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

Module 5: Rates, Equilibrium And PH Flashcards

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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
Orders of reaction How to find order of a reactant from table
Find multiplier difference in conc of reactant Find multiplier difference in rate Find relationship between multiplier, becomes order
26
Orders of reaction What does the rate equation give
Mathematical relationship between the conc of reactants and the reaction rates
27
Orders of reaction What would the rate equation for reactants A and B be
Rate = k [A]^m [B]^n
28
Orders of reaction What is the rate constant k in the rate equation
Proportionality constant Number that mathematically converts between rate of reactions and concentration and orders
29
Orders of reaction What is the overall order of reaction
Gives overall effect of the concentrations of all the reactants
30
Orders of reaction How is the overall order calculated
Sum of orders with respect to each reactant
31
Orders of reaction What is the overall order for equation rate = k [A]^m [B]^n
Overall order = m + n
32
Orders of reaction How do you find the rate constant k
Rearrange equation to make k the constant Substitute values of concentration and initial rate and solve
33
Orders of reaction What does the units of rate constant k depend on
Number of concentration terms in rate equation
34
Orders of reaction How do you work out units of rate constant k
Rearrange to make k subject Substitute units into expression Simplify
35
Orders of reaction What is the units of rate
Mol dm^-3 s^-1
36
Concentration time graphs How can rate be measures from colour change
Using colorimeter
37
Concentration time graphs How does colorimeter measure change in colour
Wavelength of light passing through coloured solution is controlled using filter and amount of light absorbed is measured
38
Concentration time graphs What is the gradient for concentration time graph
Rate of reaction
39
Concentration time graphs How can the order with respect to a reactant be deduced from these graphs
The shape
40
Concentration time graphs What shape is conc time graph for a zero order reaction
Straight line with negative gradient
41
Concentration time graphs Why is a zeri order reaction linear negative
Reaction rate(gradient) does not change throughout
42
Concentration time graphs What is shape of a first order reaction
Downward curve with decreasing gradient over time
43
Concentration time graphs Why is first order reaction graph downward curve shaped
As gradient (rate) decreases with time reaction slows down
44
Concentration time graphs What is a half life
Time taken for the conc of a reactant to half its original value
45
Concentration time graphs Shape of a second order graph
Steeper downward curve tailing off more slowly (will not be expected to analysis)
46
Concentration time graphs What is true of the half life for a first order reaction
Will be constant
47
Concentration time graphs Symbol for half life
t1/2 (small 1/2)
48
Concentration time graphs What is the pattern of constant half life called
Exponential decay
49
Concentration time graphs Do the half lives have to be concurrent to be constant and first order
No
50
Concentration time graphs What are type two ways to calculate k for first order reaction
Using rate equation Using Half life
51
Concentration time graphs How to calculate k in first order reaction from graph
Find rate at specific point with tangent Use rate and concentration at same point to find k using rate equation
52
Concentration time graphs What is equation for finding k using half life
K= ln 2 / t1/2
53
Rate Concentration graphs Why are they important
Offer route into direct link between rate and concentration in rate equation
54
Rate Concentration graphs What can be shown by shape of graph
Order with respect to a reactant
55
Rate Concentration graphs Shape of zero order graph
horizontal straight line with zero gradient
56
Rate Concentration graphs Rate equation for zero order
Rate=k
57
Rate Concentration graphs What is the y intercept on zero order graph
Rate constant k
58
Rate Concentration graphs Why is zero order graph a horizontal line
Rate does not change with increasing conc
59
Rate Concentration graphs What is shape of first order graph
Straight line graph with positive gradient through origin
60
Rate Concentration graphs Rate equation of first order graph with respect to a
Rate = k [A]
61
Rate Concentration graphs Why is first order that shape
Rate directly proportional to concentration
62
Rate Concentration graphs How can rate constant k be determined for first order graph
Measuring gradient
63
Rate Concentration graphs What shape is a second Oder graph
Upward curve with increasing gradient through origin
64
Rate Concentration graphs Rate equation with respect to a
Rate = k[A]^2
65
Rate Concentration graphs Can k be calculated from second order graph
No
66
Rate Concentration graphs What can be done to find k from a second order graph
Plot another graph with rate against concentration squared and should be straight line through origin with gradient being k
67
Rate determining step What is a rate determining step
In reaction with multiple step[s its the slowest step
68
Rate determining step How is rds linked to rate equation
All of reactant involved in slowest step and steps before are in rate equation
69
Rate determining step Why is it unlikely some reactions happen in one step
Reactants unlikely to colllide correctly if lots eg one molecule h2o2 two I- ions and two H+ ions
70
Rate determining step What is more likely for a reaction like this
Take place in a series of steps
71
Rate determining step What is the series of steps that makes up an overall reaction called
Reaction mechanism
72
Rate determining step What two things have to be for a reaction mechanism to be correct
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
73
Rate determining step Can rds be predicted from just overall equation
No not normally
74
Rate determining step Is the RDS always the first step
No
75
Arrhenius equation is the equation given
yes on data sheet
76
Arrhenius equation what is the raw equation
k=A x e^-Ea/RT
77
Arrhenius equation what is the ln equation
ln k = -Ea/RT + ln A
78
Arrhenius equation wgat is k and units
rate constant units dependent on rate eq
79
Arrhenius equation what is A and units
pre exponential factor units dependent on rate eq links proportionality
80
Arrhenius equation what is e
e is e mathematical function
81
Arrhenius equation what is Ea and units
activation energy J mol^-1
82
Arrhenius equation what is R and units
gas constant 8.314 J mol^-1 K^-1 on data sheet
83
Arrhenius equation what is T and units
temperature K
84
Arrhenius equation how to change C to K
+273
85
Arrhenius equation how to change kJ mol^-1 to J mol^-1
x 1000
86
Arrhenius equation what does increasing T do to k and why
increasing T increases k more particles have E>Ea increases exponential
87
Arrhenius equation what does increasing Ea do to k
increasing Ea decreases k less particles have E>Ea decreases exponential
88
Arrhenius equation what does a bigger rate constant mean
faster rate of reaction
89
Arrhenius equation what does the natural log form of the equation equate to
equation of straight lie graph y=mx+c
90
Arrhenius equation what is x axis of natural log eq graph
1/T
91
Arrhenius equation what is the y axis of the natural log eq graph
ln k
92
Arrhenius equation what is the gradient on ln eq graph
-Ea/R
93
Arrhenius equation what is the y intercept on ln eq graph
ln A
94
Arrhenius equation what can be found by plotting a graph of ln k versus 1/T
activatio. energy Ea and Arrhenius constant A
95
Arrhenius equation why does changing the temperature change the rate constant
according to rate equation, rate depends only on rate constant and reactant concentrations
96
Arrhenius equation when does k apply
to particular reaction at certain temperature
97
Equilibrium What is K
Equilibrium constant
98
Equilibrium What is Kc
Equilibrium constant with respect to concentration
99
Equilibrium What is Kp
Equilibrium constant with respect to pressure
100
Equilibrium What is pressure measured in
Pa/kPa/MPa/atm
101
Equilibrium What is mole fraction
Proportion of gas compared to total number of miles in the gas mixture
102
Equilibrium How do you work out mole fraction
Number of moles of particular gas/ total number of moles
103
Equilibrium What is partial pressure
Proportion of total pressure that a particular gas contributes to
104
Equilibrium What symbol is partia;l pressure
p
105
Equilibrium How do you work out partial pressure
Mol fraction x total pressure
106
Equilibrium What does a rice table stand for
Reaction Initial Change Equilibrium
107
Equilibrium How is a rice table applied to Kp
Same as Kc but use moles
108
Equilibrium What do you do after found moles at equilibrium In rice table
Covert each to mol fraction then partial pressures
109
Equilibrium What do you do with partial; pressures to find Kp
Substitute into Kp expression
110
Equilibrium What are the units for Kp
Change according to number of pressure terms
111
Equilibrium In exam what do u do if there are no units
Write unit less
112
Equilibrium What is the equilibrium law
Ratio of products to react at raised to power of molar ratio remains a constant at a given temperature
113
Equilibrium What does the equilibrium constant represent
Where position of equilibrium is
114
Equilibrium What does a K value of more than 1 signify
More towards products /right
115
Equilibrium What does a K value of less than 1 signify
More towards products /left
116
Equilibrium What does a K value of 1 signify
Positions of equilibrium directly in centre
117
Equilibrium What factprs can alter then value of K
Temperature ONLY
118
Equilibrium What factors don’t affect value of K
Conc pressure catalyst
119
Equilibrium What is a homogenous reaction
All products and reactants are in same state
120
Equilibrium What is a heterogenous reaction
Reactants and products are in different states
121
Equilibrium What do you include in the equilibrium constant if a home goes reaction
All reactants and products
122
Equilibrium What do you put into equilibrium constant if heterogenous reaction
Only gas and aqueous substances and leave out any solids or liquids
123
Equilibrium What is the exception for heterogenous reactions equilibrium constant
If solvent isn’t aqueous need to include any water
124
Equilibrium What is the total pressure if a gas mixture
Sum of all partial pressures of individual gases
125
Equilibrium What do all mole fractions add to
1
126
Equilibrium What do partial pressures add to
127
Equilibrium Why is there no square brackets for Kp
Because they’re partial pressures not concentration
128
Equilibrium How do you write partial pressure of N2
p(N2)
129
Equilibrium What do you include if writing Kp for a heterogenous equilibrium
Only include gases not liquids or solids
130
Equilibrium What happens to value of Kp if increase temperature of an exothermic forward reaction
As. POE shifts to left then less product is formed and if less product numerator decreases and therefor Kp decreases
131
Equilibrium Why does only temperature change value of K
Definition of K is given at a certain temp
132
Equilibrium What does changing the pressure and concentration affect
Shifts the position of equilibrium to counter the change to keep the values of K the same
133
Equilibrium What does changing the temperature affect
Alters amount of reactants and products at equilibrium and also value of K
134
Equilibrium What affects do catalysts have
No effect on POE or value of K Can’t increase yield but just mean equilibrium is approached faster
135
Acids and bases What is a Bronsted-Lowry acid
Species that can donate a proton
136
Acids and bases What is a bronsted lowery base
Species that can accept a proton
137
Acids and bases What does amphoteric mean
Can act as either acid or base
138
Acids and bases What do H+ ions always combine with in water
HA(aq) + H2O(l) —> H3O+(aq) + OH-(aq)
139
Acids and bases What do bases do I water
B(aq) + H2O —> BH+(aq) + OH-
140
Acids and bases What are monoprotic acids
Only have one proton they can release into solution
141
Acids and bases What is a diprotic acid
Release 2 protons in solution
142
Acids and bases Dissociation equation for sulfuric acid
H2SO4(aq) <==> 2H+ (aq) + SO4^2-(aq)
143
Acids and bases What is a triprotic acid
Can release 3 protons in solution
144
Acids and bases what are conjugate pairs
Linked by the transfer of a proton
145
Acids and bases Where are conjugate pairs always on an equation
Opposites sides of the reaction
146
Acids and bases What is the conjugate base pair
Lost a proton
147
Acids and bases What is a conjugate acid
Gained a proton
148
Acids and bases How do you label conjugate acid base pairs
Acid 1 + base 1 —> acid 2 + base 2
149
Acids and bases What is a bronsted Lowry acid base reaction
Reaction involving transfer of a proton
150
Acids and bases Ionic equation for metal and acid (Ca)
Ca(s) + 2H+(aq) —>Ca2+ (aq) + H2(aq)
151
Acids and bases Ionic equation for acid and carbonate
CO3 ^2-(aq) +2 H+ (aq) —> H2O (l) + CO2 (g)
152
Acids and bases Ionic equation for acid and alkali
H+(aq) + OH-(aq) —>H2O(l)
153
Acids and bases Ionic equation. For acid and oxide
2H+ (aq) + O^2-(s) —> H2O(l)
154
Acids and bases What three stages important in development in acid base theory
Lavoisier Arrhenius Bronsted Lowry
155
Acids and bases What did Lavoisier do
Didn’t have many formulas but knew sulfuric and nitric so proposed all acids have oxygen in them but later disproved
156
Acids and bases What did Arrhenius do
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
Acids and bases What did bronsted and lowery
Worked more on arrenhius came up with modern definitions which could explain observations
158
pH What is pH a measure of
How acidic or basic something is by measuring hydrogen ion concentration
159
Acids and bases pH What dies the pH scale use
A logarithmic scale
160
Acids and bases pH What does it mean when solution si neutral (2)
pH 7 Or [H+]=[OH-]
161
Acids and bases pH What is the equation for pH
pH=-log[H+]
162
Acids and bases pH What is pH range
0(very acidic 14(very basic
163
Acids and bases pH Equation for conc of hydrogen ions
[H+]= 10^-pH
164
pH strong acids What do strong acids do in solution
Fully ionise in solution
165
pH strong acids What si [H+] the same as
Conc of a strong acid
166
pH strong acids What do you do for conc of strong acid if given [H+] but diprotic
Divide by 2 ad each released two protons
167
pH for strong acid How do u do strong acid ph dilution calc
Find moles in original then find conc in new dilly open volume
168
Kw What is Kw
Ionic product of water
169
Kw What is the dissociation of water equation
H2O <==> H+ + OH-
170
Kw Kc expression for dissociation of water
[H+][OH-]/[H2O]
171
Kw Where does positions of equilibrium lay in dissociation of water and why
Very far to left as percentage which dissociates is very small
172
Kw What does th POE being so far to the left mean for concentration of H2O
Remains constant before equilibrium and after
173
Kw So what is Kw mathematically
Kw=Kc[H2O]
174
Kw What is the expression for Kw
Kw=[H+][OH-]
175
Kw What is the units for Kw
Mol^2 Dm^-6
176
Kw What is the value of Kw and where can u find t
1 x 10^-14 In data sheet
177
Kw How do you prove the pH of pure water is 7
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
Kw As temp rises pH of water decreases is the water now acidic alkaline or still neutral
Solution is neutral as [H+]=[OH-] still
179
Kw Is the ionisation of water endothermic or exothermic
Endothermic
180
Kw Explain why the ionisation water is endothermic in terms of Kw
As temp increases, Kw increases, more towards products, POE more to right and temp increase favours endothermic
181
Kw What does water actually dissociate into
2 H2O (l) <==> H3O+ (aq) + OH- (aq) Hydroxium and hydroxide ion
182
Kw Does changing the conc of [H+] or [OH-] cjnage Kw
No effect as equilibrium will shift to keep original value
183
Kw Dies Kw change at different temps
Yes as dissociation of water is endo process
184
Kw Dies the value change for pure water or aqueous
No is there same
185
Kw What does p mean
-log
186
Kw What does pKw mean
-log(Kw)
187
Kw What is Kw useful for
Finding pH of strong bases
188
pH of strong bases What will base produce in solution
OH- ions
189
pH of strong bases What does a strong base mean
Will fully ionise in water
190
pH of strong bases What it’s important for these calculations
Number of OH- ions per compound is dependant of number of OH
191
pH of strong bases What is [OH-] in 0.1 mol dm-3 of KOH
0.1 mol dm-3
192
pH of strong bases What is [OH-] from 0.1 mol dm-3 Mg(OH)2
0.2 mol dm-3
193
pH of strong bases How do you work out pH of a strong base of given conc
Find [OH-] from conc of base Use rearranged Kw expression to find [H+] Use pH=-log[H+]
194
pH of strong bases What is also true for pOH and pH
pOH + pH =14 always in a base
195
Weak acids What are weak acuds
Acids which do not fully dissociate in water
196
Weak acids What does weak acids not fully ionising mean for the [H+]
Not the same as the acid concentration
197
Weak acids What is Ka
Equilibrium constant with respect to acids
198
Weak acids Acid dissociation equation for weak acid HA
HA <=> H+ + A-
199
Weak acids What is an example of weak acid
Ethanoic CH3COOH
200
Weak acids What is the Ka expression
Ka=[H+][A-]/[HA]
201
Weak acids What is the units of Ka
Mol dm^-3
202
Weak acids What is pKa
-log Ka
203
Weak acids How do you find Ka from pKa
10^-pKa
204
Weak acids What does the assumption that dissociation of acid is much greater then water mean you van asssume
All H+ ions in solution come from the acid so [H+]=[A-]
205
Weak acids How else could you write Ka expression when only weak acid in aq and nothing else added
Ka=[H+]^2/[HA]
206
Weak acids Since only small amount of HA dissociates what do we assume
[HA(start)]=[HA(equilibrium)]
207
Weak acids How do you calculate pH of weak acid when given Ka and [HA]
Find [H+] using [H+]^2=Ka x [HA] FindpH using -log[H+]
208
Weak acids How do you find conc of weak acid given ph and Ka
Find [H+] from 10^-pH Find conc using [HA]=[H+]^2/Ka
209
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
1 mol A- produced No OH- or HA left
210
Weak acids If 3 mol HA and 1 mol OH- how much A- produced and OH- or HA left
1 mol A- produced 2 mol HA left
211
Weak acids and strong base reaction If 3 mol HA and 10 mol OH- how much A- produced and OH- or HA left
3 A- produced 7 mole OH- left
212
Finding ph in reactions between weak acids and strong bases What happens when a weak acid reacts with a strong base
For every one mole of OH- assed one mole of HA is used up and one mole of A- is formed
213
Finding ph in reactions between weak acids and strong bases What is always the first three steps of these calculations
Calculate moles of HA Calculate mole of OH- Calculate moles of excess HA or OH-
214
Finding ph in reactions between weak acids and strong bases What do you do if there is excess HA (acid)
4. Calculate moles of HA left and A- formed 5. Calculate HAconc left over and conc of A- formed 6. Use Ka to find conc H+ 7 find ph
215
Finding ph in reactions between weak acids and strong bases What do you do if there is excess OH-
4. Calculate conc OH- 5. Use Kw to find conc H+ Find PH
216
Finding ph in reactions between weak acids and strong bases What do you do if mole HA = OH-
Find ph using Ka of weak acid
217
PH curves and titrations What can you use to measure the ph of a solution
A ph meter
218
PH curves and titrations How do ph meters measure ph
Have a probe that is put into solution and a digital display which shows reading bulb at end of probe
219
PH curves and titrations What do you have to do before using a probe meter
Calibrate it
220
PH curves and titrations How do you calibrates a ph meter
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
PH curves and titrations What should you do between every measurement using a ph probe
Rinse with distilled water
222
PH curves and titrations What ph ph curves plot
Ph against volume of acid or base added
223
PH curves and titrations What do titrations allow you to do
Find out exactly how much alkali or acid is needed to neutralise a quantity of acid or base
224
PH curves and titrations What apparatus is used to generate ph curves
Ph meter and burette
225
PH curves and titrations Why do ph curves vary in shape
Depending on the strength of acid and base used
226
PH curves and titrations How do you find which acid or base is in the burette or conical flask
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
PH curves and titrations Why do ph curves have a flat start
Addition of small amount of acid or base has little impact on the ph of the solution
228
PH curves and titrations What is the equivalence point on graph
Vertical section of the graph
229
PH curves and titrations What is the equivalence point on graph
Vertical section of the graph
230
PH curves and titrations What is the equals valence point
When [H+]=[OH-] neutralisation has occurred
231
PH curves and titrations What is an end point in a titration
Point when indicator changes colour
232
PH curves and titrations What does an indicator need to do
Change colour to show when sample has been neutralised
233
PH curves and titrations When do you need your indicator to change colour
At the equivalence point of the titration
234
PH curves and titrations How do you pick an indicator based on a PH curve
One which changes colour over a narrow ph range which lies entirely in the vertical part of the ph curve
235
PH curves and titrations Which two indicators do we have to know about
Methyl orange and phenolpthalien
236
PH curves and titrations What colour is methyl orange is a low ph
Red
237
PH curves and titrations What colour is methyl orange in a high ph
Yellow
238
PH curves and titrations What colour is phenolpthalien in low ph
Colourless
239
PH curves and titrations What colour is phenolpthalien at a high ph
Pink
240
PH curves and titrations What is the appoximate ph of colour change for methyl orange
3.1-4.4
241
PH curves and titrations What is the approx ph of colour changed for phenolpthalien
8.3-10
242
PH curves and titrations Which indicator should you use for a strong acid strong base titration and why
Either as there’s a rapid ph change over the range for both indicators
243
PH curves and titrations Which indicators should you use for a strong acid weak base titration and why
Methyl orange as ph changes rapidly across the range for methyl orange but not for phenolpthalien
244
PH curves and titrations Which indicator should you use for a weak acid strong base titration and why
Phenolpthalien as the ph changes rapidly across phenolpthalien range but not methyl orange
245
PH curves and titrations What indicator should you use for a weak acid weak base titration and why
Neither as there’s not sharp ph change so neither of these indicators wold work
246
PH curves and titrations What indicators would you use instead for a weak acid weak base titration
Ph meter
247
PH curves and titrations What can indicators be thought of as and why
Weak acids as r they have differently colour conjugate pairs
248
PH curves and titrations What happens as the colour changes
The equilibrium concentrations of the conjugate pairs also change this depend on whether the indicators is mainly protonated or deprotonated
249
PH curves and titrations What is the ph of the end point equivalent to
The pKa value of HA
250
Buffers What is a buffer
Solution that minimises changes in ph when small amounts of acid or base are added
251
Buffers What volume do buffers work for
Only very small amounts of acid or base
252
Buffers What are the two types of buffers and which ones do we need to know about
Acidic and basic Only need to know acidic
253
Buffers What do acidic buffers contain
A weak acid and a conjugate base
254
Buffers What ph do buffers have
Less than 7
255
Buffers How are acidic buffers a made
Setting up an equilibrium between a weak acid and its conjugate base
256
Buffers What two ways can a buffer be made
Mix a weak acid with the salt of its conjugate base Mix an excess of weak acid with a strong alkali
257
Buffers How does the mixing a weal acid with the salt of its conjugate base work
Salt fully dissociates into its ions when it dissolves Weak acid only slightly dissociates
258
Buffers How does mixing an excess of weak acid with a strong alkali work
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
Buffers What is the mix of a buffer eg with Ethanoic acid
CH3COOH(aq) <==> H+(aq) + CH3COO-(aq) Mix of both undissociate weak acid and conjugate base
260
Buffers What does the equilibrium solution contain
Lots of undisscooated acid (HA) Lots of the acuds conjugate bae (A-) Enough H+ ions to make the solution acidic
261
Buffers What is the job of the conjugate pair
To control the ph of the buffer to solution
262
Buffers How does the conjugate base help control ph
Mops up excess of the H+ ions
263
Buffers What does the conjugate acid do to help control the ph
Released H+ if too much base
264
Buffers What happens to the equilibrium if you add a small amount of acid
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
Buffers How i there such large volumes of the conjugate base in the equilibrium
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
Buffers What happens if you have a small amount of alkali added
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
Buffers What is used to find the ph of a buffer solution
Ka
268
Buffers What is the equation to find the H+ ion conc
[H+] = (Ka[Acid])/[salt]
269
Buffers Therefore what ratio does the ph of buffers depend on
Ratio of acid to salt
270
Buffers Why cannot Ka just be simplified to [H+]^2
Have added excess OH-
271
Buffers What two assumption have been made to calculate ph of buffer
[HA] remains constant was such small proportion dissociates [A-] comes entirely from salt concentration

Alevel Chemistry OCR A (18 decks)

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