Metal finishing Q&As since 1989

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Mono vs. dibasic acids

Hello all,

I am also an A level student working on my investigation right now and it is basically the same as some previous questions, but I do not understand the answers given. I will like to know a suitable description of monobasic and dibasic acids (apart from one have one hydrogen atom and the other having 2).. and I will also like to know WHY dibasic acids have a higher or lower activation energy.

Thank you,

Ademola [last name deleted for privacy by Editor]
- London
2002

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I don't really understand what you want to know about mono and di basic acids. The fact that one has one replaceable hydrogen and the other has two is about all you really need to know. However, you may want to consider the pKa values of the two systems. The reactability of an acid is, in part, dependent on the ease of dissociation - if the acid doesn't want to "give up" its hydrogen, it won't react. Remember that dibasic acids can form "half salts" such as sodium bisulphate (sodium hydrogen sulphate). I would strongly suggest you ask your chemistry teacher to help you through this problem and you will get a one-to-one response that you can develop at your own speed and to meet your own needs.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2002

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Hey, I am also an A-level student doing the same investigation! It seams that apart from this question site, there are NO resources on mono and di basic acids. And if our exam boards are the same, our teachers aren't allowed to tell us the answers we have to find them for ourselves, problem! The main problem that I am having is justifying using suitable theory WHY the extra hydrogen ion in dibasic acids decreases the activation energy.

Thanks for you help!

Tara W [last name deleted for privacy by Editor]
- Watford, Herts, England
2003

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First of two simultaneous responses

I am an antique (ancient) chemist. I have never heard the term activation energy. Please reply with a book definition and I will see if I can find anything that might help.

James Watts
- Navarre, Florida
2003

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Second of two simultaneous responses

A molecule is an acid if it can give off an H+ ion when dissolved in water. 5 moles of basic acid HCl can give off 5 moles of H+ ions. Dibasic acids are different because you can get a lot more H+ ions than the number of acid molecules originally present. 5 moles of dibasic H₂SO₄ can give off 10 moles of H+ ions. That's what makes dibasic different from basic acids. There really isn't more than that as far as the definitions go.

Activation energy is high when a reactant molecule is more stable, and low when it is less stable. A basic acid only has one hydrogen to hold on to. A dibasic has to try and hold on to two of them. Dibasic acids, in general, have more complicated structures, and are therefore less stable. So, it usually requires lower activation energy to remove a H+ from a dibasic acid. However, once the first hydrogen is gone, it is usually not as easy to get the second one off.

Tim Neveau
Rochester Hills, Michigan
2003

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Is the order of reaction with respect to monobasic acid (HCl) the same as that of dibasic acid (H₂SO₄)? If yes why? If no why? Is the activation energy the same? If yes why? If no why?

Thank you,

Sam S [last name deleted for privacy by Editor]
- Oxford, England
2003

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I have just completed looking at the results I got for my a level investigation (the reaction b/w magnesium and acids). I found that the order with respect to hydrochloric acid was two, but the order with respect to sulfuric acid was one. I don't understand this, as I thought that the dibasic acid would have a higher order than the monobasic. I can't find any information anywhere and my investigation is in for Friday.......oh dear! can anyone tell me why it is that the order is two for HCl and one for H₂SO₄ ? is it maybe something to do with the fact that HSO4- is hard to ionise?

Thanks!

Lily M [last name deleted for privacy by Editor]
- Bath, UK
2003

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Sam, I could be wrong but I think you just posted your homework verbatim without even reading the page first :-)

Lily, I don't know exactly what "the order with respect to" means. But Tim has explained that a molecule of HCl will release only one hydrogen ion whereas H₂SO₄ will release two. No, I don't believe sulfuric acid is hard to ionize.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

A am also an A level student writing my course work on the same reaction of the order of reaction in respect to monobasic and dibasic acids. The order of reaction is involved in the rate equation Rate=k[A]^a[B]^b with the order being a and b. I am studying the order in respect to the acid (di and mono) so its something like rate=k[acid]^a. As the chemical equation does not indicate what the order of reaction should be I would like help to as why the orders are different for mono and di basic acids as there is practically no other information apart from "that's what it is".

Also I would like to know the difference between continuous and initial rate methods of measuring the rate of reaction.

Thank you,

David W [last name deleted for privacy by Editor]
Student - Hong Kong, China
2004

Hey. I am on A levels also and the understanding I have is that: you can look at rates by plotting the graph of the reaction./ the initial rate is the straight line part at the front of the graph,this is where there is a constant reaction between the acid and the metal. With regards to continuous it is the final amount of gas evolved divided by the total time that the reaction took.

Ashley B [last name deleted for privacy by Editor]
- UK
2004

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Hi, I am currently in my second year of A-levels, studying the edexcel Nuffield course. In Unit 5, the course work, I am studying the order of reaction for sulfuric and hydrochloric acid. I would be very grateful if someone knows, as I could then apply some logic to my illogical results.

Thanks

Bob M [last name deleted for privacy by Editor]
Student - Taunton, Somerset, England
2004

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ok, yet another one, doing the course work,

firstly, if you has the choice of doing weak and strong acids orders or mono and di, the sad truth is weak and strong turns out to be much easier as we know more at this level. sadly I have only just found that out.

as far as reacting with magnesium went, I calculated the rate by timing how long it took 10 cc of hydrogen to be produced. I got order one for the sulfuric acid and order 2 for hydrochloric acid.

I saw commented on here that this happens as even though the sulfuric is less stable and the first ion is easily pulled off, the second ion is held on much stronger.

what I would like to know is the process of the dissociation, as I am sure the mechanism is the best way to explain the results.

any help would be very very appreciated.

Tom C [last name deleted for privacy by Editor]
- Chalfont St. Giles, UK
2005

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Hi,

I'm another Nuffield student. I found that the reaction between magnesium and H₂SO₄ is order 2 at low concentrations then changes to order 1 at 0.75 M n above. Nuffield mentions this on their website so just thought I'd let you know.

I got an activation energy for 0.5 M HCl of 10.14 kJ/mol and 7.63 kJ/mol for H₂SO₄. Are these really low? everyone else seems to have much higher so just wondering if you could tell me if I've gone wrong!

Harriet T [last name deleted for privacy by Editor]
- Kenilworth, Warwickshire, UK
2006

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WHAT is the activation energy for the reaction of magnesium with hydrochloric acid. I want to compare it with the result I got in my experiment.
thanks,

Desha W.
- UK
2006

Ed. note: C'mon, Desha, this is like pulling teeth -- what answer did you get in your experiment?

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well I am doin the OCR Plan for as level as well and the question for us was to show that sulfuric acid is dibasic using both titration and gas collecting method.

for titration(dibasic) I was thinking of reacting H₂SO₄ with a base like sodium or potassium hydroxide and so the acid will be completely neutralized by 2 moles of those bases but for a monobasic acid(e.g. HCl) only 1 mole will be needed which will prove that H₂SO₄ is dibasic...am I correct in doing this?

for gas collection I will first react a carbonate(MgCO₃) with a monobasic acid(HCl)and the ratio of acid used to CO₂ produced with be 2:1 but for a diprotic acid it will be 1:1. So I will only need half the amount of H₂SO₄ compared to HCl to produce the same amount of CO₂ which will again show that H₂SO₄ is dibasic...

are those two methods correct than?

Bilal T [last name deleted for privacy by Editor]
- Pontefract, England
April 18, 2008

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April 20, 2008

the question from me is to show that sulfuric acid is dibasic using both titration and gas collecting method.

for titration(dibasic) I was thinking of reacting H2SO4 with a base like sodium or potassium hydroxide and so the acid will be completely neutralized by 2 moles of those bases but for a monobasic acid(e.g. HCl) only 1 mole will be needed which will prove that H2SO4 is dibasic...am I correct in doing this?

for gas collection I will first react a carbonate(MgCO3) with a monobasic acid(HCl)and the ratio of acid used to CO2 produced with be 2:1 but for a diprotic acid it will be 1:1. So I will only need half the amount of H2SO4 compared to HCl to produce the same amount of CO2 which will again show that H2SO4 is dibasic...

are those two methods correct than?

Bilal T.
Student - Pontefract, England

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April 28, 2008

Hi there I am also doing this ocr planning exercize and was needing a bit of help.
I have been researching for days and none of it seems to make any sense to me, I'm a simple girl so would appreciate simple language, I have done many titrations in class but just got given the acid and alkali not had to choose which to use... Am I right in saying that I have to react sulfuric acid (1mol dm-3) with HCl and when my indicator changes colour, I have to calculate the molecular formula, emperical formula, moles etc to work out that 2 moles of hydrogen were given off?

Help would be greatly appreciated
Thank you

Chili S.
- Sheffield, England

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Some of the answers given were too precise, please try to give more differences, and work examples on mono basic and di basic. Because my country lacks good science teachers.
Thanks.

Mohamed Shekuba kamara
- Freetown, Sierraleone
March 13, 2012