O-Level Chemistry

[KiausuLormee]

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[KiausuLormee]

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[hterbin]

KiausuLormee:

Need help to explain this question.

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Hi, is it possible that you take a better picture? Part of the question got cut off.

[hterbin]

KiausuLormee:

Need help to explain this question.

http://i57.tinypic.com/11sjpcp.jpg\">

In the question as shown, there are 4 metals, W, X, Y and Z. Now, W, X, Y reacts with cold water while Z does not. W and Y floats on water, so it must be either: Lithium, Sodium or Potassium as only these 3 metals are less dense than water.

Metal X is very unique. Metal X is denser than water and has less vigorous reaction than W and Y. From this we can infer the following - Metal X cannot be a Group I metal and is less reactive than Li, Na or K. Metal X is less reactive than Li, Na or K as it has less vigorous reactions than Metal W and Y, which has to be either Li, Na or K. That being said, the metal cannot be a group I metal as, it cannot be Li, Na or K, and any metal denser than water in Group I, Rb, Cs and Fr are all more reactive than W and Y. Hence, X cannot be a Group I metal. X must be therefore be Calcium since it can react with water and is not a Group I metal.

To question (a) the answer would be:
Metal X is Calcium
Ca + 2H2O -> Ca(OH)2 + H2

Metal Z does not react with cold water, and produce a reddish brown substance upon leaving in water. Whenever there is a presence of a reddish-brown compound, it is most probably the presence of Iron(III) ions. Z reacts with water to form rust, which is a reddish brown substance, also known as hydrated iron(III) oxide. Therefore metal Z is Iron. Now iron has a very unique characteristic, it has a reversible reaction: 3Fe(s) + 4H2O(g) <> Fe3O4(s) + 4H2(g). It reacts with steam to form Iron(II, III) oxide and hydrogen gas, and its oxide reacts with hydrogen gas to form elemental iron and steam. Since iron is ranked above hydrogen ion in the metal reactivity series, it reacts with hydrochloric acid.

To question (bi) the answer would be:
Metal Z is Iron
Reaction with steam: 3Fe + 4H2O -> Fe3O4 + 4H2
Reaction with hydrochloric acid: Fe + 2HCl -> FeCl2 + H2

Now, the reddish brown substance formed can either be iron(III) oxide or iron(III) hydroxide, however for simplicity sake, lets take it as iron(III) oxide. Oxidation involves the losing of electron, i.e. the charge of the species becomes more positive. E.g. Mn loses 2 electrons to form Mn2+, the oxidation state of Manganese increases from 0 in elemental manganese to +2 in manganese(ii) oxide. Note that oxidation state is written as +n and charge is written as n+.

To question (bii) the answer would be:
Iron gains 3 electrons from oxygen to form Iron(III) oxide. Since iron gained electrons to form iron(III) ions, the reaction is an oxidation reaction.

All the explanations are based off the best of what I can guess and see from the photo which cuts off half the questions.

[KiausuLormee]

Thanks hterbin,


Have been trying to figure out some parts of the answers. Thanks a lot.

[tutor76]

Electrolysis question sharing


from collin ,experienced tutor

an electrolysed circuit is being setup

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[tutor76]

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[tutor76]

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[tutor76]

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