Higher Chemistry Unit 1 Section 3

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Higher Chemistry Unit 1 Section 3 Reaction Rates
Learning objectives By the end of this section
all learners have a clear understanding of ?
Collision theory ? Catalysts ? Factors affecting
reaction rate ? How to follow a reactions
progress ? The energy associated with reactions.
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3.1 Collision Theory
Collision Theory states Substances only react
with each other if ? the particles collide with
each other with the correct orientation. ?
they have enough energy to make successful
collisions.
http//www.absorblearning.com/chemistry/demo/units
/LR1501.htmlCollisiontheory
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The greater the concentration of acid ions, the
more collisions. The more collisions, the faster
the reaction.
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3.1 Collision Theory
Concentration and collisions
http//www.sciencepages.co.uk/keystage4/GCSEChemis
try/m3ratesofreaction.phprate5 http//courses.sc
holar.hw.ac.uk/vle/scholar/session.controller?acti
onviewContentcontentGUID95cfaecc-a4c8-0706-50ef
-c98bdd386030
Collect discussion poster 1 and follow the
instructions.
Absorb chemistry animation
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3.1 Collision Theory
Particle size and collisions
http//www.sciencepages.co.uk/keystage4/GCSEChemis
try/m3ratesofreaction.phprate7 http//courses.s
cholar.hw.ac.uk/vle/scholar/session.controller?act
ionviewContentcontentGUIDb05fc614-3b3e-1a98-6df
3-3cb0d1770e71
Collect discussion poster 2 and follow the
instructions.
Absorb chemistry animation
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Collisions only take place on the surface of the
particle. Several pieces have a larger surface
area than one piece (of same mass). The
larger the surface area, the more collisions, the
faster the reaction.
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3.1 Collision Theory
Temperature and collisions
http//www.sciencepages.co.uk/keystage4/GC
SEChemistry/m3ratesofreaction.pprate6 http//cou
rses.scholar.hw.ac.uk/vle/scholar/session.controll
er?actionviewContentcontentGUIDfcbbb4e1-0053-63
b9-cdd5-ad62d851bff9
Collect discussion poster 3 and follow the
instructions.
Absorb chemistry animation
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Increasing temperature causes reactant particles
to move faster. This means they collide more
often and collide with greater kinetic energy.
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3.2 Catalysts at work
Catalytic converters
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3.2 Catalysts at work
Catalytic converters are fitted to the exhausts
of all cars to get rid of polluting gases
produced by burning petrol in car engines.
O2
Unreacted hydrocarbons
CO2
CO
H2O
NO2
N2
NO
polluting gases
non polluting gases
Converter animation
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3.2 Catalysts at work
Catalyst Mechanism
http//courses.scholar.hw.ac.uk/vle/scholar/sessio
n.controller?actionviewContentcontentGUID2113a8
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3.2 Catalysts at work
Catalytic Mechanism
Product molecules leaving surface
How do catalysts work?
Reactants adsorbed on surface of catalyst
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Catalytic converters Convert the harmful gases
into harmless ones
NOx ( nitrogen oxides) O2
N2 O2
CO ½ O2
CO2
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Molecules of the reactants form bonds with the
catalyst (adsorption). This weakens the bonds
within the reactant molecules, making them more
reactive towards each other.
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3.2 Catalysts at work
Catalyst poisoning
http//courses.scholar.hw.ac.uk/vle/scholar/sessio
n.controller?actionviewContentcontentGUID2113a8
54-de91-6647-9202-c15250817768
Catalytic converters are fitted only to exhausts
that run on unleaded petrol. This is because lead
atoms stick to the surface of the catalyst and
prevent adsorption. This is called poisoning of
the catalyst.
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3.2 Catalysts at work
Heterogeneous catalysis
A Heterogeneous catalyst is a catalyst in a
different state from the reactants. The catalysts
are often solid and they work by
adsorption. http//courses.scholar.hw.ac.uk/vle/sc
holar/session.controller?actionviewContentconten
tGUID41ae10b5-d760-63b7-c711-fc4a9f67a57d
Youtube platinum ammonia
Regenerating the Catalyst. In catalytic
cracking, a deposit of carbon appears on the
surface of the catalyst which reduces its
efficiency. This is removed by heating it and
blowing air through it - regenerating the
catalyst.
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3.2 Catalysts at work
Homogeneous Catalysis
A Homogeneous catalyst is a catalyst in the same
state as the reactants. http//courses.scholar.hw.
ac.uk/vle/scholar/session.controller?actionviewCo
ntentcontentGUIDd8b3c014-aced-081a-cd63-e06d5087
c8a0
Video link to reaction
Hydrogen peroxide Rochelle salt
No reaction
Fast reaction, gas given off
Reaction complete, turns pink again
heat
Cobalt chloride
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3.2 Catalysts at work
Enzymes
Enzymes are biological catalysts. They are
complex protein molecules which speed up specific
chemical changes by homogeneous
catalysis. http//courses.scholar.hw.ac.uk/vle/sch
olar/session.controller?actionviewContentcontent
GUID1df381b0-2fc8-257e-01f5-759f835e4b08
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3.3 Concentration and Rate
PPA 1 Effect of concentration changes on
reaction rate
In acidic conditions, hydrogen peroxide reacts
with potassium iodide to form water and
iodine H2O2(aq) 2H(aq) 2I-(aq) ?
2H2O(l) I2(aq) To monitor the reaction
thiosulphate ions are added so that as the iodine
molecules are produced they immediately react
with the thiosulphate ions and are converted back
to iodide ions I2(aq) 2S2O32-(aq) ? 2I-(aq
) S4O62-(aq)
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3.3 Concentration and Rate
During this period the reaction remains
colourless. But once the thiosulphate ions have
been used up, a blue/black colour suddenly
appears because the iodine molecules now get the
chance to react with the starch. Since the
quantity of thiosulphate ions initially present
will be the same in each experiment, the
appearance of the blue/black colour will always
represent the same extent of reaction.
Rate 1/t
t is the time taken for the black colour to
appear.
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3.3 Concentration and Rate
POINTS FOR DISCUSSION ? Aim of the experiment ?
What variables need to be controlled? ? What
measurements are to be made? ? How is the
concentration of potassium iodide to be
changed? ? Errors Safety
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Reaction rate
Concentration
Reaction rate versus concentration of reactant
always has this pattern
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3.4 Temperature and Rate
PPA 2 Effect of temperature changes on
reaction rate
5(COOH)2(aq) 6H(aq) 2MnO42-(aq)
2Mn2(aq) 10CO2(g) 8H2O(l) In the
above reaction the reaction mixture is initially
purple due to the permanganate ions, at the end
of the reaction the purple colour disappears.
This allows the course of the reaction to be
followed to completion.
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3.4 Temperature and Rate
If the quantity of permanganate ions initially
present is the same for each temperature, then
the point at which the purple colour disappears
will always represent the same extent of
reaction. If t is the time it takes for the
colour change to occur then the rate can be
calculated by
Rate 1/t
t is the time taken for the purple colour to
disappear.
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3.4 Temperature and Rate
POINTS FOR DISCUSSION ? AIM of experiment ?
Factors to be kept constant ? Measurements to
be made ? What temperatures should be used? ?
Errors Safety
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Reaction rate
Temperature
Reaction rate versus Temperature
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0.03
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3.5 Reaction Progress
Monitoring the progress of a reaction requires
the measurement of one reactant or one product
over the time the reaction is proceeding.
CaCO3 HCl
CaCl2 H2O CO2
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faster
The steeper the graph the _______ the rate of
reaction.
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3.5 Reaction Progress
Calculating the Rate The rate of reaction is the
change in quantity of a reactant or product per
unit of time.

• Change in concentration
• Change in volume
• Change in mass

Can be in minutes or seconds
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3.5 Reaction Progress
The units that rate is measured in depends on
what type of quantity is involved.
Collect the cut out Rates from progress graphs
and calculate the change in volume and average
rate.
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3.5 Reaction Progress
Comparing Reaction Progress Collect a copy of the
task sheets and complete them in your group.
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If the reaction is speeded up the the reactants
are used up __________
quicker
faster
The products are formed __________
Remember the shapes of the graph!
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Reaction rate
Temperature
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Reaction rate
Concentration
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Reaction rate
Time
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Reaction rate
Temp (enzyme)
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3.6 Activation Energy
Potential energy
Reaction Progress
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3.6 Activation Energy
Activated complex
Potential energy (kJ mol-1)
EA activation energy
EA
reactant
molecules
Product molecules
Reaction Progress
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3.6 Activation Energy
un-catalysed
Potential energy (kJ mol-1)
reactant
molecules
Product molecules
Reaction Progress
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3.7 Energy Distribution
The temperature of a substance is a measure of
the average kinetic energy of its particles
No. of molecules with a given kinetic energy
B
A
C
Kinetic energy
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3.7 Energy Distribution
Effect of temperature on energy distribution
No. of molecules with a given kinetic energy
Kinetic energy
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With some chemical reactions, light can be used
to increase the number of particles with an
energy greater than the activation energy.
e.g. photosynthesis Hydrogen and chlorine
reacting explosively in light H2 Cl2
2HCl
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Effect of temperature on energy distribution
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