Oxidation combustion and redox reactions

🧪 Experiment 1: The Tealight & Beaker (Combustion & Oxidation)

Concepts: Hydrocarbon combustion, limiting reactants, and complete vs. incomplete oxidation.

Procedure

1. Light a tealight candle on a heatproof mat.

2. Observe the flame for a few seconds.

3. Invert a 100 ml beaker directly over the burning tealight.

4. Record how long the flame lasts and any changes inside the beaker.

What is Happening?

• The Reaction: Candle wax (paraffin hydrocarbons, e.g., \(C_{25}H_{52}\)) reacts with oxygen in the air.

• Chemical Equation (Complete Combustion):
  \(C_{25}H_{52}+38O_{2}\rightarrow 25CO_{2}+26H_{2}O\)

• Observations:

  • The flame goes out as oxygen (\(O_{2}\)) is depleted inside the beaker.

  • Condensation (water vapor, \(H_{2}O\)) forms on the cold inside walls of the beaker.

  • If oxygen runs low before the flame dies, soot (carbon) may deposit, showing incomplete combustion.

🧪 Experiment 2: Burning Magnesium Ribbon (Redox & Intense Oxidation)

Concepts: Rapid oxidation, electron transfer (redox), and basic oxide formation.

Procedure (blues glass)

1. Cut a 2–3 cm strip of magnesium ribbon.

2. Grip one end firmly with tongs.

3. Hold the other end in the hottest part of the Bunsen burner flame (blue flame).

4. As soon as it ignites, do not look directly at the glare. Hold it over a heatproof mat or beaker to catch the ash.

What is Happening?

• The Reaction: Magnesium reacts violently with oxygen in the air to form magnesium oxide.

• Chemical Equation:
  \(2Mg(s)+O_{2}(g)\rightarrow 2MgO(s)\)

• The Redox Breakdown:

  • Oxidation: Magnesium loses electrons: \(Mg \rightarrow Mg^{2+} + 2e^-\)

  • Reduction: Oxygen gains electrons: \(O_2 + 4e^- \rightarrow 2O^{2-}\)

• Observations: A blinding white light is emitted, leaving behind a brittle white ash (MgO).

Optional Extension: Dissolve the white ash in a little water and add a drop of universal indicator. It will turn blue/purple, showing that magnesium oxide is a basic oxide (\(Mg(OH)_2\)).

💥 Demonstration 3: The Hydrogen Balloon (Explosive Redox)

Concepts: Exothermic redox, rapid combustion, and synthesis of water.

Procedure (TEACHER ONLY)

1. Secure a balloon filled with hydrogen gas to a heavy retort stand using a string.

2. Attach a splint to the end of a meter ruler.

3. Light the splint using a Bunsen burner.

4. Standing at arm's length, extend the burning splint to touch and pop the hydrogen balloon.

What is Happening?

• The Reaction: Hydrogen gas mixes rapidly with atmospheric oxygen. The energy from the splint provides the activation energy for a highly exothermic redox reaction.

• Chemical Equation:
  \(2H_{2}(g)+O_{2}(g)\rightarrow 2H_{2}O(g)\)

• The Redox Breakdown:

  • Oxidation: Hydrogen increases its oxidation state from \(0\) (in \(H_{2}\)) to \(+1\) (in \(H_{2}O\)).

  • Reduction: Oxygen decreases its oxidation state from \(0\) (in \(O_{2}\)) to \(-2\) (in \(H_{2}O\)).

• Observations: A sharp, loud "pop" or bang accompanied by a brief yellow/orange flash of fire as water vapor is instantly created.

📝 Core Definitions for Students

• Combustion: A chemical reaction where a substance reacts rapidly with oxygen, releasing heat and light.

• Oxidation: The gain of oxygen or the loss of electrons (OIL: Oxidation Is Loss).

• Reduction: The loss of oxygen or the gain of electrons (RIG: Reduction Is Gain).

• Redox: A reaction where both oxidation and reduction happen simultaneously.