module 5: acids, bases and cells Flashcards
how does the arrhenius vs brønsted lowry model define acids and bases
arrhenius:
acids –> H+
alkali –> OH-
brønsted lowry:
acids are protons donors
bases are proton acceptors
when do protons dissociate
they can only dissociate in solution because they form hydroxonium ions in water (H3O+)
eg. HCl + H2O <–> H3O+ + Cl-
often the H3O is simplified to H+ in equations
what is a conjugate acid-base pair
A conjugate acid-base pair is two species that are different from each other by an H+ ion
the acid vs base can be determined using Ka - the larger Ka value will be had by the acid and the smaller one by the base
how do you define an acid by the number of H+ that are replaced
monobasic - one proton eg HCl
dibasic - two protons eg H2SO4
tribasic - three protons eg H3PO4
how do you write ionic equations
write out the symbol equation
convert only aqueous stuff into ions
simplify by cancelling when on both sides
how is pH linked to [H+]
pH = -log[H+]
[H+] = 10^-pH
(remember to check if the acid is dibasic)
in a strong acid what is the relationship between [HA] (acid concentration and [H+]
they are equal because strong acids completely dissociate in solution
how is the dissociation of a weak acid represented
what is the relationship between Ka and pKa
what are 2 approximations that have to be made when calculating Ka
- HA dissociation forms equal [H+] and [A-]
- the change in [HA] in negligible, so [HA] at equilbrium = [HA] at start
what calculation is used to calculate Ka
what calcuation is then used to calculate pH
what are the issues with Ka calculations
there are issues with the approximations made
- at pH >6 there is significant water dissociation, so it doesn’t;t work for very dilute solutions or very dilute acids
- if [H+] is significant, [HA] at equilbrium ≠ [HA] at start, so doesn’t work for stronger weak acids with Ka > 0.01 moldm^-3 or very dilute solutions(?)
what is Kw
the equilibrium constant for the self-ionization reaction of water
Ka x [H2O] = [H+][OH-]
Ka for water is constant and the dissociation of water is minimal so the water stays constant too
this means:
Kw = [H+][OH-]
Kw has a constant value which is given on the data sheet
(but still varies with temp)
what is the product of the two ion concentration in Kw
so the indices of the concentrations of [H+] and [OH-] add to -14 at any pH
how are new pH’s calculated with dilutions
- divide original volume by new total volume to get the dilution factor
- dilution factor x Kw = new [H+]
- -log[new[H+]]
how is pH of a base determined
strong bases dissociate completely in solution so the conc of the reactant = conc of a product
- find [OH-], will be equal to solutions overall []
- find [H+]
Kw=[H+][OH-] so, [H+]=Kw/[OH-] - find pH
pH=-log[[H+]]
what is a buffer solution
a solution that resists pH change when a small amount of acid or alkali is added
a buffer can consist of weak acid-conjugate base or weak base-conjugate acid
what is an acid-base indicator
a weak acid that dissociates to give a different colour
how does an acid-base indicator work
the colour of the solution depends on the relative concentrations of each species
in a more acidic solution this one would shift left and and colour 1 would dominate and vice versa
so the colour depends on the solution’s pH, and will change gradually dependant on the indicators Ka
at the endpoint, there’s balance between [HIn+] and [In-] and a neg log of both sides shows pKa to equal pH
how is a suitable indicator chosen
pH changes very rapidly around a titration’s equivalence point
indicators change colour over a narrow pH range approximately surrounding the indicator’s pKa
appropriate indicator: its pH (aka pKa) range falls within the rapid change for that titration (where its straight up on a graph)
what reactions is methyl orange used for
strong acid - strong base
strong acid - weak base
what reactions can phenophthalein indicate for
weak acid-strong base
what indicator is used for weak acid-weak base reactions
none because there is no sharp change in pH, it’s very gradual
outline the shorthand of drawing an electrochemical cell
outline how you would find out how many manganate(VII) ions (MnO4^-) are needed to react with a reducing agent
- measure out a quantity of the reducing agent with a pipette and put in a conical flask (eg Fe2+)
- add some excess dilute sulfuric acid; provides extra H+ to ensure reduction of the oxidising agent
- gradually add the MnO4^- (oxidising agent) to the reducing agent using a burette and swirling as you go
- stop when the flask mixture just becomes tinted with the colour of the oxidising agent and record the volume of it added
- repeat and calculate mean titre
outline an iodine-thiosulfate redox titration
- measure out a certain volume of KIO3 (oxidising agent) and add to excess acidified KI which oxides some iodide ions into iodine
- to find out how many moles of iodine were produced:
add sodium thiosulfate solution to that above glass drop by drop from a burette - the end point is hard to see, so when the solution is pale yellow add 2cm^3 starch solution to detect iodine; it will o dark blue which shows iodine present
- keep adding sodium thiosulfate dropwise until the blue disappears
record volume and conc of sodium thiosulfate added - calculate moles of iodine
outline a hydrogen fuel cell
anode is left and cathode is right
evaluate electrochemical cells
+ more efficient than conventional combustion engines at energy production because less is wasted as heat
+ less pollution
– producing the cells involves toxic chemicals which have to be disposed of once the cell reaches the end of it’s lifespan
– chemicals used are highly flammable
what is a fuel cell
if uses energy from the reaction of a fuel with oxygen to create a voltage
the fuel and oxygen flow into the cell and the products flow out whilst the electrolyte remains in the cell
they do not have to be recharged and can operate continuously provided that the fuel and oxygen are supplied into the cell
many fuels can be used but hydrogen is the most common with the only product being water
what is a secondary cell
rechargeable as the cell reaction producing electrical energy is reversible, allowing chemicals to be regenerated and the cell to be reused
what is a primary cell
non rechargeable cells where electrical energy is produced by oxidation and reduction at the electrodes until chemicals are used up and voltage is no longer produced
how can standard cell potentials tell you whether a reaction is feasible?
the more positive the standard cell potential, the more feasible (likely to occur) a reaction is and it will tend to proceed in the forward direction
how do you calculate standard cell potential
more positive electrode potential - more negative electrode potential
how do you combine half equations when dealing with electrochemical cells
the half equation/redox system with a more negative standard electrode potential will be flipped and written from right to left as it acts as the reducing agent and so gets oxidised
the e- in both equations are then balanced and the balanced equations are combined, cancelling out where necessary
draw an experimental set up that could be used in the laboratory to measure the standard cell potential of an electrochemical
if the species is aq on both sides, the electrode is platinum
if one is solid and one is aq, the electrode will be the metal
how do you combine half equations
make the e- in each equation equal snd then add the reactants and products in each equation whilst cancelling out like species
what is a buffer solution
a solution that resists pH change when a small amount of acid or alkali is added
a buffer can consist of weak acid-conjugate base or weak base-conjugate acid
what is a common buffer solution and how does it work (savemyexams)
aqueous mixture of CH3COOH and CH3COONa
CH3COOH is a weak acid and partially ionises in solution to form a low conc of ethanoate ions
CH3COONa is a salt which fully ionises in solution
There are reserve supplies of the acid (CH3COOH) and its conjugate base (CH3COO-)
buffer solution has relatively high concentrations of CH3COOH and CH3COO- , ethanoic acid is in equilibrium with H+ and CH3COO-
adding H+: equ. shifts left as H+ react with CH3COO- to form CH3COOH until equilibrium is re-established
there is a large reserve supply of CH3COO- so conc of CH3COO- in solution doesn’t change much
As a result, the pH remains reasonably constant
adding OH-: OH- reacts with H+ to form water
equ. shifts right and more CH3COOH molecules ionise to form more H+ and CH3COO- until equilibrium is re-established
there’s a large reserve supply of CH3COOH so CH3COOH conc in solution doesn’t change much when CH3COOH dissociates to form more H+ ions
so pH stays mostly constant
when does [H+]=Ka and so pH=pKa
when concentrations of the acid and salt in a buffer solution are equal
how do you calculate the pH of a buffer solution
how can you calculate Kc
aA + bB <—> cC + dD
the [] indicates that concentrations are in moldm-3
units should be calculated as such
how will Kc change with temperature
if forwards is endothermic, increased temp will increase Kc
if forwards is exothermic, increased temp will decrease Kc
what do certain values of k indicate about the equilibrium position
1 = equally between reactants and products
100 = well within the products
0.01 = well within the reactants
what factors can change the value of K
ONLY temperature
not pressure/concentration etc.
outline the effect of temperature on the equilibrium and K
if the reaction is exothermic. increasing temp decreases K
this leaves the product:reactant ratio greater than K so the product conc decreases and reactant conc increases to reach K
yield decreases
outline the effect of increasing concentration of reactant on the equilibrium and K
it does not alter K, so equilibrium position has to change to accommodate
reactant conc has to decrease and product conc has to increase to maintain the Kc value
yield increases
outline the effect of increasing pressure on the equilibrium and K
it doesn’t alter K, but does increase each partial pressure
Kp stays constant but the product:reactant ratio is greater than Kp SO,
reactant pressure increases and product pressure decreases
yield decreases
how is Kp calculated in terms of partial pressure
what is the sum of mole fractions of all components
always 1
define mole fraction
the fraction of the total moles that each chemical in a reaction is responsible for
define partial pressure
the part of the total pressure that each chemical in a reaction is responsible for
what cell potentials are found with oxidation and reduction
oxidation (e- formed) = more -ve cell potential
reduction (e- react) = more +ve cell potential
what does a monobasic, dibasic or tribasic acid mean
the number of protons it has 😜
how do you calculate the pH for a new solution
- find out whether the acid or alkali is in excess and therefore the H+ or OH-
- divide the excess amount by total volume to get concentration
- sub into Kw if OH- was in excess to get [H+]
- -log to get pH
how is blood pH controlled
CO2 reacts with H2O to form an equilibrium
what value of delta G shows a feasible reaction
equal to or less than 0
what steps are needed to perform an accurate titration
- make sure there are no air bubbles in the end of the burette
- measure at eye level from bottom of meniscus
- place white tile under conical flask to see endpoint clearly
- rinse sides of flask with distilled water to wash unreacted solutions back down
- add solution drop by drop at the end point
what is an end point vs equivalence point
end point: colour change
equivalence point: chemical reaction done
where is the equivalence point on a pH curve
bang in the middle of the steep bit
how do you talk about a reaction favouring one side or the other when it involves electrode potentials
say that it shifts left/right
what’s the difference between an acid hydrogen fuel cell and an alkali hydrogen fuel cell
acid: the electrolyte is H+ and they go left to right
alkali: the electrolyte is OH- and they go right to left
where does oxidation and reduction occur in a fuel cell
cathode reduces the fuel
anode oxidises the fuel
why might an acid have a measured pH different to its calculated pH
a SIGNIFICANT difference in concentration at equilibrium vs initial
why does pH of a buffer solution not change when a small volume of water is added
the ratio of [HA]:[A-] stays the same because the both change by the same amount
how can accuracy be improved
repeat titrations and get a mean of concordant titres
use a 3 decimal place balance
what is unique about fuel cells
the fuel reacts with oxygen to give electrical energy
outline the method of finding the pH of a buffer solution
- find the total moles of acid
- find the total moles of base
- subtract moles of base from moles of acid to get excess acid
- divide them by the total volume to get respective concentrations
- sub into Ka to get [H+]
outline an energy storage cell
essentially a battery, they work just like an electrochemical cell
in a fuel cell, which is the positive electrode and what occurs at each one
cathode is positive and reduction occurs
anode is negative and oxidation occurs
explain an iodine-thiosulfate titration
- when the oxidising reagent is added excess of acidified KI, it causes the I- to get oxidised to I2
oxagent + I- + H+ –> I2 + H2O - resulting solution is titrated with Na2S2O3 of a known conc.
I2 + 2S2O3^2- –> 2I- + S4O6^2-
how do you explain the impact of factors other than temp on Kp or Kc
equ. shifts in given direction
the impact this has on values in the equation (eg denominator is increased)
how Kp/Kc is restored; eg numerator increases
how do two step equation questions work with electrode potential
more -ve is oxidised and flips so the system for it shifts left and vice versa
even tho that system shifted left, that doesn’t affect the E and it can shift left Again when compared with a new electrode potential
do you include e- and H+ in the solution in fuel cell diagrams
only H+ but DONT FORGEY HIM!!!!
how do you calculate the half equation for one electrode in a fuel cell when given the overall equation
subtract one of the half equations from the table from the overall equation
only subtract form the side of the equation that it’s from
why is a buffer solution formed when excess weak acid is added to a base
the acid is in excess and ions of the base are produced
what does a buffer equ. look like
acid —> H+ + A-
when are oxidation numbers used for balancing
with redox equations
