Acids and bases Yr2 and pH Flashcards
What are bronsted-lowry acids?
substance that donates a proton
What is a bronsted-lowry base?
substance that can accept a proton
What happens when you mix bronsted-lowry acids with water?
hydrogen ions are released
What ions are formed when acids dissolve in water?
H3O+ and acid ion
What happens when you mix a base with water?
react with H+ ions to form OH-
Is H3O+ acidic or basic?
acidic
What are polyprotic acids?
acids that can donate more than one proton
How does a base accept a proton explain in terms of bonding?
a base uses a lone pair of electrons to form a dative bond to a H+
Why is acid and metal reactions not an acid-base reaction?
the metal has no lone pairs and is not accepting protons
the H+ are being reduced to H2 where in A-B reactions there is no change of oxidation states
What is an example of a monoprotic acid?
HNO3
1 mole of HNO3 will produce 1 mol of H+ ions
What is an example of a diprotic/dibasic acid?
H2SO4
1 mol of H2SO4 = 2 mol of H+ ions
What is an example of a triprotic/tribasic acid?
H3PO4
1 mol H3PO4 : 3 mol H+
What is a conjugate acid?
any species that has gained a proton
What is a conjugate base?
any species that has lost a proton
What does the Bronsted-Lowry theory state happens during all acid-base reactions?
the transfer of a proton from an acid to a base
What is the conjugate acid when water reacts with acid?
H3O+ - has gained a proton from acid
What is the conjugate base when water reacts with base?
OH- = water has lost a proton
What is a strong acid?
an acid that is totally ionised in aq solution
What is an example of a strong acid?
HCl
H2SO4
HNO3
What is a strong base?
a base that is totally ionised in aq solution
What is an example of a strong base?
NaOH and KOH
What is a weak acid?
an acid that only partially ionises in aq solution
What is an example of a weak acid?
carboxylic acids
What is a weak base?
a base that only partialy ionises in aq solutions
What is an example of a weak base?
aq ammonia and amines
What dissociation reaction direction is favoured for strong acids/ bases?
forwards = lots of H+/ OH- produced
What dissociation reaction direction is favoured for weak acids/ bases?
backwards = less H+/ OH- produced, less dissociates
What is the equilibrium equation for the dissociation of water?
2H2O = H3O+ + OH-
Why is the concentration of H2O ignored in Kw equation?
assume it has a constant value = water dissociates into its ions very weakly, so there is a lot more H2O than its ions
What is the formula equation for Kw and the units for Kw?
Kw = [H+] [OH-]
units = mol2 dm-6
What is the standard value of Kw at 25 degrees?
1.00 x 10-14 mol2 dm-6
What is the expression for Kw for pure water and explain why it is this?
Kw = [H+]^2
pure water has an equal conc of H+ and OH-
What happens when you react H2SO4 and HNO3 together?
H2SO4 is a stronger acid so it will donate a proton to HNO3
forms HSO4- and H2NO3+
What is the basicity of an acid?
the number of H+ ions that can be released from 1 molecule of the acid
What is pH?
a logarithmic scale that measures the concentration of H+ ions in solution
What is assumed when calculating pH of strong acids?
that they dissociate fully
What is assumed when you calculate pH for a strong base?
it dissociates fully
What is Ka?
the acid dissociation constant
When would you use Ka equations?
for working out pH for weak acids
What is the units for Ka?
moldm-3
What is Kw?
the ionic product of water
What are the 2 assumptions of the expression of Ka so it can be simplified?
- only a small amount of weak acid dissociates so we can assume that the conc of acid at the start of reaction = conc of acid at equilibrium
- the dissociation of acid leads tp equal numbers of H+ and salt- ions so [H+] = [salt]
What is the root of Ka x [acid] equivalent to?
[H+]
What is the limitation of assuming that [H+] = [salt] ?
it ignored H+ formed by the ionisation of H2O
it is a reasonable assumption because the no of H+ from ionisation of acid is greater than the number of H+ from H2O
What is the limitation of assuming that the conc of undissociated acid is equal to the tot conc of acid?
it ignores that some of the acid is broken up into H+ and salt-, so the actual conc of undisociated [salt] is less than the overall acid conc
What is pKa?
a way of measuring the strength of an acid
What value of pKa suggests a stronger acid?
a lower value
What is a buffer?
a chemical that resists the change in pH when small amounts of acid or base are added
What are the 2 types of buffers?
acidic
basic
What are acidic buffers?
resist the change in pH in order to keep the solution below pH 7
What are acidic buffers made of?
a weak acid and its salt (the conjugate base)
What 2 equilibrium reactions are taking place within an acidic buffer solution? and where do their equilibriums lie?
the dissociation of the acid - equilib lies far to the left
the dissociation of the salt - equilib lies far to the right
What happens when you add H+ (acid) to an acidic buffer solution?
the H+ ions will react with the negative salt ions
there is a high conc of negative salt ions from the dissociation of the salt
more acid from buffer solution is produced and equilib shifts to the left
What happens when you add OH- (base) to an acidic buffer solution?
the OH- react with the H+ ions from the dissociation of the acid in solution
there is a low conc of H+ but they can be reproduced from a high conc of acid to counter the change
equilibrium shifts to the right to replace the reacted H+ ions
What happens to the moles of buffer acid when you add a strong acid to solution?
the moles of buffer acid will increase by the number of moles of strong acid added
What happens to the moles of buffer acid SALT when you add a strong acid to solution?
the moles of buffer salt will decrease by the number of moles of strong acid added
What is an example of an acidic buffer solution?
ethanoic acid and sodium ethanoate
Under what condition is a buffer solution most effective?
when pH = pKa of the acid
the [acid] = [salt]
What happens to the value of Kw when you increase temperature?
value of Kw increases = equilib shifts to the right in endo direction
H2O = H+ + OH-
What is the relationship of the pH of a buffer and the pKa of the acid?
pH is within +/- 1 of pKa
when a buffer is most effective - equal amounts of acid and salt
What makes up a basic buffer?
a weak base and its salt (conjugate acid)
What is an example of a basic buffer solution?
NH3 and NH4+
Why is buffering important in biological systems?
enzymes are highly sensitive to pH
What is an example of a buffered biological system?
blood
What is the overall equation for blood buffering?
H2CO3 + H2O –> HCO3- + H3O+
What is H2CO3?
carbonic acid
How is carbonic acid formed?
CO2 dissolved in water or in the blood
