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A-Level Chemistry > 5.1 Rates, equilibrium and pH > Flashcards

5.1 Rates, equilibrium and pH Flashcards

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1
Q

rate of reaction definition

A

change in concentration (of product or reactants) over time

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2
Q

order definition

A

power to which a concentration (of a reactant) is raised

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3
Q

overall order definition

A

sum of powers in the rate equation

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4
Q

rate constant definition

A

probability constant

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5
Q

half-life definition

A

time taken for concentration of a reactant to halve

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6
Q

rate-determining step definition

A

slowest step in a reaction mechanism

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7
Q

zero order

A

concentration of that reagent doesn’t affect the rate

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8
Q

first order

A

concentration of that reagent is proportional to the rate of reaction

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9
Q

second order

A

rate of reaction is proportional to the square of concentration of this reagent

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10
Q

rate constant relationship with constant half-life

A

kt1/2 = ln2 = 0.693
t1/2 = constant half-life
only for first order reactions where half-life is constant

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11
Q

plotting concentration against time

A

0 order = linear, negative gradient (decreasing half-life)
1st order = curved, negative gradient becomes less steep (constant half-life)
2nd order = curved, steeper but becomes less steep faster (increasing half-life)

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12
Q

plotting rate against concentration

A

0 order = flat horizontal line
1st order = linear line
2nd order = exponentially increased curved line

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13
Q

how to determine number of and what molecules are in rate determining step

A

0 order = not involved
1st order = 1 molecule of this reactant
2nd order = 2 molecules of this reactant
etc.

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14
Q

what number of different reactants in rate determining step means

A 
1 = decomposition
2 = collision
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15
Q

why Arrehenius is important

A

standard rate equation doesn’t take into account temperature

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16
Q

Arrehenius units

A 
T = temperature (Kelvin)
R = gas constant (8.314 J K^-1 mol)
EA = activation energy (J mol^-1)
A = pre-exponential factor, takes into account number of molecules that exceed activation energy
k = rate constant
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17
Q

why Arrehenius is important

A

standard rate equation doesn’t take into account temperature

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18
Q

Arrehenius units

A 
T = temperature (Kelvin)
R = gas constant (8.314 J K^-1 mol)
EA = activation energy (J mol^-1)
A = pre-exponential factor, takes into account number of molecules that exceed activation energy
k = rate constant
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19
Q

mole fraction formula

A

mole fraction = number of moles/total number of moles of gas

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20
Q

factors affecting Kc

A

only temperature

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21
Q

partial pressure formula

A

partial pressure = mole fraction x total pressure

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22
Q

formula of Kp

A

exact same as formula of Kc but with partial pressures

only gaseous reactants/products

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23
Q

mole fraction formula

A

mole fraction = number of moles/total number of moles of gas

24
Q

mole fraction formula

A

mole fraction = number of moles/total number of moles of gas

25
Q

ph formula

A

pH = -log[H+]
pH = negative log of concentration of H+
can only work directly with strong acid

26
Q

pH scale

A

logarithmic

pH1 is 10 times stronger than pH2

27
Q

phenolphthalein

A

colourless (0-8) to pink (9-14)

28
Q

methyl orange

A

red/orange (0-3) to yellow (4-14)

29
Q

bromophenol blue,

A

yellow (0-3) to blue (4-14)

30
Q

conjugate acid-base pair definition

A

conjugate acid base pairs differ by the presence or absence of a transferable proton

31
Q

amphoteric definition

A

can act as proton donor or proton acceptor

32
Q

monobasic definition

A

donates 1 proton

33
Q

dibasic definition

A

donates 2 protons

34
Q

tribasic definition

A

donates 3 protons

35
Q

why Kc is always low during acid dissociation

A

[H2O] will always be close to 55 mol dm^-3

much higher in comparison to [H3O^+] and [A^-]

36
Q

Ka formula

A

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

[H2O] removed as it remains near constant

37
Q

ionic product of water expression

A

Kw = [H^+] [OH^-]

38
Q

Brønsted-Lowry acid definition

A

proton donor

39
Q

Brønsted-Lowry base definition

A

proton acceptor

40
Q

calculating pH of weak acids

A

write expression of Ka
assume [H^+] = [A^-]
calculate [H^+] then pH

41
Q

why [HA] at start = [HA] at equil. can be assumed during weak acid dissociation

A

dissociation is very small

any decrease in [HA] is negligible

42
Q

why [H^+] at equil. = [A^-] at equil. can be assumed during weak acid dissociation

A

even though some water does dissociate, it is very small compared to [H^+]

43
Q

buffer solution definition

A

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

44
Q

composition of buffer solutions

A

a weak acid and a salt of that acid

e.g. ethanoic acid and sodium ethanoate

45
Q

what happens to equilibria when acid is added example

A
  1. CH3COOH <=­> CH3COO-­ + H+
  2. CH3COONa <=­> CH3COO­- + Na+
    [H+] increases
    position of equilibrium 1 shifts left
    H+ ions react with ethanoate ions, removing H+
    minimal change in pH
46
Q

what happens to equilibrium when alkali is added example

A
  1. CH3COOH <=> CH3COO-­ + H+
  2. CH3COONa <=> CH3COO-­ + Na+
    [OH-] increases
    reacts and removes H+, decreasing [H+]
    pos. of equilibrium shifts right
    more ethanoic acid dissociates, replacing missing protons
47
Q

assumptions when calculating pH of buffers

A

salt has fully dissociated

weak acid has not dissociated

48
Q

factors of pH of buffer

A

Ka of weak acid

concentration ratio of weak acid:conjugate base or salt

49
Q

titration curves

A

shows how pH of solution changes when different volumes of different acids and bases are neutralised

50
Q

strong acid strong base titration curve shape

A

high starting pH
very gradual change until equivalence point
very sudden drop
gradual change until pH of acid

51
Q

strong acid weak base titration curve

A 
pH not as high
slightly steeper curve until equivalence point
sharp drop (shorter)
gradual change until pH of acid
52
Q

weak acid strong base titration curve

A 
gradual change until equivalence point 
sharp drop (shorter)
slightly steeper change to pH of acid
53
Q

weak acid weak base titration curve

A

slightly steeper change until equivalence point
less steep drop
steeper change until pH of acid

54
Q

importance of titration curves to indicators

A

helps choose appropriate indicator to show equivalence point

55
Q

Haber process conditions and why

A 
N2 (g) + 3H2 (g) ⇌ 2NH3(g)
forward reaction is exothermic, reverse is endothermic
medium temperature (400-450°C) as high temp. increases rate of reaction but decreases yield
high pressure (200/250 atm.) to increase rate and increase yield
iron catalyst (increase rate of forward and backward reaction)

A-Level Chemistry (12 decks)

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