Summary
Summary Notes 3.1.12 - Acids and bases (A-level only)
- Course
- Institution
Summary Notes 3.1.12 - Acids and bases (A-level only)
[Show more]
Seller
Follow
Taylorsnotes
Content preview
Taylor Notes Acid-Base EquilibriaBrønsted-Lowry Theory
When hydrogen bromide is dissolved in water, it acts as an acid
HBr + H2O ⇌ H3O+ + Br-
HBr acts as a Brønsted-Lowry acid because it donates protons
H2O acts as a Brønsted-Lowry base because it accepts protons
The proton donor and acceptor on the right-hand side of the equation are called the conjugate acid and conjugate
base
Br- acts as the conjugate base because it accepts a proton
H3O+ acts as the conjugate acid because it donates a proton
Equation Acid Base Conjugate Acid Conjugate Base
H2O + NH3 ⇌ OH− + NH4+ H2O NH3 NH4+ OH−
H2O + HCl ⇌ Cl− + H3O+ HCl H2O H3O+ Cl−
OH- + HCOOH ⇌ HCOO- + H2O HCOOH OH- H2O HCOO-
H2SO4 + NH3 ⇌ HSO4− + NH4+ H2SO4 NH3 NH4+ HSO4−
CH3COOH + HCl ⇌ CH3COOH2+ Cl- HCl CH3COOH CH3COOH2+ Cl-
pH Calculations
The H+ ion is responsible for a substance being acidic The higher the concentration of this ion, the greater the
acidity
pH = -log10[H+] [H+] = 10-pH
Strong Acids
A strong acid is one which fully dissociates HCl (aq) H+ (aq) + Cl- (aq)
The hydrogen chloride shown above is a strong acid because the reaction is irreversible (goes to completion)
As strong acids fully dissociate, the concentration of H + ions can easily be calculated from the concentration of the
acidic molecule, HX
[HX] = [H+] 1 mol dm-3 of HCl 1 mol dm-3 H+
Calculate the pH of the following solutions:
a) 0.10 mol dm-3 HCl
b) 0.010 mol dm-3 HNO3
Calculate the concentration of the following solution:
a) pH of HCl = 2.51
b) pH of HNO3 = 1.10
The examples above are all monoprotic strong acids as they fully dissociate to produce one H + ion per molecule
Sulphuric acid is an example of a strong diprotic acid H2SO4 (aq) 2H+ (aq) + SO42- (aq)
Calculate the pH of the following solution a) 0.15 mol dm -3 H2SO4
, Taylor Notes Acid-Base Equilibria
Weak Acids
A weak acid is one which partially dissociates in solution HA (aq) ⇌ H+ (aq) + A- (aq)
The chemical equation shows that HA is a weak acid because the reaction is reversible, therefore it partially
dissociates
As weak acids partially dissociate, the concentration of H + ions cannot be assumed to be the same as the
concentration of the acidic molecule, HA
To perform pH calculations with weak acids, we must use the acid dissociation constant, K a
[H+][A−]
Ka =
[HA]
A stronger acid will have a higher value of K a because the equilibrium position will lie further to the right
A ‘weaker’ weak acid will have a lower value of K a because the equilibrium position will lie further to the left
For pure weak acids [H+] = [A-] or [H+] = √ Ka x [ HA ]
[H+]2 or [H+] = √ Ka x [ HA ]
Ka =
[HA]
Calculate the pH of the following solutions of ethanoic acid (K a = 1.75 x 10-5)
a) 2 mol dm-3 of CH3COOH
b) 0.200 mol dm-3 CH3COOH
Calculate the value of Ka for the following weak acids based on their pH and concentrations
a) HNO2 of concentration 0.100 mol dm-3 with a pH = 2.17
b) HBrO of concentration 1.00 mol dm-3 with a pH of 4.35
Ka values are small and inconvenient Sometimes pKa is used instead as it gives a more convenient number in the
same way that pH values are easier than [H +]
pKa = -log10(Ka) or Ka = 10-Ka
Calculate the pKa of the following weak acids
a) CH3COOH (Ka = 1.75 x 10-5)
A stronger weak acid will have a higher value of K a because the equilibrium position will lie further to the right This
means it will have a lower value of pKa
A ‘weaker’ weak acid will have a lower value of K a because the equilibrium will lie further to the left This means it
will have a higher value of pKa
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller Taylorsnotes. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $6.46. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
53340 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now