问题 选择题

在一定温度下,可逆反应N2(g)+3H2(g)⇌2NH3(g)达到化学反应限度的标志是(  )

A.NH3的生成速率与H2的生成速率相等

B.单位时间内生成nmolN2同时生成3nmolN2

C.N2、H2、NH3的浓度不再变化

D.N2、H2、NH3分子数之比为1:3:2

答案

A、化学平衡是动态平衡,当反应达到平衡状态时,NH3的生成速率与H2的生成速率之比为2:1,而不是相等,故A错误;

B、单位时间内生成n mol N2同时生成3n mol N2,都表示的逆向,与正向无关,故B错误;

C、当体系达平衡状态时,N2、H2、NH3的浓度不再变化,说明正逆反应速率相等,故C正确;

D、当体系达平衡状态时,N2、H2、NH3分子数之比为1:3:2,也可能不是1:3:2,与各物质的初始浓度及转化率有关,故D错误;

故选C.

阅读理解

阅读理解。

                                                     When milk arrived on the doorstep

     When I was a boy growing up in New Jersey in the 1960s, we had a milkman delivering milk to our

doorstep. His name was Mr. Basil. He wore a white cap and drove a white truck. As a 5-year-old boy, I

couldn't take my eyes off the coin changer fixed to his belt. He noticed this one day during a delivery and

gave me a quarter out of his coin changer.

     Of course, he delivered more than milk. There was cheese, eggs and so on. If we needed to change

our order, my mother would pen a note-"Please add a bottle of buttermilk next delivery"-and place it in the

box along with the empty bottles. And then, the buttermilk would magically appear.

     All of this was about more than convenience. There existed a close relationship between families and

their milkmen. Mr. Basil even had a key to our house, for those times when it was so cold outside that we

put the box indoors, so that the milk wouldn't freeze. And I remember Mr. Basil from time to time taking

a break at our kitchen table, having a cup of tea and telling stories about his delivery.

     There is sadly no home milk delivery today. Big companies allowed the production of cheaper milk, thus

making it difficult for milkmen to compete. Besides, milk is for sale everywhere, and it may just not have

been practical to have a delivery service.

     Recently, an old milk box in the countryside I saw brought back my childhood memories. I took it home

and planted it on the back porch (门廊). Every so often my son's friends will ask what it is. So I start telling

stories of my boyhood, and of the milkman who brought us friendship along with his milk.

1. Mr. Basil gave the boy a quarter out of his coin changer ____.

A. to show his magical power.

B. to pay for the delivery

C. to satisfy his curiosity.

D. to please his mother.

2. What can be inferred from the fact that the milkman had the key to the boy's house?

A. He wanted to have tea there.

B. He was a respectable person.

C. He was treated as a family member.

D. He was fully trusted by the family.

3. Why does home milk delivery no longer exist?

A. Nobody wants to be a milkman now.

B. It has been driven out of the market.

C. Its service is getting poor.

D. It is forbidden by law.

4. Why did the author bring back home an old milk box?

A. He missed the good old days.

B. He wanted to tell interesting stories.

C. He missed it for his milk bottles.

D. He planted flowers in it.

单项选择题