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OCR A A-Level chemistry > module 5 > Flashcards

module 5 Flashcards

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1
Q

define rate of reaction

A

change in conc. of a product or reactant over a certain time

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2
Q

how can you calculate rate of reaction

A

change in conc/time (s)

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3
Q

what does it mean if a reactant is in zero order?

A

changing conc. had no effect on the rate

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4
Q

what does it mean if a reactant is in first order?

A

the rate is directly proportional to the concentration

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5
Q

what does it mean if a reactant is in second order?

A

change in rate = change in conc. squared

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6
Q

what is the generic rate equation

A

rate = k [X]^x [Y]^y

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7
Q

do zero order reactants appear on the rate equation? why?

A

no, as they do not affect rate

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8
Q

how is overall order determined

A

adding all of the orders

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9
Q

when the overall order is 3 what would the units of the rate be?

A

dm^6 mol^-2 s^-1

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10
Q

how can you measure rate experimentally?

A

use colorimeter at suitable intervals and see if colour changes
if gas involved use a gas syringe to collect or use a scale to measure mass lost

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11
Q

what is a half life

A

time taken for concentration of a reactant to reduce by half

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12
Q

what is the symbol for a half life

A

t1/2

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13
Q

what is the relationship between first order reactants and half life?

A

they have a constant half life

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14
Q

which equation is used to find rate constant using the half life in a first order reaction?

A

k = ln2/ t1/2

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15
Q

what is the relationship between rate and time

A

rate is directly proportional to 1/t

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16
Q

how to find rate constant from a rate conc. graph of first order

A

k = rate/conc.

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17
Q

what is the rate determining step?

A

the slowest step in a reaction with multiple steps

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18
Q

how does the rate determining step relate to the species involved in the rate equation?

A

any species involved in the RDS appear in the rate equation - species only involved after RDS do not appear in rate equation

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19
Q

for a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the RDS?

A

the order of the reactant

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20
Q

what conditions affect the value of the rate constant

A

temperature

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21
Q

what is the effect of a 10 degree increase in temp on the rate of reaction

A

double

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22
Q

what is the Arrhenius equation? what does each term mean?

A 
k = Ae^-Ea/RT
k = rate constant
A= pre-exponential factor
e= mathematical quantity
Ea = activation energy (j)
R = gas constant
T = temp (in kelvin)
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23
Q

how can you convert the Arrhenius equation into a useful form to plot a graph? how can you find each term?

A

Lnk = -Ea/RT - LnA
graph of Lnk against 1/T
gradient = -Ea/R and y Intercept is Ln A

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24
Q

what does it mean when a reaction is in equilibrium

A

the rate of the forward reaction is equal to the rate of the backward reaction

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25
Q

what are the methods to measure equilibrium?

A
  • measure change in colour/colour intensity using colorimeter
  • use pH probe
  • measure electrical conductivity
  • titration
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26
Q

what does mole fraction mean?

A

the amount of a given component in a reaction mixture

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27
Q

what equation is used to find the mole fraction?

A

mole fraction = no. moles of substance A/total mol.

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28
Q

what does partial pressure mean?

A

the pressure exerted by a single species in a reaction vessel

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29
Q

what equation is used to find partial pressure?

A

partial pressure = mol. fraction x total pressure

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30
Q

what is the relationship between conc. of a substance and its partial pressure?

A

conc. is proportional to partial pressure

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31
Q

in the reaction aA+bB=cC+dD what would the kp expression be?

A

kp = pC^c x pD^d / pA^a x pB^b

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32
Q

what does it mean if k (eq. constant) is greater than 1?

A

reaction favours product (right)

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33
Q

what does it mean if k (eq. constant)is less than 1?

A

reaction favours reactants (left)

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34
Q

what is the effect of increasing temp on k (eq. constant)

A

will shift in endothermic direction

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35
Q

what is the effect of decreasing temp on k (eq. constant)

A

will shift in exothermic direction

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36
Q

what is the only factor that effects k?

A

temperature

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37
Q

what is the effect of increasing temperature on k for a forward endothermic reaction?

A

k increases as temp increases

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38
Q

what is the effect of increasing temperature on k for a forward exothermic reaction?

A

k decreases as temp increases

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39
Q

what effect do catalysts have on k?

A

catalysts do no effect the equilibrium - only an effect on rate of reaction

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40
Q

what is a bronsted lowery acid?

A

proton donor

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41
Q

what is a bronsted lowery base?

A

proton acceptor

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42
Q

define Lewis acid

A

electron pair acceptor

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43
Q

define Lewis base

A

electron pair donor

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44
Q

what ion causes a solution to become acidic?

A

H+/ H3O+

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45
Q

what ion causes a solution to be alkaline?

A

OH-

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46
Q

what is the equation for the ionisation of water?

A

2H2O <=> H3O+ + OH-

or H2O <=> H+ + OH-

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47
Q

give an example of a mono basic acid

A

HCl

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48
Q

give an example of a dibasic acid

A

H2SO4

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49
Q

give an example of a tribasic acid

A

H3PO4

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50
Q

identify the acid base pairs for the following reaction:

CH3COOH + H20 <=> CH3COO- + H3O+

A

acid 1: CH3COOH
acid 2: H3O+
base 1: H2O
base 2: CH3COO-

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51
Q

define a strong acid

A

dissociate completely

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52
Q

give 3 examples of strong acids

A

sulphuric acid, hydrochloric acid, nitric acid

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53
Q

what is the difference between concentrated and strong?

A 
conc = many mol per dm3
strong= amount of disassociation
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54
Q

define a weak acid

A

partially dissociates

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55
Q

give an example of any weak acid

A

methanol acid, (any organic acid)

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56
Q

what is the symbol for this acid dissociation constant

A

Ka

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57
Q

what does a larger Ka value mean?

A

Larger the Ka = greater extent of dissociation

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58
Q

what is the equation for pKa from Ka

A

pKa = -log10 Ka

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59
Q

what is the equation for Ka from pKa

A

Ka = 10^-pKa

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60
Q

what is the relationship between pKa and the strength of the acid

A

the smaller the pKa the stronger the acid

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61
Q

what is the equation to find pH from [H+]

A

pH = -log[H+]

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62
Q

what is the equation to find [H+] from pH

A

[H+] = 10^-pH

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63
Q

why is a pH scale useful rather than using H+ conc.

A

allows for a wide range of [H+] conc. to be expressed as simple positive values

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64
Q

what is the relationship between pH and [H+]

A

high pH value = small [H+]

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65
Q

if two solutions have a pH difference of 1 what is the difference of [H+]?

A

factor of 10

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66
Q

[H+] of strong acid is equal to what?

A

[HA]

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67
Q

what equation is used to find the [H+] of weak acids

A

[H+] = (sq. route)Ka x [HA]

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68
Q

what is the assumption made when calculating pH of weak acids

A

conc of acid at equilibrium = conc of acid after dissociation - because v little acid dissociates

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69
Q

what is the expression for ionic product of water - Kw

A

Kw = [H+][OH]-

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70
Q

what are the units for Kw

A

mol2dm-6

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71
Q

what is the value of Kw at 298k

A

1 x 10^-14

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72
Q

what physical values affect the values of Kw and how?

A

temp only - if temp is increased, equilibrium will move right, Kw will increase and pH of pure water will decrease

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73
Q

indices of [H+] and [OH]- should add up to what?

A

-14

74
Q

define the term strong base

A

base that fully dissociates in water

75
Q

give 3 examples of strong bases

A

NaOH
KOH
Ca(OH)2

76
Q

give an example of a weak base

A

ammonia

77
Q

what is the equation to find [H+] of strong bases?

A

[H+] = Kw/[OH}-

78
Q

define a buffer solution

A

a mixture that minimises pH change on addition of small amounts of acid or base

79
Q

what are the two ways a buffer can be made?

A

weak acid and conjugate base

weak acid and strong alkali

80
Q

in which direction does the equilibrium shift when an acid is added to a buffer solution? why?

A

equilibrium shifts left because [H+] increases and conjugate base reacts with H+ to remove most of it

81
Q

in which direction does the equilibrium shift when an alkali is added to a buffer solution? why?

A

equilibrium shifts right because [OH-] increases and the small conc of H+ reacts with it OH-. to restore H+ ions HA dissociates shifting the equilibrium

82
Q

what equation is used to find [H+] of a buffer solution

A

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

83
Q

what buffer system maintains blood pH at 7.4? what happens when acid/alkali is added?

A

H+ + HCO3- <=> CO2 + H2O
add OH- = reacts with H+ to form H20, shifts equilibrium left to restore H+ lost
add H+ = equilibrium shifts right, removing excess H+

84
Q

what is a titration?

A

the addition of an acid/base of known conc. to acid/base to determine conc. an indicator is used to show when neutralisation occurs, as is a pH meter

85
Q

what is an equivalence point

A

the point at which the exact volume of base has been added to neutralise acid or vice versa

86
Q

what is end point

A

the point at which pH changes rapidly

87
Q

what are the properties of a good indicator of a reaction?

A

sharp colour change - no more than one drop of acid/alkali needed for the colour change

end point same as equivalence point (otherwise titration gives wrong answer)

distinct colour change - obvious when endpoint has been reached

88
Q

what indicator would you use for a strong acid - strong base titration?

A

phenolphthalein or methyl orange

89
Q

what indicator would you use for a strong acid - weak base titration?

A

methyl orange

90
Q

what indicator would you use for a weak acid - strong base titration?

A

phenolphthalein

91
Q

what colour is methyl orange in an acid, base and at end point

A

acid - red, base - yellow, end point -orange

92
Q

what colour is phenolphthalein in acid and base

A

colourless in acid and red in base

93
Q

what colour is bromothymol in acid and base

A

acid - yellow, base - blue

94
Q

how do you use a pH meter?

A

remove pH probe from storage solution and rinse with distilled water
dry probe and place into solution with unknown pH
let probe stay in solution until it gives settled reading

95
Q

define lattice enthalpy

A

the formation of one mole of an ionic lattice from gaseous ions under standard conditions

96
Q

what does a more exothermic lattice enthalpy mean

A

more exothermic = stronger ionic bonds

97
Q

why is it not possible to measure lattice enthalpy directly

A

it is not possible to form one mole of ionic solids from its gaseous ions

98
Q

define enthalpy change of solution

A

enthalpy change that takes place when one mole of a solute is completely dissolved in water under standard conditions

99
Q

define enthalpy change of hydration

A

the enthalpy change that takes place when dissolving one mole of gaseous ions in water

100
Q

what are the factors that impact the size of lattice enthalpy

A

size of ions involved
charges on the ions
ionic bond strength

101
Q

which ions have more negative lattice enthalpy values? why?

A

smaller ions because they can get closer, hence stronger attraction

102
Q

describe hydration

A

when an ionic lattice is broken its ions become part of the solution
positive ions attracted to slightly negative oxygen and negative ions attracted to slightly positive hydorgeen

103
Q

what are the factors that impact the magnitude of the enthalpy of hydration?

A

size of ion

charge on ions

104
Q

define entropy

A

a measure of the dispersal of energy in a system which is greater when they system is more disordered

105
Q

what is the symbol of entropy

A

s

106
Q

which is more disordered, solid or gas?

A

gas

107
Q

what is the unit of standard entropy

A

J K-1 mol-1

108
Q

how does the temperature effect entropy?

A

greater temp = more energy = more movement
arrangement of particles will = more random
higher temp = higher entropy

109
Q

when a solid ionic lattice is dissolved in solution, what happens to entropy?

A

entropy increases because ions are more disordered

110
Q

how does a change in the no. of gas molecules in a reactions effect entropy

A 
increase = increase in entropy 
decrease = decrease in entropy
111
Q

what is the equation to calculate entropy

A

entropy = entropy (products) - entropy (reactants)

112
Q

what is the gib’s free energy equation and what is each symbol for?

A 
(delta)G = (delta)H - T(delta)S
G = free energy
H = enthalpy change
T = temp in k
s = entropy
113
Q

when H is negative and S is positive what will G and the feasibility be

A

G will be negative and the reaction will be feasible

114
Q

when H is positive and S is negative what will G and the feasibility be

A

G is positive and the reaction is not feasible

115
Q

when H is positive and S is positive what will G and the feasibility be

A

G is negative (and therefore feasible) at high temperatures

116
Q

when H is negative and S is negative what will G and the feasibility be

A

G is negative (and therefore feasible) at low temperatures

117
Q

for a reaction to occur should (delta)G be positive or negative?

A

negative

118
Q

what are the limitations of predictions of feasibility made by using (delta)G

A

reaction may have high activation energy

reaction may have low rate

119
Q

define an oxidising agent

A

a species that is reduced in a reaction and causes another to be oxidised

120
Q

define a reducing agent

A

a species that is oxidised in a reaction and causes another to be reduced

121
Q

define oxidation

A

loss of electrons

122
Q

define reduction

A

gain of electrons

123
Q

what happens in a redox reaction

A

a reaction where electrons are transferred from one species to another

124
Q

define standard electrode potential

A

the e.m.f of a half cell compared with a standard hydrogen half cell measured at 298k with 1 mol dm -3 and a gas pressure of 100kPa

125
Q

what happens when a rod of metal is dipped in a solution of its own ions

A

an equilibrium is set up between the solid metal and aqueous metal ions

126
Q

what is a standard hydrogen half cell made of

A

Hydrochloric acid 1moldm-3
Hydrogen gas at 100kPa
inert platinum electrode

127
Q

why is a hydrogen half cell used as a standard half cell

A

easy to control purity and reproducibility

128
Q

how do you make a simple salt bridge?

A

soak a piece of filter paper in KNO3, NH4NO3

129
Q

why are salt bridges necessary

A

to complete the circuit by connecting two solutions - enables charge to be transferred between half cells and don’t react with electrodes

130
Q

why may you use other standards electrodes occasionally

A

they are cheaper/easier/quicker and can be just as good

platinum is expensive

131
Q

if an E value is more negative, what does it mean in terms of oxidising/reducing power?

A

Better reducing agent (easier to oxidise)

132
Q

if an E value is more positive, what does it mean in terms of oxidising/reducing power?

A

better oxidising agent (easier to reduce)

133
Q

how do you calculate emf from E values

A

Ecell = Epositive - Enegative

134
Q

when would you use a platinum electrode?

A

when both oxidised and reduced form of the metal are in aqueous form

135
Q

why is platinum chosen for electrodes

A

inert and good conductor to complete circuit

136
Q

how can you predict if a reaction will occur according to Ecell

A

find E cell and if more than 0.4v reaction is feasible

137
Q

what are the 3 main types of electrochemical cell

A

fuel
rechargeable
non-rechargeable

138
Q

how do non-rechargeable cells work

A

provide electrical energy until all electrochemical cells have been reacted

139
Q

how do rechargeable cells work?

A

chemicals In cell will provide electrical energy when recharging reactions of cells can be reversed

140
Q

give an example of a rechargeable cell

A

lithium ion battery

lithium polymer battery

141
Q

why is lithium used in laptop batteries

A

lithium has low density so electrode is light and very reactive

142
Q

what are the drawbacks of lithium batteries

A

toxic if ingested

can cause fire/explosions due to rapid discharge of current

143
Q

how do fuel cells work

A

cell uses external supplies of fuel and an oxidant, these external supplies need to be continuously supplied

144
Q

modern fuel cells are based on what type of fuels?

A

hydrogen

hydrogen rich fuels (e.g. methanol)

145
Q

what reactions take place at the two electrodes in an alkaline fuel cell

A

2H2 + 4OH- –> 4H2O + 4e-

O2 + 2H2O + 4e- –> 4OH-

146
Q

what are the disadvantages of fuel cells?

A

hydrogen is a flammable gas with a low b.p. –> hard and dangerous to store and transport –> expensive to buy
fuel cells have a limited lifetime and use toxic chemicals in their manufacture

147
Q

why can’t some cells be recharged

A

reaction of cell not reversible - product produced either dissipates or cannot be converted back into reactants

148
Q

why may the e.m.f of a cell change?

A

conc. of ions change - reagents used up

149
Q

how can e.m.f be kept constant?

A

reagents supplied constantly, conc. of ions constant, E remains constant

150
Q

how do you write ionic equations

A

remove spectator ions from aqueous compounds

151
Q

what is the rule regarding state of compounds in ionic equations

A

must be aqueous to be split into ions - remains in compound if sol, liquid or gas.

152
Q

what is formed when an acid reacts with ammonia

A

ammonium salt

153
Q

what are the units of Kw?

A

mol^2 dm^-6

154
Q

what does a lower pKa suggest

A

stronger acid

155
Q

what is a monoprotic acid

A

an acid which dissociates to produce one H+ ion for every acid molecule - conc. of acid = conc. of H+

156
Q

how do you find the pH of a diprotic strong acid

A

-log(conc. x 2)

157
Q

what are the assumptions made when using Ka

A
  • only small amount of acid dissociates so can assume that [HA] at equilibrium is equal to [HA] at start of reaction
  • dissociation of acid is greater than dissociation of water present in solution - assume all [H+] is from acid - so [H+] is equal to [A-]
158
Q

how do you use a pH meter

A
  • place in distilled water - should read 7.0
  • repeat with standard solutions of pH 4.0 and 10
  • rinse with distilled water between each solution
159
Q

why can’t you use an indictor to measure the pH change with a weak acid and weak base

A

no sharp pH change

160
Q

what is the name given to Kw?

A

ionic product

161
Q

what factors determine the pH of a buffer solution

A
  • Ka/pKa/acid strength / amount of dissociation
  • temperature
  • ratios of weak acid: conjugate base
162
Q

which direction of reaction does a higher temperature favour

A

endothermic

163
Q

what are two features in dynamic equilibrium

A
  • conc. of reactants and products are constant but are constantly changing
  • the rate of both forward and backward reaction are equal
164
Q

if you need to write an equation which is not redox what should you make sure happens to the oxidation state of each element

A

remains the same

165
Q

what are the acid equations for a fuel cell

A

H2 –> 2H+ + 2e-

4H+ + O2 + 4e- –> 2H2O

166
Q

what are the alkali equations for a fuel cell

A

O2 + 2H2O + 4e- –> 4OH-

4OH- + H2 –> 4H2O + 4e-

167
Q

what happens to ions in acid/alkali fuel cells?

A

alkali - OH- ions move right to left

acid - H+ ions move left to right

168
Q

what is the equation for the MnO4 titration with iron

A

MnO4(-) + 8H(+) + 5Fe(2+) –> Mn(2+).+ 5Fe(3+) + 4H2O

169
Q

what are the equations for the iodine - sodium thiosulphate titration

A

IO3(-) + 5I(-) + 6H(+) –> 3H2O + 3I2

I2 + 2S2O3(2-) –> S4O6 (2-) + 2I(-)

170
Q

what is the colour change in the iron MnO4 titration?

A

purple –> colourless

171
Q

define the ionic product (Kw)

A

conc. of H+ ions multiplied by conc. of OH- ions

172
Q

why is it unlikely for a reaction to occur between HNO3 and H2SO4 if it is in water?

A

water is a stronger base (more likely to accept a proton) than HNO3

173
Q

what will the pKa value of HNO3 be if the pKa value of H2SO4 is -3 (H2SO4 is stronger)

A
  • any value less negative than -3

- HNO3 is weaker and will therefore have a larger pKa

174
Q

what are two common assumptions when calculating pH of weak acids

A
  • [H+] and [A-] are equal

- [HA] is the same at equilibrium and undissociated

175
Q

what should be written in the question asking why a buffer solution is formed

A
  • write equation

- some acid is in excess

176
Q

how do you find A in an Arrhenius graph

A
  • y intercept = lnA

- A = e^lnA

177
Q

how do you find Ea from an Arrhenius graph

A
  • find gradient

- multiply by negative gas constant (as -Ea/RT)

178
Q

how do you rearrange the Arrhenius equation to find the activation energy

A

Ea = (lnA - lnK) RT

179
Q

how do you rearrange the Arrhenius equation to find the temperature

A

T = Ea/R(lnA-lnK)

180
Q

what does the gradient equal in a first order graph

A

k

181
Q

if rate = k [A] ^2 [B] and A and B are both tripled what factor will the rate increase by

A

27

OCR A A-Level chemistry (3 decks)

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