问题 选择题

如图是可逆反应X2+3Y2⇌2Z2在反应过程中的反应速率(Ʋ)与时间(t)的关系曲线,下列叙述正确的是(  )

A.t1时,只有正方向反应

B.t2时,反应到达限度

C.t2-t3,反应不再发生

D.t2-t3,各物质的百分含量相等

答案

从图象中知:横坐标表示时间、纵坐标表示反应速率.该反应开始时只有X2(g)、Y2(g),

A、0-t1,向正反应方向进行,反应物的浓度不断减少,V不断减小,同时产物Z2(g)不断增大,V由0逐渐增大.故A错误;

B、t2时,V=V≠0,达到平衡状态,即反应到达限度.故B正确;

C、t2时,达到平衡状态,t2-t3,反应未停止,各浓度不变,V=V≠0.故C错误;

D、t2-t3,根据V=V≠0,判断出该反应已达平衡状态,平衡时各物质的浓度不再发生变化,各物质的百分含量不再发生变化,但是各物质的百分含量并不相等.故D错误;

故选B.

单项选择题
单项选择题

Watching movie stars light up on screen may increase the odds a child or young teen will try smoking, study findings suggest.

In this latest study, researchers at Dartmouth Medical School in Lebanon, New Hampshire found that the odds of a child smoking increased in tandem (一前一后,紧密相连) with the number of films with smoking scenes he or she had seen.

This pattern remained regardless of other smoking risk factors such as having parents or friends who smoked, poor school performance or a self-reported "rebellious" nature.

Dr. James D. Sargent and his colleagues report the findings in the December 15th issue of the British Medical Journal.

In response to the findings, the British Medical Association (BMA) called on the UK film industry to "stub out" on-screen smoking.

"When smoking is glamorized in movies, young people are encouraged to experiment with a lethal (致命的) habit, " the BMA’s Dr. Vivienne Nathanson said in a statement.

And an editorial accompanying the journal report made the same challenge to Hollywood. "It is time for the entertainment industry to accept responsibility for its actions and stop serving the interests of tobacco companies, " writes Dr. Stanton A. Glantz of the University of California. San Francisco.

Sargent’s team based the findings on a survey of nearly 5 000 Vermont and New Hampshire students aged 9 to 15. Students were asked whether they had seen any of 50 recent movies that the researchers had screened for the number of smoking scenes. They also answered questions about environmental risk factors for smoking, their personality traits and perceptions of their parents’ views on smoking.

The investigators found that the number of smoking instances students had seen in the movies was correlated with their odds of having tried smoking. Kids who had seen 51 to 100 smoking occurrences were 70% more likely than those who had seen fewer to have smoked. And the odds were nearly three times greater among those who had seen 150-plus smoking scenes.

According to the researchers, the effects of on-screen smoking were similar to those of having parent or sibling who smoked.

"T he results indicate that exposure to tobacco use in films is pervasive, " Sargent’s team writes. "More importantly, such exposure is associated with trying smoking, which supports the hypothesis that films have a role in the initiation of smoking. "

In his editorial, Glantz agrees, writing that the study "provides powerful new evidence" that smoking in movies does contribute to adolescent smoking.

The subjects of Dr. Sargent’s study are ().

A. college students

B. high school graduates

C. pre-school children

D. elementary and high school students