acids, bases and pH Flashcards
What are acids
dissociate and release H+ ions in an aqueous solution
What are alkalis
dissociate and release OH- ions in aqueous solution
it is also a soluble base
Wat is the ionic equation for a neutralisation reaction
H+(aq) + OH-(aq) -> H2O(l)
What is a bronsted lowry acid
proton donor
What is a bronsted lowry base
proton acceptor
What is the equation for the dissociation of hydrochloric acid
HCl(l) reversible arrow H+(aq) +OH-(aq)
Why can an equilibrium sign show dissociation of HCl even though it is a strong acid
- equilibrium is well to the right hand side
- a single arrow can be used to indicate that the forward reaction effectivley goes to completion
What are HCl and Cl-
are a conjugate acid base pair
What is a conjugate acid base pair
- contains two species that can be interconverted by transfer of a proton
Describe how HCl forms it conjugate acid base pair
- in forward direction HCl releases a proton to form its conjugate base Cl-
- in the reverse direction Cl- accepts a proton to form its conjugate acid HCL
Describe the dissociation of HCl in a neutralisation reaction
forward direction
HCl is an acid as it donates H+
OH- is a base a it accepts H=
reverse direction
H2O is an acid as it donates H+
Cl- is a base as it accepts H+
What is the equation for neutralisation of HCl and name the ocnjugate acid base pairs
HCl(aq) + OH- (aq) reversible sign H2O (l) + Cl-
HCl = acid 1
Oh- =base 2
H2O =acid 2
CL- = base 1
In an aqueuous solution what does dissociation require
a proton ot be transferred from an acid to a base
What are the requirement for dissociation to occur
does not take place unless water is present
What is the equation for HCl and H2O and state the conjugate acid base pairs
HCl (aq) + H2O(l) reversible sign H3O+(aq) +Cl (aq)
HCl =acid 1
H2O = base 2
H3O+ =acid 2
Cl- =base 1
How is hydronium ion formed and where is it important
- H2O has accepted a proton to form its conjugate acid the hydronium ion H3O+(aq)
- important as it is the active acid ingredient in any aqueous acid
What are the two neutralisation equations
H3O+(aq) +OH-(aq) ->2H2O(l)
H+(aq) + OH- (aq) ->H2O(l)
What do monobasic, dibasic and tibasic acids refer to
the total number of hydrogen ions in the acid that can be replaced per molecule in an acid base reaction
typically replacement of protons by metal ions or ammonium ions to form a salt
what happens to the volume of alkali with increasing number of hydrogen
as number of hydrogen increases the volume increases by the same amount
2[H+] - volume doubles
How do you form an ionic equation
- write out full equation
- cancel spectator ions
What are spectator ion
ions that do not change during the reaction
in equations they can simply be cancelled out
What do dilute acids undergo with some metals and write the equation
redox reactions
acid + metal -> salt + hydrogen
What are carbonates an example of
a base
what do carbonates form when reacting with acid and what type of reaction is it
neutralisation
acid + carbonate -> salt + water + carbon dioxide
What is formed with the reaction of acids and metal oxides/ hydroxides and what reaction is this
neutralisation
acid + base -> salt +water
What type of reaction is acids with alkalis and write the equation
neutralisation
acid + alkali -> salt + water
What was the test before figuring out the pH scale
- measured as colours using indicators
What is a more accurate way than using pH indicator paper or universal indicator
- pH probe
- measuring hydrigen ions using an electrochemical cell
What is the advantage of using pH indicator paper or universal indicator
easier and cheaper
What does a low value of H+ show
high value of pH
what does a high value of H+ show
low value of pH
What is the equation to find out pH
-log[H+]
How do you find out [H+]
10^-pH
What happens to the hydrogen ion concentration as the pH goes up by one
10 times increase
What is the equationfor a strong monobasic acid HA
HA(aq) -> H+(aq) + A-(aq)
What is the hydrogen concentration equal to for a strong acid
[H+]=[HA]
How is the pH of a strong acid calculated
concentration of an acid
What are the steps for calculating pH form acid concentration
- convert [HA] into [H+]
- use calculator to find pH
What are the steps for calculating acid concentration from pH
- use calculator to find [H+]
- convert [H+] into [HA]
What are the steps for calculating pH changes on dilution
- find concentration of diluted acid
- find pH values before and after dilution
What is a strong acid
it completley dissociates in an aqueous solution
What is a weak acid
partially dissociates in an aqueous solution
What is Ka
acid dissociation constant
shows acid strength
What is the equation for the dissociation of a weak acid
HA reversible sign H+ + A-
What are the units for Ka
moldm-3
What is the equation for Ka
[H+][A-]/HA]
What does Ka change with and describ how
temperature
larger numerical value of Ka the further equilbrium is to the right
larger Ka value greater the dissociation and greater the acid strength
What is the equation for PKa and hence what is the equation for Ka
PKa=-logKa
Ka=10^-pKa
Describe the trend with the strength of acid,Ka and PKa
the stronger the acid the larger the Ka value and the smaller the pKa value
the weaker the acid the smaller the Ka value and the larger the pKa value
What are pka values used for
comparing strengths of weak acids particularly in biological systems
what does [H=] depend on in a weak acid
concentration of acid [HA]
Ka acid dissociation constant
How is the Ka expression simplified for calculations
ka =[H+]^2/[HA]
What are the two approximations when calculating concentrations of pH of a weak acid
Ha dissociates to produce equilibrium concentrations of H+ and A- that are equal also be a very small concentration of H+ from the dissociation of water but thi will be extremley mall and can be neglected compared with the H+ concentration from the acid
[H+]=[A-]
equilibrium concentration of HA is smaller than the undissociated concentration the dissociation of weak acids is small [Ha]»[H+] and can neglect any decrease in concentration of HA from dissociation [HA]equilibrium=[HA] start
How do you calculate pH of a weak acid
calculate [H+] from Ka and [HA]
use calculator to find pH
How is Ka determined for a weak acid eperimentally
preparing a standard solution of the weak acid of known concentration
measuring pH of a standard solution using a pH meter
How is Ka calculated for a weak acid
use calculator to find [H=}
calculate Ka from [H+] and [HA]
What are the approximations in calculations involving weak acids
- dissociation of water is negligble
- [H+] - [A-]
- [H+] from the dissociation of water will be significant compared with dissociation of the eak acid this approximation breaks down for vry weak acids or very dilute solutions
- concentration of acid is much greater than h+ concentration at equilibrium
- [HA] equilibrium={HA] start
- this approxumation holds for weak acid with small kA values breaks down when [H+] becomes significant and there is a real difference [HA]equilibrium and [HA] start - [H+] equilibrium
- approximation is not justified for stronger weak acids with Ka.10^-2 and for very dilute solutions
describe the ionisation of water
- water ionises very slightly acting as both an acid and as a base
- setting up the acid base equilbrium
what is the equation for the ionisation of water
- H2O(l)+H2O- revesible -> H3O+(aq) + Oh- (aq)]
- H2O (l) -> reversible H+(aq) + OH- (aq)
describe the dissocaition of water
- 1dm3 (1000g0 of water is mainly undissociated H2O(l)
- so [H2O(l)]=1000/18=55.6moldm-3
what is kw
- ionic product of water
- the ions in water (H+ and OH-) multplied togeter
- Kw=[H+(aq)][OH-(aq)]
what is the value of Kw at 298K
1x10-14 mol2dm-6
what does the value of Kw vary with
temperature
what is the significance of Kw
- significance of Kw habing a value of 1x10^-14 mol2dm-6
- neutral point of the pH scale
- Kw controls the concentration of H+(aq) and OH-(aq) ions in aquous solutions
what is the pH of pure water at 25 degrees celcius
- on dissociaion water is neutral - produces same number of H+(aq) and OH-(aq) ions
- [H+(aq)]=[Oh-(aq)]
- Kw=[H+(aq)]^2=1x10^-14
- ph=-log[H+(aq)]=7
what is the ionic product of water Kw
essentially an equilbrium constant that controls the concentration of H+(aq) and OH- (aq) in aqueous solutions
Desvribe the pH of a solution regarding [H+] and [OH-] ions
- solution is acidic when [H+(aq)]>[OH-(aq)]
- solution is neutral when [H+(aq)]=[OH-(aq)]
- solution is alkaline when [H+(aq)]<[OH-(aq)]
what does the value of Kw control
concentrations of H+(aq) and OH-(aq)
decribe how you work out the pH values of whole numbers
- H+ and OH- concentrations as the indices for [H+(aq)] and [OH-(aq)] add up to -14
what is an alkali
- a soluble base that releases OH- ions aqueous solutions
what is a strong base
an alkali that completley dissociates in a solution
what type of base is NaOh
- monoacidic
- each mole of NAOH releases one mole of OH-(aq)ions
How is the pH of a strong base calculated
- concentration of the base
- ionic product of water Kw
what is a weak base and give an example with equation
- an alkali
- ammonia gas
- dissolves in water releasing OH- ions from water molecules
- NH3(aq) +H2O(l) reversible sign NH4+(aq)+OH- (aq)
