一道化學反應和能量ACT科學題

最新 06-22

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化學反應和能量ACT科學題

Flameless ration heaters (FRHs) are used by combat soldiers to heat their meals in the field. The heat is produced when magnesium reacts with water:

The rate of the reaction increases in the presence of iron (Fe) and sodium chloride (NaCl) . The following experiments were done to determine how to design FRHs.

Experiment 1

A 1.0-mole (mol) piece of Mg ribbon, 0.10 mol of Fe powder, and 0.10 mol of NaCl were added to 25 mL of H2O at 20°C in an insulated container. The mixture was stirred constantly and its temperature was measured every 50 sec. The experiment was repeated with Mg ribbon cut into 100 pieces and with Mg powder (see Figure 1).

Experiment 2

Powdered Mg (0.10 mol) was mixed with 0.10 mol of NaCl. The mixture was added to 100 mL of H2O at 20°C in an insulated container. The mixture was stirred constantly and the maximum temperature increase that occurred within 15 min was recorded. The procedure was repeated with different amounts of Fe powder added to each mixture (see Table 1).

Experiment 3

Powdered Mg (0.10 mol) was mixed with 0.50 mol of Fe powder and added to 100 mL of H2O at 20°C in an insulated container. The mixture was stirred constantly and the maximum temperature increase that occurred within 15 min was recorded. The procedure was repeated with different amounts of NaCl (see Table 2).

1. Which of the following is the most likely reason that amounts greater than 0.125 mol of NaCl were not tested in Experiment 3 ? The results showed that:

F. adding more NaCl no longer increased the reaction rate.

G. adding more NaCl would decrease the reaction rate.

H. the boiling point was reached, so no further data could be gathered with this procedure.

J. more Fe would need to be added for any greater

2. Based on the results of Experiment 3, one can reasonably conclude that as the amount of NaCl added increased from 0 mol to 0.100 mol, the maximum rise in temperature within 15 min of the start of the reaction:

A. increased only.

B. increased, then stayed the same.

C. decreased only.

D. decreased, then stayed the same.

3. If a trial had been done in Experiment 3 with 0.060 mol of NaCl added, the maximum temperature increase of the mixture that would have occurred within 15 min would have been closest to:

F. 34°C.

G. 42°C.

H. 50°C.

J. 62°C.

4. Which of the following factors affecting the reaction of Mg and H2O was studied in Experiment 1, but not in Experiments 2 or 3?

A. Reaction temperature

B. Addition of NaCl

C. Addition of Fe

D. Surface area of Mg

5. An engineer is designing an FRH, to be used with 100 mL of H2O, that will most rapidly generate the greatest amount of heat with the least amount of materials. Based on the results of Experiments 1, 2, and 3, which of the following specifications should she choose?

F. 0.50 mol Fe powder, 0.125 mol NaCl, and 0.10 mol of Mg powder

G. 0.50 mol Fe powder, 0.125 mol NaCl, and 0.10 mol of Mg ribbon

H. 0.70 mol Fe powder, 0.125 mol NaCl, and 0.10 mol of Mg powder

J. 0.70 mol Fe powder, 0.125 mol NaCl, and 0.10 mol of Mg ribbon

6. It has been observed that as Mg and H2O react, the Mg(OH)2 (magnesium hydroxide) that is produced forms an unreactive coating on the Mg surface. Which of the following models for why NaCl speeds up the reaction is most consistent with this observation and the results of the experiments?

A. NaCl absorbs heat produced in the reaction, causing the Mg(OH)2 to melt off of the Mg surface.

B. NaCl binds with Fe to inhibit the reaction of Mg with H2O.

C. NaCl reacts with Mg(OH)2 to form a stronger barrier against H2O.

D. NaCl reacts with Mg(OH)2, removing the coating from the Mg, allowing more contact with H2O.

難度係數：

題目來源：真題

考察內容: Chemical reaction and energy

解題過程

綜述：這是一道research summary 題（實驗總結）。

講述的是FRHs的化學反應原理

金屬鎂和水的置換反應產生的熱量來給食物加熱。在文章的開頭也點名了鐵和氯化鈉的存在可以增加這種化學反應的速率。粗看三個實驗的圖標，可以大致的猜出，三個實驗分別是，金屬鎂的物質形狀，鐵的含量，氯化鈉的含量，對這個化學反應的結果，主要是產生的熱量大小的影響

1. 對於題目（1）高於0.125 mol of NaCl 為什麼沒有用於對實驗3的測試的可能原因，本題的解題關鍵是，當使用0.125 mole of NaCl時，其實驗結果是溫度是85°C （solution is boiling ）, 說明此時的混合物液體已經沸騰，從物理熱學我們得知，當液體沸騰時，物體進入相變階段，此階段的物體的溫度保存不變，即使再增加NaCl的含量，混合物液體的溫度保存不變，但產生的熱能還是增加，因此The key is H.達到沸點，溫度保持不變，無法進一步測得其他數據。如果熟悉物理熱學中的相變定義，此題較簡單。如果不熟悉的話，此題屬於有中等偏難。

2. 這是通過實驗結果得出實驗結論題，NaCl added increased from 0 mol to 0.100 mol ，從實驗3可以看出，其產生的熱能（混合物）溫度一直上升，所以The key is A.

3. 此題也是屬於簡單推理，0.060 mol of NaCl 產生熱能的溫度應該在 0.050 mol of NaCl 和 0.075 mol of NaCl 產生的熱量溫度之間，在34 °C 和50°C之間，所以The key is G (42°C)

4. 只在實驗1而不在實驗2和3中研究的實驗參數，應該是，金屬鎂的物質形狀，The key is D.

5. 想要利用最少物質產生最大熱量的實驗組合方案應該是，0.50 mol Fe powder, 0.125 mol NaCl, and 0.10 mol of Mg powder, 這是綜合三個實驗的結果，對於實驗3，應該是使用0.125 mol of NaCl，對於實驗1 ，應該是使用0.10 mol of Mg powder, 關鍵點對於實驗2，當Fe 的含量超過0.50 mol時，混合物物體的溫度，也就是化學反應的熱能保持不變，因此最優的實驗方案是使用0.50 mol，The key is F.

6.這是一個化學知識點，Mg(OH)2 是難溶於水的物質，所以單用水和金屬鎂反應的話，產生的Mg(OH)2會附著在金屬鎂的表面，阻攔了與水的接觸，從而減慢了化學反應速度，加入了NaCl, 其很容易和Mg(OH)2反應，也就是溶解了金屬鎂的表面附著物，從而加速化學反應進程，The key is D.