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

5.1 rates, equilibrium and pH Flashcards

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1
Q

define rate of reaction

A

change of concentration of reactant or product per unit of time

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2
Q

give the equation for overall rate of reaction

A

change in conc of reactant or product ÷ time

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3
Q

what is order of a chemical?

A

order with respect to a chemical shows how changing concentration of the chemical affects rate of reaction

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4
Q

what is overall order?

A

the orders of each reactant added up

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5
Q

what is the rate determining step?

A

the slowest step in a chemical reaction

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6
Q

zero order:

  • describe the conc-time graph a chemical with this order
  • how does changing the conc of a chemical of this order affect rate?
  • how does the half life on a conc-time graph change over time?
A
  • linear graph, negative gradient
  • changing conc does not affect rate
  • half life decreases over time
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7
Q

first order:

  • describe the conc-time graph a chemical with this order
  • how does changing the conc of a chemical of this order affect rate?
  • how does the half life on a conc-time graph change over time?
A
  • curved, negative gradient, starts steep and gets shallower
  • changing conc changed rate by the same factor
  • half life is constant
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8
Q

second order:

  • describe the conc-time graph a chemical with this order
  • how does changing the conc of a chemical of this order affect rate?
  • how does the half life on a conc-time graph change over time?
A
  • just like first order graph but starts VERY steep
  • changing conc changes rate by the same factor squared
  • half life increases with time
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9
Q

what would be the rate equation of

A + B → C

A

rate = k[A]ˣ[B]ʸ

where x and y are the orders of reactants A and B

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10
Q

how do you find the rate constant and its units?

A
  • work out order of all reactants
  • substitute in known values of the rate and concentrations of reactants
  • rearrange to find k
  • units are derived from units of rate and concentrations to the power of their orders
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11
Q

describe the rate-concentration graph of a zero order chemical

A

horizontal line

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12
Q

describe the rate-concentration graph of a first order chemical

A

linear
starting at origin
positive gradient

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13
Q

describe the rate-concentration graph of a second order chemical

A

exponential
starting at origin
positive gradient

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14
Q

how do you find the rate constant from a first order rate-concentration graph?

A

k = gradient

ONLY for first order

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15
Q

how do you find the rate constant from a first order concentration-time graph?

A

k = ln2 ÷ half life

ONLY for first order

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16
Q

what are two rules regarding order, rate equations and the rate determining step?

A
  1. if a chemical is NOT in the rate equation it is NOT in the RDS.
  2. the order of a chemical = the number of molecules in the RDS.
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17
Q

in reaction mechanisms, what are intermediates?

A

species formed that do not appear in final equation, they are catalysts

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18
Q

give the 2 equations of the arrhenius equation

name each variable and give the units

A 
k = Ae^-Ea/RT
lnk = -Ea/RT  +  lnA
k = rate constant (units to be determined)
A = pre-exponential factor (same units as k)
Ea = activation energy (Jmol-1)
R = gas constant 8.314 (JK-1mol-1)
T = temp in KELVIN
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19
Q

what can the arrhenius equation be approximated to? what do each part of the arrhenius equation represent in the approximation?

A 
equation of straight line - y = mx + c
y = lnk
m = -Ea/R
x = 1/T
c = lnA
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20
Q

plotting arrhenius equation:

what goes on the x and y axis?

A

y axis = lnk

x axis = 1/T

21
Q

plotting arrhenius equation

how do you find Ea from the graph?

A
  • find gradient of the line of best fit using a triangle
  • multiply gradient by gas constant 8.314 to find Ea in Jmol-1
  • divide by 1000 to find it in kJmol-1
22
Q

plotting arrhenius equation

how do you find A or the pre-exponential factor from the graph?

A
  • extrapolate backwards to the y axis and find the y intercept
  • lnA = y intercept so put this number to the power of e
    OR
  • once you have found the Ea, substitute data from the table into either of the equations
  • rearrange to find A
23
Q

what is a homogenous system?

A

a reaction where all reactants and products are the same state

24
Q

what is a heterogenous system?

A

a reaction where more than one state of matter is present in reactants and products

25
Q

in the equation:
aA + bB ⇋ cC + dD
what is the Kc equation?

A

Kc =[C]^c[D]^d / [A]^a[B]^b

product conc / reactants conc

26
Q

what states of matter are included in the Kc equation?

A

only aqueous and gases concentrations are included

27
Q

how do you find the moles/concentration of substances in equilibrium from a reaction?

A

using RICE tables:
R - reaction and molar ratios go here
I - initial moles of reactants and initial mol of products = 0
C - change of moles: changes with molar ratios: opposite operation happens to products and reactions (eg. if change is +ve for reactants its -ve for products)
E - equilibrium moles in this row

28
Q

what does the equilibrium constant, Kc, indicate?

A

the position of equilibrium

29
Q

where is the position of equilibrium when:

  1. Kc = 1 ?
  2. Kc < 1 ?
  3. Kc > 1 ?
  4. Kc &laquo_space;1 ?
  5. Kc&raquo_space; 1 ?
A
  1. in the middle of products and reactants
  2. favours reactants
  3. favours products
  4. lies far to the left
  5. lies far to the right
30
Q

what is the only factor that affects the equilibrium constant?

A

temperature

31
Q

what effect does increasing/decreasing temperature do to position of equilibrium?

A

increasing - to endothermic direction

decreasing - to exothermic direction

32
Q

what happens to Kc when you change concentration or pressure? why?

A

Kc doesn’t change because the system works to restore Kc to it’s original value

33
Q

what is Kp

A

same constant as Kc but instead of concentration, partial pressure of gases are used

34
Q

define mole fraction and give the equation

A

proportion of given substance present in the reaction mixture

mole fraction = moles of compound/total moles of products and reactants

35
Q

define partial pressure and give the equation

A

the pressure exerted by an individual gas if it occupied the whole vessel on its own

partial pressure = mole fraction x total pressure

36
Q

give the equation of Kp given the reaction
aA + bB ⇋ cC + dD
what must you use in the equation?

A

partial pressure of each gas raised to the power of their stoichiometry
Kp = (Pc)^c (PD)^d / (PA)^a (PB)^b
must use ROUND brackets

37
Q

how do you find the value of Kp?

A

use a rice table but add rows M (mole fraction) and P (partial pressure)

38
Q

give the bronsted lowry (BL) definitions of an acid and base

A

acid - proton donor (loses H+)

base - proton acceptor (gains H+)

39
Q

what are three names given to acids? what do these names refer to?

A

monobasic/protic
dibasic/protic
tribasic/protic
names refer to how many H+ ions the acid loses

40
Q

what is a conjugated acid and base pair?

A

two species that can transform into each other by loss or gain of H+ ions

41
Q

in this reaction
NH₃ + HCl ⇋ NH₄⁺ + Cl⁻
what are the conjugated acid-base pairs?

A

NH₃ is the BL base and NH₄⁺ is the conjugate acid because it can LOSE a H+ ion to become NH₃

HCl is the BL acid and Cl⁻ is the conjugate base because it can GAIN a H+ ion to become HCl

42
Q

how do you find the concentration of H+ ions when given the pH?

A

[H⁺] = 10 ^-pH

10 to the power of -pH

43
Q

how do you find pH of a solution when given the concentration of H⁺ ions?

A

pH = -log[H⁺]

negative log of the conc of H+ ions

44
Q

pH increases = [H⁺] ions _______

A

decreases

45
Q

pH decreases = [H⁺] ions _______

A

increases

46
Q

what is assumed about the concentration of a strong acid?

A

in this strong acid HA —> H⁺ + A-

[HA] = [H⁺] is assumed because we assume all H+ ions completely disassociate

47
Q

derive the equation involving the equilibrium constant for acids

A

HA —> H⁺ + A-

Ka = [H⁺][A-] / [HA]

[H⁺] = [A-] so numerator can be written as [H⁺]²

multiply by [HA] and square root to get
[H⁺] = √(Ka[HA])

48
Q

how do you find the pKa value?

A 
pKa = -log(Ka)
Ka = 10^-pKa
49
Q

what does Kw symbolise?

A
  • Kw is the equilibrium constant for the dissociation water
  • the ionic product of water
  • Kw = [H⁺][OH-]]
  • Kw = 1x10^-14

Chemistry A-Level (13 decks)

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