Theories of Acids and Bases 08

Question 1

Arrhenius theory of acids and bases

Answer 1

An acid is a substance that dissolves in water to produce H+

A base is a substance that dissolves in water to produce OH-

Question 2

Bronsted Theory of acids and bases

Answer 2

An acid is defined as any species that donates a proton, H+

A base is defined as any species that accepts a proton OH-

Question 3

A bronsted acid-base reaction

Answer 3

involves the transfer of an electron from an acid to a base

Question 4

Where do bronsted reactions occur

Answer 4

aqueous solution, gases, non-aqueous solutions and homogenous mixtures

Question 5

how to form a conjugate base

Answer 5

an acid after donating a proton

Question 6

how to form a conjugate acid

Answer 6

a base after accepting a proton

Question 7

Why is the reversible reaction of a conjugate base of a strong acid (HCl) negligible?

Answer 7

Conjugate bases of strong acids have a lower tendency to accept protons

Question 8

Why is does the conjugate base of a weak acid have a reversible reaction?

Answer 8

Conjugate bases of weak acids have a higher tendency of accepting protons

Question 9

formula to find pH

Answer 9

-lg[H+]

Question 10

formula to find concentration of H+

Answer 10

[H+]=10^-pH

Question 11

formula to find pOH

Answer 11

-lg[OH-]

14-(-lg[OH-])

Question 12

formula to find concentration of [OH-]

Answer 12

10^-pOH

Question 13

Equilibrium constant, Kc is

Answer 13

[H+][OH-]/[H2O]

Question 14

Kw, ionic product of water is:

Answer 14

Kw=[H+][OH-]
=Kc x [H2O]

Kw depends only on temperature, and as always constant at a constant temperature

Question 15

acid dissociation constant, Ka

Answer 15

Ka=H+A-/HA

the value of Ka indicates the extent at which the acid dissociates in water

a larger Ka and smaller pKa =stronger acid

Question 16

pKa

Answer 16

-lg[Ka]

Question 17

what does Ka and Kb depend on?

Answer 17

temperature only

Question 18

base dissociation constant, Kb

Answer 18

Kb= BH+OH-/[B]eqm

larger Kb and smaller pKb =stronger base

Question 19

pKb

Answer 19

-lg Kb

Question 20

degree of dissociation, a

Answer 20

a
=amount ionised/initial amount
=[acid]dissociated/[acid]initial

for strong acids, a is close to 1
for weak acids, a is &laquo_space;1

Question 21

*how to determine the POE and determination of the weaker/stronger acid/base

Answer 21

if the Kc value is very small, the POE lies on the EXTREME LEFT, the reaction favours the backward reaction, hence the reactants on the right side is the stronger base/acid and is a better proton acceptor/donor

Question 22

*definition of a buffer solution

Answer 22

it resists pH changes upon addition of a small amount of acid or base.

an acidic buffer contains a weak acid and its conjugate base

a buffer contains a WEAK bronsted acid and a WEAK bronsted base, forming a conjugate acid-base pair.

Question 23

• how does a buffer work?

Answer 23

when a SMALL amount of H+ is added to the solution, the H+ ions react with the large reservoir of xxxOH-, the conjugate base, hence, the added H+ ions are removed and the pH remains almost constant.

Question 24

strong acid-strong base titration suitable indicator

Answer 24

methyl orange or thymol blue

Question 25

weak acid-strong base indicator

Answer 25

thymol blue

Question 26

strong acid-weak base indicator

Answer 26

methyl orange

Question 27

at the equivalence point:

Answer 27

there is a sharp increase in pH as the acid has been completely neutralised

Question 28

why is there a buffer solution formed in weak-acid strong base but no buffer solution formed in strong acid-weak base?

Answer 28

once the strong base is added and some acid is neutralised, a weak acid and its salt (conjugate base) forms, therefore buffer solution is formed. the buffer solution causes the part before the steep increase at the equivalence point have a gentle upward gradient.

when the weak base is added, the H+ ions from the strong acid neutralises it, hence there is no weak base to form a buffer solution

Question 29

working range of methyl orange and thymol blue

Answer 29

MO: 3.1-4.4, from red to yellow
TB: 8.0-9.6, from yellow to blue

Question 30

Kw

Answer 30

[H+][OH-]

pKw= pH + pOH

affected only by temperature