Chemical Reactions

🔹 Physical & Chemical Changes

• Physical change: No new substance, reversible (e.g. melting ice).

• Chemical change: New substance formed, irreversible (e.g. rusting).

• Signs of chemical reaction:

  • Gas produced (bubbling/fizzing)

  • Colour change

  • Temperature change

  • Precipitate forms

• Example equation:

  • Word: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

  • Balanced: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

• 🔹 Synthesis & Decomposition Reactions

  • Synthesis: 2 or more substances → 1 product

  • Two or more reactants combine to form one product

    • Example: 2H₂(g) + O₂(g) → 2H₂O(l)

  • Decomposition: 1 compound → 2 or more simpler substances
    One compound breaks into simpler substances

    • Example: 2H₂O₂(aq) → 2H₂O(l) + O₂(g)

  • Observations: colour change, gas produced, temperature change.

  • aqueous: dissolves in water/soluble
    

  • 🔹 Precipitation Reactions

    • Precipitate: Insoluble solid formed in a liquid reaction.

    • Identification: Cloudy mixture or solid appears.

    • Use solubility table: If one product is insoluble → precipitate forms. if not than it’s aqueous

    • Example: NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq)

    • Observation: White precipitate (AgCl).

Double displacement reaction:

The anions swapped spots

Each product must be written again from scratch

AX + BY → AY + BX

usually precipitate

🔹 Observations in Double Displacement Reactions

1. Precipitate Formation

• Most common evidence of double displacement.

• A solid (precipitate) appears when two clear solutions are mixed.

• Observation examples:

  • Solution turns cloudy.

  • Solid particles form and settle at the bottom.

  • Colour change in the mixture.

Example:
NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq)
Observation: White precipitate of AgCl forms.

2. Gas Formation

• Some double displacement reactions produce a gas that bubbles or fizzes.
  Example:
  Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
  Observation: Bubbling or fizzing (CO₂ gas released).

3. Temperature Change

• The reaction can be exothermic (gives off heat) or endothermic (absorbs heat).
  Observation:

• Beaker feels warm → exothermic.

• Beaker feels cold → endothermic.

4. Colour Change

• Colour of the solution changes due to new ions or precipitate formation.
  Examples:

• Blue → green when copper ions are replaced.

• Clear → coloured (or vice versa).

5. Change in Clarity

• Solution becomes cloudy or opaque when a precipitate forms.

• Or becomes clear again if the precipitate dissolves (in rare cases).

🔹 Acid Reactions

1. Acid + Metal → Salt + Hydrogen
 Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
2. Acid + Metal Hydroxide → Salt + Water
 HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
3. Acid + Metal Carbonate → Salt + CO₂ + H₂O
 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + CO₂(g) + H₂O(l)

🔹 Reaction Energy & Collision Theory

• Exothermic: Releases heat → feels hot (e.g. combustion). Total chemical energy of the products is less than the total chemical energy of the reactants.

  Excess energy is released into the surrounding environment generally as heat

• Endothermic: Absorbs heat → feels cold (e.g. photosynthesis). Total chemical energy of the products is greater than the total chemical energy of the reactants. Additional energy is absorbed from the surrounding environment for the reaction to occur.

• Activation energy: Minimum energy needed for particles to react. Energy needed to break bonds between atoms of reactants so products can form. During a reaction, old bonds must be broken and new bonds formed. . Breaking bonds requires energy — this is why particles need enough activation energy. If particles don’t have enough energy, they simply bounce off each other without reacting.

• Reaction rate: Speed with which reactants become products, also depends on how frequently particles successfully collide.

• Collision theory:

  • “According to collision theory, reactions occur when particles collide with enough energy and the correct orientation. Increasing the number of successful collisions increases the rate of reaction.”

  • Unsuccessful collision   Particles bounce off each other or don't react due to low energy or poor orientation (not aligned correctly)

    Successful collision     Have enough kinetic energy to break bonds within the reactant particles. Requires particles to collide in the right direction

  • 🔹 Factors Affecting Reaction Rate

    1. Temperature ↑ → faster particles → more collisions.

    2. Concentration ↑ → more particles → more collisions.

    3. Surface area ↑ → more exposure → faster rate.

    4. Catalyst present → lowers activation energy → faster rate.

• 

🔹 How We Know the Chemical Formula for Sulfuric Acid

1. Name Tells You the Ions

• The name “sulfuric acid” comes from the sulfate ion (SO₄²⁻).

• All acids contain hydrogen ions (H⁺) because acids release H⁺ in water.

So sulfuric acid contains:
→ H⁺ (hydrogen ion)
→ SO₄²⁻ (sulfate ion)

2. Balance the Charges

• The sulfate ion has a –2 charge.

• Each hydrogen ion has a +1 charge.

To make the total charge = 0 (neutral compound):
→ You need 2 H⁺ to balance one SO₄²⁻.

Therefore:
H₂SO₄

3. Check Naming Pattern