topic eight/eighteen Flashcards
the bl theory defines acids and bases as either
proton donors or proton acceptors
what is an amphiprotic species
able to act as both a bronsted lowry acid and a bronsted lowry base depending on what its reacting with
Benzoic acid
C6H5COOH (s)
phosphoric acid
H3PO4 (aq)
carbonic acid
H2CO3 (aq)
ethanoic acid
CH3COOH (aq)
sulfuric acid
H2SO4 (aq)
nitric acid
HNO3 (aq)
ethanoic acid is
monoprotic despite having four hydrogen atoms
Ethanamine
CH3CH2NH2 (g)
A conjugate acid-base pair
differ by a proton (H+).
amphiprotic applies only to what
the BL theory
the term amphoteric refers to
substances that can act as either acids or bases, but its use is not limited to the Brønsted–Lowry theory
metal + acid →
salt + hydrogen
metal oxide + acid →
salt and water
metal hydroxide + acid →
salt and water
what is a neutralisation reaction
an acid reacts with a base or alkali to produce a salt and water
ingredients in most antacids
The active ingredients of most antacid tablets are metal carbonates or hydrogen carbonates, NaHCO3, CaCO3, MgCO3, for instance, or insoluble metal hydroxides such as Mg(OH)2 or Al(OH)3. These react with excess stomach acid in neutralisation reactions to relieve the symptoms of heartburn.
purpose of an acid base titration
to determine the unknown concentration of an acidic or basic solution using a solution of known concentration
steps of a titration
The burette is filled with an alkali of known concentration (the titrant). The titrant is added to the acid of unknown concentration (the analyte) until the end-point is reached. The end-point is signified with the use of an indicator; in the example below, the indicator is phenolphthalein. Note that the colour of phenolphthalein in an alkaline solution can be described as pink/purple and can fade to colourless in strong alkali if left to stand.
thermometric titration
Heat is released when an acid and base react together, which means it is an exothermic reaction. The enthalpy change that occurs is known as the enthalpy change of neutralisation; it is the enthalpy change when one mole of water is formed in the reaction of an acid and a base.
pH formula
pH = −log[H+(aq)]
the higher the conc of H+ ions in solution,
the lower th pH
a change in one pH unit is equal to
ten times the change in hydrogen ion concentration
An acid–base indicator is
a weak acid or a weak base in which the dissociated and undissociated forms have different colours
litmus comes in two colours
red and blue
in an acidic sol, blue litmus urns to
red
in an alkaline sol red litmus turns to
blue
universal indicator
red in acid
purple in alkali
pH probe
more accurate method of measuring pH
Water molecules do dissociate however, but only to a very small extent; this is known as the auto-ionisation of water. This reaction can be represented by the following equation:
H2O (l) ⇌ H+ (aq) + OH– (aq)
The equilibrium constant expression (Kc) for the auto ionisatin of water is
Kc=[H+][OH−]/[H2O]
The position of equilibrium for the dissociation of H2O lies very far to the left, so the concentration of the water is effectively constant. Therefore, we can write a new expression which is known as the ionic product of water (Kw):
Kw=[H+][OH−]
ionic product of water
Kw=[H+][OH−]
kw
1 x 10^-14
The strength of an acid refers to its
degree of dissociation (or ionisation) in aqueous solution.
Strong acids and bases completely
dissociate (or ionise) in solution
weak acids and bases only
partially dissociate (or ionise) in solution
A solution of a strong acid will contain
virtually no undissociated HA molecules
organic acids are often
weak acids
three common strong bases
sodium hydroxide, potassium hydroxide and barium hydroxide
metaly hydroxides of group 1
for the disosociation of weak bases the eq lies very far to the
left
Kc is very small
remember to use an equilibrium sign (⇌) for
weak acids and bases.
equimolar solutions
ones that have equal concentrations
active metals are those
above hydrogen in the activity series
which have higher elec conductivity
strong bases
In general, strong acids have relatively weak conjugate bases and weak acids have relatively strong conjugate bases
Acid deposition can be classified as
dry deposition or wet deposition
Natural, unpolluted rainwater is acidic, having a pH of approximately
5.6
acid rain pH
less than 5
how is acid rain formed
when acidic gases such as sulfur dioxide (SO2) and the oxides of nitrogen (NOx) dissolve in the water in the atmosphere to produce sulfuric acid or nitric acid.
how can nitrogen monoxide be formed
during lightning storms by the reaction of nitrogen and oxygen, the two predominant atmospheric gases. The second is by the reactions that take place in internal combustion engines. Nitrogen reacts directly with oxygen at high temperatures to produce nitrogen monoxide
benefit of a catalytic converter
reduces the level of these pollution emissions. This reaction also takes place in the jet engines of aircraft.
how is sulfur dioxide formed
occurs naturally in the atmosphere as it is released during volcanic eruptions
sulfur dioxide photochemical oxidation in the atmosphere
2SO2 (g) + O2 (g) → 2SO3 (g)
what does sulfur dioxide form with water
Sulfur dioxide and sulfur trioxide react with water in the atmosphere to form sulfurous and sulfuric acid:
SO2 (g) + H2O (l) → H2SO3 (aq)
SO3 (g) + H2O (l) → H2SO4 (aq)
what is dry deposition
acidic particles and gases fall to the ground via dust and smoke in the absence of precipitation. This form of deposition can be washed into streams, lakes and rivers, causing harm to biological systems.
strong acids
These four acids (nitric, nitrous, sulfuric and sulfurous acids) are all much stronger than carbonic acid, H2CO3.
environmental impact of acid deposition
Acid deposition can enter aquatic ecosystems either directly (e.g. precipitation as rain) or indirectly as run-off. At a pH of below 5.5, fish such as salmon cannot survive and a low pH can also prevent fish eggs from hatching.
When aluminium ions (Al3+) are leached from the soil by acid rain, they enter rivers and streams, where they have harmful effects on fish’s gills. Fish exposed to aluminium ions secrete excess mucus around the gills, preventing oxygen uptake and leading to death by asphyxiation.
Aluminium is present in soil at high pH values in the form of insoluble aluminum hydroxide (Al(OH)3). When the pH falls, the aluminium ions are released into the soil where it can damage the roots of plants, preventing them from taking up water. Acid deposition also has the potential to leech (remove) essential plant nutrients from the soil.
harms biodiversity
acid rain on statues
detrimental because calcium carbonate can react with sulfuric acid and nitric acid
CaCO3 (s) + H2SO4 (aq) → CaSO4 (aq) + H2O (l) + CO2 (g)
CaCO3 (s) + 2HNO3 (aq) → Ca(NO3)2 (aq) + CO2 (g) + H2O (l)
acid rain on humans
can adversely affect the mucous membranes and lungs, causing irritation and possibly exacerbating the symptoms for people with asthma and other respiratory conditions.
how to resuce acid deopisition
pre combustion methods: removing the sulfur before the coal is combusted
post comubstin methods: removing the sulfur oxides from exhaust gases once they have been formed by reacting with a base.
hydrodesulfurisation
a pre-combustion method by which the sulfur is removed from refined petroleum products such as gasoline, jet fuel, kerosene and diesel fuel before combustion. This process takes place at high temperatures and pressures (300-400 oC at pressures of between 300000 - 1300000 Pa) in the presence of a catalyst. The sulfur is removed from the product in the form of hydrogen sulfide (H2S).
flue gas desulfurisation
The levels of sulfur dioxide emissions in the flue gases can be reduced by passing them into a flue gas desulfurisation tower, also known as a ‘scrubber’
what happens in the scrubbing tower
the gases are passed through a sprayed aqueous suspension of calcium carbonate and calcium oxide. The product is calcium sulfite (sulfate(IV)) which is further oxidised to produce calcium sulfate.
hwo to treat soil and lakes with lower pH due to acid deposition
treat with calcium hydroxide to neurtralise the acidity and raise the pH
H2SO4 (aq) + Ca(OH)2 (s) → CaSO4 (aq) + 2H2O (l)
H2SO4 (aq) + CaCO3 (s) → CaSO4 (aq) + H2O (l) + CO2 (g)
lewis acid
electron pair acceptor
lewis base
electron pair doner
when are complex ions formed
when a ligand uses a lone pair of electrons to form a coordinate covalent bond with a central metal ion
lewis acids are
electrophiles
lewis bases are
nucleophiles
At 298 K, the value of Kw =
1.00 × 10^–14.
when is the pH of water 7
at 298K
what happens to the pH of water as temp increases
it decreases
how does the PH of water never change
At higher temperatures, the [H+] increases and the pH decreases. However, the solution is still neutral as [H+] = [OH–].
pKw =
pH + pOH
why is the degree of dissocaition or ionisation not complete in weak acids and bases
there exists an equilibrium between the dissociated and undissociated forms.
it only partially dissociates
Weak acids such as ethanoic acid partially dissociate in solution. Unlike strong acids and strong bases, we cannot determine the [H+] or [OH−] from their initial concentrations. This is because each weak acid or weak base has a different degree of dissociation. Just like equilibrium reactions, the dissociation of a weak acid or weak base has a constant associated with it, which tells us the extent of this dissociation and where the position of equilibrium lies. For weak acids, this is the acid dissociation constant, Ka.
what is the acid dissocation constant Ka
This is because each weak acid or weak base has a different degree of dissociation. Just like equilibrium reactions, the dissociation of a weak acid or weak base has a constant associated with it, which tells us the extent of this dissociation and where the position of equilibrium lies. For weak acids, this is the acid dissociation constant, Ka.
Ka=
[H3O+][A−]/[HA]
[H+][A–] / [HA]
a high Ka value
stronger and more dissoacted
a low Ka value
weaker and less dissoacted
what is Kb
Kb, the base dissociation constant, has a fixed value for a particular base at a specific temperature. The value of Kb tells us about the strength of the base and therefore the degree of dissociation. A weak base with a high Kb value is stronger and more dissociated than a weak base with a low Kb value.
pKa =
–log10Ka
pKb =
–log10Kb
Ka =
= 10–pKa
Kb =
10–pKb
The relationship between Ka and pKa with acid strength,
and the equivalent relationship for bases.
For any conjugate acid-base pair:
Ka × Kb = Kw
pKa + pKb =
pKw
pKa + pKb = (number)
14.00 at 298 K
why can we use the initial concentration of the acid or base to calculate the pH or pOH
When calculating the pH of strong acids and bases we assume full dissociation into ions in aqueous solution
If the acid is weak, the equilibrium concentration of the undissociated acid molecules can be taken as the initial concentration of the acid, [HA]eq =
[HA]initial.
The assumption here is that very few acid molecules have actually dissociated. A similar assumption is made for the concentration of the undissociated molecules of a weak base at equilibrium; namely, that the equilibrium concentration of the weak base is approximately equal to the initial concentration of the weak base, [B]eq = [B]initial.
When answering exam questions, it can be assumed that [HA]eq =
[HA]initial and [B]eq = [B]initial for weak acids and bases
In a titration what happens
a solution of known concentration is added to a known volume of a solution with an unknown concentration. An indicator is used to determine the equivalence point when the acid and base have been reacted in stoichiometrically equivalent amounts. At that point, the solution contains only salt and water.
waht is the equivalence point
occurs when stoichiometrically equal amounts of acid and base have reacted and the solution contains only salt and water. The pH of the solution at the equivalence point depends on the combination of parent acid and base that have reacted together (see section 18.3.2). It is worth noting that the equivalence point is only at pH 7.00 in the titration of a strong acid and a strong base.
what is the point of inflection
the sharp increasae or decrease in a pH curve ina titration when something starts to be added
what is the buffer region
where the solution contains HA (the weak acid) and a significant amount of A– (the conjugate base of the weak acid). At this point, the solution acts as a buffer solution - meaning that it has the ability to resist a change in pH on the addition of a small amount of acid or base.
how to find the half equivalence point
found on the x axis on the point which is half the volume added at the equivalence point
what is true at the half equivalence point
the pH is equal to the pKa of the acid
can you have multiple equivalence points
yes as new substances form
conductometric titration
follow the change in conductivity as an acid reacts with a base
As the acid and base react, the conductivity of the solution decreases as H+ and OH− ions react to form water molecules
what are indicators
weak acids or bases with the dissociated or undissocated forms having different colours
Like all weak acids and weak bases, they exist in an equilibrium between the two forms, which are represented as HIn (the undissociated form) and In− (the dissociated form).
Hn form of phenolphtalein
colourless
ln- form of phenolphtalein
pink
Increasing [H+] by adding an acid will shift the equilibrium to the left, so phenolphthalein appears
colourless
Increasing [OH−] by adding a base will shift the equilibrium to the right, so phenolphthalein appears
pink
The end-point is
the pH when the indicator changes colour.t
the equivalenc epoint is when
acid and base have reacted in stochiometrically equivalent amounts.
buffer sol
resists changes in pH when small amounts of acid or base are added.
An acidic buffer can be made by mixing together
a weak acid and the salt of a weak acid and a strong base
A basic buffer can be made by mixing together
the same amounts of a weak base and the salt of a weak base and a strong acid
an alternative approach to make a bufferis
An alternative approach, which results in an acidic buffer solution, is to start with a weak acid and add half as many moles of strong base. Similarly, a basic buffer can be prepared by starting with a weak base and adding half as many moles of strong acid.
the key points of making buffers
The key point in any of the above methods is that the initial solution must be a weak acid or base, otherwise the starting acid or base would already be 100% dissociated into ions. Both components of a conjugate acid–base pair have to be present in the solution for it to be able to neutralise any added acid or base.
The pH of a buffer is determined by the interactions of its components. Specifically it depends on:
the pKa or pKb of its weak acid or base;
the ratio of the concentrations of weak acid and salt, or weak base and salt, used to prepare the buffer solution.
ultimately how is a buffer sol prepared
by using a weak acid or base with a pKa or pKb value that is as close to the required pH or pOH of the buffer as possible.
why do salts dissolve to produce acidic or alkaline solutions
This occurs because one of the constituent ions of the salt reacts with water to release an excess of either hydroxide or hydrogen ions. This phenomenon is referred to as salt hydrolysis and it occurs with salts that are formed from either weak acids or weak bases. The pH of an aqueous salt is dependent on the strength of the conjugate acid and bases formed and their ability to hydrolyse water, producing H+ or OH− ions.
When a strong acid and strong base react, the resulting conjugate acids and bases are
both weak, which means that neither is able to hydrolyse water.
a solution of a salt formed by the reaction of a weak acid and a strong base
is strong enough to hydrolyse water
a salt formed by the reaction of a weak acid and a weak base depends on the Ka and Kb values of the acids and bases involved. If the parent acid and base have approximately the same value,
neither ion will hydrolyse and the salt produced will have a pH of 7
In the reaction of a strong acid and a weak base, the conjugate acid of the weak base is
strong enough to hydrolyse water
Strong acid + strong base
neiher ion hydrolyses water
Weak acid + strong base
hydrolyses water
Strong acid + weak base
hydrolyses water
Weak acid + weak base
both ions hydrolyse water
