Transcript Double bonds or not - St Mary's College, Wallasey

Double bonds or not
• A saturated fat has no
C=C double bonds
(alkene functional
groups) and is usually a
solid fat like margarine
or animal fat.
• An unsaturated fat has
C=C double bonds and
is usually an oil like
vegetable oil.
1
Example question
2
Mark scheme
3
Making margarine
• To make margarine we have to saturate
vegetable oil by bubbling hydrogen gas
through the oil.
• This process is called hydrogenation
4
Is a fat or oil saturated or not?
• We can test for this by adding bromine water.
• If there are double bonds present the bromine
water changes from
to
.
5
Example question
6
Mark scheme
7
Hydrolysis
• When an ester is hydrolysed it goes back to an
acid and alcohol
• We can hydrolyse by adding acid or alkali
(NaOH).
8
Example question
9
Mark scheme
10
Energy changes in chemistry
C7.2
11
Quiz
• When a chemical reaction takes place heat
may be given out or taken in.
1. Can you remember the word we use when
heat is given out?
2. Can you remember the word we use when
heat is taken in?
12
What do I need to know?
1. Recall and use the terms ENDOTHERMIC and
EXOTHERMIC
2. Describe examples of ENDOTHERMIC and
EXOTHERMIC reactions.
3. Use simple energy level diagrams to
represent ENDOTHERMIC and EXOTHERMIC
reactions.
13
Change in energy
• Chemical reactants have a certain amount of
stored within them.
• When the reaction has taken place they have
either
within them
than before.
14
Definitions
(exothermic) then the
than they did before.
They have lost it to the surroundings.
(endothermic) then the
than they had
before. They have taken it from the
surroundings.
15
Energy level diagrams
Which diagram do you think is
which is
?
Heat taken in
16
Heat given out
and
Energy level diagrams
Endothermic
Exothermic
Heat taken in
Energy level of products is higher
than reactants so heat taken in.
17
Heat given out
Energy level of products is lower
than reactants so heat given out.
Example question
18
Mark scheme
19
Bond enthalpies
C7.2
20
Quick quiz
1. Reactions where the products are at a lower energy
than the reactants are endothermic (TRUE/FALSE)
2. Activation energy is the amount of energy given out
when a reaction takes place (TRUE/FALSE)
3. A reaction which is exothermic transfers heat energy
to the surroundings (TRUE/FALSE)
4. How can we tell if a reaction is exothermic or
endothermic?
5. Sketch the energy profile for an endothermic
reaction.
6. When methane (CH4) burns in oxygen (O2) bonds
between which atoms need to be broken?
21
Answers
1.
2.
3.
4.
5.
6.
22
Reactions where the products are
at a lower energy than the
FALSE
reactants are endothermic
(TRUE/FALSE)
Activation energy is the amount
of energy given out when a
FALSE
reaction takes place
(TRUE/FALSE)
A reaction which is exothermic
transfers heat energy to the
TRUE
surroundings (TRUE/FALSE)
How can we tell if a reaction is
exothermic or endothermic? Measure the
Sketch the energy profile for an
endothermic reaction.
When methane (CH4) burns in
oxygen (O2) bonds between
which atoms need to be broken?
temperature change
C—H bonds and O=O bonds
What do I need to know?
1. Recall that energy is needed to break
chemical bonds and energy is given out when
chemical bonds form
2. Identify which bonds are broken and which
are made when a chemical reaction takes place.
3. Use data on the energy needed to break
covalent bonds to estimate the overall energy
change for a reaction.
23
Activation energy revisited
• What is the activation energy of a reaction?
• The energy needed to start a reaction.
• BUT what is that energy used for and why
does the reaction need it if energy is given out
overall?
• The activation energy is used to break bonds
so that the reaction can take place.
24
Burning methane
Consider the example of burning methane gas.
CH4 + 2O2  CO2 + 2H2O
This reaction is highly exothermic, it is the
reaction that gives us the Bunsen flame.
However mixing air (oxygen) with methane is
not enough. I need to add energy (a flame).
25
What happens when the reaction gets
the activation energy?
H H H H
Energy in chemicals
C
O
Bond
Breaking
H
C
H
H
H
O
O
O
Bond
Forming
O
O
O
O
O
H
O
Progress of reaction
26
C
O
H
O
H
H
Using bond enthalpies
By using the energy that it takes to break/make
a particular bond we can work out the overall
enthalpy/energy change for the reaction.
Sum (bonds broken) – Sum (bonds made) =
Energy change
27
BIN MIX
Breaking bonds is ENDOTHERMIC energy is
TAKEN IN when bonds are broken
Making bonds is EXOTHERMIC energy is GIVEN
OUT when bonds are made.
28
Bond enthalpies
Bond
Bond enthalpy (kJ)
Bond
Bond enthalpy (kJ)
C—H
435
Cl—Cl
243
C—C
348
C—Cl
346
H—H
436
H—Cl
452
H—O
463
O=O
498
C=O
804
C=C
614
29
Can you work out the energy change
for this reaction?
30
The answer is -120 kJ
31
Example question part 1
32
Question part 2
33
Question part 3
34
Mark scheme
35
Challenge question
• The true value for the energy change is often
slightly different from the value calculated
using bond enthalpies.
• Why do you think this is?
36
Example question
The calculated value is 120 kJ
37
Mark scheme
38
Definitions
Write each of these phrases in your book with a
definition in your own words:
•
•
•
•
•
39
Exothermic reaction
Endothermic reaction
Activation energy
Catalyst
Bond energy/enthalpy
How did you do?
Exothermic reaction
A reaction which gives energy out to the surroundings.
Endothermic reaction
A reaction which takes in energy from the surroundings.
Activation energy
The energy required to start a reaction by breaking bonds in the
reactants
Catalyst
A substance that increases the rate of a reaction by providing an
alternative pathway with lower activation energy. It is not used up in
the process of the reaction
Bond energy/enthalpy
The energy required to break a certain type of bond. The negative
value is the energy given out when that bond is made.
40
Popular exam question
1. Explain why a reaction is either exothermic or
endothermic?
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
41
Popular exam question
1. Explain why a reaction is either exothermic or
endothermic?
42
① In a chemical reactions some bonds are broken and
some bonds are made.
② Breaking bonds takes in energy.
③ Making bonds gives out energy.
④ If the energy given out making bonds is higher than
the energy needed to break them the reaction is
exothermic.
⑤ If the energy needed to break bonds is higher than the
energy given out making them the reaction is
endothermic.
Chemical Equilibria
C7.3 Reversible Reactions &
Dynamic Equilibria
43
What do I need to know?
1. State that some chemical reactions are
reversible
2. Describe how reversible reactions reach a
state of equilibrium
3. Explain this using dynamic equilibrium
model.
44
Reversible or not reversible
Until now, we were careful to say that most
chemical reactions were not reversible –
They could not go back to the reactants
once the products are formed.
45
Example
In the case of the vast majority of chemical
reactions this is true, the reaction of
methane and oxygen for example:
It is almost impossible to return the carbon
dioxide and water to the original methane
and oxygen.
46
Reversible
• Some chemical reactions, however, will go
backwards and forwards depending on the
conditions.
• CoCl2·6H2O(s)  CoCl2(s) + 6H2O(l)
pink
blue
47
How do we write them down?
• This is the symbol for used for reversible
reactions.
CoCl2·6H2O(s)
48
CoCl2(s) + 6H2O(l)
What is equilibrium?
• Reversible reactions reach a balance point,
where the amount of reactants and the
amount of products formed remains
constant.
49
Dynamic Equilibrium.
• In
the forward and
backwards reactions continue at equal rates
so the concentrations of reactants and
products do not change.
• On a molecular scale there is
.
• On the macroscopic scale
. The system needs to be closed
– isolated from the outside world.
50
Example question
51
Mark scheme
52
Dynamic Equilibria
C7.3 Controlling equilibria
53
What do I need to know?
1. Recall that reversible reactions reach a state
of dynamic equilibrium.
2. Describe how dynamic equilibria can be
affected by adding or removing products and
reactants.
3. Explain the difference between a “strong” and
“weak” acid in terms of equilibria
54
Position of the equilibrium
• Equilibrium can “lie” to the left or right.
• This is “in favour of products” or “in favour of
reactants”
• Meaning that once equilibrium has been
reached there could be more products or
more reactants in the reaction vessel.
55
Le Chatelier’s principle
• If you remove product as it is made then
equilibrium will move to the right to counteract
the change
• If you add more reactant then equilibrium will
move to the right to counteract the change.
• In industry we recycle reactants back in and
remove product as it is made to push the
equilibrium in favour of more product.
56
Complete
When a system is at__________ to make more
product you can_________ product or add more
__________ for example by recycling them back
in.
To return to reactants you ______ product or
remove_________.
[equilibrium, add, reactant, remove, product]
57
Strong and weak acids
A strong acid is one which is FULLY IONISED in water. It
will have a high hydrogen ion concentration
A weak acid is one which is NOT fully ionised and is in
equilibrium. It has a low hydrogen ion concentration
Caution – weak and strong are not the same as concentration.
58
59
Mark scheme
60
Example question
61
Mark scheme
62
Practicing definitions
Write each of these phrases in your book with a
definition in your own words:
•
•
•
•
•
63
Reversible reaction
Dynamic equilibrium
Position of equilibrium
Strong acid
Weak acid
How did you do?
Reversible reaction
A reaction that can proceed in the forward or reverse directions (represented
by two arrows in an equation).
Dynamic equilibrium
The point where the rate of the forward reaction = rate of the reverse
reaction.
Position of equilibrium
The point where there is no further change in the concentration of either
reactants or products. The position can lie to the left (favouring reactants) or
right (favouring products).
Strong acid
An acid that is completely dissociated in water
Weak acid
An acid that is only partly dissociated in water because the reaction is in
dynamic equilibrium and favours the reactants (LHS).
64
Popular exam question
1. Ethanoic acid (CH3COOH) is a weak acid but
hydrochloric acid is a strong acid. Use ideas
about ion formation and dynamic equilibrium
to explain this difference.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------65
Popular exam question
Ethanoic acid (CH3COOH) is a weak acid but hydrochloric acid is a
strong acid. Use ideas about ion formation and dynamic
equilibrium to explain this difference.
① Hydrochloric acid ionises completely
② So hydrogen ion concentration is high
③ Ethanoic acid only partly dissociates because the reaction
is reversible
④ Equilibrium is mainly to the left
⑤ So hydrogen ion concentration is low.
66
Analysis
C7.4 – Analytical Procedures
67
What do I need to know?
1. Recall the difference between qualitative and
quantitative methods of analysis.
2. Describe how analysis must be carried out on a
sample that represents the bulk of the material
under test
3. Explain why we need standard procedures for the
collection, storage and preparation of samples for
analysis
68
Qualitative vs. Quantitative
• A qualitative test is
. It can give
vital information without needing to wait too
long for it.
• A
, for
example for a concentration in Moldm-3
tests include universal
indicator, silver nitrate for halide ions and
bromine water for unsaturation.
tests include titration,
chromatography and spectroscopy.
69
Which sample should I test?
• It is important that the sample
of the material under test.
• You may chose to
from a range of points to ensure that you have
.
• For example
? Are their
pockets of higher concentration/different
composition?
70
Chemical industry
• Analysis of samples is crucial to the chemical
industry to ensure the
of the chemicals
they are manufacturing. Some are analysed
numerous times a day or even within an hour.
• To maintain consistency it is essential that we use
to:
o collect the sample
o store the sample
o prepare the sample for analysis
o analyse the sample.
71
Chromatography
C7.4 – paper chromatography
72
Chromatography
73
Solvents
1. The mobile phase is the solvent – the part
that moves
2. In paper chromatography it is water or
ethanol
74
Paper/column
1. The stationary phase is the paper in paper
chromatography or the column in gas
chromatography.
2. In thin layer chromatography it is silica gel on
a glass plate
3. The stationary phase does not move.
75
How does the technique work?
In chromatography, substances are separated by
movement of a mobile phase through a stationary
phase.
Each component in a mixture will prefer either the
mobile phase OR the stationary phase.
The component will be in dynamic equilibrium
between the stationary phase and the mobile
phase.
76
Substance A
• This is substance A
• Substance A prefers the stationary phase and
doesn’t move far up the paper/column.
• The equilibrium lies in favour of the stationary
phase.
77
Substance B
• This is substance B
• Substance B prefers the mobile phase and moves
a long way up the paper/column
• The equilibrium lies in favour of the mobile phase
78
Using a reference
• In chromatography we can sometimes use a
known substance to measure other
substances against.
• This will travel a known distance compared to
the solvent and is known as a standard
reference.
79
Advantages of TLC
TLC has a number of advantages over paper
chromatography.
It is a more uniform surface chromotograms are
neater and easier to interpret
Solvent can be used which is useful if a
substance is insoluble in water.
80
Past Paper Questions
81
Past paper question
82
Mark scheme
83
Describing how chromatography
works – exam definition
• stationary phase is paper and mobile phase is
solvent / mobile phase moves up through
stationary phase (1)
• for each compound there is a dynamic
equilibrium between the two phases (1)
• how far each compound moves depends on its
distribution between the two phases (1)
84
Using an Rf value
• In order to be more precise we can use
measurements on the TLC plate to compare
the distance travelled by our substance (the
solute) with the distance travelled by the
solvent.
• The Rf value is constant for a particular
compound.
• The distance travelled however could be
different on different chromatograms.
• The Rf value is always less than 1.
85
Rf value
Distance travelled by spot
Rf value =
Distance travelled by solvent
86
Example question
This question relates to the chromatogram shown in the earlier question. Refer back…
87
Mark scheme
88
Example question
89
Mark scheme
90
Past paper question
91
92
93
94
Mark scheme
95
Gas-liquid chromatography
C7.4 GLC
96
What do I need to know?
1. recall in outline the procedure for separating
a mixture by gas chromatography (gc);
2. understand the term retention time as
applied to gc;
3. interpret print-outs from gc analyses.
97
Gas chromatography
• The mobile phase is an unreactive gas known
as the carrier gas this is usually nitrogen
• The stationary phase is held inside a long
column and is lots of pieces of inert solid
coated in high bp liquid.
• The column is coiled in an oven
• The sample to be analysed is injected into the
carrier gas stream at the start of the column.
98
GC
99
GC analysis
• Each component of the sample mixture has a
different affinity for the stationary phase
compared with the mobile phase
• Therefore each component travels through the
column in a different time.
• Compounds favouring the mobile phase (usually
more volatile) emerge first.
• A detector monitors the compounds coming out
of the column and a recorder plots the signal as a
chromatogram
100
GLC Chromatograph
101
Interpretation
• The time in the column is called the retention
time
• Retention times are characteristic so can
identify a compound
• Area under peak or relative heights can be
used to work out relative amounts of
substances
102
The key points – revise this!
• the mobile phase carries the sample (1)
• components are differently attracted to the
stationary and mobile phases (1)
• the components that are more strongly
attracted to the stationary phase move more
slowly (1)
• the amount of each component in the
stationary phase and in the mobile phase is
determined by a dynamic equilibrium (1)
103
Past paper question
104
105
106
107
Mark scheme
108
Titration
C7.4
109
What do I need to know?
1. Calculate the concentration of a given
volume of solution given the mass of solvent;
2. Calculate the mass of solute in a given
volume of solution with a specified
concentration;
3. Use the balanced equation and relative
formula-masses to interpret the results of a
titration;
110
Concentration
• We can measure the concentration of solution in
grams/litre. This is the same as g/dm3
• 1dm3 = 1000cm3
• If I want to make a solution of 17 g/dm3 how much will
I dissolve in 1dm3.
• 17 g
• If I want to make a solution of 17g/dm3 but I only want
to make 100cm3 of it how much will I dissolve?
• 1.7g
111
Making standard solutions
• For a solution of 17g/dm3
• First I will measure 17g of solid on an
electronic balance
112
Making standard solutions
• Now I must dissolve it in a known
1dm3 of water.
• I transfer it to a volumetric flask and fill
up with distilled water to about half
the flask.
• I then swirl to dissolve
• Top up with a dropping pipette so that
the meniscus is on the line.
113
How much to dissolve?
• Worked example:
• I want to make 250cm3 of a solution of
100g/dm3.
• How much solid do I transfer to my 250cm3
volumetric flask?
114
How much to dissolve?
Worked example:
I want to make 250cm3 of a solution of
100g/dm3.
1. Work out the ratio of 250cm3 to 1000cm3
250/1000 = 0.25
2. I therefore need 0.25 of 100g in 250cm3 which
is 0.25x100=25g
115
General rule
3
3
Volume(cm )xConcentration(g / dm )
Mass(g) =
1000
116
Practie - how much to dissolve?
• I want to make 250cm3 of a solution of
63.5g/dm3.
• How much solid do I transfer to my 250cm3
volumetric flask?
• 250/1000 x 63.5 = 15.9 g
117
Practice - how much to dissolve?
• I want to make 100cm3 of a solution of
63.5g/dm3.
• How much solid do I transfer to my 100cm3
volumetric flask?
• 100/1000 x 63.5 = 6.35 g
118
Concentration from mass and
volume
We need to rearrange this:
3
3
Volume(cm )xConcentration(g / dm )
Mass(g) =
1000
To give
Mass(g)x1000
Concentration(g / dm ) =
3
Volume(cm )
3
119
What is the concentration of?
1. 12g dissolved in 50cm3
2. 50g dissolved in 100cm3
3. 47g dissolved in 1000cm3
4. 200g dissolved in 250cm3
120
What is the concentration of?
1. 12g dissolved in 50cm3
= 1000/50 x 12
= 240g/dm3
2. 50g dissolved in 100cm3
=1000/100 x 50
= 500g/dm3
3. 47g dissolved in 1000cm3
= 1000/1000 x 47
= 47g/dm3
4. 200g dissolved in 250cm3
1000/250 x 200
= 800g/dm3
121
Solutions from stock solutions
Stock solution
• highest
concentration
• use to make
other solutions
122
Extract a
portion of
stock solution
• as calculated
Dilute with
distilled water
• Making a known
volume of a
lower
concentration
Making solutions from stock
solutions
If I have a solution containing 63g/dm3, how do I make
up 250cm3 of a solution of concentration 6.3g/dm3?
To make 1dm3 of 6.3g/dm3 I would need 100cm3
To make 250cm3 of 6.3g/dm3 I would therefore need
25cm3 and make it up to 250cm3 with distilled water
Final concentration(g/dm3 )
3
3
xSize
of
flask(cm
)=Amount
to
add(cm
)
3
Initial concentration(g/dm )
123
Working out masses
• We can use the useful relationship
Mass1 Mass2
=
Mr1
Mr2
• Where Mr is the molecular mass
• eg Mr of NaOH is (23 + 16 + 1) = 40
• This can help us to calculate an unknown mass
124
Titration calculations
• In a titration we have added a known amount
of one substance usually an acid (in the
burette) to a known amount of another
substance usually an alkali (in the conical
flask).
• The amount added allows us to determine the
concentration of the unknown.
125
Titration equipment
126
Using a table
• It can be helpful to sketch a table to keep track
of information you know…
Value
Volume (cm3)
Mass (g)
Concentration (g/dm3)
Molecular weight (Mr)
127
Acid
Alkali
128
Mark scheme
129
130
Mark scheme
131
Uncertainty
• Uncertainty is a quantification of the doubt
about the measurement result.
• In a titration the uncertainty is the range of
the results.
• If results are reliable then it will be within
0.2cm3
• NOTE THAT THIS IS RELIABLE NOT
NECESSARILY ACCURATE
132
133
Mark scheme
134
135
Mark scheme
136
C7.5 Green Chemistry
The Chemical Industry
137
What do I need to know?
1. Recall and use the terms 'bulk' (made on a
large scale) and 'fine' (made on a small scale)
in terms of the chemical industry with
examples;
2. Describe how new chemical products or
processes are the result of an extensive
programme of research and development;
3. Explain the need for strict regulations that
control chemical processes, storage and
transport.
138
Bulk processes
• A bulk process manufactures large
quantities of relatively simple
chemicals often used as feedstocks
(ingredients) for other processes.
• Examples include ammonia, sulfuric
acid, sodium hydroxide and
phosphoric acid.
• 40 million tonnes of H2SO4 are made
in the US every year.
139
Fine processes
• Fine processes manufacture
smaller quantities of much more
complex chemicals including
pharmaceuticals, dyes and
agrochemicals.
• Examples include drugs, food
additives and fragrances
• 35 thousand tonnes of
paracetamol are made in the US
every year.
140
Example question
141
Mark scheme
142
Research and Development
• All chemicals are produced following an
extensive period of research and
development.
• Chemicals made in the laboratory need to be
“scaled up” to be manufactured on the plant.
143
Research in the lab
144
Examples of making a process
viable
• Trying to find suitable conditions –
compromise between rate and equilibrium
• Trying to find a suitable catalyst – increases
rate and cost effective as not used up in the
process.
145
Catalysts
• Can you give a definition of a catalyst?
• A substance which speeds up the rate of a
chemical reaction by providing an alternative
reaction pathway.
• The catalyst is not used up in the process
• Catalysts can control the substance formed eg
Ziegler Natta catalysts.
146
Regulation of the chemical industry
• Governments have strict regulations to control
chemical processes
• Storage and transport of chemicals requires
licenses and strict protocol.
• Why?
• To protect people and the environment.
147
Example question
148
Mark scheme
149
Process development
150
Example question – part 1
151
Example question part - 2
152
Mark scheme
153
Example question – part 3
154
Mark scheme
155
Factors affecting the sustainability of a
process
energy inputs
and outputs
type of waste
and disposal
environmental
impact
health and
safety risks
atom economy
renewable
feedstock
156
Sustainability
social and
economic
benefits
Example question
157
Mark scheme
158
Atom economy
Mr of desired product
% atom economy =
x 100
Mr of reactants
159
Atom economy calculation
For example, what is the atom economy for making
hydrogen by reacting coal with steam?
Write the balanced equation:
C(s) + 2H2O(g) → CO2(g) + 2H2(g)
Write out the Mr values underneath:
C(s) + 2H2O(g) → CO2(g) + 2H2(g)
12
2 × 18
44
2×2
Total mass of reactants 12 + 36 = 48g
Mass of desired product (H2) = 4g
% atom economy = 4⁄48 × 100 = 8.3%
160
161
Example question
Example question – part 2
162
Mark scheme
163