Energy from fuels (combustion) practice questions.

Define the standard enthalpy of combustion of a substance.

• The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
• The enthalpy change when one mole of a substance is formed from its elements under standard conditions
• The energy required to break all the bonds in one mole of a gaseous substance
• The enthalpy change when one mole of water is formed from H+ and OH- ions

Answer: The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions

Standard enthalpy of combustion (ΔHc°) is defined per mole of substance burned completely in oxygen under standard conditions; it is always exothermic (negative).

Which set of coefficients (C₃H₈ : O₂ : CO₂ : H₂O) balances the equation for the complete combustion of propane, C₃H₈?

• 1 : 5 : 3 : 4
• 1 : 4 : 3 : 4
• 1 : 6 : 3 : 3
• 2 : 7 : 6 : 8

Answer: 1 : 5 : 3 : 4

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O balances C (3), H (8) and O (10 on each side), giving coefficients 1 : 5 : 3 : 4.

A student uses a spirit burner of ethanol to heat water in a copper calorimeter but obtains an experimental ΔHc° much less exothermic than the data-book value. Suggest the most likely reason.

• Heat loss to the surroundings and incomplete combustion
• The water had too high a specific heat capacity
• The ethanol released more energy than expected
• The thermometer over-read the temperature rise

Answer: Heat loss to the surroundings and incomplete combustion

In open calorimetry much heat escapes to the air/apparatus and combustion is often incomplete (soot), so less energy reaches the water and the measured value is smaller in magnitude than the literature value.

Calculate the heat absorbed by 300.0 g of water when its temperature rises by 20.0 °C. (c = 4.18 J g⁻¹ K⁻¹.)

• 25.1 kJ
• 2.51 kJ
• 12.5 kJ
• 250.8 kJ

Answer: 25.1 kJ

q = mcΔT = 300.0 × 4.18 × 20.0 = 25 080 J ≈ 25.1 kJ.

The complete combustion of methane in the gas phase is CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Using bond enthalpies (C–H 414, O=O 498, C=O 804, O–H 463 kJ mol⁻¹), determine ΔH for this reaction.

• -808 kJ mol⁻¹
• -698 kJ mol⁻¹
• +810 kJ mol⁻¹
• -286 kJ mol⁻¹

Answer: -808 kJ mol⁻¹

Bonds broken = 4(414)+2(498) = 2652; bonds made = 2(804)+4(463) = 3460; ΔH = 2652 − 3460 = −808 kJ mol⁻¹. Bond-enthalpy calculations only apply to gaseous species, hence H₂O(g).

Which statement explains why incomplete combustion of a hydrocarbon is hazardous to health?

• Carbon monoxide, because it binds strongly to haemoglobin and prevents oxygen transport
• Carbon dioxide, because it is extremely toxic to humans even at very low concentrations in air
• Water vapour, because it displaces oxygen from the lungs
• Soot, because it is a greenhouse gas

Answer: Carbon monoxide, because it binds strongly to haemoglobin and prevents oxygen transport

Limited oxygen produces CO, which binds to haemoglobin far more strongly than O₂, blocking oxygen transport in the blood.