Acids and Alkalis help

Why does an acid for H+ ions in water when there is no ionic bond occuring?? same with an alkali with OH- ions. Or am I majorly not understanding. Because in terms of H+ + OH- ====> H2O they are both non metals? Thanks :smile:

(edited 7 years ago)

Reply 1

brainzistheword

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Original post by JackLeggett

Why does an acid for H+ ions in water when there is no ionic bond occuring?? same with an alkali with OH- ions. Or am I majorly not understanding. Because in terms of H+ + OH- ====> H2O they are both non metals? Thanks :smile:

Feel free to correct me but I think they don't necessarily form H+ but more release them, because pH is the percentage of Hydrogen it contains so a stronger acid basically gives out more H+ ions than a weaker one. I'm not sure but I don't think bonding here is something you need to bother about, particularly not for GCSE anyway. :smile:

Reply 2

JackLeggett

OP

9

Original post by brainzistheword

Feel free to correct me but I think they don't necessarily form H+ but more release them, because pH is the percentage of Hydrogen it contains so a stronger acid basically gives out more H+ ions than a weaker one. I'm not sure but I don't think bonding here is something you need to bother about, particularly not for GCSE anyway. :smile:

Oh OK thanks starting A Levels this september so wanted to make sure I Know everything. THanks Againg :smile:

Reply 3

brainzistheword

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Original post by JackLeggett

Oh OK thanks starting A Levels this september so wanted to make sure I Know everything. THanks Againg :smile:

Ah cool - what subjects? I've just done GCSE this year too - are you looking forward to results day :wink:

?

Reply 4

JackLeggett

OP

9

Original post by brainzistheword

Ah cool - what subjects? I've just done GCSE this year too - are you looking forward to results day :wink:

?

Yeah not really that nervous though, what about you?? Im going to be doing Maths, Chemistry, Biology and maybe government and politics. How about you :wink:

Reply 5

GabbytheGreek_48

11

acids and bases is not til A2 that im aware of atleadt on the new specs I think

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Reply 6

username1539513

21

Original post by JackLeggett

Why does an acid for H+ ions in water when there is no ionic bond occuring?? same with an alkali with OH- ions. Or am I majorly not understanding. Because in terms of H+ + OH- ====> H2O they are both non metals? Thanks :smile:

It's to do with how well a given acid of any sort dissociates in solution. Generally speaking mineral acids dissociate more than organic acids hence why the proportion of H+ ions is higher in stronger acids than weaker ones

Reply 7

Tom1699

2

From what I have been taught. An acid, take Hydrochloric, for example, has H+ ions and Cl- ions. This means that it isn't ionised as the charges become stable. The difference between a strong and a weak acid is the concentration (the amount) of theseH+ ions. So if there is more H+ ions, the more acidic it is and it is more concentrated. The same applies to alkalis but it is OH- ions. I am not 100% sure, but I think it is something along those lines. Hope it helps anyway. :smile:

Reply 8

Hazard82

2

Original post by JackLeggett

Why does an acid for H+ ions in water when there is no ionic bond occuring?? same with an alkali with OH- ions. Or am I majorly not understanding. Because in terms of H+ + OH- ====> H2O they are both non metals? Thanks :smile:

A little intro to acid-base equilibria. In reality, H⁺ions are H₃O⁺ ions which are formed by water molecules (H₂O) accepting a proton (H⁺) with its lone pair of electrons on its Oxygen atom via dative covalent bonding.The reaction below is reversible:

H₂O + H⁺(Proton) ⇌ H₃O⁺[Water acting as a base]

For a chemical species to form ions in solution the species doesn’t have to be an ionic compound at all. Acids such as Ethanoic acid (CH₃COOH) behave in similar way and dissociate in solution: CH₃COOH ⇌ CH₃COO^- + H⁺

[Ethanoic acid behaving as an acid in the forward reaction]

The reason for this dissociation is because of bond fission(bond breaking) which occurs in solution. Ionic compounds such as NaCl just split (almost) completely into Na⁺ and Cl^- ions in solution that’s why you are probs used to seeing:

NaCl (aq) --> Na⁺ + Cl^- [Not in equilibrium]

Hope this answers your q

Reply 9

L33t

10

Original post by JackLeggett

Why does an acid for H+ ions in water when there is no ionic bond occuring?? same with an alkali with OH- ions. Or am I majorly not understanding. Because in terms of H+ + OH- ====> H2O they are both non metals? Thanks :smile:

Hi, I have just completed my A2 year of "OCR A" chemistry (which is sadly no longer with us :frown:

due to reform) and have a fairly simple explanation for you that I hope makes sense and explains your qualms in relation to this problem.

If an acid is introduced to water then yes it will dissolve into it's ions. This is called "dissociation" which i'm sure you've heard of before (the reason WHY water and acids react will be explained last). Strong acids like hydrochloric, sulfuric, phosphoric and nitric are 100% dissociated in aqueous solution i.e 1 mole, which is 6.02x10^23 molecules, of HCl will form (in theory) 1 mole of H+ ions and 1 mole of Cl- ions. Weak acids 'partially' dissociate meaning 1 mole of say propanoic acid, CH3CH2COOH, would maybe only produce 0.1 moles of H+ ions and 0.1 moles of CH3CH2COO- ions. When you do A-Level chemistry you will discover that each acid has what is called an "acid dissociation constant", Ka, which is a numerical figure that can be calculated to show to what extent an acid has dissociated in water i.e a bigger dissociation constant=more dissociation in an aqeous solution. This is based on the equilibrium constant, Kc, which tells you numerically whether your equilibrium lies to the right or to the left of a reaction.

Now after that background lets get into what happens when HCl is added to water to create a solution. We know that HCl is a strong acid (you might be told in exams whether something is strong or weak but you should know as there are very few strong acids most of which were mentioned above the rest you can safely assume are weak) so all its H+ ions will be in solution, but how does this happen? Well, upon addition of HCl to water a H+ ion from HCl combines with a water molecule to form a hydronium (H3O+) ion leaving the Cl- ion behind in the solution:

H2O + HCl ==> H3O+ + Cl-

The reason for this is because a H+ ion is VERY reactive and won't stay on its own for long at all, so it combines with the water molecule for stability. Now in theory there should be an equilibrium sign as the arrow in that equation but, because it is a strong acid it is correct to put either. Essentially when ANY protonic (H+ producing) acid is dissolved in water this hydronium-anion equilibrium is set up. In general for any acid, strong or weak:

HA + H2O <==> H3O+ + A-

A weak acid will have an equilibrium that lies far to the left of this reaction (very few H3O ions and A-ions), a weak acid will have a position of equilibrium that lies to the far right of the above eqilibrium. At A-Level you will only look at protonic acids but there are acids that do not contain hydrogen! We shant get into this because at A-Level you will likely only use the Bronsted-Lowry definition of an Acid and a Base whereby an acid is a "proton donor" and a base is a "proton acceptor". HOWEVER as chemists we tend to ignore this fundemental chemistry and simply write:

HA ==> H+ + A-

Why? well because its easier of course! We can do lots more with this equation.

Also a quick mind f**k and reason behind all this: water is itself dissociates making it essentially a very very very very very weak acid which is WHY this all happens. Acids do and can react with other acids you have just never been taught about this before. The general rule is whichever dissociates more will donate the H+ ion (proton) and seeming only 1x10^-14 water molecules in each litre of water releases a proton H2O ==> H+ +OH- water is ALWAYS the weaker acid hence why this reaction occurs! Hope this helps and isn't too complicated but get used to it if you wanna study chemistry at advanced level lol

If you would like any more information here (http://www.chemguide.co.uk/physical/acidbaseeqia/acids.html) is an excellent page that explains the mathematics behind this tricky physical chemistry concept. May I also say well done for being intrigued! Questions and thinking like this will get you very, very far in A-level chemistry and your other A-levels as you are thinking outside the box already. Good luck to you!

EDIT: with bases and alkalis nearly all of them are strong like NaOH KOH etc. so they 100% dissolve in water regardless because they are ionic lattices that break down into their constituent ions- simple as.

(edited 7 years ago)

Reply 10

brainzistheword

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22

Original post by JackLeggett

Yeah not really that nervous though, what about you?? Im going to be doing Maths, Chemistry, Biology and maybe government and politics. How about you :wink:

That's good! And they are cool subjects too. I'm quite nervous about results day but we will see how it goes. I'd be looking at doing Maths, Further Maths, Chemistry and Physics :smile:

Acids and Alkalis help

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