draw conclusions.

  • State the law of conservation of matter.

Demonstrations and Examples

  • Experiment 1: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH)

    • Starting mass: 288.65 g

    • Mix acid and base → water + sodium chloride (NaCl)

    • Final mass: 288.65 g (mass remains the same)

  • Experiment 2: Hydrochloric acid (HCl) + Sodium carbonate (Na₂CO₃)

    • Initial System: Open

    • Sodium carbonate (solid) is added to hydrochloric acid (liquid), causing fizzing (gas release).

    • Starting mass of sodium carbonate and hydrochloric acid: 155.59 g

    • Gas Released (Carbon Dioxide)

    • The mass decreases because the gas (carbon dioxide) escapes.

    • Final mass: 154.75 g (mass decreases)

  • The law of conservation of mass is not broken because the released gas accounts for the missing mass.

Chemical Changes

  • Chemical changes create new substances.
  • Example: Carbon monoxide + sugar + water → cold (a new substance is formed).
  • Breaking and forming bonds requires energy.
    • Endothermic: Needs energy/heat to happen.
    • Exothermic: Releases energy/heat.
  • Mass and atoms stay the same.
  • Molecules can change; the compound's structure changes.
  • Example: H2 + Cl2 \rightarrow 2HCl. Add heat, and hydrogen and chlorine separate and then bond with themselves.

Chemical Equations

  • Chemical changes make products from reactants, shown in a chemical equation.
  • Unbalanced equation example: H2O2 \rightarrow H2O + O2 (hydrogen peroxide breaks down into water and oxygen).
  • Balancing is important.
  • Example: 2H2O2 \rightarrow 2H2O + O2
  • Each element must have the same number of atoms on both sides.

States of Matter in Equations

  • Use these abbreviations to show the state of matter:
    • (l) for liquid
    • (s) for solid
    • (g) for gas
    • (aq) for aqueous solution
  • Aqueous solutions (aq) are when a solid is dissolved in water. This is a physical change.
  • Example: Coffee or copper chloride solution.
  • Using an aqueous solution in a reaction makes it a chemical change.

Conservation of Atoms and Mass

  • Law of Conservation of Matter: In a balanced equation, the number of atoms of each element is the same on both sides.
  • Example: Methane (CH₄) burns in oxygen (O₂), making carbon dioxide (CO₂) and water (H₂O).
  • Unbalanced: CH4(g) + O2(g) \rightarrow CO2(g) + H2O(l)

Balancing Chemical Equations

  • Example
    • Reactants: Carbon (C), Hydrogen (H), Oxygen (O)
    • Products: Carbon (C), Hydrogen (H), Oxygen (O)
    • Unbalanced atoms:
    • Reactant side: 1 C, 4 H, 2 O
    • Product side: 1 C, 2 H, 3 O
    • Balanced equation: CH4(g) + 2O2(g) \rightarrow CO2(g) + 2H2O(l)

Applying Heat and States of Matter

  • Heat is applied (endothermic), shown as
    \Delta above the arrow.
  • States: (g) for gas, (l) for liquid.
  • The number of atoms in the reactant equals the number in the product; this is conservation of mass.
  • A balanced equation means the total atoms of reactants equals the atoms of the product for each element.

Conservation of Mass - Calculations

  • Check conservation of mass by calculating the mass of reactants and products using the periodic table.
  • Reactant side: CH₄ + 2O₂
    • Carbon: 1 atom * 12 u = 12 u
    • Hydrogen: 4 atoms * 1 u = 4 u
    • Oxygen: 4 atoms * 16 u = 64 u
    • Total reactant mass: 12 + 4 + 64 = 80 u
  • Product side: CO₂ + 2H₂O
    • Carbon: 1 atom * 12 u = 12 u
    • Hydrogen: 4 atoms * 1 u = 4 u
    • Oxygen: 4 atoms * 16 u = 64 u
    • Total product mass: 12 + 4 + 64 = 80 u
  • Reactant mass equals product mass (80 u = 80 u), showing conservation of mass.

Practical Applications and Observations

  • The practical shows that mass is conserved.
  • Definition: In a balanced equation, the total number of atoms of each element in the reactant equals the total number in the product.
  • Average calculation: Add all measurements and divide by the number of measurements.

Important Concepts for Practical

  • Write balanced equations from word descriptions.
  • Know formulas for sodium hydroxide and hydrochloric acid.
  • Distinguish between chemical and physical changes.
  • Write ratios of compounds and compare them.
  • Explain observations and draw conclusions.
  • State the law of conservation of matter.

Demonstrations and Examples

  • Experiment 1: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH)
    • Starting mass: 288.65 g
    • Mix acid and base → water + sodium chloride (NaCl)
    • Final mass: 288.65 g (mass stays the same)
  • Experiment 2: Hydrochloric acid (HCl) + Sodium carbonate (Na₂CO₃)
    • Initial System: Open
    • Add sodium carbonate (solid) to hydrochloric acid (liquid), and it fizzes (gas release).
    • Starting mass: 155.59 g
    • Gas Released (Carbon Dioxide)
    • The