unit 4 section 4 Acids, Bases and PH Flashcards
what is a Brønsted-lowry acid
they are proton donors. they release hydrogen ions when mixed with water.
what is a Brønsted-lowry base
they accept protons. when in a solution they take hydrogen ions from water molecules.
what does dissociation mean
it means that they break up into their positive and negative ions.
what dictates the amount of dissociation in a reaction between an acid and an alkali
how weak or strong an acid or base is.
- strong acids almost completely dissociate in water
- strong bases ionise almost completely in water
- weak acids dissociate very slightly in water and a small amount of H+ ions are released ( equilibrium lies to the left )
- weak bases only slightly dissociate in water too ( equilibrium lies to the left )
what can water dissociate into and show the formula for this reaction and the ionic formula for this reaction
only a very small amount of water can dissociate, they dissociate into hydroxonium ions and hydroxide ion.
H2O + H2O = H3O+ + OH-
the ionic equation:
H2O = H+ + OH-
since only a small amount of water dissociates, where does the equilibrium lie and explain why.
(3)
the equilibrium lies way over to the left. this is because there is a huge amount of water compared to the H+ & OH- ions that the amount of water is considered to be constant.
what is the equation for the ionic product of water and how is it dervided
(2)
ionic product of water
Kw = [H+] [OH-]
it is dervided by times the kc for water by the concentration of water Kc of water = [H+] [OH-] / [H2O]
kw = [H+] [OH-] / [H2O] X [H2O] = [H+] [OH-]
Kw = [H+] [OH-]
what are the conditions that give you the same value of Kw of a aqueous solution, and what is that value
(2)
temp - 298k or 25 degrees
the same value that is given is
1.00x10^-14 mol^2 dm^-6
what is the formula for the kw of pure water and explain why there is a difference
(2)
Kw = [H+]^2
this is because pure water there is always 1 H+ ion for each OH- ion.
what is the PH scale
(1)
it is the measure of the hydrogen ion concentration of a solution
what is the equation for PH
(1)
PH = -log10 [H+]
-the 10 next to the log is the base so it goes at the bottom on the calculator
- [H+] concentration of H+ ions in solution in mol dm^-3
how do you calculate the concentration of hydrogen ions (H+) when you are given the PH.
(1)
you use the equation
[H+] = 10^-PH
what is a monoprotic acid and give 2 examples
(3)
a monoprotic acid is an acid that for every molecule of the acid, it will release 1 proton when it dissociates. this means that the H+ concentration is the same as the concentration of the acid.
hydrochloric acid (HCL) & nitric acid (HNO3)
what is a diprotic acid and give an example of it.
(2)
a diprotic acid is an acid that for every molecule of acid, it will release 2 protons when it dissociates. this means the concentration of the [H+] ions is double the concentration of the acid
- sulfuric acid (H2SO4)
why do you have to use the Ka formula instead of the Kw formula when you are dealing with a weaker base or acid
(2)
this is because weak acids and bases only slightly dissociate in water, this means that the concentration of the H+ ions is not the same as the concentration of the reaction.
what is the formula for Ka (the acid dissociation constant )
(1)
Ka = [H+] [ A-] / [HA]
this can be simplified to (because we can assume that all the H+ comes from the acid)
Ka = [H+]^2 / [HA]
what is pKa and why is it used, also give the formula for pKa
(3)
pKa is calculated from Ka in the same way that PH is calculated from [H+]. they use pKa because the value of Ka varies massively between different acids, this can make the numbers hard to deal with.
pKa = -log10 (ka)
what is
a buffer and what are the two types
(1)
a buffer is a solution that resists changes in ph when a small amount of acid or alkali is added to the solution
- acidic buffers & base buffers
what is an acidic buffer
(3)
it is a buffer that has a PH lower than 7, they contain a mixture of a weak acid and one of its salt. they can resist change when an acid or a base is added to it.
what is a base buffer
(3)
they have a PH that is higher than 7, they contain a mixture of a weak base and 1 of its salts. they can resist change when an acid or a base is added to it.
why can buffer resist change even when they are diluted in water
(3)
this is because if a small amount of water is added to the buffer, the water only slightly dissociates. so the extra H+ ions and the OH- ions push the equilibrium the same amount in both directions, therefore the the equilibrium is left unchanged.
what are some of the applications of buffers
(3)
- they are used in shampoo, this is to maintain the ph whilst you wash your hair
- blood - this is in order to keep the blood at a PH very near to 7.4
- ensuring all the tissues in the body are kept at the right PH.
what are the two ways of making a buffer
(2)
- mixing a weak acid with its salts
- add a small amount of acid to a weak alkali so that only a small amount of the acid is neutralised to make a salt.
what does the HA in the acid dissociation constant represent
it represent the acid in the equation
e.g.
HA + OH = A^- + H20
what does it mean if you see pka or ka in a question
it means you are dealing with a weak acid and you make need to use the equation for Ka ( the acid dissociation constant)
identify two strong bases
NaOH
KOH
give an example of a weak acid and what weak acids normally are
ethanoic acid
- weak acids tend to be carboxylic acids
identify a weak base
NH3 ammonia
what is the generic formula for an acid reacting with a base
HA + B = BH^+ + A^-
where does the equilibrium lie in a reaction with a strong acid and water
it lies far to the right
where does the equilibrium lie in a reaction between a weak acid and water
the equilibrium lies far to the left
what equation do you use when you are working out the PH of a weak acid that doesn’t contain a buffer
ka = [H+]^2 / [HA]
we can use this equation as we can assume that all of the H+ ions comes from the acid
what equation do you use when you are working out the PH of a weak acid that contains a buffer
Ka = [H+] [A-] / [HA]
what is a titration used for
to work out the concentration of an acid or base
what are the axis on a titration graph
ph - y axis
volume of base - x axis
how does the titration curve of a strong base strong acid look like
-the graph starts at around ph 1 as there is an excess of a strong acid
- ends at ph 13 as there is now an excess of a stong base
- the graph has an S shape with a sharp increase in gradient at the middle of the graph
how does the titration curve of a strong base and a weak acid look like
- the graph starts at PH 1 as there is an excess of strong acid
- the graph ends at a PH of around 9 as there is now an excess of weak base
- the graph has an S shape and there is a steep gradient around the middle of the graph
how does the titration curve of a weak acid and a strong base look like
- the graph starts at a PH around 5 as there is an excess of weak acid
- the graph ends at PH 13 as there is now an excess of strong base
- the graph has an S shape and it has a steep gradient at the middle of the graph
how does a titration curve between a weak acid and a weak base look like
- the graph starts at a PH around 5 as there is an excess of weak acid
- the graph ends at a PH around 9 as there is now an excess of weak base
- the graph has an S shape and still has a steep gradient but isn’t as steep as the over curves
what is the equivalence point or the end point on a titration curve
it is the point where the acid has been fully nuetralised by the base. this is the point on the graph where there is a sharp change in PH ( steep gradient )
what is the half nuetralisation point on a titration curve and what can it be used to do
it is the half way point between 0 and the equivalence point on the curve. it can be used to calculate the pKa of a weak acid by taking the PH at this point.
what is an indicator used for in a titration
it is used to determine the end point of a reaction
what are the two most common indicators and at what PH do they change colour
- methyl orange : PH 3 - 4.5
- phenolpthalein : PH 8.2-4.5
how do you decide which indicator to use for a titration when you are given the titration curve
you at equivalence point/ end point of the reaction. this is where the solution is fully neutralised and there is a steep change in PH. you then look for the indicator that will change colour around that point
what is the colour change for methyl orange and phenolpthalein
methyl orange - red to yellow
phenolpthalein - colourless to pink
what indicator should be used in a reaction between a weak acid and a weak base
no indicator can be used as there is no sharp pH change, therefore a pH meter must be used
explain what a titration curve of a diprotic acid looks like
for diprotic acids, they neutralise in two steps. this is because the 2 H+ ions leave separately, they do not leave at the same time. this means that there are two equivalence points, one for each proton that is dissociated
