SNS - General Chemistry - Acids and Bases Flashcards
Acids and Bases
Bronsted Lowry Definition
An acid is a substance that donates protons. A base is a substance that accepts protons In a solution where an acid donates a proton, the acid becomes a conjugate base and the base a conjugate acid A-H + B B-H + A- Acid Base Conjugate Acid Conjugate Base
Acids and Bases Arrhenius’s Definition
An acid is a substance that, in the presence of an aqueous solution, donates an H+ to that solution. A base is a substance that, in the presence of an aqueous solution, produces OH-. The amound of dissociation defines the strength of the acid or base. A substance that dissociates completely is considered to be a strong acid/base (large Ka or Kb) and one that dissociates partially is considered to be weak (small Ka or Kb) Ka = [A] [H]/[HA]
Acids and Bases Lewis Definition
An acid is a substance that accepts a pair of electrons. A base is a substance that donates a pair of electrons
Acids and Bases Amphoteric
or Amphiprotic Refers to a substance that can function as a base or an acid depending on its environment
Acids and Bases pH
Measures hydrogen ion concentration pH = -log [H+]
Acids and Bases pOH
Measures hydroxide ion concentration pOH = -log [OH-]
Acids and Bases
Equilibrium Constant
Expression for the Autoionisation of Water
K[H2O] = Kw = [H+] [OH-] = 1 x 10⁻14
Rewriting this in logarithmic form: pH + pOH = 14
Acid Base Calculations Neutralisation Formula
VOLacid x NORMALITYacid = VOLbase x NORMALITYbase
Acid Base Calculations
Buffer
Solution containing a mixture of a weak acid and its salt (which consists of a conjugate base and a cation) or a mixture of a weak base and its salt (which consists of a conjugate acid and its anion).
Undergoes a limited change in pH upon addition of a small amount of acid or base
For example, a buffer solution of acetic acid and sodium acetate:
CH3COOH ⇔ H+ + CH3COO-
When a small amount of NaOH is added to the buffer, OH- ions from the NaOH react with H+ present in the solution causing more CH3COOH to dissociate. When a small amount of HCl is added, its H+ ions react with the acetate ions to form acetic acid
Acid Base Calculations Buffering Capacity
Refers to the amount of acid or base a buffer can neutralise before the pH begins to change to an appreciable degree
Acid Base
Calculations
Henderson Hasselbach equation
Used ot estimate the pH of a buffer in a region where the concentrations of the species and its conjugate are present in approximately equal concentrations
Acid Base
Calculations
Henderson Hasselbach equation Weak acid buffer solution
pH = pKa + log ( [congugate base] / [weak acid] )
Therefore when [congugate base] = [weak acid], pH = pKa since log 1 = 0
Acid Base Calculations
Henderson Hasselbach equation
Weak base buffer solution
pOH = pKb + log ( [congugate acid] / [weak base] )
Arrhenius Acids
Nomenclature
Anions ending in -ide
Acids formed from anions ending in -ide have the prefix hydro- and the ending -ic
eg Cl- : HCl Hydrochloric acid
Br- : HBr Hydrobromic acid
Arrhenius Acids
Nomenclature
Anions ending in -ite
Oxyacids
Acids end in -ous acid
eg ClO- Hypochlorite : HClO Hypochlorous acid
ClO2- Chlorite : HClO2 Chlorous acid
Arrhenius Acids
Nomenclature
Anions ending in -ate
Oxyacids
Acid ends in -ic acid
ClO3- Chlorate : HClO3 Chloric acid
ClO4- Perchlorate : HClO4 Perchloric acid
NO3- Nitrate : HNO3 Nitric acid
Calculate the concentration of H+ in a 2M aqueous solution of CH3COOH (Ka = 1.8 x 10-5)
CH3COOH ⇔ CH3COO- + H+
Ka = [H+] [CH3COO-] / [CH3COOH = 1.8 x 10-5 = [x][x] / [2-x]
Since acetic acid only weakly dissolves in water, 2-x can be approximated to 2
x = 6 x 10-3M
Acid + Base Reaction
Strong Acid, Stong Base
Salt and water
Resulting solution pH7
Ions formed don’t react with water
Acid + Base Reaction
Strong Acid, Weak Base
Forms a salt but usually no water since weak bases are usually not hydroxides
The cation of the salt will react with the water solvent, reforming the weak base - hydrolysis
Acid + Base Reaction
Weak Acid, Stong Base
Forms a basic solution due to the hydrolysis of the salt to reform the acid with theconcurrent formation of an OH= ion from the hydrolysed water molecules
Acid + Base Reaction
Weak Acid, Weak Base
Froms a solution whose pH depends on the relative strengths of the reactants
For example, HClO has Ka = 3.2 x 10-8 and NH3 has Kb = 1.8 x 10-5. Thus an aqueous solution of HClO and NH3 is basic since Ka for HClO is less than Kb for NH3
Polyvalence
An acid or base which liberates more than one acid or base equivalent per mole
Acid and Base Equivalents
Equal to one mole of H+ or OH- ion respectively
Find the equivalent weight of H2SO4
Gram molecular weight = 98 g/mol
Releases two equivalents per mole
Equivalent weight = 98/2 = 49g
Amphoteric Species
Examples
- Water: H2O + B- ⇔ BH + OH-, H2O + HA ⇔ H3O+ + A-
- Partially dissociated conjugate bases of polyprotic acids, since they can either gain or lose a proton
- Hydroxides of certain metals - Al, Zn, Pb, Cr
- Species that can act as both oxidising and reducing agents
Titration of:
- Strong Acid with a Strong Base
Sigmoid curve with equivalence point at pH7. End point close to, but not precisely equal to the equivalence point. At the early part of the curve, when acidic species predominates, sddition of small amounts of OH- has little effect on pH. Likewise in the later part of the curve where basic species predominates. The addition of one species has the greatest effect around the equivalence point and thus pH changes most drastically in this region
Titration of:
Weak Acid with a Strong Base
Initial pH is much higher than for a strong acid. pH changes most significantly early on and the equivalence point is in the basic range
Titration of:
Base with a Acid to form a Polyprotic Acid
Divides into five regions:
- Little acid added and the predominant species is basic.
- More acid has been added and the basic species and partially dissociated conjugate base of the polyprotic acid are the predominant species in approximately equal concentrations. Flat part of the curve is the first buffer region corresponding to the pKa of the congugate base
- Contains the equivalence point at which all of the original base has reacted with H+ from the acid added. In the latter part of this region, the polyprotic acid congugate base is the predominant species
- Acid has neutralised approximately half of the polyprotic acid conjugate base. The polyprotic acid and its congugate base are present in roughly equal concentrations. The flat part of this curve corresponds to the pKa of the polyprotic acid
- The equivalence point for the entire titration is reached, as all of the congugate base is converted to polyprotic acid. Again a rapid change in pH is observed near the equivalence point as acid is added
