IB chemistry R3.1

what is a bronsted lowry acid

a proton donor

what is a bronsted lowrly base

a proton accepter

how do you represent a proton in an aqueous solution

H+ or H3O+

what is a conjugate acid-base pair

a pair of species differing by a single proton

in the conjugate acid and base pair, which one has one more proton

the acid

what is an alkali

a soluble base that can dissolve in water and release a hydroxide ion

what is an amphiprotic species

substances that can act as both an acid and a base

must possess both a lone pair of electrons and hydrogen

what is the pH used to describe

the H+ of a solution.

the pH number is ——- related to the [H+]

inversely

a change in one unit of pH is a —- change in

10 fold

what is Kw

the ionic constant of water [H+][OH-]

in pure water, what is Kw

[H+]=[H+], so H+=Kw²

Kw depends on —— because ——

temperature, equilibrium constant

what is a strong acid

one that fully dissociates

a strong acid has a —- conjugate base, and vice versa

weak

what is a weak acid

one that partially dissociates

which conjugate do dissociation reactions favour

the weaker one

what is a strong base

one that is fully ionized (dissociates fully)

which acids are strong acids

HCl, HBr, HI, HNO3, H2SO4, group 17 exceot HF

which bases are strong

LiOH, NaOH, KOH, all of the bases made from alkali metals

what is the periodic trend of acid strength

as you go down a group, the acid strength increases because there is an increase in the H-halogen bond, which is weaker and needs less energy to break. so, the strength of the hydrogen halide decreases down the group

to distinguish between a strong and a weak acid and base, you need

the same concentration are compared at the same temperature. the one with a higher concentration of ions is stronger

how can you measure pH with electrical conductivity

it depends on the concentration of mobile ions. strong acids and bases will have a higher conductivity than weak acids and bases if they have the same concentration of acid/ base. this is measured using a conductivity meter or probe

how is rate of reaction affected by the strength

the stronger the acid, the faster the rate because it depends on the concentration of H+ ions

acids react with bases in ——— reactions

neutralization

a — base and a —- acid make a salt

parent

acid + base →

salt + water

acid + carbonate →

salt + water + carbon dioxide

what is the point of inflection

when there is a big jump in pH

what is the equivalence point

when the acid and base have exactly neutralized each other, so there is a big jump in pH

what are the characteristics of a strong base strong acid pH curve (5)

initial pH at 1

pH changes gradually until equivalence

very sharp jump at equivalence from pH 3 to 11

after equivalence the curve flattens out at a high value

pH at equivelance is 7

what is pOH

it describes the [OH-] of the solution

when you write pH or pOH, you write —- decimal places

2

look at this

what are Ka, Kb

acid/ base dissociation constants and can describe the strengths of weak acids and bases, fixed at specific temperatures

for a conjugate acid base pair, Kw=

Ka*Kb

what are the key points to know when doing calculations with Ka and Kb (ice)

the given concentration of an acid or base is it’s initial concentration

the pH or pOH refers to the concentration of those ions at equilibrium

the concentration values substituted into the expression for Ka and Kb must be the equilibrium values for all reactants and products

when Kb or Ka is small, the acid/ base is roughly equal to the acid/ base concentration at equilibrium

what are pKa and pKb

logarithmic expressions of Ka and Kb,

and they have an inverse relationship with them.

depend on temperature

for a conjugate acid base pair, pKw=14=

pKa+pKb

the pH of a salt solution depends on

the relative strength of the parent acid and the parent base because it depends on the extent to which they react with water and release OH- and H+ ions

weak acid + strong base

the conjugate base is enough to cause hydrolysis, so it releases OH-. which causes the pH to increase, and the solution becomes basic, with a pH > 7 at 298

salt of strong acid and weak base

the conjugate base of the acid is still able to cause hydrolysis and release H+, causing the concentration of the pH to decrease, so pH< 7 at 298 K

salt of weak acid and weak base

both the conjugates are strong enough to carry out hydrolysis, so it depends on the Ka and Kb values

pH curve of weak acid and strong base 5 characteristics

initial pH is relatively high

pH stays relatively constant till equivalence (buffer region)

jump in pH at equivalence from about pH 7-11

after equivalence, it flattens out at a high pH

pH at equivalence > 7

what is the half equivalence point

represents where half of the acid has been neutralized by the base and is converted into salt while the other is not reacted. so, it is known as a buffer

at the half equivalence point (buffer region), acid=

salt at equilibrium

at the half equivalence point, Ka = —- because

[H+], Ka=[H+][salt]/[acid], but [acid]=[salt], so they cancel out

pH curve of strong acid and weak base characteristics (5)

initial pH is 1

pH stays relatively constant through the buffer region to equivalence

jump in pH from 3-7

after equivalence it flattens out at a low pH

pH at equivalence < 7

pH curve of a weak acid and a weak base at

initial pH is high

addition of base causes it to rise steadily

change in pH at the equivalence point is not sharp

after the equivalence point, it flattens out at a low pH

does not have a well defined equivalence point

the relationship between pH and the volume added is —-

non-linear

acid base indicators are

weak acids where the components of the conjugate acid base pair have different colors

an appropriate indicator for titration

has an endpoint range that coincides with the pH at the equivalence point

how would Hind respond to adding H+ or decreasing H+

according to Le Chatelier’s principle, adding H+ would shift it lefte in favour of Hind, color A dominates at low pH

decreasing H+ would shift it right in favor of ind-, color B at high pH

what is the endpoint of an indicator

the point where there is a color change

how can you use the endpoint to find the pH

becasue Hind=ind- because it is in the middle of it’s color change, they cancel out, so Ka=[H+], pKa=pH

what are indicators used for and how

to signal the equivalence point in titrations because they give a visible cue of when pH changes

how do you choose which indicator is best (3)

determine the combination of weak and strong acid and base reacting

deduce the pH of the salt solution

consult data tables to choose an indicator with an endpoint in the range of the equivalence point

what is a buffer solution

it is a solution that resists the change in pH on the addition of small amounts of acid or alkali

how are buffers made in detail (like how the chemistry works for weak acid)

a buffer solution is made by mixing an aqueous solution of a weak acid with a solution of its salt of a strong alkali

there is an equilibrium of the dissociation of the acid, which produces H+

the soluble salt fully dissociates in water, producing a A-

so there will be high concentration of the acid and the anion, which is the conjugate base

how do buffers work for weak acid

if H+ is added, it will combine with the conjugate base, taking it from A- to HA, removing most of the H+

if OH- is added, it will combine with the acid, HA, forming a conjugate base and water, removing OH-

how are buffers made in detail (like how the chemistry works for weak base)

a weak base is added to a solution with its strong conjugate acid

this means it will dissociate and make OH- in water

the salt fully dissociates in the solution

there is high concentrations of the base and the conjugate acid

how do buffers work for weak base

if H+ is added, it will combine with the base

if OH- is added, it will combine with the acid and produce water

what approximations can we make when doing calculations with buffers

the dissociation of the weak acid/ base is so small it is negligible, so [HA[=[HA],

the salt is fully dissociated so [MA]=[A-]

at equilibrium, for buffers, pH=,

pKa+log[salt]/[acid]

at equilibrium, for buffers, pOH=,

pKb+log[salt]/[base]

you can also create a buffer by….

mixing double the concentration of a weak acid/base with it’s strong conjugate

which factors influence buffers

the dilution, which does not affect Ka and Kb, but affects the buffering capacity

temperature, which affects Ka and Kb, so it needs to be constant