Physical Chemistry 1.2 - Chemical Equilibrium - Acids and Bases, Salts, Buffers and Indicators Flashcards
What is an acid?
What is a base?
a proton donor
a proton acceptor
Water is amphetoric. What does this mean?
it can act as an acid and a base
The equation for the ionisation of water demonstrates that water is ___.
H2O + H2O ⇌ H3O+ + OH-
amphetoric
Another word for a hydrogen ion is a ___.
proton
For every acid there is a ___ base, formed by the ___ of a proton.
For every base there is a ___ acid, formed by the ___ of a proton.
conjugate, loss.
conjugate, gain
A strong acid or base is one that…
A weak acid or base is one that…
completely dissociates into ions in solution (→).
only partially dissociates into ions in solution, and is therefore in equilibrium (⇌)
Since acids donate protons, they react to form _ve ions.
Since bases accept protons, they react to form _ve ions.
-ve, +ve
At the end of a reaction involving a strong acid or base, there will be no ___ left.
reactant (acid/base)
Name 3 strong acids and 3 weak acids.
Strong: Hydrochloric (HCl), Nitric (HNO3), Sulfuric (H2SO4)
Weak: Ethanoic (CH3COOH), Carbonic (H2CO3), Sulfurous (H2SO3)
Name some strong bases and weak bases.
Strong: any solution of metal hydroxides: Solidum hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH)
Weak: Ammonia (NH3), Amines (NH2R, NHR2)
What is a conjugate acid-base pair?
a pair of species - one an acid and the other a base - they create each other by transferring a proton when the reaction takes place
Since weak acids and bases only partially dissocate, they are a type of ___ reaction.
This means they can be expressed by __.
equilibrium.
K
What is the equation for the dissocate of an acid?
What does K look like for this equation?
HA + H2O ⇌ A- + H3O+
Ka = [H3O+] [A-] / [HA]
Why is H2O not included in the expression for Ka?
What is A-?
it’s a liquid (and therefore has a value of 1).
the conjugate base of the acid
The relative dissocation of an acid can also be represented by…
pKa.
(pKa = - log (Ka)
Since pKa is the - log of Ka, it provides an inverse value, meaning as Ka decreases, pKa ___.
increases.
The pH of a weak acid can be calculated by pH = ?
*in db
pH = 1/2 pKa - 1/2 log (c)
*where c is concentration of acid in mol L-1
What is pH a measure of?
What is a neutral solution?
the concentration of hydrogen ions in solution.
a solution in which the concentration of H+ ions is equal to the concentration of OH- ions.
Are neutral solutions always pH 7?
Why is this?
Nope.
Because neutrality is defined by an equal concentration of H+ and OH- ions, and does not care how many there are, as long as they’re the same.
pH on the other hand is purely a measure of H+ ion concentration (in mol/L), and does not care how many OH- ions there are. pH only cares about the number of moles of H+ ions in a certain volume.
What are salts made of?
an acid and a base
What will the relative strength of the parent acid and base determine in a salt?
it’s pH (when it dissocates into ions in solution)
A strong acid and strong base will produce…
A strong acid and weak base will produce…
A weak acid and strong base will produce…
A weak acid and weak base will produce…
a neutral salt.
an acidic salt.
an alkaline (basic) salt
nothing
All salts will fully ___ in water.
dissociate
Why will a salt made of a strong acid and a strong base create a pH neutral solution?
because the conjugate partners of the acid and base are present in equal amounts, so they react with H+ and OH- ions the same amount
The equilibrium of ___ is always present in solutions.
water
A strong acid plus a weak base will produce an ____ salt because…
acidic, because OH- ions are being reacted with, and therefore removed from the water equilibrium.
The equilibrium of water then…
shifts to the right to replenish the OH- ions, creating more H+ in the process
When explaining why salt solutions are acidic or alkali, you must refer to the water equilibrium, explaining that the ions produced by ___ of the salt are reacting with either __ or __.
dissocation, H+ or OH-
In an alkaline salt, the concentration of _ ions is greater than that of _ ions.
This is because the ___ ions of the salt are ___ with the _ ions, removing them from solution.
This shifts water’s ___ to the product side, producing more ___ and ___ (in a __ ratio).
OH-, H+.
conjugate, reacting, H+.
equilibrium, H+ and OH-, 1:1
Salts are named after…
their parent base, and acid
What is the name of the acid and base which reacted to produce sodium ethanoate?
The name of the ___ comes first.
sodium hydroxide, Ethanoic acid.
base
What is a buffer solution?
a solution in which the pH remains approximately constant when a small amount of acid or alkali is added
What are the two types of buffer and how are they made?
acidic and alkali - made by reacting a weak acid or alkali with one of its salts
When explaining how the pH of a buffer remains constant, which equations should you write out and make reference to?
- The salt (which completely dissocates into +ve and -ve ions, one of which is reactive, the other unreactive)
- The weak acid/base (this is an equilibrium reaction - which will shift to the right (products) should the H+/OH- on the products side react with added OH-/H+)
- Water (the equilibrium of water is far to the left, but will shift right if H+/OH- is reacted with, but will produce H+ and OH- in equal amounts, not affecting pH
You should then suggest which ions would react with the added __ or __ ions, to keep the __ constant.
H+, OH-, pH
Take the buffer of ammonia and ammonium chloride:
NH3 + H2O ⇌ OH- + NH4+
NH4+Cl- -> NH4+ + Cl-
1. Is this buffer acidic or alkali?
2. If a small volume of acid is added, what will happen?
3. What about an alkali?
- alkali.
- the H+ ions produced by the acid will react with the OH- ions produced by the weak base dissocating, which will cause its equilibrium to shift to the right to replenish them
- The OH- ions produced by the alkali will react with the +ve ions of the salt (NH4+)
How does an acidic buffer solution cope with added acid?
How about a base?
- the H+ ions from the acid react with the -ve ions of the salt
- the OH- ions from the base will react with the H+ ions from the weak acid’s equilibrium, which will be replaced by the shifting of the equilibrium to the right
Which equation tells you the pH of a buffer?
*in db
pH = pKa - log(acid/salt).
The pKa of an acid is the - log of Ka, and has values provided in the db
What are indicators?
weak acids with the equation:
HIn + H2O ⇌ In- + H3O+
The colour of an indicator molecule is ___ from the colour of its ion.
different
The colour of an indicator solution is determined by the ratio of ___ to ___.
reactants, products
What is the acid dissocation constant? (indicator edition)
KIn = [In-] [H3O+] / HIn
What is the theoretical point at which colour change occurs?
At what point does the colour change become visible?
when the concentration of HIn = In- or when KIn = H3O+, (given that: KIn = [H3O+][In-] / [HIn] )
Orrr when pKIn = pH.
when the concentration of HIn and In- differ by a factor of 10
The pH range over which a colour change occurs can be estimated by which equation? *in db
pH = pKIn ±1
How can you tell if an indicator is suitable for use in a solution?
the pH range of the indicator fits within the vertical section of the tritration curve
Why do indicators change colour?
because the addition of H+ or OH- shifts their equilibrium, changing the concentrations of the coloured HIn and In- molecules.
(i think the other reactants and products make up the difference so that the ratio of reactants to products (K) remains the same?)
If KIn = H3O+, what does pKIn =?
pH
Different indicators have different ___ ___ (there is a table on pg 14 of the data booklet)
pH ranges
What is the pH range of an indicator?
the range of pHs over which it changes colour
Over, or beneath an indicator’s pH range, it will appear a ___ colour, because the equilibrium is very far to one ___.
uniform, side
Explain fully why a solution of the salt sodium 4-hydroxybenzoate has a pH greater than 7.
(2 marks)
Conjugate base of a weak acid,
removes/reacts with H+ from the water
Causing the water equilibrium to shift to the right hand side
