[Rayan]

hello.
i just want to know why do elements of group IA (H-Li-Na...) react vegorously with water.
thanks.

[Grafter]

All of the group 1 metals have one electron in their outer shell. This makes it very easy to remove.
How do you think this relates to their ability to react with water?
Which will react most quickly out of Li, Na and K?
Can you write out a balanced equation for the reaction with water?

Na + H₂O --> ?

[Rayan]

starting from what you said, since they all have one electron in the outer shell, and since it is easy to remove, then it will be even easier to remove this electron if it's far away from the nucleus because then it will be less attracted to it. then i think that K will react most quickly with water.
as for the balanced equation, i'm not sure but i think:

Na + 2H₂O --> NaOH + 1/2 H₂

waiting your reply as soon as possible. thank you

[Grafter]

Nearly,

Na + H₂O --> NaOH + 1/2 H₂

Can you write out the half equations for the reaction?
Which will be a more alkaline solution, LiOH, NaOH or KOH?
Will this reaction happen for group II metals in the same way? What will the products be?

[Rayan]

The half equations for the reaction are:

Na ---> Na⁺ + 1e

2H₂O + 2e---> 2HO^- + H₂

(The first equation should be multiplied by 2 when writing the overall reaction.)

Li ---> Li⁺ + 1e

2H₂O + 2e---> 2HO^- + H₂


K ---> K⁺ + 1e

2H₂O + 2e---> 2HO^- + H₂

Ca ----> Ca²⁺ + 2e

2H₂O + 2e---> 2HO^- + H₂

I can't see the difference between these reactions!
They seem to be the same for me. But I know that Li reacts slowly, Na reacts vigorously, K reacts violently. As for Ca, it reacts quite quickly. I know this because we learned it but I dont know why!
I need help.

[Grafter]

Thats right,

Li will form LiOH
Na will form NaOH
K will form KOH
Ca will form Ca(OH)₂

Why do you think Ca forms a Ca²⁺ ion instead of a Ca⁺ ion?
How does NaOH act as a base in water? Can you write a dissociation equation?
What makes something a good base?

[jdurg]

For everyone involved in this thread, great job!  You're doing a great job of leading the OP towards the answer Grafter, and rayan is picking up on the proper answers as she's going along.  This is what I like to see.  

So rayan, take a look at the periodic table and compare the observations you have made to the location of those elements on the periodic table.  For the elements that have reacted the quickest, what common trait to they seem to have?  Remember that a chemical reaction involves the movement of electrons.  So if something doesn't want to get rid of its electrons, it's not going to react as quickly.  

Where is lithium on the periodic table?
How many electrons does lithium have?
How many electrons does it need to give up in order to have a full outer shell?  (By full outer shell, I mean how many electrons must it lose in order to have the same number of electrons as the noble gas that precedes it?  In this case it would be helium).  
That electron(s) that it needs to give up; how far away from the positively charged nucleus is it/are they?
So how tightly is the nucleus holding onto that electron(s) compared to say potassium or rubidium?
Since reactions involve the movement of electrons from one substance to another, why did you see that potassium and sodium reacted with water faster than lithium did?

With calcium, where is calcium on the periodic table?  (I.E. what column?)
Based on its position, how many electrons do you think it has to give up in order to have a full outer shell?
What do you think would be easier, removing one electron from something or removing two electrons?
Since chemical reactions involved the movement (transfer is probably a better word) of electrons, how do you think the group two metals would react with water in relation to the group 1 metals?  Would it be faster, slower, or the same?

[Rayan]

hello..
Li belongs to group I, thus it has one valence electron and 7 other electrons to completet its octet, so it loses one electron. all the elements of this group tend to lose one electron too. but since as we go down a group, the number of shells increases, so this electron that will be lost will be more far away as we go down, so K loses its electrone easier than Na, which also loses its electon easier than Li.
Ca has to lose two electrons, and i suppose that it is harder than to lose one electron and that's why reactions of the elements of group II with water are slower than those of elements of group I with water  
so?!?!

[Grafter]

In calcium its not so much that its easier to loose 2 than it is to loose 1. The second ionisation energy will be greater than the first. Its more that the energy benefit from going 2+ instead of 1+ is more than enough to compensate for having to ionise twice.

[GCT]

hello.
i just want to know why do elements of group IA (H-Li-Na...) react vegorously with water.
thanks.

I guess the answer is related to effective nuclear charge, that is the strength of the nucleus in holding on to its electrons, IA elements have relatively lower proton numbers.

Grafter is right in saying, that the second ionization energy of magnesium would be disfavorable for the formation of the ionic compound, you'll need to consider the lattice energy (which also consists of neighboring anions and cations, such as those designated through a cubic cell).

You can also visualize the stability in terms of a PE diagram as a function of the internuclear spacing between the anion and cation.

edit: wrong trend in atomic radii

[Donaldson Tan]

H being an exception.

Something I never understood during my course of pre-u study in chemistry is that the metal reactivity always described with reference to its atomic nature and not the metallic structure. Does the 'sea of electrons' somehow contribute towards metal reactivity? Remember the harpoon mechanism?

[Rayan]

"Its more that the energy benefit from going 2+ instead of 1+ is more than enough to compensate for having to ionise twice. "
i didnt get that..!

[jdurg]

hello..
Li belongs to group I, thus it has one valence electron and 7 other electrons to completet its octet, so it loses one electron. all the elements of this group tend to lose one electron too. but since as we go down a group, the number of shells increases, so this electron that will be lost will be more far away as we go down, so K loses its electrone easier than Na, which also loses its electon easier than Li.
Ca has to lose two electrons, and i suppose that it is harder than to lose one electron and that's why reactions of the elements of group II with water are slower than those of elements of group I with water  
so?!?!

Correct!  As you move down the first group of elements, that lone outer electron feels less and less of an attraction to its nucleus than the outer electron of the elements above it.  Therefore, when the atom comes across another substance that is willing to pull that electron off, it will readily give it up.  Water is a highly polar molecule since the electrons are attracted more to the central O atom than the two H atoms on the outside.  As a result, these outer hydrogen atoms will pull that lone electron away from the Group I metal and begin the chemical reaction.  If you put sodium, cesium, potassium, etc. into a completely non-polar solvent, like mineral oil, no reaction takes place because there is nothing there to pull that lone electron away.  This is why these metals are stored under a noble gas or completely non-polar solvent.

With the group IA metals, the atoms have two electrons in their outer shell, thus filling up their S-shell.  This provides a little bit more stability than having one outer electron like the group I elements have.  As a result, it requires a little more energy to pull those electrons away.  What Grafter is saying is that while it takes more energy to remove two electrons from a Group IA atom, the 'stability' it gains by forming a stable structure as a 2+ ion makes up for the energy needed to pull away those two electrons.  

In reality, however, the group IA atoms react a bit slower in water because they have a tighter 'grip' on their two outer electrons.  They aren't willing to give them up as easily unless they have a good reason to do this.  Because chemical reactions are all about the transfer of electrons, if the electrons aren't transfered easily or quickly, the overall reaction will slow down.  Group I metals release their outer electron very easily and very quickly, so their reaction with water is quite vigorous.  Group IA metals don't release their electrons as easily or as quickly, so their reactions are generally much slower.  (In fact, Beryllium won't even react with water at any temperature, and Mg requires a high temperature to get it to react.  The other metals react very slowly at normal temperatures but do increase their rate as the temperature increases).