pH Flashcards
What is a bronsted lowery acid
any substance that can donate a proton
What is a bronsted lowery base
any substance that can accept a proton
what did Joseph Priestly and Karl Scheele report
they reported findings that suggested the existence of the gas we know now as oxygen
what did antonine-Laurent de Lavoiser suggest
he suggested that oxygen was important in rusting and was the source of acidity, this was an early step towards the understanding of acids
what did Humphrey Davy do
he suggested that some acidic substances such as HCl did not actually contain oxygen
What did Justin Liebig do
he defined an acid as a substance containing hydrogen that could be replaced by a metal
What did Svante Arrhenius propose
he proposed that acids dissociate in water to form hydrogen ions and that bases dissociate in water to form hydroxide ions, this is true for many acids and bases and they can be referred to as Arrhenius acids and bases
- the model breaks down when the acids and bases are not solutions in water or when the bases are not soluble hydroxides
what is a lewis acid
describes an acid as an electron-pair acceptor
what is a lewis base
describes a base as a electron pair donor
Whats a monobasic acid
a monobasic acid is when each molecule can release 1 proton
for example
HCl = H+ + Cl-
Whats a dibasic acid
this is when each molecule can release 2 protons
For example
H2SO4 = HS04- + H+
HSO4- = H+ + SO42-
Whats a tribasic acid
this is when each molecule can release 3 protons
H3PO4 = H+ + H2PO4-
H2PO4- = H+ + HPO42-
HPO42- = H+ + PO43-
What is a conjugate acid base pair
these are a set of 2 species that transform into each other by gain or loss of a proton
differ by … H+
What is an alkali
a base that dissolves in water forming OH- ions
What is neutralisation
is a chemical reaction in which an acid and a base react together to form a salt and water
What is a strong acid
is an acid that completely dissociates in solution
What is a weak acid
an acid that partially dissociates in solution, it forms an equilibrium that sits towards the left this means that there is a little amount of hydrogen ions released
What does Ka do
show us the strength of an acid, write out the equation of Ka with HA
define pKa
-log10Ka
define Ka
10 -pKa
Draw the hydronium ion
DRAW IT
How do acids react with carbonates
- form carbon dioxide and water
- draw full equation for reaction between hydrochloric acid and solid calcium carbonate, then draw the ionic equation
- draw full equation for reaction between aqueous sodium carbonate and hydrochloric acid, then draw the ionic equation
How do acids react with bases
- form a salt and water
- full equation of reaction between nitric acid and magnesium oxide
- ionic equation of reaction between nitric acid and magnesium oxide
How do acids react with alkalis
- forms a salt and water
- full equation of reaction between sulphuric acid and potassium hydroxide
- ionic equation between sulphuric acid and potassium hydroxide
reaction between acid and metal
- redox reaction so does not fit in with the acid-base model
- form a slat and hydrogen
- write full equation of reaction between hydrochloric acid and magnesium
- write ionic equation of reaction between hydrochloric acid and magnesium
Name the strong acids
HCL HNO3 H2SO4 HBr HI HCLO4
describe strong acids
100% dissociate in aqueous solution
- HA= H+ + A-
describe weak acids
- only partially dissociate in aqueous solution, set up an equilibrium
- equilibrium positions to the left, this means that there are only small concentrations of dissociated ions such as CH3COO- and H+ compared with the lagre concentration of CH3COOH which is undissociated
- draw equation for weak acid dissociating such as ethanoic acid
What does a large Ka indicate
A large Ka value indicates a large dissociation the acid is strong
What does a small Ka value indicate
A small Ka value indicates a small extent of dissociation the acid is weak
What is a low value of Ka
This matches a high value of pKa
What does a high value of Ka match
A low value of pKa
The smaller the pKa
The stronger the acid
Why do we use the pH scale over hydrogen ion concentrations
Hydrogen ions have a large range
Define pH
-log(H+)
Define Hydrogen ions
10-pH
What does the Bronsted-Lowry model of acids and bases state
it states that reactions between acids and bases involve the transfer of hydrogen ions known as protons
what is the acid dissociation constant
- this is the actual extent of a dissociation which is measured by the acid dissociation constant
a large Ka value indicates…
a large extent of dissociation and the acid is strong
A small Ka value indicates ….
a small extent of dissociation and the acid is weak
why do we use a pH scale and not the hydrogen ion concentration
- the pH scale is a smaller range than the concentrations of hydrogen ions
- easier to read
What does a low pH value mean
a high concentration of hydrogen ions
What does a high pH value mean
a low concentration of hydrogen ions
What does a pH change of 1 indicate
this changes hydrogen ion concentration by 10 times
How do you calculate a pH of strong acids
- H+ = HA
- therefore -log(H+)= pH
- this means that the hydrogen ion concentration of a strong acid is equal to the concentration of the acid
How do you calculate the pH of weak acids
- weak acids are in equilibrium
- we can no longer assume that the hydrogen ion concentration is equal to the concentration of the acid therefore we need to work out the Ka for the reaction
- Ka = [H+][A-] / [HA]
- for weak acids the hydrogen ion concentration is less than [HA] as the extent of dissociation is small, but when HA molecules dissociate there is an equal number of H+ ions = A- ions therefore [H+] and [A-] can be considered equal so in Ka it becomes [H+]2
- any water present will have dissociated to such an negligible amount that it will not affect the concentration of H+
What are the two things that you need to assume
- for weak acids the hydrogen ion concentration is less than [HA] as the extent of dissociation is small, but when HA molecules dissociate there is an equal number of H+ ions = A- ions therefore [H+] and [A-] can be considered equal so in Ka it becomes [H+]2
- any water present will have dissociated to such an negligible amount that it will not affect the concentration of H+
What are the limitations of the scientific approach
- when calculating pH for a weak acid it is assumed that so little of the original acid has dissociated that the concentration of the acid at equilibrium is effectively the same as the concentration of the original amount of acid
- this is an example of an approximation
Define Kw
- ionic product of water
- Kw = [H+(aq)][OH-(aq)]
- 1.00 x10 -14
What is the significance of Kw
- Kw is only affected by change in temperature
- the balance between hydrogen ions and hydroxide ions can be affected by the addition of extra hydrogen ions or hydroxide ions
- equilibrium will shift in order to minimize change
Calculating pH for strong bases
- do it like you do acids
What is a strong base
this is an alkalis that 100% dissociate in aqueous solution releasing OH- ions
