SNS - General Chemistry - Acids and Bases

Question 1

Acids and Bases

Bronsted Lowry Definition

Answer 1

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

Question 2

Acids and Bases Arrhenius’s Definition

Answer 2

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]

Question 3

Acids and Bases Lewis Definition

Answer 3

An acid is a substance that accepts a pair of electrons. A base is a substance that donates a pair of electrons

Question 4

Acids and Bases Amphoteric

Answer 4

or Amphiprotic Refers to a substance that can function as a base or an acid depending on its environment

Question 5

Acids and Bases pH

Answer 5

Measures hydrogen ion concentration pH = -log [H+]

Question 6

Acids and Bases pOH

Answer 6

Measures hydroxide ion concentration pOH = -log [OH-]

Question 7

Acids and Bases

Equilibrium Constant

Expression for the Autoionisation of Water

Answer 7

K[H2O] = Kw = [H+] [OH-] = 1 x 10⁻¹⁴

Rewriting this in logarithmic form: pH + pOH = 14

Question 8

Acid Base Calculations Neutralisation Formula

Answer 8

VOLacid x NORMALITYacid = VOLbase x NORMALITYbase

Question 9

Acid Base Calculations

Buffer

Answer 9

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

Question 10

Acid Base Calculations Buffering Capacity

Answer 10

Refers to the amount of acid or base a buffer can neutralise before the pH begins to change to an appreciable degree

Question 11

Acid Base

Calculations

Henderson Hasselbach equation

Answer 11

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

Question 12

Acid Base

Calculations

Henderson Hasselbach equation Weak acid buffer solution

Answer 12

pH = pKa + log ( [congugate base] / [weak acid] )

Therefore when [congugate base] = [weak acid], pH = pKa since log 1 = 0

Question 13

Acid Base Calculations

Henderson Hasselbach equation

Weak base buffer solution

Answer 13

pOH = pKb + log ( [congugate acid] / [weak base] )

Question 14

Arrhenius Acids

Nomenclature

Anions ending in -ide

Answer 14

Acids formed from anions ending in -ide have the prefix hydro- and the ending -ic

eg Cl- : HCl Hydrochloric acid

Br- : HBr Hydrobromic acid

Question 15

Arrhenius Acids

Nomenclature

Anions ending in -ite

Answer 15

Oxyacids

Acids end in -ous acid

eg ClO- Hypochlorite : HClO Hypochlorous acid

ClO₂- Chlorite : HClO₂ Chlorous acid

Question 16

Arrhenius Acids

Nomenclature

Anions ending in -ate

Answer 16

Oxyacids

Acid ends in -ic acid

ClO₃- Chlorate : HClO3 Chloric acid

ClO4- Perchlorate : HClO4 Perchloric acid

NO3- Nitrate : HNO3 Nitric acid

Question 17

Calculate the concentration of H+ in a 2M aqueous solution of CH3COOH (Ka = 1.8 x 10-5)

Answer 17

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

Question 18

Acid + Base Reaction

Strong Acid, Stong Base

Answer 18

Salt and water

Resulting solution pH7

Ions formed don’t react with water

Question 19

Acid + Base Reaction

Strong Acid, Weak Base

Answer 19

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

Question 20

Acid + Base Reaction

Weak Acid, Stong Base

Answer 20

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

Question 21

Acid + Base Reaction

Weak Acid, Weak Base

Answer 21

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

Question 22

Polyvalence

Answer 22

An acid or base which liberates more than one acid or base equivalent per mole

Question 23

Acid and Base Equivalents

Answer 23

Equal to one mole of H+ or OH- ion respectively

Question 24

Find the equivalent weight of H2SO4

Answer 24

Gram molecular weight = 98 g/mol

Releases two equivalents per mole

Equivalent weight = 98/2 = 49g

Question 25

Amphoteric Species

Examples

Answer 25

1. Water: H2O + B- ⇔ BH + OH-, H2O + HA ⇔ H3O+ + A-
2. Partially dissociated conjugate bases of polyprotic acids, since they can either gain or lose a proton
3. Hydroxides of certain metals - Al, Zn, Pb, Cr
4. Species that can act as both oxidising and reducing agents

Question 26

Titration of:

1. Strong Acid with a Strong Base

Answer 26

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

Question 27

Titration of:

Weak Acid with a Strong Base

Answer 27

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

Question 28

Titration of:

Base with a Acid to form a Polyprotic Acid

Answer 28

Divides into five regions:

1. Little acid added and the predominant species is basic.
2. 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
3. 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
4. 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
5. 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