Acids and Bases
7.1 The nature of acids and Bases
Acids are characterized by a sour flavour, whereas bases are characterized by bitter
taste.
Acids produce hydrogen ions in aqueous solutions, and bases produce hydroxide
ions. Arrhenius. This concept is limited because it only refers to hydroxide bases and
aqueous solutions.
In terms of Brønsted-lowry concept of acid base, an acid is proton (H+) donor and
base is proton acceptor.
H3O+ is known as hydronium ion. This is usually formed when an acid dissociated in
water.
A conjugate base is everything that remains of the acid molecule after a proton is
lost.
A conjugate acid is formed when proton is transferred to the base.
A conjugate acid-base pair consists of two substances related to each other thru the
donation and acceptance of a proton.
Ka is the acid dissociation constant. The activity of water is said to be 1, so it is not
included in Ka.
The Brønsted-lowry concept of acid can be applied to gaseous medium.
An acid-base reaction based on one concept may not be considered Acid-base if
judged by another concept.
7.2 Acid strength
Strength of an acid is determined by the equilibrium position of its dissociation
reaction.
A strong acid is one that has its equilibrium position far to the right. A strong acid
yields a weak conjugate base
A weak acid is one that has its equilibrium position far to the left. A weak acid yields
a strong conjugate base.
Common strong acids:
- H2SO4
- HCl
- HClO3
- HNO3
Sulfuric acid is a diprotic acid indicating that the acid has two protons.
Most acids are oxyacids such as the ones mentioned above and H3PO4, HOCl,, HNO2.
Organic acids are those with a carbon atom backbone, these types of acids are
usually weak.
When trying to measure the Ka for strong acids, the concentration of the acid cannot
be accurately measured due to the minimal presence of these molecules; leading to
uncertain Ka.
Amphoteric substances are those that have the capacity to behave as acid and base.
, Autoionization involves transfer of proton from one molecule to another to produce
hydroxide ion and hydronium ion.
The equilibrium expression for autoionization of water is Kw and called ion-product
constant, which equals 1.0 x 10-14 M at 25º
Ion-product constant always equals 1.0 x 10-14 independent of whether [H+] is more,
less or equal to [OH-].
7.3 The pH Scale
pH = -log[H+]
The number of decimal places in the log is equal to the number of significant figures
in the original figure.
pH decreases as [H+] increases
Each pH value is 10x stronger in H+ concentration as the one before.
pH is measured using pH meter
7.4 Calculating the pH of strong acid solution
Major species are those which are present at relatively large amounts.
The very first step in solving acid-base problems is writing out all the major species
of acids and bases present.
The pH of a 1M HCl in aqueous solution is 0.00 because the H + produced by the
autoionization of water is negligible as Le Châtelier principle causes all the H +
produced from HCl and H2O to tend to the left direction. Thereby, -log [1] is 0.00
which is the pH of the acid.
7.5 Calculating the pH of weak acid solution
The second step in solving acid-base problems is evaluating which of the major
species provides *the most* H+ ions.
Later, the initial concentration of each species is written (concentration before the
reaction has achieved equilibrium.)
The next step is to determine how many moles are required to reach equilibrium
(represented by the net change)
Using the ICE method and equate the quadratic equation formulated to the Ka value
for the acid, we can find the value of the “change” in moles that has taken place to
reach equilibrium.
This “change” represents the concentration of [H+] as it’s the number of moles that
has being gained (in products) in order to reach equilibrium; knowing that the initial
moles of H+ were negligible because it is a weak acid.
As we know the equilibrium concentration of H+, we can find the pH of the acid.
If there are multiple weak acids together, we use the strongest weak acid to find the
concentration of H+ and then calculate the pH.
Percent dissociation is the amount of weak acid that has dissociated and is a ratio
between the initial concentration and amount dissociated.
For a given acid, the percent dissociation increases as acid becomes more dilute.
For a given acid, the concentration of its species decreases as it becomes more dilute
7.1 The nature of acids and Bases
Acids are characterized by a sour flavour, whereas bases are characterized by bitter
taste.
Acids produce hydrogen ions in aqueous solutions, and bases produce hydroxide
ions. Arrhenius. This concept is limited because it only refers to hydroxide bases and
aqueous solutions.
In terms of Brønsted-lowry concept of acid base, an acid is proton (H+) donor and
base is proton acceptor.
H3O+ is known as hydronium ion. This is usually formed when an acid dissociated in
water.
A conjugate base is everything that remains of the acid molecule after a proton is
lost.
A conjugate acid is formed when proton is transferred to the base.
A conjugate acid-base pair consists of two substances related to each other thru the
donation and acceptance of a proton.
Ka is the acid dissociation constant. The activity of water is said to be 1, so it is not
included in Ka.
The Brønsted-lowry concept of acid can be applied to gaseous medium.
An acid-base reaction based on one concept may not be considered Acid-base if
judged by another concept.
7.2 Acid strength
Strength of an acid is determined by the equilibrium position of its dissociation
reaction.
A strong acid is one that has its equilibrium position far to the right. A strong acid
yields a weak conjugate base
A weak acid is one that has its equilibrium position far to the left. A weak acid yields
a strong conjugate base.
Common strong acids:
- H2SO4
- HCl
- HClO3
- HNO3
Sulfuric acid is a diprotic acid indicating that the acid has two protons.
Most acids are oxyacids such as the ones mentioned above and H3PO4, HOCl,, HNO2.
Organic acids are those with a carbon atom backbone, these types of acids are
usually weak.
When trying to measure the Ka for strong acids, the concentration of the acid cannot
be accurately measured due to the minimal presence of these molecules; leading to
uncertain Ka.
Amphoteric substances are those that have the capacity to behave as acid and base.
, Autoionization involves transfer of proton from one molecule to another to produce
hydroxide ion and hydronium ion.
The equilibrium expression for autoionization of water is Kw and called ion-product
constant, which equals 1.0 x 10-14 M at 25º
Ion-product constant always equals 1.0 x 10-14 independent of whether [H+] is more,
less or equal to [OH-].
7.3 The pH Scale
pH = -log[H+]
The number of decimal places in the log is equal to the number of significant figures
in the original figure.
pH decreases as [H+] increases
Each pH value is 10x stronger in H+ concentration as the one before.
pH is measured using pH meter
7.4 Calculating the pH of strong acid solution
Major species are those which are present at relatively large amounts.
The very first step in solving acid-base problems is writing out all the major species
of acids and bases present.
The pH of a 1M HCl in aqueous solution is 0.00 because the H + produced by the
autoionization of water is negligible as Le Châtelier principle causes all the H +
produced from HCl and H2O to tend to the left direction. Thereby, -log [1] is 0.00
which is the pH of the acid.
7.5 Calculating the pH of weak acid solution
The second step in solving acid-base problems is evaluating which of the major
species provides *the most* H+ ions.
Later, the initial concentration of each species is written (concentration before the
reaction has achieved equilibrium.)
The next step is to determine how many moles are required to reach equilibrium
(represented by the net change)
Using the ICE method and equate the quadratic equation formulated to the Ka value
for the acid, we can find the value of the “change” in moles that has taken place to
reach equilibrium.
This “change” represents the concentration of [H+] as it’s the number of moles that
has being gained (in products) in order to reach equilibrium; knowing that the initial
moles of H+ were negligible because it is a weak acid.
As we know the equilibrium concentration of H+, we can find the pH of the acid.
If there are multiple weak acids together, we use the strongest weak acid to find the
concentration of H+ and then calculate the pH.
Percent dissociation is the amount of weak acid that has dissociated and is a ratio
between the initial concentration and amount dissociated.
For a given acid, the percent dissociation increases as acid becomes more dilute.
For a given acid, the concentration of its species decreases as it becomes more dilute
