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ocr chem > module 5.1 - rates, equilibrium and pH > Flashcards

module 5.1 - rates, equilibrium and pH Flashcards

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1
Q

what is a brønsted-lowry acid

A
  • species that donates a proton
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2
Q

what is a brønsted-lowry base

A
  • species that accepts a proton
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3
Q

what is different in an equation of a weak acid compared to a strong acid

A
  • weak acid equation = reversible arrow
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4
Q

what is a conjugate acid-base pair

A
  • pairs of molecules that are interchangeable by H+
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5
Q

what are monobasic, dibasic and tribasic acids

A

acids that can be classified by the number of bases that they can donate protons to in a reaction

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6
Q

what is the acid dissociation constant/ equilibrium constant of acids

A

Ka

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7
Q

difference between strong and weak acids

A

strong = fully ionise
weak = partially ionise

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8
Q

what is the extent of dissociation of weak acid determined by

A

Ka

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9
Q

rate equation for Ka

A

[products] / [reactants]

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10
Q

low Ka =

A

high pKa (weak acid)

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11
Q

high Ka =

A

low pKa (strong acid)

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12
Q

pKa =

A
  • log (Ka)
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13
Q

Ka =

A

10 -pKa

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14
Q

what does pH depend on

A
  • concentration of acid [HA]
  • Ka - constant
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15
Q

approximations

A
  • HA dissociates to produce equal concentrations of H+ and A-
  • assume that weak acids don’t dissociate to a great extent so the overall concentration of HA doesn’t change much
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16
Q

generic equation of a weak acid

A

HA -><- H+ + A-

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17
Q

Ka equation using concentrations

A

[H+]2 / [HA]

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18
Q

calculating pH of a weak acid

A

[H+] = √ Ka x [HA]
pH = -log [H+]

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19
Q

pH =

A
  • log [H+]
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20
Q

[H+] =

A

10 -pH

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21
Q

Kw valuw

A

1 x10 -14

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22
Q

Kw equation

A

Kw = [H+][OH-]

23
Q

using Kw to find pH steps

A
  1. find conc of OH-
  2. use Kw = [H+][OH-] to find [H+]
  3. calculate pH
24
Q

pOH method to find pH

A
    • log [OH-]
  2. 14 - ans = pH
25
Q

what is a buffer

A

system that minimises pH changes on addition of small amounts of an acid or base

26
Q

what does a buffer contain

A
  • a weak acid
  • the conjugate base of the acid
27
Q

what are the ways of preparing a buffer

A

route 1; mixture of weak acid and one of its salts. e.g ethanoic acid and sodium ethanoate
route 2; mixture of an excess of weak acid and a strong base (partial neutralisation) e.g excess ethanoic acid and sodium hydroxide

28
Q

what experiment is used to find initial rate of a reaction

A

disappearing cross method

29
Q

effect of zero order reactants

A
  • changing concentration of reactant has no effect on rate
30
Q

effect of first order reactants

A

changing concentration of reactant has a directly proportional effect on rate

31
Q

effect of second order reactants

A

changing concentration of reactant changes rate by the square of the change

32
Q

rate equation

A

rate = k[A]^a[B]^b

33
Q

rules for rate equation

A
  • only reactants appear in rate equation; not products
  • rate equation does not have to match overall equation
34
Q

what is on the x axis of a concentration time graph

A

time/s

35
Q

what is on the y axis of a concentration time graph

A

[A] mol/dm^3

36
Q

zero order concentration time graph

A

straight line down

37
Q

first order concentration time graph

A

slight curve down

38
Q

second order concentration time graph

A

steeper curve down

39
Q

half life definition

A

time taken for half a reactant to be consumed

40
Q

equation for half life

A

t1/2 = ln(2) / k

41
Q

equation for rate constant using half life

A

K = ln(2) / t 1/2

42
Q

zero order concentration rate graph

A

straight line across
- y intercept = k

43
Q

first order concentration rate graph

A

straight line up
- gradient = k

44
Q

second order concentration rate graph

A

curve upwards

45
Q

what is the rate determining step

A

slowest step in a reaction

46
Q

steps to writing rate determining step

A
  1. write rate equation
  2. use rate equation to write out another equation and create products that are found in equation
    e.g rate = k[H2O2][I-] so H2O2 + I- -> H2O + IO-
  3. find intermediates (if any) and use immediately in next reaction
    e.g IO- = intermediate as it is not in the products of original equation
    so IO- + I- + 2H+ -> I2 + H2O
  4. find overall equation by crossing out intermediates/ substances that appear on both side of equation
47
Q

arrhenius equation

A

K = Ae ^-Ea/RT

48
Q

what is A in the arrhenius equation

A
  • frequency factor
  • how frequently molecules collide in the correct orientation
49
Q

how does increasing temperature increase rate in arrhenius equation

A
  • if T increases, Ea/RT decreases and e ^-Ea/RT increases so k increases and so does rate
50
Q

how does adding a catalyst increase rate in arrhenius equation

A
  • if Ea decreases (as a catalyst allows a reaction to occur by finding a lower activation energy), then Ea/RT decreases so e ^-EA/RT increases. and rate and k also increase
51
Q

how to determine equation for a straight line using arrhenius equation

A

ln(k) = -EA/R x 1/T + ln(A)
y = m x + c

52
Q

what is ln(a) in the arrhenius equation

A

y-intercept = ln(a)

53
Q

what is -EA/R in the arrhenius equation

A

gradient

54
Q

how to calculate Ea in graph version of arrhenius equation

A

Ea = gradient x R

Ea = kJmol so divide by 1000

ocr chem (8 decks)

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