(三)用于金属的防护：使被保护的金属制品做原电池正极而得到保护。例如，要保护一个铁质的输水管道或钢铁桥梁等，可用导线将其与一块锌块相连，使锌做原电池的负极。

(二)比较金属活动性强弱：两种金属分别做原电池的两极时，一般做负极的金属比做正极的金属活泼性强。例如，有两种金属a、b，用导线连接后插入到稀H₂SO₄中，观察到a溶解，b极上有气泡产生或a、b之间用导线连上灵敏电流计，则推测a为负极，b为正极，则金属活动性a＞b。

(一)加快氧化还原反应的速率：一个自发进行的氧化还原反应，设计成原电池时反应速率大。例如，在Zn与稀H₂SO₄反应时加入少量CuSO₄溶液能使产生H₂的反应加快。

(二)电极反应式的书写：书写电极反应式时，要根据电极材料、电解质溶液分析电极反应和电极产物，写出电极反应式，还要注意电子守恒。

1、负极为活泼金属时，一般失去电子形成可溶性稳定价态阳离子，或进入溶液，或与电解质发生反应；若为燃料电池，负极一般不参与反应，可燃气体失去电子，形成稳定的氧化产物，或进一步与电解质溶液反应。2、正极上，一般是溶液中氧化性最强的微粒获得电子被还原，生成稳定的还原产物，或进一步与电解质溶液反应。3、正、负电极反应式在确保得失电子数目相等的前提下，加和可得电池反应总方程式。

20.They _____ on the program for almost one week before I joined them, and now we _____ on it as no god results have come out so far.

　 A. had been working; are still working　　　 B. had worked; were still working

　 C. have been working; have worked　　　　 D. have worked; are still working

19.--Hi, Tracy; how are you doing?

--I’m tired out, for I ______my living room all day.

A．painted　　　　　　　　　　　 　B．had painted

C．have painted　　　　　　　 　D．have been painting

18.On returning home , I found I ______ umbrella in the office. A had left　　　　　　　　　　　 B. have left

C. left　　　　　　　　　　　　　 　D. would leave　

17.He said that his bike __________stolen and he __________have to telephone the police.

A. was; would　　　　　　　　　　 B. has; will

C. has been; will　　　　　　　　　 D. had been; would

16.　The train _____ because of an accident; many passengers are now waiting anxiously at the railway station.

　 A. has been delayed　　　　　　　　　　 B. delayed　　　　

　 C. has delayed　　　　　　　　　　　 　　 D. had been delayed