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# solution9_pdf - spano velasco(ds28282 hw 09...

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spano velasco (ds28282) – hw 09 – Fakhreddine – (52395) 1 This print-out should have 13 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 1.0 points Under certain conditions, the reaction 3 A + 2 B 4 C was observed to proceed at a rate of 0 . 00199 M · s - 1 . What was the corresponding rate of change in reactant A? 1. 0 . 00398 M · s - 1 2. - 0 . 000663333 M · s - 1 3. - 0 . 00398 M · s - 1 4. 0 . 00597 M · s - 1 5. 0 . 000663333 M · s - 1 6. - 0 . 00597 M · s - 1 correct 7. 0 . 000995 M · s - 1 8. 0 . 00199 M · s - 1 9. - 0 . 00199 M · s - 1 10. - 0 . 000995 M · s - 1 Explanation: Reactant A is reacting at 3 times the stated rate of the overall reaction. A is also a reac- tant, so the concentration is DECREASING with time, which corresponds to a NEGA- TIVE rate. 002 1.0 points The reaction N 2 + 3 H 2 2 NH 3 is proceeding under conditions that 0.150 moles of NH 3 are being formed every 20 sec- onds. What is the rate of disappearance of H 2 ? 1. 1 . 125 × 10 - 2 moles/sec correct 2. The question cannot be answered from the information given; we need to know the volume of the container. 3. The question cannot be answered from the information given; we need to know the rate law for the reaction. 4. 2 . 25 × 10 - 1 moles/sec 5. 7 . 5 × 10 - 3 moles/sec Explanation: n NH 3 = 0.150 mol t = 20 s The rate of appearance of NH 3 is Rate = 1 2 Δ[NH 3 ] Δ t We can consider this to be Rate = 1 2 · 0 . 150 mol 20 s = 0 . 00375 mol / s The rate of disappearance of H 2 is Rate = - 1 3 · Δ[H 2 ] Δ t 0 . 00375 mol / s = - 1 3 · Δ[H 2 ] Δ t Δ[H 2 ] Δ t = - 0 . 01125 mol / s Thus H 2 disappears at a rate of 1 . 125 × 10 - 2 mol/s. 003 1.0 points What is the rate of the reaction 4 NH 3 + 5 O 2 4 NO + 6 H 2 O when 0.0500 mol/L of NH 3 is being consumed per second? 1. - 0.0125 M · sec - 1 2. 0.200 M · sec - 1 3. - 0.200 M · sec - 1 4. 0.0125 M · sec - 1 correct

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spano velasco (ds28282) – hw 09 – Fakhreddine – (52395) 2 5. One of the other answers has the right number but the units are wrong. Explanation: Rate = - 1 4 Δ[NH 3 ] Δ t parenleftbigg = - 1 5 Δ[O 2 ] Δ t = 1 4 Δ[NO] Δ t = 1 6 Δ[H 2 O] Δ t parenrightbigg = - 1 4 ( - 0 . 0500 M) 1 . 0 s = 0 . 0125 M · s - 1 004 1.0 points The rate of the reaction 2 O 3 3 O 2 is equal to 1. - Δ[O 3 ] Δ t . 2. + 1 2 Δ[O 3 ] Δ t . 3. - 1 3 Δ[O 2 ] Δ t . 4. - 1 2 Δ[O 3 ] Δ t . correct 5. - Δ[O 2 ] Δ t . Explanation: The rate of a chemical reaction can either be expressed as the rate of disappearance of a reactant (as in this case) or as the rate of appearance of a product. However, this rate is multiplied by the inverse of the coef- ficient of the species in question. The nega- tive sign here reminds us that as the reaction progresses, this species is consumed and de- creases in concentration.
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