combustion:

Combustion is a chemical reaction in which a substance reacts with oxygen, releasing energy as heat and light.

• Flash point:
  • The lowest temperature at which a substance produces enough vapour to ignite briefly.
• Ignition temperature:
  • The minimum temperature required for a substance to continue burning.
• Complete combustion:
  • Occurs when there is excess oxygen.
  • Produces carbon dioxide and water.
  • Releases the maximum amount of energy.
• Word equation (methane):
  • Methane + oxygen → carbon dioxide + water
  • CH₄ + 2O₂ → CO₂ + 2H₂O
• Incomplete combustion:
  • Occurs when oxygen is limited.
  • Produces carbon monoxide and/or carbon (soot) plus water.
  • Releases less energy than complete combustion.
  • Carbon monoxide is toxic.
• Example equations:
  • 2CH₄ + 3O₂ → 2CO + 4H₂O
  • CH₄ + O₂ → C + 2H₂O
• Importance of combustion:
  • Main source of energy for fuels.
  • Used in electricity generation, transport, and heating.

enthalpy:

• Enthalpy (H):

  • The total energy stored in a chemical system.

• Enthalpy change (ΔH):

  • The energy absorbed or released during a chemical reaction.

• Standard enthalpy change of reaction (ΔH°):

  • The enthalpy change when reactants form products under standard conditions (298 K, 1 atm, 1 mol dm⁻³).

• Average bond enthalpy:

  • The average energy required to break a bond in gaseous molecules.

• Where chemical energy comes from:

  • Energy is stored in chemical bonds.
  • Breaking bonds requires energy.
  • Forming bonds releases energy.

• Hess’ Law:

  • The total enthalpy change of a reaction is the same, regardless of the pathway taken.

• Calculating ΔH using bond enthalpies:

  

Thermochemical equations:

• Chemical equations that include an enthalpy change.