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Quick Question On Acids - HSN forum

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Quick Question On Acids


16 replies to this topic

#1 englisher

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Posted 27 May 2007 - 04:41 PM

Hey,

Looking for a definition here - what constitutes a concentrated acid and what is a dilute acid?

Hydrochloric acid 0.1mol l-1 , for example,

Surely that is 1x10-1 moll-1 of acid, then surely as it fully dissociates that is 1x10-1 moll-1 of H+ ions?? Is that not pH 1, hence concentrated?


Any help appreciated.

At the moment I'm thinking just to say that 1moll-1 and greater is concentrated and anything less is dilute? (This is only because I'm getting them wrong).

#2 BuckminsterFullerene

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Posted 27 May 2007 - 04:44 PM

I myself find such ambiguous, but would assume stuff like 1mol/l or above would be considered strong, seen many of those types in MChoice papers.
Chemical Engineering - Edinburgh

Conditional

#3 ginneswatson

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Posted 27 May 2007 - 05:30 PM

Concentrated acids usually mean 'pure acid'.

HCl is a gas. concentrated HCl is water with as much HCl disolved in it as is possible. HCl is very soluble though that is a lot.

H2SO4 and HNO3 are covalent liquids. However HNO3 is usually made by dissolving NO2 gas in water (at end of Ostwald Process) so again 'concentrated' signifies 'as much dissolved as possible'.

H2SO4 is made by dissolving SO2. However, to further confuse things, SO2 turns out to be not that soluble in water so it is dissolved in 98% sulphuric acid which goes up to 99% sulphuric acid.

The significance of the word 'concentrated' is that these 'pure' acids can often have different properties from the normal dilute acid.

For example, dilute acids react with metals, but only those above hydrogen in the reactivity series. In these reactions it is the H+ that oxidises the metal and H2 gas is produced.

Concentrated H2SO4 and HNO3 can react with any metal, including copper, silver & gold. However, it is the SO4 group and the NO3 group that oxidise the metal and SO2 and NO2 are produced respectively.

Your teacher may have used conc. HNO3 to produce brown NO2 during the equilibrium Topic.

Concentrated H2SO4 is also a dehydrating agent. React it with a carbohydrate and it 'removes water' from each molecule to leave only carbon.

Concentrated H2SO4 is used to help esterification (PPA). By removing the water produced, it helps slow down the reverse reaction and, therefore, increases the yield of ester. Other condensation reactions (Polymers) often use Concentrated H2SO4 for the same reason.

1M, 2M, 4M, 10M etc are getting steadily more concentrated but they are all still dilute acids.

Hope this has helped clear up that confusion. wink.gif


#4 englisher

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Posted 27 May 2007 - 10:49 PM

Thanks. So basically unless it's ridiculously concentrated it's going to be dilute?

Cheers

#5 verticalforce

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Posted 27 May 2007 - 11:05 PM

QUOTE(englisher @ May 27 2007, 11:49 PM) View Post
Thanks. So basically unless it's ridiculously concentrated it's going to be dilute?

Cheers


I used to think that even 0.1 mol/l of acid is concentrated because hey...it's pH 1 right? What can be more concentrated than that. laugh.gif

#6 will_789

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Posted 28 May 2007 - 12:24 AM

QUOTE(verticalforce @ May 28 2007, 12:05 AM) View Post
QUOTE(englisher @ May 27 2007, 11:49 PM) View Post
Thanks. So basically unless it's ridiculously concentrated it's going to be dilute?

Cheers


I used to think that even 0.1 mol/l of acid is concentrated because hey...it's pH 1 right? What can be more concentrated than that. laugh.gif

Haha...You've kinda got the wrong idea there...

#7 ginneswatson

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Posted 28 May 2007 - 07:10 AM

Solubility determines the limits of the pH scale. Most acids are soluble enough to form a 10 M solution (pH = -1) but none are soluble enough to dissolve 100 moles in a litre, 100 M (pH =-2) so the lowest pH is somewhere between -1 and -2.

Similarly for bases but they tend to be less soluble, so probably between pH = 15 and pH = 16.

Sorry, but I can't resist this. If you go onto Advanced Higher, be prepared for more 'surprises'. pH is temperature dependent so at a higher temperature, for example, the water constant is 10-16, meaning that H+ and OH- (they are still equal) are 10-8 each. In other words, neutral water has a pH = 8.


#8 kamran

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Posted 28 May 2007 - 10:43 AM

sorry but im a bit confused

""""""I used to think that even 0.1 mol/l of acid is concentrated because hey...it's pH 1 right? What can be more concentrated than that""""""""

im thinking the same as the person above, but what am i missing?? i dont understand why he's wrong???

#9 englisher

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Posted 28 May 2007 - 12:18 PM

Read the third post.

#10 bikerdude

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Posted 28 May 2007 - 12:35 PM


here are the basic defanitions of strengths and concentrations of acids or alkali's which are easily confused moreover important to know.

STRONG (ACID) -- fully dissociates in aqueous solution
WEAK (acid) -- not full dissociated in aq solution

CONCENTRATED (acid) -- a lot of solute in a little water... eg 2mol/l
DILUTE (acid) -- a little solute in alot of water....eg 0.1 mol/l

#11 bikerdude

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Posted 28 May 2007 - 12:42 PM

In addition to my last post,

It must be noted that in neutralisation reactions weak acids or alkalis can neutralise the same amount of solution as strong acids/alkalis!
It's to do with equilibrium, in a weak acid/alkali the equilibrium shifts to the right to pruduce more OH- or H+ ions as the lost OH- and H+ ions have formed water.

#12 kamran

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Posted 28 May 2007 - 12:48 PM

thx for tht, but where would you draw the line to which one is concentrated and which is dilute

would this be okish

if its less than 1, then its dilute, but if its greater or equall to one itss aight to say that its concentrated

#13 englisher

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Posted 28 May 2007 - 02:32 PM

QUOTE(bikerdude @ May 28 2007, 01:35 PM) View Post
here are the basic defanitions of strengths and concentrations of acids or alkali's which are easily confused moreover important to know.

STRONG (ACID) -- fully dissociates in aqueous solution
WEAK (acid) -- not full dissociated in aq solution

CONCENTRATED (acid) -- a lot of solute in a little water... eg 2mol/l
DILUTE (acid) -- a little solute in alot of water....eg 0.1 mol/l


I know about the strong and weak part but the thing you've said about 2mol/l isn't correct as ginnewatson has just said that it needs to be a lot more concentrated than 2mol/l to be "concentrated".

#14 englisher

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Posted 28 May 2007 - 02:38 PM

Further to above, 1mol/l is described as a "dilute acid" in one of the past paper questions in 2004 or 2005. Can't remember which.

#15 bikerdude

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Posted 28 May 2007 - 02:39 PM

Well iIwould hope it is correct. it came straight from the topic 3 HSN notes pg13.
Can anyone who is truley in the know clarify this! ah!

#16 englisher

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Posted 28 May 2007 - 03:05 PM

Haha oh no.

The third post down is from someone in the know so read that and see what you make of it then.

#17 ginneswatson

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Posted 28 May 2007 - 04:19 PM

All I can do is repeat what I've said above. However, for those who haven't 'met' me before I should 'confess' that I am a Chemistry teacher with about 25 years experience of teaching Higher & Advancrd Higher Chemistry.

A dilute acid is an acid dissolved in water (of whatever concentration, 0.1 M, 1M, 10M). The properties of a dilute acid are based on H+ ions
A concentrated acid is (as close as possible) the pure covalent molecular form of the acid. It will have different properties.

The pH scale does not stop at pH = 1. This is only 0.1 mol l-1.
1 mol l-1 would have a pH = 0.
10 mol l-1 would have a pH = -1
100 mol l-1 would have a pH = -2. (However not even acids are soluble enough to dissolve 100 moles in one litre of water. By that stage you would be more accurate to talk about dissolving water in the acid!)

When considering safety implications a distinction is sometimes made between acids of concentration greater than 2M and acids of concentration less than 2M. However, this does not marks some sort of boundary between 'dilute' and 'concentrated'.

Hopefully, this will 'nail' this thread. ph34r.gif








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