Chapt 11 - Chemical Changes Notes (ans)

Topic 1: Chemical Changes

1.1 Physical Change

• Definition: A change that does not alter the original chemical properties or composition of substances.

  • No new substances/products are formed.

  • The change is reversible.

• Examples:

  • Dissolving solute in solvent (ex: salt in water).

  • Changes of state (solid to liquid, liquid to gas, etc.).

1.2 Chemical Change

• Definition: A change that occurs when a substance combines with another to form new substances or decomposes into different substances.

  • New substances/products are formed as atoms are rearranged.

  • The change is usually irreversible.

• Types of Reactions:

  • Exothermic: Produces heat (ex: burning fuel).

  • Endothermic: Absorbs heat (ex: photosynthesis).

• Evidence of Chemical Change:

  • Change of odor (e.g., pungent smell).

  • Change of color.

  • Change in temperature (test tube feels hot or cold).

  • Change in form (e.g., powder dissolves in acid).

  • Heat, light, or sound is given off.

  • Effervescence (bubbles).

  • Formation of precipitate (insoluble substance, e.g., bubbling carbon dioxide into limewater).

  • Decomposition of organic matter (e.g., rotting meat).

1.3 Initiating Chemical Reactions

• Chemical reactions can be initiated in four ways:

  1. Mixing.

  2. Heating.

  3. Exposure to light.

  4. Passing electricity.

• Word equations represent chemical reactions:

  • Example 1: Reactant 1 + Reactant 2 ➔ Product 1 + Product 2.

  • Example 2 (decomposition): Reactant 1 ➔ Product 1 + Product 2.

1.3.1 Chemical Reactions Caused by Mixing

• Oxidation: When a substance gains oxygen in a chemical reaction.

  • Examples:

    1. Cellular respiration: Glucose + Oxygen ➔ Carbon Dioxide + Water.

    2. Corrosion of copper: Copper + Oxygen ➔ Copper Oxide.

    3. Rusting: Iron + Oxygen ➔ Iron Oxide.

• Acids and Alkalis:

  • Acids: pH < 7 (e.g., hydrochloric acid, nitric acid, sulfuric acid).

    • Produce chloride, nitrate, and sulfate salts, respectively.

  • Alkalis: pH > 7 (e.g., potassium hydroxide, sodium hydroxide).

  • Salts: Ionic compounds with neutral pH (7).

  • Metals react with acids except for copper, platinum, silver, and gold.

• Neutralization:

  • General equation: Acid + Alkali ➔ Salt + Water.

  • Examples:

    1. Hydrochloric acid + Sodium hydroxide ➔ Sodium chloride + Water.

    2. Nitric acid + Sodium hydroxide ➔ Sodium nitrate + Water.

    3. Sulfuric acid + Sodium hydroxide ➔ Sodium sulfate + Water.

1.3.2 Chemical Changes Caused by Heating

• Combustion: When a substance is heated in the presence of oxygen to form new substances.

  • Heat and light are released.

  • Examples:

    1. Carbon + Oxygen ➔ Carbon Dioxide (complete combustion).

    2. Methane + Oxygen ➔ Carbon Dioxide + Water Vapour.

    3. Hydrogen + Oxygen ➔ Water Vapour.

• Thermal Decomposition: Breakdown of a substance into simpler substances upon heating.

  • Equation: 1 Reactant ➔ Many Products.

  • Examples:

    1. Calcium Carbonate ➔ Calcium Oxide + Carbon Dioxide.

    2. Sugar ➔ Carbon + Water Vapour.

1.3.3 Chemical Reactions Caused by Exposure to Light

• Photosynthesis:

  • In the presence of light, chlorophyll (in plants) combines carbon dioxide with water to produce glucose and oxygen.

  • Equation: Carbon Dioxide + Water ➔ Glucose + Oxygen.

• Photodecomposition: Certain substances break down into simpler substances when exposed to light.

  • Examples:

    1. Silver Bromide ➔ Silver + Bromine Gas.

    2. Silver Chloride ➔ Silver + Chlorine Gas.

1.3.4 Chemical Reactions Caused by Electricity

• Electrolysis: Decomposition of compounds into simpler substances using electrical energy.

  • Example: Water ➔ Oxygen Gas + Hydrogen Gas.

• Electroplating: Coating a metallic object with a different metal layer using electrical means to prevent corrosion and improve appearance.