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Alevel Chemistry OCR A > Module 5: Rates, Equilibrium And PH > Flashcards

Module 5: Rates, Equilibrium And PH Flashcards

1
Q

Rate of reaction
What 4 ways can you measure rate of reaction

A

Gas collection
Mass loss
Disappearing cross
Colorimeter

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2
Q

Rate of reaction
Two ways to measure gas collection

A

Over water
Gas syringe

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3
Q

Rate of reaction
Negative of sometimes using gas collection over water

A

Some gases can dissolve in water

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4
Q

Rate of reaction
What is used to measure rate in mass loss

A

Mass balance and loss of mass recorded

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5
Q

Rate of reaction
How is disappearing cross done for measuring rate

A

As solid forms increasing turbidity and can’t see cross

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6
Q

Rate of reaction
How is colorimeter used to measure rate

A

Measures changes in colour through amount of light absorbed

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7
Q

Orders of reaction
What are orders used for

A

Finding which reactant impacts rate the most

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8
Q

Orders of reaction
How are orders found

A

Change conc of each reactant independently

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9
Q

Orders of reaction
What may happen as you increase concentration of any one reactant

A

May increase rate
May have no effect

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10
Q

Orders of reaction
What is rate always proportional to

A

Change in concentration of a particular reactant

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11
Q

Orders of reaction
Symbols for proportionality

A

Rate § [reactant]^x

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12
Q

Orders of reaction
What are the most common orders

A

Zero order, first order, second order

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13
Q

Orders of reaction
What is the power when zero order

A

0

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14
Q

Orders of reaction
What does a zero order with respect to the reactant mean

A

Concentration of a reactant has no effect on the rate

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15
Q

Orders of reaction
Why does zero order mean not influences rate

A

Any thing to power of 0 = 1

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16
Q

Orders of reaction
What is the power (x) in first order

A

1

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17
Q

Orders of reaction
What is first order with respect to the reactant

A

When rate depends on conc raised to power of 1
(Directly proportional )

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18
Q

Orders of reaction
In first order if conc of a is doubled whayt happens to rate

A

Rate increases by a factor of 2^1 = 2

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19
Q

Orders of reaction
What is power (x) in second order

A

2

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20
Q

Orders of reaction
What’s second order

A

When rate depend on it’s reaction raised to the power of two

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21
Q

Orders of reaction
In a second order reaction what happens if the conc of a is doubles

A

Reaction rate increases by factors of 2^2 = 4

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22
Q

Orders of reaction
Proportional symbols for zero order

A

Rate § [A]^0

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23
Q

Orders of reaction
Proportional symbols for first order

A

Rate § [A]^1

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24
Q

Orders of reaction
Proportional symbols for second order

A

Rate § [A] ^2

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25
Q

Orders of reaction
How to find order of a reactant from table

A

Find multiplier difference in conc of reactant
Find multiplier difference in rate
Find relationship between multiplier, becomes order

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26
Q

Orders of reaction
What does the rate equation give

A

Mathematical relationship between the conc of reactants and the reaction rates

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27
Q

Orders of reaction
What would the rate equation for reactants A and B be

A

Rate = k [A]^m [B]^n

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28
Q

Orders of reaction
What is the rate constant k in the rate equation

A

Proportionality constant
Number that mathematically converts between rate of reactions and concentration and orders

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29
Q

Orders of reaction
What is the overall order of reaction

A

Gives overall effect of the concentrations of all the reactants

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30
Q

Orders of reaction
How is the overall order calculated

A

Sum of orders with respect to each reactant

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31
Q

Orders of reaction
What is the overall order for equation rate = k [A]^m [B]^n

A

Overall order = m + n

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32
Q

Orders of reaction
How do you find the rate constant k

A

Rearrange equation to make k the constant
Substitute values of concentration and initial rate and solve

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33
Q

Orders of reaction
What does the units of rate constant k depend on

A

Number of concentration terms in rate equation

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34
Q

Orders of reaction
How do you work out units of rate constant k

A

Rearrange to make k subject
Substitute units into expression
Simplify

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35
Q

Orders of reaction
What is the units of rate

A

Mol dm^-3 s^-1

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36
Q

Concentration time graphs
How can rate be measures from colour change

A

Using colorimeter

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37
Q

Concentration time graphs
How does colorimeter measure change in colour

A

Wavelength of light passing through coloured solution is controlled using filter and amount of light absorbed is measured

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38
Q

Concentration time graphs
What is the gradient for concentration time graph

A

Rate of reaction

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39
Q

Concentration time graphs
How can the order with respect to a reactant be deduced from these graphs

A

The shape

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40
Q

Concentration time graphs
What shape is conc time graph for a zero order reaction

A

Straight line with negative gradient

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41
Q

Concentration time graphs
Why is a zeri order reaction linear negative

A

Reaction rate(gradient) does not change throughout

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42
Q

Concentration time graphs
What is shape of a first order reaction

A

Downward curve with decreasing gradient over time

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43
Q

Concentration time graphs
Why is first order reaction graph downward curve shaped

A

As gradient (rate) decreases with time reaction slows down

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44
Q

Concentration time graphs
What is a half life

A

Time taken for the conc of a reactant to half its original value

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45
Q

Concentration time graphs
Shape of a second order graph

A

Steeper downward curve tailing off more slowly (will not be expected to analysis)

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46
Q

Concentration time graphs
What is true of the half life for a first order reaction

A

Will be constant

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47
Q

Concentration time graphs
Symbol for half life

A

t1/2 (small 1/2)

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48
Q

Concentration time graphs
What is the pattern of constant half life called

A

Exponential decay

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49
Q

Concentration time graphs
Do the half lives have to be concurrent to be constant and first order

A

No

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50
Q

Concentration time graphs
What are type two ways to calculate k for first order reaction

A

Using rate equation
Using Half life

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51
Q

Concentration time graphs
How to calculate k in first order reaction from graph

A

Find rate at specific point with tangent
Use rate and concentration at same point to find k using rate equation

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52
Q

Concentration time graphs
What is equation for finding k using half life

A

K= ln 2 / t1/2

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53
Q

Rate Concentration graphs
Why are they important

A

Offer route into direct link between rate and concentration in rate equation

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54
Q

Rate Concentration graphs
What can be shown by shape of graph

A

Order with respect to a reactant

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55
Q

Rate Concentration graphs
Shape of zero order graph

A

horizontal straight line with zero gradient

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56
Q

Rate Concentration graphs
Rate equation for zero order

A

Rate=k

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57
Q

Rate Concentration graphs
What is the y intercept on zero order graph

A

Rate constant k

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58
Q

Rate Concentration graphs
Why is zero order graph a horizontal line

A

Rate does not change with increasing conc

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59
Q

Rate Concentration graphs
What is shape of first order graph

A

Straight line graph with positive gradient through origin

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60
Q

Rate Concentration graphs
Rate equation of first order graph with respect to a

A

Rate = k [A]

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61
Q

Rate Concentration graphs
Why is first order that shape

A

Rate directly proportional to concentration

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62
Q

Rate Concentration graphs
How can rate constant k be determined for first order graph

A

Measuring gradient

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63
Q

Rate Concentration graphs
What shape is a second Oder graph

A

Upward curve with increasing gradient through origin

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64
Q

Rate Concentration graphs
Rate equation with respect to a

A

Rate = k[A]^2

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65
Q

Rate Concentration graphs
Can k be calculated from second order graph

A

No

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66
Q

Rate Concentration graphs
What can be done to find k from a second order graph

A

Plot another graph with rate against concentration squared and should be straight line through origin with gradient being k

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67
Q

Rate determining step
What is a rate determining step

A

In reaction with multiple step[s its the slowest step

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68
Q

Rate determining step
How is rds linked to rate equation

A

All of reactant involved in slowest step and steps before are in rate equation

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69
Q

Rate determining step
Why is it unlikely some reactions happen in one step

A

Reactants unlikely to colllide correctly if lots eg one molecule h2o2 two I- ions and two H+ ions

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70
Q

Rate determining step
What is more likely for a reaction like this

A

Take place in a series of steps

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71
Q

Rate determining step
What is the series of steps that makes up an overall reaction called

A

Reaction mechanism

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72
Q

Rate determining step
What two things have to be for a reaction mechanism to be correct

A

Rate equation only included reacting involved in the rate determining steps and reactions before
Orders in rate equation match number of species involved in the rate determining step

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73
Q

Rate determining step
Can rds be predicted from just overall equation

A

No not normally

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74
Q

Rate determining step
Is the RDS always the first step

A

No

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75
Q

Arrhenius equation
is the equation given

A

yes on data sheet

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76
Q

Arrhenius equation
what is the raw equation

A

k=A x e^-Ea/RT

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77
Q

Arrhenius equation
what is the ln equation

A

ln k = -Ea/RT + ln A

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78
Q

Arrhenius equation
wgat is k and units

A

rate constant
units dependent on rate eq

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79
Q

Arrhenius equation
what is A and units

A

pre exponential factor
units dependent on rate eq
links proportionality

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80
Q

Arrhenius equation
what is e

A

e is e
mathematical function

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81
Q

Arrhenius equation
what is Ea and units

A

activation energy
J mol^-1

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82
Q

Arrhenius equation
what is R and units

A

gas constant
8.314 J mol^-1 K^-1
on data sheet

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83
Q

Arrhenius equation
what is T and units

A

temperature
K

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84
Q

Arrhenius equation
how to change C to K

A

+273

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85
Q

Arrhenius equation
how to change kJ mol^-1 to J mol^-1

A

x 1000

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86
Q

Arrhenius equation
what does increasing T do to k and why

A

increasing T increases k
more particles have E>Ea
increases exponential

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87
Q

Arrhenius equation
what does increasing Ea do to k

A

increasing Ea decreases k
less particles have E>Ea
decreases exponential

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88
Q

Arrhenius equation
what does a bigger rate constant mean

A

faster rate of reaction

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89
Q

Arrhenius equation
what does the natural log form of the equation equate to

A

equation of straight lie graph y=mx+c

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90
Q

Arrhenius equation
what is x axis of natural log eq graph

A

1/T

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91
Q

Arrhenius equation
what is the y axis of the natural log eq graph

A

ln k

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92
Q

Arrhenius equation
what is the gradient on ln eq graph

A

-Ea/R

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93
Q

Arrhenius equation
what is the y intercept on ln eq graph

A

ln A

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94
Q

Arrhenius equation
what can be found by plotting a graph of ln k versus 1/T

A

activatio. energy Ea and Arrhenius constant A

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95
Q

Arrhenius equation
why does changing the temperature change the rate constant

A

according to rate equation, rate depends only on rate constant and reactant concentrations

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96
Q

Arrhenius equation when does k apply

A

to particular reaction at certain temperature

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97
Q

Equilibrium
What is K

A

Equilibrium constant

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98
Q

Equilibrium
What is Kc

A

Equilibrium constant with respect to concentration

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99
Q

Equilibrium
What is Kp

A

Equilibrium constant with respect to pressure

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100
Q

Equilibrium
What is pressure measured in

A

Pa/kPa/MPa/atm

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101
Q

Equilibrium
What is mole fraction

A

Proportion of gas compared to total number of miles in the gas mixture

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102
Q

Equilibrium
How do you work out mole fraction

A

Number of moles of particular gas/ total number of moles

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103
Q

Equilibrium
What is partial pressure

A

Proportion of total pressure that a particular gas contributes to

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104
Q

Equilibrium
What symbol is partia;l pressure

A

p

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105
Q

Equilibrium
How do you work out partial pressure

A

Mol fraction x total pressure

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106
Q

Equilibrium
What does a rice table stand for

A

Reaction
Initial
Change
Equilibrium

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107
Q

Equilibrium
How is a rice table applied to Kp

A

Same as Kc but use moles

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108
Q

Equilibrium
What do you do after found moles at equilibrium In rice table

A

Covert each to mol fraction then partial pressures

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109
Q

Equilibrium
What do you do with partial; pressures to find Kp

A

Substitute into Kp expression

110
Q

Equilibrium
What are the units for Kp

A

Change according to number of pressure terms

111
Q

Equilibrium
In exam what do u do if there are no units

A

Write unit less

112
Q

Equilibrium
What is the equilibrium law

A

Ratio of products to react at raised to power of molar ratio remains a constant at a given temperature

113
Q

Equilibrium
What does the equilibrium constant represent

A

Where position of equilibrium is

114
Q

Equilibrium
What does a K value of more than 1 signify

A

More towards products /right

115
Q

Equilibrium
What does a K value of less than 1 signify

A

More towards products /left

116
Q

Equilibrium
What does a K value of 1 signify

A

Positions of equilibrium directly in centre

117
Q

Equilibrium
What factprs can alter then value of K

A

Temperature ONLY

118
Q

Equilibrium
What factors don’t affect value of K

A

Conc pressure catalyst

119
Q

Equilibrium
What is a homogenous reaction

A

All products and reactants are in same state

120
Q

Equilibrium
What is a heterogenous reaction

A

Reactants and products are in different states

121
Q

Equilibrium
What do you include in the equilibrium constant if a home goes reaction

A

All reactants and products

122
Q

Equilibrium
What do you put into equilibrium constant if heterogenous reaction

A

Only gas and aqueous substances and leave out any solids or liquids

123
Q

Equilibrium
What is the exception for heterogenous reactions equilibrium constant

A

If solvent isn’t aqueous need to include any water

124
Q

Equilibrium
What is the total pressure if a gas mixture

A

Sum of all partial pressures of individual gases

125
Q

Equilibrium
What do all mole fractions add to

126
Q

Equilibrium
What do partial pressures add to

127
Q

Equilibrium
Why is there no square brackets for Kp

A

Because they’re partial pressures not concentration

128
Q

Equilibrium
How do you write partial pressure of N2

129
Q

Equilibrium
What do you include if writing Kp for a heterogenous equilibrium

A

Only include gases not liquids or solids

130
Q

Equilibrium
What happens to value of Kp if increase temperature of an exothermic forward reaction

A

As. POE shifts to left then less product is formed and if less product numerator decreases and therefor Kp decreases

131
Q

Equilibrium
Why does only temperature change value of K

A

Definition of K is given at a certain temp

132
Q

Equilibrium
What does changing the pressure and concentration affect

A

Shifts the position of equilibrium to counter the change to keep the values of K the same

133
Q

Equilibrium
What does changing the temperature affect

A

Alters amount of reactants and products at equilibrium and also value of K

134
Q

Equilibrium
What affects do catalysts have

A

No effect on POE or value of K
Can’t increase yield but just mean equilibrium is approached faster

135
Q

Acids and bases
What is a Bronsted-Lowry acid

A

Species that can donate a proton

136
Q

Acids and bases
What is a bronsted lowery base

A

Species that can accept a proton

137
Q

Acids and bases
What does amphoteric mean

A

Can act as either acid or base

138
Q

Acids and bases
What do H+ ions always combine with in water

A

HA(aq) + H2O(l) —> H3O+(aq) + OH-(aq)

139
Q

Acids and bases
What do bases do I water

A

B(aq) + H2O —> BH+(aq) + OH-

140
Q

Acids and bases
What are monoprotic acids

A

Only have one proton they can release into solution

141
Q

Acids and bases
What is a diprotic acid

A

Release 2 protons in solution

142
Q

Acids and bases
Dissociation equation for sulfuric acid

A

H2SO4(aq) <==> 2H+ (aq) + SO4^2-(aq)

143
Q

Acids and bases
What is a triprotic acid

A

Can release 3 protons in solution

144
Q

Acids and bases what are conjugate pairs

A

Linked by the transfer of a proton

145
Q

Acids and bases
Where are conjugate pairs always on an equation

A

Opposites sides of the reaction

146
Q

Acids and bases
What is the conjugate base pair

A

Lost a proton

147
Q

Acids and bases
What is a conjugate acid

A

Gained a proton

148
Q

Acids and bases
How do you label conjugate acid base pairs

A

Acid 1 + base 1 —> acid 2 + base 2

149
Q

Acids and bases
What is a bronsted Lowry acid base reaction

A

Reaction involving transfer of a proton

150
Q

Acids and bases
Ionic equation for metal and acid (Ca)

A

Ca(s) + 2H+(aq) —>Ca2+ (aq) + H2(aq)

151
Q

Acids and bases
Ionic equation for acid and carbonate

A

CO3 ^2-(aq) +2 H+ (aq) —> H2O (l) + CO2 (g)

152
Q

Acids and bases
Ionic equation for acid and alkali

A

H+(aq) + OH-(aq) —>H2O(l)

153
Q

Acids and bases
Ionic equation. For acid and oxide

A

2H+ (aq) + O^2-(s) —> H2O(l)

154
Q

Acids and bases
What three stages important in development in acid base theory

A

Lavoisier
Arrhenius
Bronsted Lowry

155
Q

Acids and bases
What did Lavoisier do

A

Didn’t have many formulas but knew sulfuric and nitric so proposed all acids have oxygen in them but later disproved

156
Q

Acids and bases
What did Arrhenius do

A

Suggested acids release H+ in solution and bases OH- and reacted to form water and salt
Works for most but some not eg ammonia

157
Q

Acids and bases
What did bronsted and lowery

A

Worked more on arrenhius came up with modern definitions which could explain observations

158
Q

pH
What is pH a measure of

A

How acidic or basic something is by measuring hydrogen ion concentration

159
Q

Acids and bases pH
What dies the pH scale use

A

A logarithmic scale

160
Q

Acids and bases pH
What does it mean when solution si neutral (2)

A

pH 7
Or [H+]=[OH-]

161
Q

Acids and bases pH
What is the equation for pH

A

pH=-log[H+]

162
Q

Acids and bases pH
What is pH range

A

0(very acidic
14(very basic

163
Q

Acids and bases pH
Equation for conc of hydrogen ions

A

[H+]= 10^-pH

164
Q

pH strong acids
What do strong acids do in solution

A

Fully ionise in solution

165
Q

pH strong acids
What si [H+] the same as

A

Conc of a strong acid

166
Q

pH strong acids
What do you do for conc of strong acid if given [H+] but diprotic

A

Divide by 2 ad each released two protons

167
Q

pH for strong acid
How do u do strong acid ph dilution calc

A

Find moles in original then find conc in new dilly open volume

168
Q

Kw
What is Kw

A

Ionic product of water

169
Q

Kw
What is the dissociation of water equation

A

H2O <==> H+ + OH-

170
Q

Kw
Kc expression for dissociation of water

A

[H+][OH-]/[H2O]

171
Q

Kw
Where does positions of equilibrium lay in dissociation of water and why

A

Very far to left as percentage which dissociates is very small

172
Q

Kw
What does th POE being so far to the left mean for concentration of H2O

A

Remains constant before equilibrium and after

173
Q

Kw
So what is Kw mathematically

A

Kw=Kc[H2O]

174
Q

Kw
What is the expression for Kw

A

Kw=[H+][OH-]

175
Q

Kw
What is the units for Kw

A

Mol^2 Dm^-6

176
Q

Kw
What is the value of Kw and where can u find t

A

1 x 10^-14
In data sheet

177
Q

Kw
How do you prove the pH of pure water is 7

A

Kw=[H+][OH-]
Assume [H+]=[OH-]
1 x 10^-14=[H+]^2
[H+]=1 x 10^-7
pH= -log(1 x 10^-7) = 7

178
Q

Kw
As temp rises pH of water decreases is the water now acidic alkaline or still neutral

A

Solution is neutral as [H+]=[OH-] still

179
Q

Kw
Is the ionisation of water endothermic or exothermic

A

Endothermic

180
Q

Kw
Explain why the ionisation water is endothermic in terms of Kw

A

As temp increases, Kw increases, more towards products, POE more to right and temp increase favours endothermic

181
Q

Kw
What does water actually dissociate into

A

2 H2O (l) <==> H3O+ (aq) + OH- (aq)
Hydroxium and hydroxide ion

182
Q

Kw
Does changing the conc of [H+] or [OH-] cjnage Kw

A

No effect as equilibrium will shift to keep original value

183
Q

Kw
Dies Kw change at different temps

A

Yes as dissociation of water is endo process

184
Q

Kw
Dies the value change for pure water or aqueous

A

No is there same

185
Q

Kw
What does p mean

186
Q

Kw
What does pKw mean

187
Q

Kw
What is Kw useful for

A

Finding pH of strong bases

188
Q

pH of strong bases
What will base produce in solution

189
Q

pH of strong bases
What does a strong base mean

A

Will fully ionise in water

190
Q

pH of strong bases
What it’s important for these calculations

A

Number of OH- ions per compound is dependant of number of OH

191
Q

pH of strong bases What is [OH-] in 0.1 mol dm-3 of KOH

A

0.1 mol dm-3

192
Q

pH of strong bases
What is [OH-] from 0.1 mol dm-3 Mg(OH)2

A

0.2 mol dm-3

193
Q

pH of strong bases
How do you work out pH of a strong base of given conc

A

Find [OH-] from conc of base
Use rearranged Kw expression to find [H+]
Use pH=-log[H+]

194
Q

pH of strong bases
What is also true for pOH and pH

A

pOH + pH =14 always in a base

195
Q

Weak acids
What are weak acuds

A

Acids which do not fully dissociate in water

196
Q

Weak acids
What does weak acids not fully ionising mean for the [H+]

A

Not the same as the acid concentration

197
Q

Weak acids
What is Ka

A

Equilibrium constant with respect to acids

198
Q

Weak acids
Acid dissociation equation for weak acid HA

A

HA <=> H+ + A-

199
Q

Weak acids
What is an example of weak acid

A

Ethanoic CH3COOH

200
Q

Weak acids
What is the Ka expression

A

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

201
Q

Weak acids
What is the units of Ka

202
Q

Weak acids
What is pKa

203
Q

Weak acids
How do you find Ka from pKa

204
Q

Weak acids
What does the assumption that dissociation of acid is much greater then water mean you van asssume

A

All H+ ions in solution come from the acid so [H+]=[A-]

205
Q

Weak acids
How else could you write Ka expression when only weak acid in aq and nothing else added

A

Ka=[H+]^2/[HA]

206
Q

Weak acids
Since only small amount of HA dissociates what do we assume

A

[HA(start)]=[HA(equilibrium)]

207
Q

Weak acids
How do you calculate pH of weak acid when given Ka and [HA]

A

Find [H+] using [H+]^2=Ka x [HA]
FindpH using -log[H+]

208
Q

Weak acids
How do you find conc of weak acid given ph and Ka

A

Find [H+] from 10^-pH
Find conc using [HA]=[H+]^2/Ka

209
Q

Reaction of weak acid and strong base
If 1 mol HA reacts with 1 mol OH- how much A- produced and how much OH- or HA left

A

1 mol A- produced
No OH- or HA left

210
Q

Weak acids
If 3 mol HA and 1 mol OH- how much A- produced and OH- or HA left

A

1 mol A- produced
2 mol HA left

211
Q

Weak acids and strong base reaction
If 3 mol HA and 10 mol OH- how much A- produced and OH- or HA left

A

3 A- produced
7 mole OH- left

212
Q

Finding ph in reactions between weak acids and strong bases
What happens when a weak acid reacts with a strong base

A

For every one mole of OH- assed one mole of HA is used up and one mole of A- is formed

213
Q

Finding ph in reactions between weak acids and strong bases
What is always the first three steps of these calculations

A

Calculate moles of HA
Calculate mole of OH-
Calculate moles of excess HA or OH-

214
Q

Finding ph in reactions between weak acids and strong bases
What do you do if there is excess HA (acid)

A
  1. Calculate moles of HA left and A- formed
  2. Calculate HAconc left over and conc of A- formed
  3. Use Ka to find conc H+
    7 find ph
215
Q

Finding ph in reactions between weak acids and strong bases
What do you do if there is excess OH-

A
  1. Calculate conc OH-
  2. Use Kw to find conc H+
    Find PH
216
Q

Finding ph in reactions between weak acids and strong bases
What do you do if mole HA = OH-

A

Find ph using Ka of weak acid

217
Q

PH curves and titrations
What can you use to measure the ph of a solution

A

A ph meter

218
Q

PH curves and titrations
How do ph meters measure ph

A

Have a probe that is put into solution and a digital display which shows reading bulb at end of probe

219
Q

PH curves and titrations
What do you have to do before using a probe meter

A

Calibrate it

220
Q

PH curves and titrations
How do you calibrates a ph meter

A

Place bulb of ph meter into distilled water and allow the reading to settle now adjust the reading so that is reads 7.0
Then repeat with a standard solution of PH 4 and PH 10

221
Q

PH curves and titrations
What should you do between every measurement using a ph probe

A

Rinse with distilled water

222
Q

PH curves and titrations
What ph ph curves plot

A

Ph against volume of acid or base added

223
Q

PH curves and titrations
What do titrations allow you to do

A

Find out exactly how much alkali or acid is needed to neutralise a quantity of acid or base

224
Q

PH curves and titrations
What apparatus is used to generate ph curves

A

Ph meter and burette

225
Q

PH curves and titrations
Why do ph curves vary in shape

A

Depending on the strength of acid and base used

226
Q

PH curves and titrations
How do you find which acid or base is in the burette or conical flask

A

If ph above 7 at start, alkali is in the conical flask and can decide whether strong or weak from ph and the acid will be shown by u a low ph at the other side
Vice versa

227
Q

PH curves and titrations
Why do ph curves have a flat start

A

Addition of small amount of acid or base has little impact on the ph of the solution

228
Q

PH curves and titrations
What is the equivalence point on graph

A

Vertical section of the graph

229
Q

PH curves and titrations
What is the equivalence point on graph

A

Vertical section of the graph

230
Q

PH curves and titrations
What is the equals valence point

A

When [H+]=[OH-] neutralisation has occurred

231
Q

PH curves and titrations
What is an end point in a titration

A

Point when indicator changes colour

232
Q

PH curves and titrations
What does an indicator need to do

A

Change colour to show when sample has been neutralised

233
Q

PH curves and titrations
When do you need your indicator to change colour

A

At the equivalence point of the titration

234
Q

PH curves and titrations
How do you pick an indicator based on a PH curve

A

One which changes colour over a narrow ph range which lies entirely in the vertical part of the ph curve

235
Q

PH curves and titrations
Which two indicators do we have to know about

A

Methyl orange and phenolpthalien

236
Q

PH curves and titrations
What colour is methyl orange is a low ph

237
Q

PH curves and titrations
What colour is methyl orange in a high ph

238
Q

PH curves and titrations
What colour is phenolpthalien in low ph

A

Colourless

239
Q

PH curves and titrations
What colour is phenolpthalien at a high ph

240
Q

PH curves and titrations
What is the appoximate ph of colour change for methyl orange

241
Q

PH curves and titrations
What is the approx ph of colour changed for phenolpthalien

242
Q

PH curves and titrations
Which indicator should you use for a strong acid strong base titration and why

A

Either as there’s a rapid ph change over the range for both indicators

243
Q

PH curves and titrations
Which indicators should you use for a strong acid weak base titration and why

A

Methyl orange as ph changes rapidly across the range for methyl orange but not for phenolpthalien

244
Q

PH curves and titrations
Which indicator should you use for a weak acid strong base titration and why

A

Phenolpthalien as the ph changes rapidly across phenolpthalien range but not methyl orange

245
Q

PH curves and titrations
What indicator should you use for a weak acid weak base titration and why

A

Neither as there’s not sharp ph change so neither of these indicators wold work

246
Q

PH curves and titrations
What indicators would you use instead for a weak acid weak base titration

247
Q

PH curves and titrations
What can indicators be thought of as and why

A

Weak acids as r they have differently colour conjugate pairs

248
Q

PH curves and titrations
What happens as the colour changes

A

The equilibrium concentrations of the conjugate pairs also change this depend on whether the indicators is mainly protonated or deprotonated

249
Q

PH curves and titrations
What is the ph of the end point equivalent to

A

The pKa value of HA

250
Q

Buffers
What is a buffer

A

Solution that minimises changes in ph when small amounts of acid or base are added

251
Q

Buffers
What volume do buffers work for

A

Only very small amounts of acid or base

252
Q

Buffers
What are the two types of buffers and which ones do we need to know about

A

Acidic and basic
Only need to know acidic

253
Q

Buffers
What do acidic buffers contain

A

A weak acid and a conjugate base

254
Q

Buffers
What ph do buffers have

A

Less than 7

255
Q

Buffers
How are acidic buffers a made

A

Setting up an equilibrium between a weak acid and its conjugate base

256
Q

Buffers
What two ways can a buffer be made

A

Mix a weak acid with the salt of its conjugate base
Mix an excess of weak acid with a strong alkali

257
Q

Buffers
How does the mixing a weal acid with the salt of its conjugate base work

A

Salt fully dissociates into its ions when it dissolves
Weak acid only slightly dissociates

258
Q

Buffers
How does mixing an excess of weak acid with a strong alkali work

A

All the base reacts with the acid and as weak acid in excess there’s still some left in solution once all the base has reacted the acid slightly dissociates

259
Q

Buffers
What is the mix of a buffer eg with Ethanoic acid

A

CH3COOH(aq) <==> H+(aq) + CH3COO-(aq)
Mix of both undissociate weak acid and conjugate base

260
Q

Buffers
What does the equilibrium solution contain

A

Lots of undisscooated acid (HA)
Lots of the acuds conjugate bae (A-)
Enough H+ ions to make the solution acidic

261
Q

Buffers
What is the job of the conjugate pair

A

To control the ph of the buffer to solution

262
Q

Buffers
How does the conjugate base help control ph

A

Mops up excess of the H+ ions

263
Q

Buffers
What does the conjugate acid do to help control the ph

A

Released H+ if too much base

264
Q

Buffers
What happens to the equilibrium if you add a small amount of acid

A

H+ ion concentration increases and thr extra H+ ions co,bind with CH3COO- ions to form CH3COOH
Thus shifts the equilibrium to the left reducing H+ concentration to close to its original value

265
Q

Buffers
How i there such large volumes of the conjugate base in the equilibrium

A

From weak acid only small proportions of H+ and A-
But on addition of A- , large proportions of A- and small volume of H+

266
Q

Buffers
What happens if you have a small amount of alkali added

A

OH- conc increases which mostly react with H+ ions to form water removing H+ ions from the solutions, this causes the equilibrium to move to the right and H+ conc increases until it’s close to its original value

267
Q

Buffers
What is used to find the ph of a buffer solution

268
Q

Buffers
What is the equation to find the H+ ion conc

A

[H+] = (Ka[Acid])/[salt]

269
Q

Buffers
Therefore what ratio does the ph of buffers depend on

A

Ratio of acid to salt

270
Q

Buffers
Why cannot Ka just be simplified to [H+]^2

A

Have added excess OH-

271
Q

Buffers
What two assumption have been made to calculate ph of buffer

A

[HA] remains constant was such small proportion dissociates
[A-] comes entirely from salt concentration

Alevel Chemistry OCR A (17 decks)

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