问题 实验题

实验题:I、下列方法适用于实验室中制备氨气的是____________________

A.固态氯化铵加热分解              B.向固体氢氧化钠中滴加浓氨水

C.氯化铵溶液与氢氧化钠溶液共热    D.固体氯化铵与氢氧化钙固体混合加热

II、为了在实验室中利用工业原料制备少量氨气,有人设计了如图所示的装置(图中夹持装置均已略去)。

实验操作如下:

①检查装置的气密性后,关闭弹簧夹A.B.C.D.e。在A中加入锌粒,向长颈漏斗注入一定量的稀硫酸。打开弹簧夹C.D.e,则A中有氢气产生。在F出口处收集到氢气并检验其纯度。

②关闭弹簧夹c,取下截去底部的细口瓶C,打开弹簧夹a,将氢气经导管B验纯后点燃,然后立即罩上无底细口瓶C,塞紧瓶塞,如图所示。氢气继续在瓶内燃烧,几分钟后火焰熄灭。

③用酒精灯加热反应管E,继续通入氢气,待无底细口瓶C内水位下降到液面保持不变时,打开弹簧夹b,无底细口瓶C内气体经D进入反应管E,片刻后F中的溶液变红。

请回答下列问题:

(1)写出反应管E中发生反应的化学方程式_______________________________________

用方程式解释F中溶液变红的原因______________________________________________

(2)C瓶内水位下降到液面保持不变时,A装置内发生的现象为____________________________,防止了实验装置中压强过大。此时再打开弹簧夹b的原因是_____________________________________,,C中气体的主要成分为.__________________________________________.

(3)为什么用酒精灯加热反应管E-段时间后再打开弹簧夹b____________________________

(4)为什么F装置不用防倒吸的装置_________________________________________________

答案

Ⅰ、bd

Ⅱ、⑴N2+3H22NH3;    NH3·H2ONH4+OH

⑵、试管内液面下降,长颈漏斗中液因上升,直到试管内液因与锌粒分离;

使气体进入反应管E,并保证氢气含量较多,提高氮气的转化率;

氮气和氢气。

⑶、升高催化剂温度,使催化剂达到最佳活性温度,以提高反应速率。

⑷、该反应为可逆反应,且在该反应条件下,生成的氨气含量较少,在进入F装置的气体中含有较多难溶洞性的气体,所以不易产生倒吸。

题目分析:Ⅰ:a方法固态氯化铵加热分解得不到氨气,因为分解生成的氨气跟氯化氢气体在温度稍低的情况下就会结合生成氯化铵;c方法氯化铵溶液与氢氧化钠溶液共热,不能得到氨气,因为氨气在水中的溶解度大。因此选b、d

Ⅱ:①完成这一步后,相当于先在整个装置中通入了氢气,先把装置中的空气进行了排除。使氢气与氨气的反应更安全。②关闭弹簧夹c,取下截去底部的细口瓶C,打开弹簧夹a,将氢气经导管B验纯后点燃,然后立即罩上无底细口瓶C,塞紧瓶塞,如图所示。氢气继续在瓶内燃烧,此时消耗的是无底细口瓶C中的氧气,几分钟后火焰因缺氧而熄灭;③步先点燃E处酒精灯,只是为了先给催化剂升温,因为催化剂也只有在适宜的温度下才能发挥最大的活性,对于铁触媒来说,温度应该为500℃,在这同时,继续通入氢气,待无底细口瓶C内水位下降到液面保持不变时,此时两种气体也混合的很均匀了,打开弹簧夹b,无底细口瓶C内气体经D进入反应管E,在这个地方氮气跟氢气进行反应生成氨气,但是这个反应是可逆反应,不可能大量的生成氨气,尽管氨气在水中的溶解度比较大,但由于该混合气体中氨气的量少,进入F装置的气体中,含有较多难溶性的气体,所以不易产生倒吸。

选择题
单项选择题

With unemployment rising and housing costs still high, cities around the country are experiencing a new and sudden wave of homelessness. Shelters are overflowing, and more people this year are sleeping on floors in dingy social service centers, living in cars or spending nights on the streets.

In New York, Boston and other cities, homelessness is at record levels, a consequence of a faltering (摇晃的) economy that has crumbled even further after the Sept.11 attacks.

A survey by the U. S. Conference of Mayors released last week found that requests for emergency shelter in 27 cities had increased an average of 13 percent over last year. The report said the increases were 26 percent in Trenton; 25 percent in Kansas City, Mo.; 22 percent in Chicago; 20 percent in Denver; and 20 percent in New Orleans.

An unusual confluence of factors seems to be responsible for the surge. Housing prices, which soared in the expansion of the 1990’s, have not gone down, even though the economy has tumbled. A stream of layoffs has newly unemployed people taking low-wage jobs that might have otherwise gone to the poor. Benefits for welfare recipients are expiring under government-imposed deadlines. And charitable donations to programs that help the disadvantaged are down considerably, officials around the country said, because of the economy and the outpouring of donations for people affected by Sept.11.

"This is an unprecedented convergence (集中) of calamities (灾难), " said Xavier De Souza Briggs, an assistant professor of public policy at the John F. Kennedy School of Government at Harvard. "It’s really a crisis. "

More than half the cities surveyed by the mayors’ group reported that in the last year people had remained homeless longer, an average of six months.

There is no total number for the homeless nationwide. Experts said it was difficult to compare the situation with statistics in previous decades, because counting methods have improved. Yet, several experts said they believed that the increases reported by cities like Boston and Chicago reflected a national trend.

"My impression is there is more homelessness now than there was 20 years ago, " Gary Burtless, an economist at the Brookings Institution, said, adding that he believed that economic factors were not the sole explanation.

"I think that there must be a greater segment of our population that has tenuous connections to family and friends, and therefore has fewer resources to fall back on when something very bad happens like when they lose their job, " he said.

The word "tenuous" in the last paragraph means ().

A. p

B. enduring

C. weak

D. bad