2025 H2 Chem P1 Q8 — Thermodynamics Solution

The answer is A (\(\Delta H\) negative, \(\Delta S\) negative, \(\Delta G\) negative).
• \(\Delta H\): forming water from hydrogen and oxygen (the same overall reaction as burning hydrogen) is strongly exothermic, so \(\Delta H\) is negative.
• \(\Delta S\): 3 mol of gas (2 mol \(\text{H}_2\) + 1 mol \(\text{O}_2\)) become 2 mol of liquid water. The number of ways of arranging the particles and their energy decreases, so \(\Delta S\) is negative.
• \(\Delta G\): the reaction in the fuel cell takes place spontaneously and does useful electrical work, so \(\Delta G\) is negative. At room temperature the large negative \(\Delta H\) outweighs the \(-T\Delta S\) term, which is positive.
• B is wrong: \(\Delta G\) is not positive, because the cell reaction is spontaneous.
• C is wrong: \(\Delta H\) is not positive, because forming water from hydrogen and oxygen is exothermic.
• D is wrong: \(\Delta S\) is not positive, because gases are converted into a smaller amount of liquid.