acidic env. problem (1 Viewer)

[onebytwo]

Mg(OH)2 is considered a strong base, right?
If so, that means it dissociates completely in solution (or almost completely).
But Mg(OH)2 has a very limited solubility, so why is it referred to as a strong base?
any help is appreciated - thankyou

 

[Riviet]

Magnesium hydroxide is a strong base, so it does dissociate almost completely in solution. I'm not sure about the limited solubility of it though, what source was this information from?

 

[pLuvia]

I've encountered the limited solubility twice on these forums yet have not learnt it at school, is it in the syllabus?

 

[onebytwo]

Chemistry Contexts 2, pg 160, states "all group 1 and 2 metal hydroxides are strong bases....The group 2 metal hydroxides, howver, have limited solubility. Magnesium hydroxide is vitually insoluble....To the extent to these compounds dissolve they are completely dissociated and are therefire strong bases." it also shows an equation for the dissociation of Mg(OH)2, suggesting it is an equilibrium reaction.
how is it a strong base and at the same time insoluble?????????? - it contradicts itself
help.

 

[pLuvia]

It states that Mg(OH)₂ is virtually insoluble yet it is to an extent it will dissolve and dissociate hence why it is a strong base as it will completely dissociate

To the extent to these compounds dissolve they are completely dissociated and are therefore strong bases

 

[pkc]

I think you've got to interpret the meaning of "strong base "as substances which dissociate completely when they dissolve.


Otherwise you end up with contradictions such as magnesium hydroxide.

Personally, I think the naming of magnesium hydroxide as a "strong base" for all practical purposes is misleading. It is used in high concentrations in stomach antacid medicines that you take by mouth - try doing that with other strong bases eg sodium hydroxide- ouch.

 

[Riviet]

It is used in high concentrations in stomach antacid medicines that you take by mouth - try doing that with other strong bases eg sodium hydroxide- ouch.

In this case, it would not be dissociating, rather it is reacting directly with the acid, which in turn neutralises it.

 

[pkc]

In this case, it would not be dissociating, rather it is reacting directly with the acid, which in turn neutralises it.

Acids don't react directly with bases as such.

It's the OH- + H+ which is the neutralisation reaction and it occurs in the solution outside both the acid and the base molecules/lattice.

Removal of the OH- however drives the dissolving reaction of Mg(OH)2 to the right causing more dissociated OH- to be produced in its place.

 

[funking_you]

Mg(OH)2 is considered a strong base, right?
If so, that means it dissociates completely in solution (or almost completely).
But Mg(OH)2 has a very limited solubility, so why is it referred to as a strong base?
any help is appreciated - thankyou

This is a very important question, so i thought i'd better clear this up.

• A substance must be soluble in water for it to ionise (i.e. form its contituent ions).

• A substance that is completely soluble in water will completely ionises .

• the solubility rules for hydroxides are as follow: [All common metals hydroxides are INSOLUBLE, except those of Group 1 and the larger members of Group 2, i.e. Ca²⁺, Ba²⁺ and Sr²⁺.

• From the above rule, we learnt that Mg(OH)₂ is INSOLUBLE in water. As such it will NOT ionise to from it constituent ions.

• Therefore Mg(OH)₂ is NOT a strong base, since it is NOT soluble in water.

• Therefore, All Group 1 and Group 2 hydroxides are strong bases except Li(OH)₂ and Mg(OH)₂.

EXTENSION:
Solubility of an ionic compound is directly related to the strength of the ionic bonds .
The strength of an ionic bond is related to the charge of the ions and the distance between the ionic radii.
The strength of an ionic bond will

• increases as the charge of the ions increases

• decreases SIGNIFICANTLY as the distance between the ions increases

So a simple reason why the magnesium hydroxide is more insoluble that calcium hydroxide is that the magnesium ion is relatively smaller in size than the calcium ion, and hence froms a much stronger ionic bond with the hydroxide ion. The stronger this bond, the harder it is to overcome and so the compound wil not ionise easily in water.

The same person that posted this question, posted another question in a different thread enquiring as to why HF(aq) is a weak acid and HCl(aq) is a strong acid. The same theory explained here is behind this phenomena as well.
HF(aq) is a weak acid, because the ionic bond here is stronger due the small size of the F- ion.

This extension theory here is BEYOND the scope of the BoS syllabus.


Cheers,
George

 

[onebytwo]

• Therefore Mg(OH)₂ is NOT a strong base, since it is NOT soluble in water.

The same person that posted this question, posted another question in a different thread enquiring as to why HF(aq) is a weak acid and HCl(aq) is a strong acid. The same theory explained here is behind this phenomena as well.
HF(aq) is a weak acid, because the ionic bond here is stronger due the small size of the F- ion.

thanks for the post,
- so is the text book (CC2) technically wrong, to an extent, in stating that Mg(OH)2 is a strong base?
- in relation to the HF weak acid stuff, if we consider the halogen gruop ie group 7 of the periodic table and the order of the elements down the group, are u saying that this order has nothing to do with whether the molecule is a strong or weak acid, because HF is weak, HCl is strong, HBr is weak, HI is weak, there seems to be no "pattern" or trend
??????/

 

[funking_you]

- so is the text book (CC2) technically wrong, to an extent, in stating that Mg(OH)2 is a strong base?

They should not have stated that Mg(OH)2 is a strong base.

I have too many criticisms on what they have written on p160 in their discussion of strong hydroxide bases, and will not bother to discuss them all here.

- in relation to the HF weak acid stuff, if we consider the halogen gruop ie group 7 of the periodic table and the order of the elements down the group, are u saying that this order has nothing to do with whether the molecule is a strong or weak acid, because HF is weak, HCl is strong, HBr is weak, HI is weak, there seems to be no "pattern" or trend
??????/

HF(aq) is a weak acid for the reasons i gave before, HCl, HBr and HI are all STRONG acids.


Cheers,
George

 

[pkc]

• Therefore Mg(OH)₂ is NOT a strong base, since it is NOT soluble in water.

This is not true.

The strength of an acid depends on its pKa only.

...And pKa depends on how much of the stuff already dissolved then goes and splits up into H+ and -ve ion.

It doesn't depend at all on how much dissolved in the first place.

Mg(OH)2 is a strong base.



"Definition of pKa :

(pK for short). The negative logarithm of the acid dissociation constant, Ka.

Just like the pH, the pKa tells you of the acid or basic properties of a substance.
pKa <2 means strong acid
pKa >2 but <7 means weak acid
pKa >7 but <10 means weak base
pKa >10 means strong base "

http://www.everythingbio.com/glos/definition.php?word=pKa

 

[Monstar]

so for the purpose of the hsc.. is Magnesium hydroxide considered a strong base?

 

[Riviet]

• A substance must be soluble in water for it to ionise (i.e. form its contituent ions).

• A substance that is completely soluble in water will completely ionises .

• the solubility rules for hydroxides are as follow: [All common metals hydroxides are INSOLUBLE, except those of Group 1 and the larger members of Group 2, i.e. Ca²⁺, Ba²⁺ and Sr²⁺.

• From the above rule, we learnt that Mg(OH)₂ is INSOLUBLE in water. As such it will NOT ionise to from it constituent ions.

• Therefore Mg(OH)₂ is NOT a strong base, since it is NOT soluble in water.

• Therefore, All Group 1 and Group 2 hydroxides are strong bases except Li(OH)₂ and Mg(OH)₂.

This is what we're expected to know for the HSC.

 

[onebytwo]

ive just googled magnesium hydroxide and all the chemistry websites have claimed it to be a strong base. The Chem Coach is saying that its not a strong base
im still confused

 

[onebytwo]

i know that its commonly stated that alkali metals are "stronger" than the alkali earths (group 2), but if both contain strong bases, that ionise completley, how can one be "stronger" than the other??????

 

[Riviet]

i know that its commonly stated that alkali metals are "stronger" than the alkali earths (group 2), but if both contain strong bases, that ionise completley, how can one be "stronger" than the other??????

To keep things simple, we do not need to know which group of metals form stronger bases, we just need to know that the two groups form strong bases, other than any insoluble substances such as magnesium hydroxide.

 

[pkc]

As stated in previous posts,

Mg(OH)2 is a strong base, because if the high extent of its dissociation when dissolved (pKb).

Where does it say that the HSC requires a different answer?

 

[pkc]

i know that its commonly stated that alkali metals are "stronger" than the alkali earths (group 2), but if both contain strong bases, that ionise completley, how can one be "stronger" than the other??????

All bases vary in the extent to which they ionise in water (dissociate). No base dissociates 100%.

The scale for measuring this is the pKb scale (0-7 for bases), where 0 is a base which "highly dissociates", and 7 is a base which is "does not dissociate".

The "pKb" values (at 25C, 0.1M) for some strong bases (pKb <4):

sodium hydroxide = 0.2
potassium hydroxide = 0.5
calcium hydroxide = 1.4 - 2.4
magnesium hydroxide = 0.8-2.6

The "pKb" values (at 25C, 0.1M) for some weak bases (pKb 4-7):
ammonium hydroxide = 4.75


[Chemical Engineers Handbook 4th Edn.]


Hope this helps.

 

[funking_you]

theorectically magnesium hydroxide is a strong base.

however, for all practical purposes, if a question asks you to name a strong base that you could use in a titration against a strong acid, i would not choose magnesium hydroxide as a strong base.