Consider the following reaction equation: $2SO_3(g) → 2SO_2(g) + O_2$(g) ∆H = +198 kJ/mol Which of the following statements about the reaction is correct?

Step 1: Analyze the question We are given a thermochemical equation with ΔH = +198 kJ/mol and asked which statement about it is correct.

Step 2: The concept The sign of ΔH tells you whether a reaction is endothermic or exothermic:

• ΔH positive (+): heat is absorbed from the surroundings → endothermic. The container feels cold.
• ΔH negative (−): heat is released to the surroundings → exothermic. The container feels warm.

Step 3: Apply it to the reaction 2SO₃(g) → 2SO₂(g) + O₂(g) ΔH = +198 kJ/mol

Since ΔH is positive:

• The reaction is endothermic.
• Energy is absorbed (198 kJ for the equation as written, i.e., for 2 moles of SO₃ decomposing).
• The surroundings lose heat, so the container would feel cool, not warm.

Step 4: Check each option

• A. 198 kJ of energy is given off: Wrong. Energy is absorbed, not released.
• B. The reaction is exothermic: Wrong. It is endothermic.
• C. The reaction container would feel warm: Wrong. It would feel cold.
• D. 198 kJ of energy is absorbed: Correct.

Answer: D

Exam tip: Remember "Positive = Put in" (heat is taken in, endothermic) and "Negative = Net out" (heat is given out, exothermic). Also, endothermic reactions feel cold because they draw heat from their surroundings.