acids and bases and PH Flashcards
what is a bronstead lowry acid
a proton donor
what is a bronstead lowry base
a proton acceptor
what is a conjugate acid
it donates a proton to form the conjugate base
what is a conjugate base
it accepts a proton to form the conjugate acid
what is the role of water in acid- base equilibria
an acid can only donate a proton if there is a base to accept it . by mixing eg HCl with warer an equilibrium is set up consisiting of two acid - base conjugate pairs and a hydronium ion forms
draw a dot and cross diagram of the hydronium ion
how would we use the hydronium ion to show neutralisation
H3O+ + OH- = 2H20
what is a polybasic acid
monobasic , dibasic and tribasic acids refer to the total number of hydrogen ions in the acid that can be replaced per molecule in an acid - base reaction
how do you calculate the amount of alkali needed for the complete neutralisation of polybasic acids
1) decide whether the acid is monobasic , dibasic or tribasic
2) write the equation using as many NAoh units needed to replace the hydrogen atoms in the acid
3) use the balanced equations to deduce the molar ratio of acid to alkali and carry out the calculations
what must we remeber about all bases
not all bases are alkalis . a base is only an alkali if it is soluble in water . eg magnesium hydroxide is a base but not an alkali
definition of a monobasic acid
one mol of a monobasic acid will release one mol of hydrogen ions in solution
give an example of a dibasic acid
carbonic acid H2CO3
give an example of a tribasic acid
H3BO3
what is sorensens PH scale
converted PH colour values to hydrogen ion concentration values
how do you work out PH using logs
PH = - log [H+]
how do you work out PH from hydrogen ion concentration
[H+] = 10^ - ph
how many times greater is the [H+] for a PH of 1 than a PH of 2
10 times greater
what do we know about [H+] with a small PH value
[H+] concentration is high and vice versa
how come we can calculate PH for a strong acid from h+ concentration
- a strong acid completely dissociates in water therefore the conentration of h+ ions is directly proportional to the acid concentration
what are the strong acids
H2SO4 , HCl and HNO3
how do we represent the dissociation of a weak acid
using a reversible arrow
HA(aq) = H+(aq) + A-(aq)
which way will the equilibrium lie for a weak acid disociation
the equilibrium will lie to the left because they only partially dissociate. a very small concentration of H+ ions and A- ions so Kc<1
what is the acid dissociation constant Ka
the extent of an acid dissociation is measured by another type of equilibrium constant Ka
- to find the equation for Ka it is the exact same as for Kp or Kc
we use it to calculate PH for a weak acid
what does a large Ka value tell us about the degree of dissociation of an acid and therefore its strength
- a large ka value would tell us it is a stronger acid because the concentration of reactants (acid) is lower than the products ( ions) and the further the equilibrum is to the right
what does a small value of ka tell us about the strength of the acid and therefore its strength
equilibrium lies to the left therefore it is a weaker acid as fewer dissociation
what changes the value of ka
temperature
why do we not use ka for strong acids
because the value would be zero for the denominator
what is pKa
pKa is used to convert ka values into easier values
what are the equations for pka
Pka = -log(Ka)
Ka= 10^ -pka
what is the correlation between the size of pka value and acid strength
the greater the pka value , the smaller the ka value and the weaker the acid
what happens to pka values with dibasic and tribasic acids
the successive dissociations have a larger pka value as ka is lower as the dissociation is less likely to happen as we are removing a H+ from a negative ion
what is the simplified expression for Ka for weak acids
ka = [ H+]2 / [HA]
what is the first approximation that allows this simplified equation
HA dissociates to produce equilibrium concentrations of H+ and A- that are equal . there will also be a very small concentration of H+ from the dissociation of water. this is extremely small and can be neglected compared with the much greater H+ concentration from the acid so [H+] = [A-] so [H+] X [A-] = [H+]2
what is the second approximation that allows this expression
the equilibrium concentration of [HA] is lower than the start concentration of [HA] but only by a very small amount as it is only partial dissociation so we can assume [HA] equilibrium = [HA] start - [H+] so [HA] equilibrium = [HA] start
how can you experimentally work out the value of Ka
prepare a standard solution of the weak acid of known concentration
and measure the PH of the solution.
when do these approximations start to break down
the first approximation assumes that the concentration of [H+] from the dissociation of water is negligible but if PH >6 then the dissociation from water will be significant compared to the dissociation of the weak acid therefore this approximation breaks down for very weak acids or very dilute solutions
when does the other approximation start to break down
the second approximation assumes that the concentration of [H+] is much smaller than the concentration of [HA] at the start . this starts to break down when [H+] becomes significant and [HA] start - [ H+] is significant so this approximation starts to break down for stronger weak acids with Ka > 10 ^ -2 and for very dilute solutions
how does water act as both an acid and a base
water ionises very slightly setting up an acid - base equilibrium .
H2O + H2O = H3O+ + OH-
acid 1 base 2 acid 2 base 1
what is Kw
Kw is the ionic product of water . this is the concentration of OH- and H+ multiplied together
what happens to Kw with temperature
Kw changes with temperature
what is the value for Kw at 298K ( 25 degrees)
1.00 x 10 ^ -14 mol2dm-6
what is an alkali
a soluble base ( all alkalis are bases - but not all bases are alkalis ) a base is only alkali if it is soluble in water
what is a strong base
a strong base is an alkali that completely dissociates in aqueous solution
what is another equation for Kw
Kw = Kc x [ H2O ] this is because Kc = [H+][OH-] / [H2O] and when we reararrange
what is another equation for Kw
Kw = [H+]2
what happens when [H+]>[OH-]
it is an acidic solution
what happens when [H+]<[OH-]
it is an alkaline solution
what happens to PH and the value of Kw at different temperatures
as temperature increases , the value of Kw increases and the value of PH decreases. therefore at a higher temperature the water is more acidic . when temperature increases the equilibrium shifts to the right . this means the forward reaction ( the ionisation of water) is endothermic
tip : think global warming , as temperature is increasing the oceans are becoming more acidic
