Acid and dynamic equilibrium......

[albertlee]

In what condition, the reaction will be a dynamic equilibrium??? ie, there are are both back and forwards reaction taken place at the same time

 

 

For ethnoic acid, we know it is weak acid, because the reaction taken place in the solution is not completely forwards, it has both back and forward, so is a dynamic equilibrium, but why it does not fully be forwards reaction, like HCl???

 

The second question reflects the questioning upon the 1st one

 

Any body can help??

 

Oh, yes, according to my knowledge, for the formula of ethonoic acid, something like CHCOOH, the positive ion is H, and the anion is CHCOO, my chemistry teacher told me that positive ion starts first in a chemical formula long time ago, how come it is not something like HCHCOO???

 

Secondly, what is the difference between bases and alkalis??

 

thx

 

Albert

[albertlee]

Can any one help me with this????

 

thx for furthur responds

[mak10]

In what condition, the reaction will be a dynamic equilibrium??? ie, there are are both back and forwards reaction taken place at the same time

 

the only conditions i can think of is that the system, where the reaction takes place, must be closed/isolated system and the influx of energy, if its there, must be constant.

 

For ethnoic acid, we know it is weak acid, because the reaction taken place in the solution is not completely forwards, it has both back and forward, so is a dynamic equilibrium, but why it does not fully be forwards reaction, like HCl???

 

the reaction is reversible but that doesn't mean all solutions of ch3cooh are in dynamic equilibrium. its a weak acid because it undergoes incomplete ionization, i.e - produces less H+ ions compared to ch3coo- ones.... the ratio is something like 1:99. and whether a reaction proceeds fuly forward or backward really depends on its electrode potential, whether oxidation or reduction is the chemical reaction's forte... i think. i can explain the latter in detail if u're interested...

 

Oh, yes, according to my knowledge, for the formula of ethonoic acid, something like CHCOOH, the positive ion is H, and the anion is CHCOO, my chemistry teacher told me that positive ion starts first in a chemical formula long time ago, how come it is not something like HCHCOO???

 

pardon me but i don't exactly understand what u're asking here. the formula is ch3cooh and yes, the positive ion is H+... so why doesn't the formula start off with the positive ion?? u can have ch3cooh as h3ccooh, but u certainly cannot have it as h4ccoo... the h of the coo which forms the positive ion must be with coo which must be written with, if not after, ch3. but u can write it as hoocch3... as long as the h+ is with the coo. and i don't recall any rule strictly stating that all formulae shud start off with the positive ion.... and even if there is, as per ur teacher, i highly doubt it being applicable to organic compunds like ethanoic acid.

 

Secondly, what is the difference between bases and alkalis??

 

anything which reacts with an acid is a base and bases that are soluble in water are alkalis.

 

-mak10

[albertlee]

whether a reaction proceeds fuly forward or backward really depends on its electrode potential, whether oxidation or reduction is the chemical reaction's forte...

 

What do you mean by its electrode potential and chemical reaction's forte????

 

produces less H+ ions compared to ch3coo- ones.... the ratio is something like 1:99.

 

How does it happen??? a positive ion should be bonded with a negative ion..... so there should be equaled amount of positive ion and negative ion....

 

 

 

 

 

 

Why does H and coo has to be written together???

 

there are only two ions H+ and ch3coo-, I dont see why the positive ion H+ has to be written with some elements (coo) of the negative ion, shown on a formula....

 

Albert

[albertlee]

mak10??? Or any one can help??? thx

[Skye]

why it does not fully be forwards reaction, like HCl???

Because it doesn't fully ionise, as was said by mak10. The amount that a substance will ionise, and the strength of the acid, depends on three things:

 

1) How polarised it is. Highly polarised molecules, like HCl where the H has a strong positive charge and the F has a strong negative charge, will dissociate into water. Ethanoic acid is relatively less polar, so it doesn't dissociate into water so easily.

 

2) Bond strength. HCl is a fairly polar molecule, however it's not as strong an acid as you might expect. This is because the strength of the H-Cl bond is also fairly high, which prevents dissociation.

 

3) The stability of the conjugate base.

 

 

A dynamic equalibrium is where there can be lots of small changes, but overall there is no change from the equilibrium. These small changes are because ions and molecules are all moving around in solution, bumping into each other and these collision can cause either dissociation or association. So for ethanoic acid, the CH3COOH molecules are constantly dissociating into CH3COO- and H+ ions, and the ions are constantly associating into molecules again. However overall, once equilibrium has been reached and there are no more changes made, the equilibrium concentrations of ions and molecules stays the same, the associations and dissociations are equal.

[albertlee]

but the there are huge amount of partially positive and partially negative ends from water molecule, which could overcome the forces of CH3COO- and H+ to dissociate them, thus how could those ions asscociate with each other??? it should cost more energy to be stable, right???

 

Albert

[mak10]

What do you mean by its electrode potential and chemical reaction's forte????

 

i dont know if i can explain electode potentials in a nutshell (you need to have fairly good idea about redox chemistry for which u can refer to any good advanced chemistry textbook) but let me give it a shot

 

any feasible set of reactions that take place, either involves loss (oxidation) or gain (reduction) of electrons. thus all reactions have an electrode potential value, which can be positive or negative, determined by electrolysis reactions (for details how its done, see here but be prepared for some agnozing moments!). if a reaction has a positive EP value, the forward part of the reaction, i.e - the product side, is more stable than the backward part, i.e - the reactant side, that is to say that the position of the equilibirum lies to the right. its the other way round if the EP is -ve. example:

 

1) Mg2+ + 2e- ===> Mg [EP = -2.37]

2) Cu2+ + 2e- ===> Cu [EP= +0.34]

 

the EP of the 1) reaction shows that the position of the equilibrium lies to the left, i.e - Mg2+ does not get easily reduced to Mg. the EP of 2) reaction shows that, compared to the EP of Mg2+, the position of equilibrium of copper lies to the right, i.e - the Cu2+ is almost fully reduced to Cu, since its easier, as the EP predicts.

 

Similarly, the EP of the reaction,

 

CH3COOH <==> CH3COO- + H+

 

is probably negative, hence the position of this equilibrium lies well to the left, as un-ionised ethanoic acid molecules, which is why its a weak acid. other factors such as the delocalisation of negative charge by oxygen lone electron pairs over the ethanoate ion "pushed" by the alky group attached (as they are an electron donating group), attracting the positive hydrogen ion and thus re-forming the un-ionised molecule most easily... are also important in determining the negative EP of the reaction.

 

here are some links u can go through to learn more, if u're interested in electrode potentials and the chemistry involved:

 

http://au.encarta.msn.com/encyclopedia_761569809/Electrochemistry.html

http://www.physchem.co.za/Redox/SEP.htm

http://www.pinkmonkey.com/studyguides/subjects/chem/chap9/c0909601.asp

 

How does it happen??? a positive ion should be bonded with a negative ion..... so there should be equaled amount of positive ion and negative ion....

 

well, there is 1 mole of ch3coo- and 1 mole of h+ in the ch3cooh molecule, by the reaction equation... but i am talking about how much of ch3coo- and h+ are ionised during the reaction... and out of the 1 mole, only 0.01 moles of H+ and 0.99 moles of ch3coo- are probably ionised at any one instant of time.

 

Why does H and coo has to be written together???

 

oxygen, in its un-ionised/neutral state, has to form two bonds.... so h is written in the cooh to indicate that one oxygen has formed double bonds with carbon and the other has formed a single bond with carbon and a single bond with hydrogen.

 

but the there are huge amount of partially positive and partially negative ends from water molecule, which could overcome the forces of CH3COO- and H+ to dissociate them, thus how could those ions asscociate with each other???

 

you are confusing the ionisation of an acid in an aqueous solution with their dissolution in dilute solutions. remember, the partially +ve end of the water is still H+.

 

-mak10

[albertlee]

you are confusing the ionisation of an acid in an aqueous solution with their dissolution in dilute solutions. remember, the partially +ve end of the water is still H+.

 

I cant figure out the meaning of dissolution and dilute solutions, but any way, is the molecule of ch3cooh non-polar???

 

Albert

[ans]

Conditions required for dynamic equilibria

-a closed system

-reversible reaction

-rate of forward reaction = rate of backward reaction

-concentration of reactants and products do not change.

[ans]

.......

[albertlee]

is the molecule of ch3cooh non-polar???

[budullewraagh]

ethanoic acid is polar...

[albertlee]

Once the ethanoic acid is ionized, at any instant time, the cations and anions should be overwhemingly attracted by the partial charges of H2O....., hence could those ions, at any instant time, form back to molecular form??

 

Albert

[budullewraagh]

technically one could say that ethanoic acid isn't a compound...

[albertlee]

Eh,... so, what's the answer to my question???

[budullewraagh]

the answer is no, because it's really an ionic compound rather than a molecule

[albertlee]

so, if we leave an ethanoic acid alone, it will be more acidic as the time passes??? because at any instant time, there are some ethonoic acid being ionised??

 

Albert

[budullewraagh]

it only acts acidic if it's wet or fused