Acids and Bases Check
1. Define Bronsted-Lowry base;
2. Identify the acid and base;
HSO4-(aq) + H2O(l) ←→ H3O+(aq) + SO42-(aq)
3. What is a dibasic base? Give an example.
4. What is the concentration of HCl if 0.5 moles arepresent in 25 cm3?
Objectives
1. Define pH;
2. Explain the difference between strong and weakacids and bases;
3. Calculate pH for strong acids.
What is pH?
pH = - log10[H+(aq)]
pH; defining and determining
[H+ (aq)] means based on H+ ionconcentration (moldm-3)
Hydrogen ions are usually represented asH+(aq) rather than the hydrated H3O+(aq)
What does pH mean?
pondus hydrogenii,
the power of hydrogen
pH values of a range of 0.1moldm-3 solutions ofsubstances measured by a pH meter
Measuring pH
1.The approximate pH of a solution can beobtained by using indicators whose colourchange with pH is known.
2.pH can also be measured electronicallyusing a pH meter.
Strong Acids and Bases
HCl(aq) → H+(aq) + Cl-(aq)
NaOH(aq) → Na+(aq) + OH-(aq)
“Virtually completely dissociated(ionised) in water.”
Weak Acids and Bases
CH3COOH(aq) ↔ H+(aq) + CH3COO-(aq)
NH3 (aq) + H2O(l) ↔ NH4+(aq) + OH-(aq)
“Partially dissociated(ionised) in water.”
There are a limited number of strong acids andbases in water.
•Hydroxide ion, OH-
Acids
Bases
•Hydrohalic acids
(HCl, HBr and HI)
•Nitric acid, HNO3
•Sulphuric acid, H2SO4
•Perchloric acid, HClO4
A strong acid does NOT have tobe 100% ionised. Any ionisationabove 50% entitles the acid tobe classed as strong.
The majority of acids are weak and are not fully ionised in water at
normal concentrations.
Examples
HCl(aq) + H2O(l) ←→ H3O+(aq) + Cl-(aq)
KOH(aq) ←→ K+(aq) + OH-(aq)
H2SO4(aq) + H2O(l) ←→ H3O+(aq) + HSO4-(aq)
HSO4-(aq) + H2O(l) ←→ H3O+(aq) + SO42- (aq)
Calculating pH forstrong acids
[H+(aq)] = 1 moldm-3
pH ?
[H+(aq)] = 2.5 moldm-3
pH ?
[H+(aq)] = 10 moldm-3
pH ?
Concentration limitation
•pH can never have very large negativevalues since these would demand veryconcentrated acid solutions
•Very concentrated solutions areimpossible to obtain!
•[H+] is limited by the solubility of the acidin water as well as by ion association insoutions
Quick recap
1.Define pH
2.Define a strong acid, with example
3.Define a weak acid, with example
4.pH can never have very large negativenumbers. Explain.
5.Calculate the pH of 0.1moldm-3 HNO3,0.2moldm-3 KOH, 0.05moldm-3 H2SO4.
6.Now, suggest a definition for a monoprotic anddiprotic acid
Is pH affected by dilution?
Strong acid
Weak acid
Concentration(moldm-3)
pH
Concentration(moldm-3)
pH
0.1
1.00
0.1
2.60
0.01
2.00
0.01
3.40
0.001
3.00
0.001
4.30
Can you explain the results?
Use HCl as an example of a strong acid andCH3COOH as the weak acid
Write dissociation eqns / reaction with water
Is pH affected by dilution?
Strong acid
Weak acid
Concentration(moldm-3)
pH
Concentration(moldm-3)
pH
0.1
1.00
0.1
2.60
0.01
2.00
0.01
3.40
0.001
3.00
0.001
4.30
HCl + H2O Cl- + H3O+
CH3COOH + H2O CH3COO- + H3O+
Dilution of a weak acid
•In 0.1M ethanoic acid, approximately 1.3% ofthe molecules are dissociated into ions.
•In 0.01M ethanoic acid, approximately 4.1%of the molecules are dissociated into ions.
•In 0.001M ethanoic acid, approximately 13%of the molecules are dissociated into ions.
Homework