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a level chemistry > module 5: acids, bases and cells > Flashcards

module 5: acids, bases and cells Flashcards

1
Q

how does the arrhenius vs brønsted lowry model define acids and bases

A

arrhenius:
acids –> H+
alkali –> OH-

brønsted lowry:
acids are protons donors
bases are proton acceptors

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2
Q

when do protons dissociate

A

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

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3
Q

what is a conjugate acid-base pair

A

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

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4
Q

how do you define an acid by the number of H+ that are replaced

A

monobasic - one proton eg HCl
dibasic - two protons eg H2SO4
tribasic - three protons eg H3PO4

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5
Q

how do you write ionic equations

A

write out the symbol equation
convert only aqueous stuff into ions
simplify by cancelling when on both sides

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6
Q

how is pH linked to [H+]

A

pH = -log[H+]
[H+] = 10^-pH
(remember to check if the acid is dibasic)

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7
Q

in a strong acid what is the relationship between [HA] (acid concentration and [H+]

A

they are equal because strong acids completely dissociate in solution

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8
Q

how is the dissociation of a weak acid represented

A
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9
Q

what is the relationship between Ka and pKa

A
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10
Q

what are 2 approximations that have to be made when calculating Ka

A
  1. HA dissociation forms equal [H+] and [A-]
  2. the change in [HA] in negligible, so [HA] at equilbrium = [HA] at start
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11
Q

what calculation is used to calculate Ka

what calcuation is then used to calculate pH

A
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12
Q

what are the issues with Ka calculations

A

there are issues with the approximations made

  1. at pH >6 there is significant water dissociation, so it doesn’t;t work for very dilute solutions or very dilute acids
  2. 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(?)
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13
Q

what is Kw

A

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)

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14
Q

what is the product of the two ion concentration in Kw

A

so the indices of the concentrations of [H+] and [OH-] add to -14 at any pH

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15
Q

how are new pH’s calculated with dilutions

A
  1. divide original volume by new total volume to get the dilution factor
  2. dilution factor x Kw = new [H+]
  3. -log[new[H+]]
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16
Q

how is pH of a base determined

A

strong bases dissociate completely in solution so the conc of the reactant = conc of a product

  1. find [OH-], will be equal to solutions overall []
  2. find [H+]
    Kw=[H+][OH-] so, [H+]=Kw/[OH-]
  3. find pH
    pH=-log[[H+]]
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17
Q

what is a buffer solution

A

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

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18
Q

what is an acid-base indicator

A

a weak acid that dissociates to give a different colour

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19
Q

how does an acid-base indicator work

A

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

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20
Q

how is a suitable indicator chosen

A

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)

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21
Q

what reactions is methyl orange used for

A

strong acid - strong base
strong acid - weak base

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22
Q

what reactions can phenophthalein indicate for

A

weak acid-strong base

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23
Q

what indicator is used for weak acid-weak base reactions

A

none because there is no sharp change in pH, it’s very gradual

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24
Q

outline the shorthand of drawing an electrochemical cell

A
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25
outline how you would find out how many manganate(VII) ions (MnO4^-) are needed to react with a reducing agent
1. measure out a quantity of the reducing agent with a pipette and put in a conical flask (eg Fe2+) 2. add some excess dilute sulfuric acid; provides extra H+ to ensure reduction of the oxidising agent 3. gradually add the MnO4^- (oxidising agent) to the reducing agent using a burette and swirling as you go 4. stop when the flask mixture just becomes tinted with the colour of the oxidising agent and record the volume of it added 5. repeat and calculate mean titre
26
outline an iodine-thiosulfate redox titration
1. measure out a certain volume of KIO3 (oxidising agent) and add to excess acidified KI which oxides some iodide ions into iodine 2. to find out how many moles of iodine were produced: add sodium thiosulfate solution to that above glass drop by drop from a burette 3. 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 4. keep adding sodium thiosulfate dropwise until the blue disappears record volume and conc of sodium thiosulfate added 5. calculate moles of iodine
27
outline a hydrogen fuel cell
anode is left and cathode is right
28
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
29
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
30
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
31
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
32
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
33
how do you calculate standard cell potential
more positive electrode potential - more negative electrode potential
34
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
35
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
36
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
37
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
38
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
39
when does [H+]=Ka and so pH=pKa
when concentrations of the acid and salt in a buffer solution are equal
40
how do you calculate the pH of a buffer solution
41
how can you calculate Kc
aA + bB <—> cC + dD the [] indicates that concentrations are in moldm-3 units should be calculated as such
42
how will Kc change with temperature
if forwards is endothermic, increased temp will increase Kc if forwards is exothermic, increased temp will decrease Kc
43
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
44
what factors can change the value of K
ONLY temperature not pressure/concentration etc.
45
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
46
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
47
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
48
how is Kp calculated in terms of partial pressure
49
what is the sum of mole fractions of all components
always 1
50
define mole fraction
the fraction of the total moles that each chemical in a reaction is responsible for
51
define partial pressure
the part of the total pressure that each chemical in a reaction is responsible for
52
what cell potentials are found with oxidation and reduction
oxidation (e- formed) = more -ve cell potential reduction (e- react) = more +ve cell potential
53
what does a monobasic, dibasic or tribasic acid mean
the number of protons it has 😜
54
how do you calculate the pH for a new solution
1. find out whether the acid or alkali is in excess and therefore the H+ or OH- 2. divide the excess amount by total volume to get concentration 3. sub into Kw if OH- was in excess to get [H+] 4. -log to get pH
55
how is blood pH controlled
CO2 reacts with H2O to form an equilibrium
56
what value of delta G shows a feasible reaction
equal to or less than 0
57
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
58
what is an end point vs equivalence point
end point: colour change equivalence point: chemical reaction done
59
where is the equivalence point on a pH curve
bang in the middle of the steep bit
60
how do you talk about a reaction favouring one side or the other when it involves electrode potentials
say that it shifts left/right
61
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
62
where does oxidation and reduction occur in a fuel cell
cathode reduces the fuel anode oxidises the fuel
63
why might an acid have a measured pH different to its calculated pH
a SIGNIFICANT difference in concentration at equilibrium vs initial
64
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
65
how can accuracy be improved
repeat titrations and get a mean of concordant titres use a 3 decimal place balance
66
what is unique about fuel cells
the fuel reacts with oxygen to give electrical energy
67
outline the method of finding the pH of a buffer solution
1. find the total moles of acid 2. find the total moles of base 3. subtract moles of base from moles of acid to get excess acid 4. divide them by the total volume to get respective concentrations 5. sub into Ka to get [H+]
68
outline an energy storage cell
essentially a battery, they work just like an electrochemical cell
69
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
70
explain an iodine-thiosulfate titration
1. when the oxidising reagent is added excess of acidified KI, it causes the I- to get oxidised to I2 oxagent + I- + H+ --> I2 + H2O 2. resulting solution is titrated with Na2S2O3 of a known conc. I2 + 2S2O3^2- --> 2I- + S4O6^2-
71
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
72
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
73
do you include e- and H+ in the solution in fuel cell diagrams
only H+ but DONT FORGEY HIM!!!!
74
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
75
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
76
what does a buffer equ. look like
acid —> H+ + A-
77
when are oxidation numbers used for balancing
with redox equations

a level chemistry (15 decks)

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