Module 4: Reactions, Energy, and Combustion

Hydrocarbon

A molecule made only of carbon (C) and hydrogen (H).

2. Molecular Structure of Hydrocarbons

Carbon atoms form chains or rings; hydrogen fills remaining bonds.
Types include alkanes (single bonds), alkenes (double bonds), and alkynes (triple bonds).

Alkanes

Hydrocarbons with only single C–C bonds (saturated).
General formula: CₙH₂ₙ₊₂.

Alkenes & Alkynes

Alkenes: have C=C double bonds (CₙH₂ₙ).
Alkynes: have C≡C triple bonds (CₙH₂ₙ₋₂).

Combustion Reaction

Reaction where a fuel reacts with oxygen to produce CO₂, H₂O, and energy.

General Combustion Formula

Hydrocarbon + O₂ → CO₂ + H₂O + energy

Balancing Combustion Reactions – Steps

1. Balance C first

2. Balance H

3. Balance O last

4. Check all atoms

Reaction Energy

Energy released or absorbed when chemical bonds break and form.

Exothermic Reaction

Releases energy (feels hot); combustion is exothermic.

Endothermic Reaction

Absorbs energy (feels cold).

Energy Released Depends On:

Bond strengths, amount of reactants, and fuel type.

Mole

A counting unit equal to 6.022 × 10²³ particles (Avogadro’s number).

Mole–Mass–Number Conversion

• moles → particles: multiply by Avogadro’s number

• particles → moles: divide by Avogadro’s number

• moles ↔ mass: use molar mass (g/mol)

Molar Mass

Mass of 1 mole of a substance (found on periodic table).

Natural Gas – Formula & Traits

Mostly methane (CH₄).
High energy per gram, burns cleanest (least CO₂, lowest pollutants).
Drawbacks: leaks of methane are potent greenhouse gases.

Gasoline – Formula & Traits

Mixture of liquid hydrocarbons (mostly C₈H₁₈).
High energy density; easy to store/transport.
Drawbacks: significant CO₂ emissions; combustion releases pollutants.

Coal – Formula & Traits

Mostly solid carbon (C) with impurities.
Very abundant and cheap.
Drawbacks: dirtiest emissions—CO₂, sulfur, mercury, particulate matter.

Energy Storage Comparison

• Natural gas: high energy per carbon atom, cleanest

• Gasoline: high energy density, portable

• Coal: lowest energy quality, highest pollution

Overall Environmental Comparison

Natural gas → least harmful
Gasoline → moderate
Coal → most harmful