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A-level Chemistry > Module 5 - Physical chemistry and transition elements > Flashcards

Module 5 - Physical chemistry and transition elements Flashcards

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1
Q

what is lattice enthalpy?

A

when gaseous ions combine to make a solid lattice, energy is given out -> lattice enthalpy

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2
Q

what is the standard lattice enthalpy?

A
  • is the enthalpy change when 1 mole of ionic lattice is formed from its gaseous ions under standard conditions. its a measure of ionic bond strength. the more negative the lattice enthalpy, the stronger the bonding
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3
Q

what does it mean if the charge of the ions is higher?

A
  • more energy is released when an ionic lattice forms. this is due to the stronger electrostatic forces between the ions. more energy release means that the lattice enthalpy will be more negative. so the lattice enthalpies for compounds with 2+ or 2- ions are more exothermic than those with less
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4
Q

what does it mean if the ionic radii of the ions involved are smaller?

A
  • the more exothermic the lattice enthalpy. smaller ions have a higher charge density and their smaller ionic radii mean that the ions can sit closer together in the lattice. both these things mean that the attractions between the ions are stronger
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5
Q

how do you calculate the lattice enthalpy using a born-haber cycle?

A
  1. you start with the enthalpy of formation
  2. then put the enthalpies of atomisation and ionisation above this
  3. the electron affinity
  4. and lattice enthalpy goes last
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6
Q

what is the enthalpy of formation?

A
  • the enthalpy change when 1 mole of a compound is formed from its elements in their standard states
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7
Q

what happens when a solid ionic lattice dissolves in water?

A
  • the bonds between the ions break to give gaseous ions, this is endothermic. the enthalpy change is the opposite of the lattice enthalpy
  • bonds between the gaseous ions and water are made, this is exothermic. the enthalpy change of hydration.
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8
Q

what is the enthalpy change of solution?

A
  • its the overall effect on the enthalpy of these two things. the effect happens because oxygen is more electronegative than hydrogen, so it draws the bonding electrons towards itself, creating a dipole
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9
Q

what is the enthalpy of hydration?

A
  • the enthalpy change when 1 mole of gaseous ions dissolves in water
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10
Q

what is the enthalpy of solution?

A
  • the enthalpy change when 1 mole of solute dissolves in water
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11
Q

when do substances generally dissolve?

A
  • if the energy released is roughly the same or greater than the energy taken in. so soluble substances tend to have exothermic enthalpies of solution
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12
Q

how do you calculate the enthalpies of hydration of ions using lattice enthalpy?

A

1 - put the ionic lattice and the dissolved ions at the top, connect them by the enthalpy change of solution
2 - connect the ionic lattice to the gaseous ions by the reverse of the lattice enthalpy. the breakdown of the lattice has the opposite enthalpy change to the formation of the lattice
3 - connect the gaseous ions to the dissolved ions by the hydration enthalpies of each ion

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13
Q

what two things affect the enthalpies of hydration?

A
  1. ions with a higher charge are better at attracting water molecules as the electrostatic attraction between the ion and the water molecules is stronger, so more energy is released when the bonds are made giving them a more exothermic enthalpy of hydration
  2. smaller ions have a higher charge density so they attract the water molecules better and have a more exothermic enthalpy.
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14
Q

what is entropy?

A
  • its a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles. the more disordered the particles are, the higher the entropy is. A large, positive value of entropy shows a high level of disorder.
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15
Q

how does physical state affect entropy?

A
  • solid particles just unable about a fixed point - there’s hardly any randomness, so they have the lowest entropy. gas particles whizz around whenever they like. they’ve got the most random arrangements of particles, so the highest entropy
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16
Q

how does the number of particles affect entropy?

A
  • the more particles you’ve got, the more ways they and their energy can be arranged and so the entropy increases
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17
Q

why are some reactions feasible?

A

substances really like disorder, they’re more energetically stable when there’s more disorder, so particles will move to increase their entropy. when reactions are feasible they just happen by themselves without addition of energy even when the enthalpy change is endothermic

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18
Q

what happens when a substance reaches its maximum entropy state?

A

its said to be thermodynamically stable. this means it wont react any further without the input of energy

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19
Q

what is the equation for entropy change of a reaction?

A

entropy change S = Sproducts - Sreactants

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20
Q

what is a positive entropy change?

A

means that a reaction is likely to be feasible, but a negative change in entropy of reaction doesnt guarantee the reaction cant happen enthalpy, temperature and kinetics play a part in whether or not a reaction occurs

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21
Q

what is the tendecny of a process to take place dependant on?

A
  • the entropy, the enthalpy and the temperature. when all of these are together you get the free energy change, which tells us if a reaction is feasible,
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22
Q

how do you calculate the free energy change?

A
  • free energy change = enthalpy change - temperature x entropy change
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23
Q

what happens when free energy change = 0?

A
  • the reaction is just feasible, so the temperture at which the reaction becomes feasible can be calculated by
    temperature = enthalpy change divided by entropy change
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24
Q

what happens when the enthalpy change is negative and entropy change is positive?

A
  • the free energy change will always be negative and the reaction is feasible
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25
Q

what happens when the enthalpy change is positive and the entropy change is negative?

A
  • the free energy change will always be positive and the reaction is not feasible
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26
Q

what happens when the free energy shows a reaction is feasible?

A
  • it might have a high activation energy or happen very slowly
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27
Q

what is the reaction rate?

A

the change in amount of reactants or products per unit time, colorimeters measure the absorbance of a particular wavelength of light by a solution

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28
Q

what is the order of reaction?

A

the order of reaction with respect to a particular reactant tells you how the reactant’s concentration affects the rate

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29
Q

what is zero order?

A

if you double the reactant’s concentration and the rate stays the same its a zero order reaction

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30
Q

what is a first order?

A

if you double the reactant’s concentration and the rate also doubles, its a first order reaction

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31
Q

what is a second order?

A

if you double the reactant’s concentration and the rate quadruples, its a second order reaction

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32
Q

what is the overall order?

A

this is the sum of the order of all the different orders

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33
Q

what do the orders look like on a rate-concentration graph?

A

zero order - a horizontal line means changing the concentration doesn’t change the rate, so its order=0
first order - straight line through the origin means rate is proportional, so its order 1
second order - a curve means its order 2

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34
Q

what is the general equation for rate?

A

rate = k[A]m[B]n

  • k is the rate constant, the bigger it is the faster it is
  • m and n are the orders of the reaction with respect to reactants A and B, they tell you how the concentration of A and B affect the rate
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35
Q

what does it mean of the overall reaction is first order?

A
  • then the rate constant is equal to the gradient of the rate-concentration graph of that reactant
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36
Q

what is the half life of the reaction?

A

is the time it takes for half life of the reactant to be used up.

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37
Q

what is the half life of a first order reaction?

A

its independent of the concentration so each half life will be the same length. this means the half life of a first order reaction can be read of its concentration-time graph, this is exponential decay

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38
Q

what is the equation to work out the rate constant using half life?

A

K= ln2 divided by t1/2

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39
Q

what happens to the rate when you increase the temperature?

A

more reactant particles will have the required activation energy for the reaction, so a greater proportion of the collisions will result in the reaction actually happening. so increasing the temperature, increases the reaction rate

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40
Q

what does a rate constant apply to?

A

it applies to a particular reaction at a certain temperature, so at a higher temperature the reaction will have a higher rate constant

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41
Q

what does Arrhenius equation?

A

it likes the rate constant K with Ea and temp

K = Ae x -Ea/RT
- K is rate constant, Ea is activation energy Jmol, T is temp is K, R is gas constant 8.314, and A is the pre-exponential factor

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42
Q

what happens as Ea gets bigger?

A

k gets smaller. so a large Ea, will mean a slow rate as if a reaction has a high Ea, then not many of the reactant particles will have enough energy to react. so only a few of the collisions will result in the reaction actually happening and the rate will be slow

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43
Q

what is Arrhenius’ equation is logarithmic form?

A

lnK= -Ea/RT + lnA

  • you can use the eqn to create an arrhenius plot by plotting lnK against 1/T. this will make a graph of -Ea/R and y-intercept of lnA.
  • You can then find Ea and A.
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44
Q

what are rate mechanisms?

A

they can have one step or series of steps. each step can have a different rate, the overall rate if decided by the step with the slowest rate which is called the RATE DETERMINING STEP

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45
Q

what happens if a rate appears or doesnt appear in the equation?

A
  • if it appears in the rate reaction, it must affect the rate. so this reactant, or something derived from it must be in the rate-determining step
  • if it doesnt appear in the rate reaction, then it isn’t involved in the rate determining step
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46
Q

what is the total pressure of a gas mixture?

A
  • is the sum of all the partial pressures of the individual gases
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47
Q

what is a mole fraction?

A
  • its the proportion of a gas mixture that is a particular gas
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48
Q

what is the equation for a mole fraction of a gas?

A

number of moles of gas divided by total number of moles of gas in the mixture

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49
Q

what is the equation for partial pressure of a gas?

A

mole fraction of a gas x total pressure of the mixture

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50
Q

what is the general equation for equilibrium of a partial pressure?

A

instead of brackets like in Kc you use p. eg p(D)d x p(E)e

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51
Q

what happens to the equilibrium when you change a condition of a reversible reaction?

A
  • if the change causes more product to form, then you say the equilibrium shifts to the right
  • if the change causes left product to form, then the equilibrium shifts to the left
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52
Q

what does the equilibrium constant for a reaction depend on?

A

temperature
1 - if changing the temp causes less product to form the equilibrium moves the left and the equilibrium constant decreases
2 - if changing the temp causes more product to form the equilibrium moves the right and the equilibrium constant increases

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53
Q

what do concentration and pressure do to the equilibrium constant (pressure)?

A

they dont affect the values of Kp or Kc but they do change the amounts of products and reactants present at equilibrium,
- catalysts have no effect on the position of equilibrium or the value of kc or kp.

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54
Q

what is the d-block?

A

the block of elements in the middle of the periodic table. most of the elements in the d-block are transition elements. a transition element is one that can form at least one stable ion with an incomplete d sub-shell

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55
Q

what can a d sub shell element do?

A

it can hold 10 electrons, transition elements must form at least one ion that has between 1&9 electrons in the d-subshell. all the period 4 d-block elements are transition elements except Sn and Zn.

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56
Q

describe chromium’s arrangement?

A

prefers to have one electron in each orbital of the 3d subshell and just one in the 4s subshell which gives it more stability
- when transition elements form positive ions, the s electrons are removed first, then the d electrons

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57
Q

describe copper’s arrangement?

A

prefers to have a full 3d subshell and just one electron in the 4s subshell. when cu2+ ion is formed it loses 2 electrons forming an incomplete d sub shell.

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58
Q

why are transition elements good catalysts?

A

they can change oxidation states by gaining or losing electrons within their d-orbitals. this means they can transfer electrons to speed up reactions.
- they are also good at absorbing substances onto their surfaces to lower the activation energy of reactions

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59
Q

what happens when you mix aq transition ions with aq NaOH or aq NH3?

A

you get a coloured hydroxide precipitate. in aqueous solutions, transition elements take the form (M(H20)6)n+.

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60
Q

describe the reactions of Copper II with NaOh and NH3?

A

1- [Cu(H20)6]2+ + 2OH- –> [Cu(OH)2 (H20)4]s + 2H2Ol
2- [Cu(H20)6]2+ + 2NH3 –> [Cu(OH)2 (H20)4]s + 2NH4+
solution goes from pale blue to a blue precipitate

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61
Q

describe the reactions of iron II with NaOH and NH3?

A

1- [Fe(H20)6]2+ + 2OH- –> [Fe(OH)2 (H20)4]s + 2H2Ol
2- [Fe(H20)6]2+ + 2NH3 –> [Fe(OH)2 (H20)4]s + 2NH4+
solution goes from pale green to a green precipitate, darkens on standing

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62
Q

describe the reactions of manganese II with NaOH and NH3?

A

1- [Mn(H20)6]2+ + 2OH- –> [Mn(OH)2 (H20)4]s + 2H2Ol

2- [Mn(H20)6]2+ + 2NH3 –> [Mn(OH)2 (H20)4]s + 2NH4+ solution goes from pale pink to a pink/buff solution

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63
Q

describe the reactions of iron III with NaOH and NH3?

A

1- [Fe(H20)6]3+ + 3OH- –> [Fe(OH)2 (H20)4] + 3H2O
2- [Fe(H20)6]3+ + 3NH3 –> [Fe(OH)2 (H20)4] + 3NH4+
solution goes from yellow to a orange precipitate

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64
Q

describe the reactions of chromium III with NaOH and NH3?

A

1- [Cr(H20)6]2+ + 3OH- –> [Cr(OH)2 (H20)4] + 3H2O
in excess NaOH it forms [Cr(OH)6]3- a dark green
2- [Cr(H20)6]2+ + 3NH3 –> [Cr(OH)2 (H20)4] + 3NH4+
in excess NH3 forms [Cr(NH3)6]3+ a dark purple
originally the solution goes from green to a grey-green

65
Q

what is a complex ion?

A

a metal ion surrounded by coordinately bonded ligands. a coordinate bond (or dative bond) is a covalent bond in which both electrons in the shared pair come from the same atom

66
Q

what is a ligand?

A

an atom,ion or molecule that donates a pair of electrons to a central metal atom or ion. the coordination number is the number of coordinate bonds that are formed with the central metal atom/ion

67
Q

what is the coordination number in most complex ions?

A

it will be 4 or 6.

  • if the ligands are small like H20, CN- or NH3, 6 can fit around the central atom. its octahedral.
  • if the ligands are larger like Cl- only 4 can fit. its tetrahedral.
68
Q

what are ligands with 1 or 2 coordinate bonds called?

A
  • ligands with one lone pair are called monodentate eg H20, Nh3, Cl- and CN-
  • ligands with two lone pairs are called bidentate, these ligands can each form two coordinate bonds with a metal ion
  • ligands with two or more coordinate bonds are called multidentate
69
Q

what do square planar and octahedral complex ions do?

A

they have at least two pairs of ligands which show cis/trans isomerism.

70
Q

what is cis-platin?

A

its a complex of platinum (II) with chloride ions and two ammonia molecules in a square planar shape. it is used as an anti cancer drug

71
Q

how does cis-platin work?

A

the two chloride ligands are easy to displace, so the cis-platin loses the and bonds to two nitrogen atoms on the DNA molecule inside the cancerous cell.
- the block on its DNA prevents the cancerous cell from reproducing by division, the cell will die since its unable to replace damage.

72
Q

what is the downside of cis-platin?

A

cis-platin also prevents normal cells from reproducing, including blood which can suppress the immune system and increase risk of infection

73
Q

what happens in ligand substitution when the ligand is the same size?

A

if its the same size eg H20, CN- or NH3, then the coordination number stays the same and so does the shape

74
Q

what happens in ligand substitution when the ligand is the a different size?

A

if they are different sizes eg H2O and Cl- theres a change of coordination number and change of shape

75
Q

what can ligand substitution sometimes be?

A

sometimes it can be partial, this only happens when there’s an excess of ammonia
[Cu(H2O)6]2+ + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 4H2O

76
Q

What does haemoglobin contain?

A

It has Fe2+ ions which form 6 coordinate bonds, four of the lone pairs come frol nitrogen atoms within a circular part of the molecule called haem

77
Q

whats the equation of [Cu(H2O)6]3+ with Cl?

A

[Cu(H2O)6]2+ + 4Cl- -> [CuCl4]2- + 6H2O
pale blue yellow
octahedral tetrahedral

78
Q

whats the equation of [Cr(H2O)6]3+ with NH3?

A

[Cr(H2O)6]3+ + 6NH3 -> [Cr(NH3)6]3+ + 6H20
violet purple
octahedral octahedral

79
Q

whats the equation of [Cu(H2O)6]2+ with NH3?

A

[Cu(H2O)6]2+ + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 4H2O

80
Q

what is the half equation for acidified potassium manganate (VII), KMnO4?

A

MnO4- + 8H+ + 5e -> Mn2+ +4H20

- in this solution Fe2+ is oxidised to Fe3+ (yellow colour)

81
Q

what is the half equation for Fe3+ being reduced to Fe2+ by iodide ions?

A
  • 2I- -> I2 + 2e
  • Fe3+ + e -> Fe2+
    colour of Fe changes from yellow in Fe3+ to Fe2+ pale green
82
Q

what happens to the Cr3+ ion when it is warmed with hydrogen peroxide, H2O2?

A
  • H2O2 + 2e -> 2OH-
  • 2Cr(OH)6 + 4OH- -> 2CrO42- +8H2O + 6e
    dark green Cr3+ ion is oxidised to yellow chromate CrO4-, yellow chromate(VI) solution
83
Q

what happens when you add dilute sulfuric acid to chromate (VI) solution?

A

2CrO42- +2H+ -> Cr2O72- + H2O

it produces the orange dichromate(VI) solution

84
Q

what happens when acidified dichromate(VI) solution is reduced by acidified zinc?

A
  • Zn -> Zn2+ +2e
  • Cr2O72- + 14H+ + 6e -> 2Cr3+ + 7H2O
    the Cr2O72-, orange is reduced to Cr3+ green
85
Q

what happens when Cu2+ is reduced to copper(I) by iodide ions?

A
  • 2Cu2+ + 4I- -> 2CuI + I2

changes from pale blue to off-white precipitate

86
Q

what does Cu+ do?

A

it is unstable and spontaneously disproportionates to produce Cu(s) and Cu2+
- 2Cu2+ -> Cu + Cu2+

87
Q

what is oxidation and reduction?

A
  • oxidation is the loss of electrons and reduction is the gain of electrons
88
Q

what are iodine-thiosulfate titrations?

A

its a way of finding the concentration of an oxidising agent. the more concentrated an oxidising agent is, the more ions will be oxidised by a certain volume of it

89
Q

what are electrochemical cells?

A

they can be made from two different metals dipped in salt solutions of their own ions and connected by a wire (the external circuit). there are always two reactions within an electrochemical cell, oxidation and reductions

90
Q

what happens to zinc in an zinc/copper electrochemical cell?

A

zinc loses electrons more easily than copper. so in the half cell on the left, zinc from the zinc electrode is oxidised to form Zn2+ ions. This releases electrons into the external circuit

91
Q

what happens to copper in an zinc/copper electrochemical cell?

A

in the other half cell, the same number of electrons are taken from the external circuit reducing the Cu2+ ions to copper atoms.

92
Q

what is the salt bridge?

A

it connects the solutions, its made from filter paper soaked in KNO3(aq) which allows ions to flow through and balance out the charges.
- electrons flow through the wire from the most reactive metal to the least reactive

93
Q

what is the cell potential or emf?

A

a voltmeter in the external circuits show the voltage between two half cells which is the emf

94
Q

what happens with half cells involving solutions of two aqueous ions of the same element?

A

eg with Fe2+ and Fe3+, the conversion from one to the other happens on the surface of a platinum electrode. platinum is used because its inert and conducts electricity

95
Q

when can electrochemical cells be made from non-metals?

A

for systems involving gas eg chlorine, this gas be bubbled over a platinum electrode sitting in a solution of its aqueous ions eg Cl-

96
Q

what measures how easily a metal is oxidised?

A

electrode potentials can. a metal that is easily oxidised has a very negative electrode potential, while one that’s harder to oxidise has a positive electrode potential.
- the zinc half cell has a more negative electrode potential, so zinc is oxidised (the reaction goes backward), while copper is reduced (the reaction goes forward).

97
Q

how do you draw electrochemical cells?

A
  • the half cell with the more negative potential goes on the left
  • the oxidised form goes in the centre of the cell diagram
    the cell potential will always be a positive voltage, because the more negative E value is being subtracted from the more positive E value
98
Q

what is the standard electrode potential?

A

the standard electrode potential of a half cell is the voltage measured under standard condtions when the half cell is connected to a standard hydrogen electrode

99
Q

what are the standard conditions?

A
  • any solutions must have a concentration of 1.00moldm3 or be equimolar (ie contain the same number of moles of ions)
  • the temperature must be 298K
  • the pressure must be 100kPa
100
Q

where is the standard hydrogen electrode shown?

A

its always shown on the left and has a value of 0.00V. the whole cell potential is equal to right hand side minus left hand side

101
Q

what does changing the equilibrium position do?

A

it changes the cell potential. standard conditions are used to measure the electrode potentials, using these conditions means you always get the same value for the electrode potential and you can compare values for different cells

102
Q

what do more negative electrode values mean?

A

more negative standard electrode potentials means they want to lose electrons, undergo oxidation and form a positive ion.

103
Q

what do more positive electrode values mean?

A

more positive standard electrode potentials means they want to gain electrons, undergo reduction, to form a negative ion

104
Q

whats a fuel cell?

A

it produces electricity by reacting a fuel, usually hydrogen with an antioxidant which is more likely to be oxygen

105
Q

what are stages 1 and 2 of the fuel cell process?

A

1- at the anode, the platinum catalyst splits the H2 into protons and electrons
2- the polymer electrolyte membrane (PEM) only allows the H+ across and this forces the electrons to travel around the circuit to get the cathode

106
Q

what are stages 3 and 4 of the fuel cell process?

A

3- an electric current is created in the cirvuit, which is used to power something like a car
4- at the cathode, )2 combines with the H+ from the anode and the electrons from the circuit to make H2O, this is the only waste product

107
Q

what are the advantages of fuel cells?

A
  • they are more efficient at producing energy than conventional combustion engines, as energy is wasted as heat during combustion
  • they produce a lot less pollution, as for hydrogen fuel cells the only waste is water
108
Q

what are the disadvantages of fuel cells?

A
  • production of the cells involves the use of toxic chemicals, which need to be disposed of
  • the chemicals used to make the cell are often flammable
109
Q

what is homogenous equilibria?

A

contains equilibrium species that all have the same state or phase.

110
Q

what is hetergenous equilibria?

A

contains equilibrium species that have different states or phases. any species that are solid or liquids are removed the kc expression

111
Q

what is the effect of temperature on equlibrium constants when the forward reaction is exothermic (negative delta H)?

A

the equilibrium constant decreases with increasing temperature, raising the temp decreases the equilibrium yield of products.

112
Q

what is the effect of temperature on equlibrium constants when the forward reaction is endothermic (positive delta H)?

A

the equilibrium constant increases with decreasing temperature, raising the temp increases the equilibrium yield of products

113
Q

when is reaction feasible?

A

when delta H is negative, delta S is positive, delta G is negative

114
Q

when is a reaction not feasible?

A

when delta H is positive, delta S is negative, delta G is positive

115
Q

what is a bronsted lowry acid?

A

it is a proton donor that releases H+ ions when mixed with water. they’re combined with H2O to form H3O+, hydroxonium ions. HA + H2O -> H3O+ + A-

116
Q

what is a bronsted lowry base?

A

it is a proton acceptor. when they are in solution. they grab hydrogen ions from water molecules
B + H2O -> BH+ + OH-

117
Q

what is a conjugate base-acid pair?

A

they contain two species that can be interconverted by transfer of a proton. the species that has lost a proton is the conjugate base and the species that has gained a proton is the conjugate acid

118
Q

describe this reversible reaction HA + B -> BH+ + A-?

A
  • in the forward reaction, HA acts as an acid as it donates a proton and in the reverse reaction, A- acts as a base and accepts a proton from the BH+ ion to form HA
  • HA and A- are a conjugate pair, HA is the conjugate acid of A- and A- is the conjugate base of the acid, HA
  • B and BH+ are a conjugate pair, the base B takes a proton to form BH+, so B is the conjugate base of BH+ and BH+ is the conjugate acid of B
119
Q

in aqueous solution what does dissociation require?

A

it requires a proton to be transferred from an acid to a base. the dissociation doesn’t take place unless water is present. it reacts with acids to form a conjugate acid (H3O+), and reacts with bases to form a conjugate base (OH-)

120
Q

what are monobasic, dibasic and tribasic acids?

A
  • monobasic has one hydrogen atom
  • dibasic has two hydrogen atoms
  • tribasic has three hydrogen atoms
121
Q

what are spectator ions?

A

they are ions that don’t change during the reaction, they can be cancelled out

122
Q

what is pH?

A

its a measure of how acidic or basic something is, it measures the concentration of hydrogen ions in solution. for any solution at 25C:

  • pH less than 7 shows increasing acidity
  • PH greater than 7 shows increasing alkalinity
  • pH 7 is neutral
123
Q

what do low and high values of H+ mean?

A
  • low value of H+ matches a high value of pH

- high value of H+ matches a low value of pH

124
Q

how do you calculate the pH?

A

pH = -log[H+]

reverse is H+ = 10-pH

125
Q

describe the logarithmic scale of pH?

A
  • a change of one pH number is equal to a 10 times difference in H+. a pH of 1 has 10 times the concentration of H+ ions as a solution with a pH of 2.
126
Q

what does it mean the larger the numerical value of Ka?

A

the greater the dissociation and the greater the acid strength. Ka cant give negative indices so we use:
pKa = -logKa
reverse is Ka = 10-pKa

127
Q

what does it mean the stronger/weaker the acid?

A
  • 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
128
Q

how do strong and weak acids dissociate?

A
  • strong acid HA completely dissociates H+ = HA

- weak acid HA partially dissociates, there is an equilibrium: HA -> H+ + A-

129
Q

what does H+ depend on?

A
  • the concentration of the acid (HA)
  • the acid dissociation, Ka
    when HA molecules dissociate, H+ and A- ions are formed in equal quantities
130
Q

how do you calculate pH using the Ka expression?

A

H+ = square root of Ka x HA

131
Q

how can the Ka for a weak acid be measured?

A

it can be determined by preparing a standard solution of the weak acid of known concentrations, measuring the pH of standard solution using a pH meter

132
Q

how does water ionise?

A

it ionises very slightly, acting as both an acid and as a base. the dissociation of water is small 1dm3

133
Q

what is Kw?

A

it is the ionic product of water, the ions in water H+ and OH- multiplied together.
Kw = H+ + OH-
- the value of Kw at 298K (25C) is 1.00 x10-14 mol2dm-6

134
Q

what does Kw do?

A

it is an equilibrium constant that controls the concentrations of H+ and OH- in aqueous solutions

135
Q

when is a solution acidic or alkaline?

A
  • its acidic when H+ is greater than OH-
  • a solution is neutral when H+ = OH-
  • its alkaline when H+ is less than OH-
136
Q

what is an alkali?

A

it is a soluble base that releases OH- ions in aqeuous solution. a strong base is an alkali that completely dissociates in solution
- as with strong bases, weak bases are also an alkali

137
Q

how can the pH of a strong base be calculated?

A
  • the concentration of the base

- the ionic product of water Kw

138
Q

what is a buffer solution?

A

it is a system that maintains pH changes when small amounts of an acid or a base are added. Buffer solutions contain two components to remove added alkali or acid.
- A weak acid and its conjugate base, the weak acid, HA, removes added alkali and the conjugate base, A-, removes added alkali

139
Q

what happens when acid or alkalis are added to a buffer?

A

the two components in the buffer solution react and will be used up. As soon as one component has reacted, the solution loses its buffering ability towards added acids and alkalis. the pH changes by a small amount

140
Q

how can a buffer be prepared by mixing a solution of ethanoic acid with a solution of one of its salts sodium ethanoate? 1

A
  • when ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small. Ethanoic acid is the source of the weak acid component of the buffer solution
    CH3COOH -> H+ + CH3COO-
141
Q

what happens to salts of weak acid? 2

A

they are ionic compounds and provide a convenient source of the conjugate base. when added to water, the salt completely dissolves. Dissociation into ions is complete and so the salt is the source of the conjugate base component of the solution
CH3COONA + aq -> CH3COO- + Na+

142
Q

how can a buffer solution be prepared by adding an aqueous solution of an alkali eg NaOH to an excess of the weak acid?

A

the acid is partially neutralised by the alkali, forming the conjugate base. some of the acid is left over unreacted. the resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid

143
Q

what happens in the ethanoic acid equilibrium?

A

the position lies towards CH3COOH. when Ch2COO- ions are added to Ch3COOH, the position shifts towards the left, reducing the small concentration of H+ ions, leaving a solution containing two components CH3COOH and CH3COO-.

144
Q

what do CH3COOH and CH3COO- act as in an ethanoic acid equilibrium?

A

they act as two reservoirs that are able to act independently to remove acid and alkali, which is achieved by shifting the buffers equilibrium either right or left

145
Q

how do conjugate bases remove acids?

A

on addition of an acid, H+:

  1. H+ increases
  2. H+ ions react with the conjugate base, A-
  3. the equilibrium position shifts to the left, removing most of the H+ ions
146
Q

how do weak acids remove alkali?

A

on the addition of an alkali, OH-:

  1. OH- increases
  2. small concentrations of H+ ions reacts with the OH- ions
  3. HA dissolves, shifting the equilibrium position to the right to restore most of the H+ ions
147
Q

what is the pH of the buffer solution the same as?

A

the pKa value of HA, the operating pH is typically over about two pH values, centred at the pH of the pKa value

148
Q

how do you calculate the pH of a buffer solution?

A

[H+] = Ka x [HA]/[A-]

149
Q

what is blood plasma?

A

it needs to be maintained between 7.35 and 7.45. the pH is controlled by a mixture of buffers, with the carbonic acid-hydrogencarbonate (H2CO3/HCO3-). if the pH falls below 7.35 people can develop a condition called acidosis. if the pH rises above 7.45 the condition is called alkalosis

150
Q

how does the carbonic acid- hydrogencarbonate buffer system work? on addition of an acid?

A
  1. H+ increases
  2. H+ ions react with the conjugate base HCO3-
  3. the equilibrium shifts to the left, removing H+ ions
151
Q

how does the carbonic acid- hydrogencarbonate buffer system work? on addition of an alkali?

A
  1. OH- increases
  2. small concentrations of H+ ions reacts with the OH- ions
  3. H2CO3 dissociates, shifting the equilibrium to the right to restore most of the H+ ions
152
Q

what does the body produce more of?

A

produces more acidic materials than alkaline, which the conjugate base HCO3- converts to H2CO3. the body prevents H2CO3 building up by converting it to CO2 which is exhaled

153
Q

how is the vertical section produced?

A

when a base is first added, the acid is in great excess and the pH is increases slightly. as the vertical section is approached, the pH is starts to increase more quickly as the acid is used up quicker. eventually pH increases rapidly during addition of a small volume of base, producing the vertical section

154
Q

what is the equivalence point?

A

it is the volume of one solution that exactly reacts with the volume of the other solution, its the centre of the vertical section of the pH titration curve

155
Q

what is an acid-base indicator?

A

it is a weak acid, HA, that has a different colour from a conjugate base, A-.
Eg methylorange. the weak acid, HA, is red. the conjugate base, A-, is yellow. at the end point, the indicator contains equal concentrations of HA and A-

156
Q

what happens in a titration in which a strong base is added to a strong acid? eg with methylorange

A

methylorange is initially red as the presence of H+ ions forces the equilibrium to the left:
on addition of a basic solution containing OH- ions:
1. OH- ions react with H+ in the indicator
2. the weak acid, HA dissociates shifting equilibrium to the right
3. colour changes at first to orange then yellow as equilibrium shifts to the right

157
Q

what happens when methylorange is added initially to a basic solutions and acid is added?

A
  1. H+ ions react with the conjugate base, A-
  2. the equilibrium position shifts to the left
  3. the colour change first to orange and finally to red when equilibrium shifts left
158
Q

what is the pH of the end point the same as?

A

it is the same as the pKa value of HA

A-level Chemistry (5 decks)

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