{ "format": "recallweave-deck/1", "title": "Reaction amounts: ratios, limits and leftovers", "attribution": "Original questions, examples and explanations by HAMON estate contributor 87eaaf0fdf63. Chemistry background: OpenStax, Chemistry 2e, sections 4.3 Reaction Stoichiometry and 4.4 Reaction Yields. https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry ; https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields . Calculations assume ideal completion of the stated reaction with no side reactions; theoretical amounts do not predict experimental yield, rate or equilibrium.", "license": "Original course wording and examples: CC0-1.0. Referenced material retains its own license. No textbook passages, figures or exercises are reproduced.", "concepts": [ "reaction-ratios", "limiting-reactants", "excess-and-scaling", "theoretical-amounts" ], "items": [ { "id": "stoich-ratio-water", "concept": "reaction-ratios", "prerequisites": [], "prompt": "For 2 H₂ + O₂ → 2 H₂O, how much water can 1 mol O₂ form if enough H₂ is available and the stated reaction completes?", "options": [ "1 mol H₂O", "2 mol H₂O", "3 mol H₂O", "4 mol H₂O" ], "answer": 1, "explanation": "The coefficient ratio is 2 mol H₂O per 1 mol O₂. Thus 1 × 2/1 = 2 mol H₂O. Hydrogen must be available in at least the required amount.", "transfer": "How much H₂ is consumed to form those 2 mol H₂O? Explain both ratios from the balanced equation." }, { "id": "stoich-coefficient-meaning", "concept": "reaction-ratios", "prerequisites": [], "prompt": "In 2 Mg + O₂ → 2 MgO, which quantities are directly compared by the coefficients 2 : 1 : 2?", "options": [ "The masses of the three substances in grams", "The volumes of the three substances in litres", "The reaction times for the three substances", "The amounts of the three substances in moles" ], "answer": 3, "explanation": "Coefficients relate amounts of substance. Molar masses would also be needed to compare masses; equal coefficients do not imply equal masses or volumes.", "transfer": "Give one reason why a 2 : 1 mole ratio need not be a 2 : 1 mass ratio." }, { "id": "stoich-ratio-ammonia", "concept": "reaction-ratios", "prerequisites": [], "prompt": "For N₂ + 3 H₂ → 2 NH₃, 6 mol H₂ are supplied with more than enough N₂. What is the theoretical amount of NH₃?", "options": [ "4 mol NH₃", "2 mol NH₃", "6 mol NH₃", "12 mol NH₃" ], "answer": 0, "explanation": "Each 3 mol H₂ supports formation of 2 mol NH₃. Six moles of H₂ support 6/3 = 2 mol of reaction extent, producing 2 × 2 = 4 mol NH₃.", "transfer": "Use the same coefficients to determine the N₂ consumed. Why is it different from the NH₃ amount?" }, { "id": "stoich-equal-moles", "concept": "limiting-reactants", "prerequisites": [ "reaction-ratios" ], "prompt": "For 2 H₂ + O₂ → 2 H₂O, the starting amounts are 2 mol H₂ and 2 mol O₂. Which statement identifies the limit correctly?", "options": [ "O₂ limits because its coefficient is smaller", "Neither remains because their mole amounts match", "H₂ limits because 2/2 is smaller than 2/1", "Both remain because neither starts in excess" ], "answer": 2, "explanation": "Compare amount divided by coefficient: H₂ supports an extent of 1 mol, while O₂ supports 2 mol. H₂ runs out at the smaller shared extent. Equal starting moles are not the required 2 : 1 ratio.", "transfer": "Keep H₂ at 2 mol. Find an O₂ amount that makes H₂ excessive, and justify your answer." }, { "id": "stoich-ammonia-limits", "concept": "limiting-reactants", "prerequisites": [ "reaction-ratios" ], "prompt": "For N₂ + 3 H₂ → 2 NH₃, start with 2 mol N₂ and 3 mol H₂. Which ideal-completion result is correct?", "options": [ "4 mol NH₃ form; no N₂ remains", "2 mol NH₃ form; 1 mol N₂ remains", "3 mol NH₃ form; 1 mol H₂ remains", "6 mol NH₃ form; no H₂ remains" ], "answer": 1, "explanation": "N₂ supports an extent of 2/1 = 2 mol. H₂ supports 3/3 = 1 mol and is limiting, even though its starting amount is larger. The reaction consumes 1 mol N₂ and forms 2 mol NH₃, leaving 1 mol N₂.", "transfer": "Why can the reactant with the larger mole amount still be limiting? Use a coefficient in your explanation." }, { "id": "stoich-fractional-excess", "concept": "limiting-reactants", "prerequisites": [ "reaction-ratios" ], "prompt": "For 2 Mg + O₂ → 2 MgO, start with 1.5 mol Mg and 1 mol O₂. How much O₂ remains after ideal completion?", "options": [ "0 mol O₂", "0.5 mol O₂", "0.75 mol O₂", "0.25 mol O₂" ], "answer": 3, "explanation": "Mg supports an extent of 1.5/2 = 0.75 mol, below O₂'s 1 mol. The reaction consumes 0.75 mol O₂, leaving 1 − 0.75 = 0.25 mol.", "transfer": "Determine MgO formed and Mg remaining. Check that each element is accounted for." }, { "id": "stoich-exact-ratio", "concept": "excess-and-scaling", "prerequisites": [ "limiting-reactants" ], "prompt": "For 2 H₂ + O₂ → 2 H₂O, start with 4 mol H₂ and 2 mol O₂. What happens under ideal completion?", "options": [ "4 mol H₂O form; neither reactant remains", "2 mol H₂O form; 2 mol H₂ remain", "4 mol H₂O form; 1 mol O₂ remains", "6 mol H₂O form; neither reactant remains" ], "answer": 0, "explanation": "Both normalized capacities equal 2 mol: 4/2 = 2/1. The supplied amounts match the equation's ratio, so both are consumed and 2 × 2 = 4 mol H₂O form.", "transfer": "Give a smaller pair of positive starting amounts with the same exact ratio." }, { "id": "stoich-zero-reactant", "concept": "excess-and-scaling", "prerequisites": [ "limiting-reactants" ], "prompt": "For 2 H₂ + O₂ → 2 H₂O, no H₂ and 5 mol O₂ are supplied. What does this reaction model predict?", "options": [ "5 mol H₂O form; all O₂ is consumed", "10 mol H₂O form; all O₂ is consumed", "No H₂O forms; all 5 mol O₂ remain", "No H₂O forms; 2.5 mol O₂ remain" ], "answer": 2, "explanation": "The hydrogen capacity is zero, so the shared extent is zero. This stated reaction consumes neither reactant and produces no water. Oxygen cannot supply the missing hydrogen atoms.", "transfer": "Explain why total reactant amount alone cannot determine product amount." }, { "id": "stoich-more-excess", "concept": "excess-and-scaling", "prerequisites": [ "limiting-reactants" ], "prompt": "For 2 H₂ + O₂ → 2 H₂O, compare 2 mol H₂ + 2 mol O₂ with 2 mol H₂ + 4 mol O₂. How does the theoretical H₂O amount change?", "options": [ "It doubles from 2 mol to 4 mol", "It stays at 2 mol in both cases", "It rises from 2 mol to 6 mol", "It falls from 2 mol to 1 mol" ], "answer": 1, "explanation": "Hydrogen remains limiting in both mixtures: its normalized capacity is 1 mol. Adding only excess oxygen cannot increase this theoretical product amount; it increases oxygen left over.", "transfer": "What must be changed to form more water in this model? State which starting amount first needs to increase." }, { "id": "stoich-scale-both", "concept": "theoretical-amounts", "prerequisites": [ "excess-and-scaling" ], "prompt": "For N₂ + 3 H₂ → 2 NH₃, 1 mol N₂ + 2.4 mol H₂ can form 1.6 mol NH₃. If both starting amounts double, what follows?", "options": [ "1.6 mol NH₃ form; N₂ becomes limiting", "1.6 mol NH₃ form; H₂ remains limiting", "3.2 mol NH₃ form; N₂ becomes limiting", "3.2 mol NH₃ form; H₂ remains limiting" ], "answer": 3, "explanation": "Doubling both amounts doubles both normalized capacities, so their ordering is unchanged. H₂ remains limiting, and the theoretical amount doubles to 3.2 mol NH₃.", "transfer": "Does doubling only one reactant always double the product? Use a counterexample from this lesson." }, { "id": "stoich-mass-to-moles", "concept": "theoretical-amounts", "prerequisites": [ "reaction-ratios", "limiting-reactants" ], "prompt": "A reaction problem supplies reactant masses in grams. What must happen before comparing amount/coefficient to identify a limiting reactant?", "options": [ "Convert each mass to moles using its own molar mass", "Compare the masses directly and choose the smaller", "Divide each mass by its balanced-equation coefficient", "Treat every substance as having the same molar mass" ], "answer": 0, "explanation": "The coefficients compare mole amounts. For each substance, n = mass/molar mass; then compare n/coefficient. Comparing raw grams can choose the wrong limit because substances have different molar masses.", "transfer": "Which extra data would you need to enter a mass-based problem into an explorer that accepts only moles?" }, { "id": "stoich-theory-boundary", "concept": "theoretical-amounts", "prerequisites": [ "limiting-reactants" ], "prompt": "A balanced-equation calculation gives 2 mol of theoretical product. Which conclusion is justified without any experimental evidence?", "options": [ "Every experiment must recover exactly 2 mol of pure product", "The product must form quickly because the equation is balanced", "The 2 mol value depends on ideal completion of the stated reaction", "A smaller recovered amount proves the coefficients were wrong" ], "answer": 2, "explanation": "The calculation assumes the stated reaction consumes the limiting reactant as specified. It does not model rate, equilibrium, side reactions or recovery losses, so it cannot guarantee the amount collected in an experiment.", "transfer": "Name a quantity the balanced equation alone cannot predict, and explain what kind of additional information would be needed." } ] }