Chemical Equilibrium

Question 1

when is a chemical reaction said to be in equilibrium?

Answer 1

the rate of the forward reaction equals the rate of the reverse reaction
concentrations of reactants and products are constant (not equal)
the composition remains constant indefinitely

Question 2

what changes the position of equilibrium?

Answer 2

concentration
pressure
temperature

Question 3

what is the effect of a catalyst on equilibrium?

Answer 3

a catalyst speeds up the rate at which equilibrium is reached but does not affect the position of equilibrium

Question 4

what is the equilibrium constant?

Answer 4

capital K

Question 5

do solids and liquids appear in the equilibrium expression?

Answer 5

no
they are given a value of 1 and are omitted

Question 6

what does the equilibrium constant indicate?

Answer 6

small value - equilibrium lies to the left
large value - equilibrium lies to the right

Question 7

what affects the equilibrium constant?

Answer 7

only temperature
concentration pressure and catalysts do not affect K

Question 8

what is the ionisation equation for water?

Answer 8

H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq)

Question 9

what is Kw and its value at 25oC?

Answer 9

the ionic product of water: Kw = [H₃O⁺][OH⁻] = 1 × 10⁻¹4

Question 10

what is the pH formula?

Answer 10

pH = -log₁₀[H₃O⁺]

Question 11

what is the relationship between pH and pOH?

Answer 11

pH + pOH = 14

Question 12

what does it mean if [H₃O⁺] = [OH⁻]?

Answer 12

the solution is neutral
(pH = 7 at 25oC)

Question 13

what is a strong acid/base?

Answer 13

fully dissociates into ions in aqueous solution

Question 14

what defines a weak acid/base?

Answer 14

only partially dissociates into ions in aqueous solution

Question 15

examples of strong acids:

Answer 15

hydrochloric acid, nitric acid, sulfuric acid

Question 16

examples of weak acid

Answer 16

ethanoic acid
carbonic acid
sulphurous acid

Question 17

strong base examples

Answer 17

solutions of metal hydroxides (sodium hydroxide)

Question 18

weak base examples

Answer 18

ammonia
amines

Question 19

what is the pH of a salt solution from a strong acid + strong base?

Answer 19

neutral - 7

Question 20

what is the pH of a salt from a strong acid + weak base?

Answer 20

acidic pH

Question 21

what is the pH of a salt from a weak acid + strong base?

Answer 21

basic pH > 7

Question 22

what is an acid?

Answer 22

a proton donor

Question 23

what is a base?

Answer 23

a proton acceptor

Question 24

what is a conjugate acid?

Answer 24

the species formed when a base gains a proton

Question 25

what is a conjugate base?

Answer 25

the species formed when an acid loses a proton

Question 26

what is the equilibrium constant for the dissociation of an acid?

Answer 26

Ka = [H⁺][A⁻] / [HA] OR pKa = -log10 Ka

Question 27

formula to calculate pH of a weak acid from its dissociation constant

Answer 27

pH = ½(pKa - log₁₀c) c is acid concentration

Question 28

what is a buffer solution?

Answer 28

a solution that resists pH change when small amounts of acid or base are added

Question 29

what is an acidic buffer made of?

Answer 29

weak acid + its salt

Question 30

what is a basic buffer made of?

Answer 30

weak base + its salt

Question 31

how do buffers work when acid is added?

Answer 31

extra H+ ions react with conjugate base equilibrium shifts left

Question 32

how do buffers work when alkali is added?

Answer 32

OH- reacts with H+ so more acid dissociates equilibrium shifts right

Question 33

what is an acid-base indicator?

Answer 33

a weak acid where the acid and its conjugate base have different colours

Question 34

indicator dissociation equation

Answer 34

HIn(aq) + H2O(l) ⇌ H3O+(aq) + In-(aq)

Question 35

what determines the colour of an indicator?

Answer 35

the ratio of [Hln] to [ln-]

Question 36

what is the pH range for colour change in indicators?

Answer 36

pH = pKIn ± 1

Question 37

when does the theoretical colour change occur with indicators?

Answer 37

[H₃O⁺] = Kw

Question 38

how to determine the pH of an acid buffer solution?

Answer 38

pH = pKa - log10[acid] / [salt]

Question 39

pH scale for strong acid strong alkali

Answer 39

36960

Question 40

pH scale for strong acid and weak alkali

Answer 40

36957

Question 41

pH scale for weak acid and strong alkali

Answer 41

37082