Chem 高手請答

When the reaction is carried out, there are several changes can be observed.

1)The colour of solution changes from blue to colourless.
2)Reddish brown solid deposits on the zinc rod.

In fact, the zinc rod would be dissolved and replace copper(II) ions in the solution. However, this is not concerned as an observation because it cannot be seen by just using our eyes. The deposited copper metal will coat the rod and hence we cannot see whether the zinc rod has any changes on size which is a very small change.

So, when choosing an answer, the best one should be chosen. Therefore the answer should be C.

Next, the answer of this question is AgCl can be easily understood. We know that AgX is insoluble in water when X is halogens. So you may think that chloride ion may not be the answer only, as there are bromide and iodide ions. But you should notice that the question has stated that the colour of the precipitate formed is white. Among AgCl, AgBr and AgI, the one has a white colour is only AgCl. The others are pale yellow and yellow in colour.

To answer questions like this type, it is so easy only by remembering the colour of the precipitate.

2007-04-08 01:07:28 補充：
I think you has something in misunderstanding. First, I think you want to say AgBr and AgI are white in colour, but not colourless.But when you consider the reactivity between this halides, you make know that they are different.

2007-04-08 01:07:47 補充：
For example, fluorine gas is pale yellow in colour, while chlorine, bromine and iodine are in yellowish-green, brown and dark black in colour respectively. Therefore, their precipitate should not be the same colour as well.

2007-04-08 01:08:04 補充：
In my chemistry text book, there has stated their colour. If you do not believe it, you can check their colours in wikipedia.

2007-04-08 04:43:31 補充：
For my information, you can click my name and see my introduction.For the publisher of my chemistry text book, it is Aristo Educational Press Ltd. and it's first published in July,2003.

2007-04-08 04:43:40 補充：
If you need to sit for HKCEE, I think that the notes given to your may have indicated. But there are much different for the teaching methods among different teachers, I do not know what you still have not known, that I cannot help you, I am sorry for this.

2007-04-09 04:39:18 補充：
no thz

option C is more precised.
as this is a displacement reaction, the reduction of copper(II) and oxidation of zinc metal are expected to be carried out at the SAME time. therefore, brown coating indicates the formation of copper metal while not all portion of zinc metal dissolves because its dissolution is impeded when the zinc rod is totally embedded into the copper coating where there is no contact between zinc and the solution. hence, the reaction stops here and trivially there is still a certain concentration of copper(II) left in the solution.

key word: WHITE ppt
AgBr and AgI appear pale yellow and yellow respectively