U9.6 · Coupled ReactionsPractice · 偶联反应
Lessons covered:U9.6 Coupled ReactionsU9.10 Electrolysis and Faraday's Law
- 1Numeric
In cells, glucose + phosphate → glucose-6-phosphate + H₂O has ΔG°′ of about +13.8 kJ/mol, which is unfavorable; ATP + H₂O → ADP + phosphate has ΔG°′ of about −30.5 kJ/mol. Coupled, glucose + ATP → glucose-6-phosphate + ADP. What is ΔG°′?
kJ/molExplanationAdding the equations cancels phosphate and H₂O (the shared intermediates), so ΔG°′ = +13.8 + (−30.5) = −16.7 kJ/mol < 0, and the coupled reaction is favorable. Cells use ATP hydrolysis to drive many unfavorable reactions this way.
Review: U9.6 Coupled Reactions
- 2Multiple choice
For a favorable reaction to drive an unfavorable one, what must be true of the two reactions?
ExplanationCoupling needs a chemical link: the favorable reaction uses up a product of (or supplies a reactant to) the unfavorable one, so the intermediate cancels when the equations are added and the ΔG° values add. Two reactions that merely share a container but no substances each reach their own equilibrium and don’t drive each other.
Review: U9.6 Coupled Reactions
- 3Numeric
Reaction ① A ⇌ B has K₁ = 2.0 × 10−5; reaction ② B ⇌ C has K₂ = 5.0 × 107 (same temperature). What is K for the coupled reaction A ⇌ C?
× 10ExplanationAdding equations adds ΔG° values and multiplies K values: K = 2.0 × 10−5 × 5.0 × 107 = 1.0 × 103. On its own ① barely goes, but coupled, A mostly turns into C.
Review: U9.6 Coupled Reactions
- 4Multiple choice
Which of these can NOT make a reaction with ΔG > 0 happen?
ExplanationA catalyst only lowers the activation energy; it supplies no energy and doesn’t change ΔG. Coupling, electrolysis, and light all bring in free energy from an outside favorable process to drive the unfavorable one.
Review: U9.6 Coupled ReactionsU9.10 Electrolysis and Faraday's Law
- 5Numeric
In making titanium, TiO₂ reacts with chlorine and coke together: TiO₂(s) + 2Cl₂(g) + 2C(s) → TiCl₄(l) + 2CO(g). ΔG°f (kJ/mol): TiO₂(s) −888.8, TiCl₄(l) −737.2, CO(g) −137.15. Calculate ΔG° at 25 °C.
kJExplanationΔG° = [−737.2 + 2 × (−137.15)] − (−888.8) = −1011.5 + 888.8 = −122.7 kJ (Cl₂ and C have ΔG°f = 0). Without coke, TiO₂ + 2Cl₂ → TiCl₄ + O₂ has a positive ΔG°; carbon combining with O₂ to make CO is very favorable, and coupling the two makes the overall reaction favorable.
Review: U9.6 Coupled Reactions
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