The chapter on Acids Bases and Salts doesn’t have to be confusing. Over the June Holidays, a couple of students have emailed me with their questions on acids. Rather than answer them individually, I thought of saving a little time and putting it all here in a blog post!
In addition to knowing the basics – definitions, examples, basic properties, here’s a list of 3 things you absolutely need to know if you want to master the chapter on Acids!
1. Strength of an acid.
Acids can be classified as Strong or Weak.
Strong acids are substances which undergo full ionisation when dissolved in water to produce H+ ions.
Weak acids are substances undergo partial ionisation when dissolved in water to produce H+ ions.
If you’re not too sure what this means, look at the diagram below!
Notice how ALL the acid molecules in a strong acid has split into ions whereas only FEW of the acid molecules in a weak acid have split?
Since all the acid molecules have split into ions, there will be more H+ ions in a strong acid than a weak acid, hence it’s pH is lower!
2. Basicity of an acid
Monobasic acids – 1 acid molecule produces 1 H+ ion upon ionisation/dissociation
E.g. Hydrochloric acid HCl, nitric acid HNO3.
HCl(aq) -> H+(aq) + Cl-(aq)
HNO3 (aq) -> H+(aq) + NO3- (aq)
Dibasic acid – 1 acid molecule produces 2 H+ ions upon ionisation/dissociation
E.g. Sulfuric acid H2SO4
H2SO4 (aq) -> 2H+(aq) + SO42-(aq)
Tribasic acid – 1 acid molecule produces 3 H+ ions upon ionisation/dissociation
E.g. Phosphoric acid H3PO4
If one mole of an acid is neutralised by 2 moles of NaOH, then the acid is dibasic!
Basicity is useful to explain why although both HCl and H2SO4 are strong acids, H2SO4 has a lower pH, and reactions involving H2SO4 are often faster than HCl!
3. Reactions of acids
Acids react with Metals (except Cu, Ag, Au in reactivity series), Carbonates and Bases – the majority of students should be able to recall this very quickly! But… do you know what substances acids DO NOT react with? =)
Acids in general DO NOT react with salts!
E.g. HCl (aq) + NaNO3 (aq)-> no reaction!
HNO3 (aq) + CuSO4 (aq) -> no reaction!
However, there is an exception to this!
Acids can ‘react’ with a salt if the product formed is an insoluble solid! (Think ionic precipitation!)
E.g. HCl (aq) + AgNO3 (aq) -> AgCl (s) + HNO3(aq)
H2SO4 (aq) + Ba(NO3)2 (aq) -> BaSO4 (s) + 2HNO3 (aq)
This usually comes out in MCQs… where students are given 4 seemingly correct options and are asked which of the following takes place!
A H2SO4 + FeCl2 -> FeSO4 + 2HCl
B 2HCl + PbSO4 -> PbCl2 + H2SO4
C 2HNO3 + MgCl2 -> Mg(NO3)2 + 2HCl
D H2SO4 + Ba(NO3)2 -> BaSO4 + 2HNO3
Do you know which is the correct option? =)
If you want to check that you really understand what you’ve read, go pull out your TYS or your assessment books and attempt some questions on Acids! I will be posting more content on Bases and Salts over the next few days! Till then, happy revising!