问题 选择题

取17.4 g MnO2固体粉末与一定体积的浓盐酸(含溶质0.4mol)共热制取氯气(假设HCl挥发损失不计),下列叙述中正确的是(   )

A.反应过程中MnO2作催化剂

B.盐酸未完全反应

C.参加反应的盐酸有一半被氧化

D.反应产生2.24L氯气(标况)

答案

答案:C

【错解分析】静态考虑反应问题,忽略盐酸浓度变化引起还原性变化的事实。错选C、D。根据该反应中量的关系:MnO2——4HCl可知,该反应中MnO2过量,因此HCl完全反应。又因该反应中只有一半的HCl作还原剂(另一半为起到酸的作用),因此参加反应的盐酸有一半被氧化(0.2mol),得到2.24L的氯气。

【正解】随反应的进行,盐酸的浓度逐渐减小,对应的还原性逐渐减弱,直至发生的氧化还原反应被停止。因此该反应过程中,虽然MnO2过量,盐酸也不可能完全反应,同样的,收集到的气体也不可能是2.24L。但是不论如何,参与反应的盐酸一定有一半被氧化(对应的量一定少于0.2mol)。因此正确选项为C。(注:许多物质的氧化(还原)性会受浓度变化的影响,并在一些具体的反应中表现出来。例如,浓硫酸具有氧化性,在加热的条件下与铜能够发生反应,随反应的进行,硫酸浓度变小,氧化性逐渐减弱,最终反应将停止;若参加反应的金属为Zn等活泼金属,则当浓度降低到一定程度(转化为稀硫酸)时,又会有H2产生。又如,浓硝酸具有强氧化性,与铜反应生成NO2,随反应进行,浓度减小,氧化性将有所减弱。当浓度减小到一定程度,反应产物变为NO。在解此类问题时,必须要打破常规,“动态”地考虑反应进行问题。这类问题也常在化学计算中出现。)

多项选择题
单项选择题

I was a single parent of four small children, working at a low paid job. Money was always tight, but we had a (1) over our heads, food on the table, clothes on our backs, and if not a lot, always (2) . Not knowing we were poor, my kids just thought I was (3) . I’ve always been glad about that.

It was Christmas time, and although there wasn’t (4) for a lot of gifts, we planned to celebrate with a family party. But the big (5) for the kids was the fun of Christmas (6) .

They planned weeks ahead of time, asking (7) what they wanted for Christmas. Fortunately, I had saved MYM120 for (8) to share by all five of us.

The big (9) arrived. I gave each kid a twenty-dollar bill and (10) them to look for gifts of about four dollars each. Then everyone scattered. We had two hours to shop; then we would (11) back at the "Santa’s Workshop".

Driving home, everyone was in high Christmas spirits, (12) my younger daughter, Ginger, who was unusually (13) . She had only one small, flat bag with a few candies—fiftycent candies! I was so angry, but I didn’t say anything (14) we got home. I called her into my bedroom and closed the door, (15) to be angry again. This is what she told me.

"I was looking (16) thinking of what to buy, and I (17) to read the little cards on the ’Giving Trees’. One was for a little girl, four years old, and all she (18) for Christmas was a doll. So I took the card off the tree and (19) the doll for her. We have so much and she doesn’t have anything. "

I never felt so (20) as I did that day.

14()

A.since 

B.after 

C.while 

D.until