Combustion of hydrocarbons Knowledge-Based Practice

Questions

1. 1.

   When comparing structural isomers of alkanes, such as linear pentane and branched 2,2-dimethylpropane, which of the following statements correctly explains why they release different amounts of heat upon complete combustion?

   • A) The branched isomer releases more heat because steric hindrance destabilizes its carbon skeleton.
   • B) Both isomers release identical amounts of heat because they possess the same molecular formula and form identical products.
   • C) The linear isomer releases less heat because its larger surface area leads to stronger intermolecular forces.
   • D) The branched isomer releases less heat because increased branching imparts greater thermodynamic stability to the reactant.
2. 2.

   Which of the following statements best explains why the complete combustion of hydrocarbons is a strongly exothermic process?

   • A) The covalent bonds within the reactant hydrocarbon molecules break spontaneously without requiring an input of energy.
   • B) A greater total number of covalent bonds is formed in the product molecules than the number of bonds broken in the reactants.
   • C) The energy released in forming the strong bonds of carbon dioxide and water exceeds the energy absorbed to break the reactant bonds.
   • D) The combustion process is driven exclusively by a large decrease in the total entropy of the gaseous reaction system.
3. 3.

   Which of the following observations indicates that a hydrocarbon is undergoing incomplete combustion rather than complete combustion?

   • A) A clean, non-luminous blue flame is sustained throughout the reaction.
   • B) Carbon dioxide and water vapor are the sole chemical products formed.
   • C) A luminous, smoky yellow flame appears and deposits of elemental carbon are formed.
   • D) The reaction releases the maximum theoretical quantity of thermal energy.
4. 4.

   In the complete combustion of one mole of an open-chain alkane having the general molecular formula $\text{C}_n\text{H}_{2n+2}$, how many moles of molecular oxygen ($\text{O}_2$) are required?

   • A) $\frac{2n + 1}{2}$
   • B) $\frac{3n}{2}$
   • C) $\frac{3n - 1}{2}$
   • D) $\frac{3n + 1}{2}$
5. 5.

   When 0.10 mol of an unknown hydrocarbon undergoes complete combustion in excess oxygen, exactly 0.40 mol of carbon dioxide and 0.50 mol of water vapor are formed. What is the molecular formula of this hydrocarbon?

   • A) $\text{C}_4\text{H}_{10}$
   • B) $\text{C}_4\text{H}_8$
   • C) $\text{C}_3\text{H}_8$
   • D) $\text{C}_5\text{H}_{10}$

Answers and step-by-step solutions appear after you finish the test.