9701 MCQ Solution 9

Question 9: [Physical > Chemical Energetics]:

The following energy cycle represents the enthalpy changes in the formation of carbon dioxide from its constituent elements in their standard states.

What substances are present at level Y in this diagram?

A C(g) + 2O(g)

B C(g) + O₂(g)

C C(s) + O₂(g)

D CO₂(g)

Reference: Past Exam Paper – November 2002 Paper 1 Q9

Solution 9:

Answer: A.

At the zero level are the starting elements in their standard states, so this immediately eliminates option C as the correct answer. Option D is carbon dioxide which is the final product of the reaction thus will be eliminated as level Y is an intermediate step, not the final. In option B the carbon changes form from solid to gas. This would require energy to break the weak forces of attraction between carbon atoms. In option A the strong covalent bond within the oxygen molecule is being broken. This will require a significantly higher amount of energy to overcome. Hence the energy level of A must be higher than the energy level of B, making option A the correct answer.