3.1 Proton transfer reactions

Arrhenius idea about acids and bases

The Arrhenius idea about acids and bases states that acids produce hydrogen ions (H+) in aqueous solution, while bases produce hydroxide ions (OH-).

the basic idea of bronsted-lowry theory

acids donate protons and bases accept protons (hydrogen ions)

what is a Bronsted-Lowry acid?

A proton (H+) donor

what is a Bronsted-Lowry base?

A proton (H+) acceptor

conjugate acid-base pairs

the pair of species that differ by the presence and absence of one proton, the one with 1 less will be the acid and will accept protons, their roles are reversed if the reaction is reversed

what is the conjugate acid of a species?

It will have one more hydrogen and +1 more to its charge

what is the conjugate base of a species?

It will have one less hydrogen and -1 from its original charge

difference between alkali and bases?

alkali are bases that dissolve in water to form hydroxide (OH-) while bases are in general substances that give out one H+.

are polyatomic anions bases or acids?

they are bases

is the ammonium ion a base or acid?

it is an acid because it has positive charge

what does amphiprotic mean?

a substance is one which can act as both ta proton donor and a proton acceptor

difference between amphiprotic and amphoteric?

amphiprotic is only within the bronsted lowry definition of acids and bases

amphoteric is a more general, Lewis definition

what does pH measure?

concentration of hydrogen ions

pH curve shape

the equivalence point is at 7pH in standard conditions;

there is a very sharp jump because the base and acid have reached equilibrium

how do you find the equivalence point from a pH curve?

the midpoint of the straight vertical line, which is 7 at standard conditions

when do indicators change colour?

at the end point when the pH is equal to the pKa

pH of a salt that is formed from a strong acid and strong base

neutral pH = 7 at standard conditions

pH of a salt that is formed with a weak acid and a strong base

more alkali start and end pH > 7

pH of a salt that is formed with a strong acid and a weak base

more acidic and end at pH < 7

how to choose a suitable indicator

pKa of the indicator should be as close as possible to the pH at the equivalence point of a titration and in the equivalence range

what happens to the equilibrium we increase H+ concentration in an indicator solution

as per chateliers principle, equilibrium will shift towards reactants

what happens to the equilibrium we decrease H+ concentration in an indicator solution

as per chateliers principle, equilibrium will shift towards products

difference between acid base titrations and in redox titration?

acid base:

proton transfer

redox:

electron transfer

may change colour themselves but not always

similarities between redox and acid base titrations?

both change colour at the equivalence point

strong acid and strong base has the equivalence range of:

3 - 11 pH

strong acid and weak base has the equivalence range of:

3- 7 pH

weak acid and strong base has the equivalence range of:

7 - 11pH

a buffer

a solution resistant to changes in pH on the addition of small amount of acid or alkali

why do we put a small amount of indicator only?

Indicator is slightly acidic so it will change the pH of the sample

how do buffers work? what are they made of?

are a micture of two solutions, composed so that they contain a pair of conjugate base-acid pair

acidic buffers are made from

a weak acid and a solution of a salt from a strong alkali, need to be conjugate:

E.g. CH3COOH(aq) and NaCH3COO(aq) The following equilibria exist

CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq) equilibrium lies to the left

NaCH3COO(aq) → Na+(aq) + CH3COO-(aq)

what is a strong base? examples

it is a salt made with a group 1 or group 2 metal hydroxides except for Mg(OH)2 and Be(OH)2

strong acid examples?

acid that dissociates fully into H+ and A-

HCl, HBr, HI, HNO₃, H₂SO₄ (first dissociation), HClO₄.

what if H+ or OH- is added to an acid buffer?

H+ will react with the base in hte buffer to form salt

OH- will react with the acid in the buffer to from salt again:

Addition of acid (H+):

H+ will react with the base CH3COO- to form CH3COOH

CH3COO-(aq) + H+(aq) ⇌ CH3COOH

Addition of base (OH-):

OH- will react with the acid CH3COOH to form CH3COO- and H2O

CH3COOH(aq) + OH-(aq) ⇌ CH3COO-(aq) + H2O(l)

basic buffers are made of

mixing a weak base with the salt of a strong acid, which are congugate pairs

how do H+ and OH- react in an basic buffer

the H+ reacts with the weak base:

Addition of acid (H+):

H+ will react with the base NH3 to form NH4+

NH (aq) + H+(aq) ⇌ NH +(aq) 34

Addition of base (OH-):

OH- will react with the acid NH4+ and form NH3 and H2o

NH4++(aq) + OH-(aq) ⇌ NH3(aq) + H O(l)

how do we figure out the number of moles to make a buffer?

we need to find the ratio which produces excess weak acid or base depending of the type of buffer and salt

approximations to do calculations of an acidic buffer

initial concentration of weak acid = eq concenration of salt

initial concentration of salt = eq concentration of salt

factors that influence buffers

dilution: dillution does not change the pH, but can change its buffering capacity

temeperature: affects the values of Ka and Kb so it affects the pH