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Chemical kinetics

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Chemical kinetics Going further; Mr Arrhenius kinetics 6.1 Rates of Reaction (2h) Rate of reaction can be defined as the decrease in the concentration of reactants ... – PowerPoint PPT presentation

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Title: Chemical kinetics


1
  • Chemical kinetics

2
kinetics
  • 6.1 Rates of Reaction (2h)
  • Rate of reaction can be defined as the decrease
    in the concentration of reactants per unit time
    or the increase in the concentration of product
    per unit time.
  • Consider the reaction
  • 2H2(g) 2NO(g) ? 2H2O(g) N2 (g)
  • The rate of disappearance of H2 is the same as
    the rate
  • of disappearance of NO. It is also the same as
    the rate
  • of appearance of H2 O and HALF the rate of
  • appearance of N2 from the stoichiometry of the
  • reaction in the balanced equation.

3
INTRODUCTION TO REACTION RATES
  • Work in your table groups to write down an
    example of
  • A very, very fast chemical reaction
  • A chemical reaction that takes about 5 10
    minutes to occur.
  • A chemical reaction that takes several years to
    happen
  • A chemical reaction that takes thousands or
    millions of years to occur. Cooking/digesting
    food are both examples of medium rate chemical
  • An example of a chemical reaction, perhaps
    related to food, that you would want to
  • slow down
  • speed up
  • Can you write down a link between how fast a
    chemical reaction is and the time it takes for
    that reaction to occur?

4
Rate is..
  • For the disappearance of a reactant
  • Change in some property/ time unit
  • The value is given a sign
  • For the appearance of a product
  • The value is given a sign

5
  • At Grade 10 level you learnt that chemical
    reactions occur when the particles in the
    reacting substances collide with each other with
    sufficient energy to produce new product
    chemicals.
  • Why do gases, liquids and solutions tend to react
    faster than solids at the same temperature?
  • Controlling reaction rates is an economic
    necessity. We need to produce chemicals as
    cheaply as possible, which usually means as
    quickly as possible.
  • Working in your table groups, try to list at
    least 5 different factors which affect how fast a
    chemical reaction occurs.

6
Collision Theory
  • As the reactants heat up, the particles move
    faster and so collide more often and with more
    energy. When they react, they must have enough
    energy to overcome the activation energy needed
    by the reaction. A rough rule of thumb which
    applies to many reactions (think about cooking!)
    is that a temperature rise of 10K approximately
    doubles the rate of the reaction. Mathematically,
    this may take some explaining.
  • Reaction Rates and Industrial Processes
  • Work in your table group and choose one of the
    following industrial processes. Produce an OHT /
    PowerPoint to help you tell the rest of the class
    the key facts about your chosen process.
  • 1. Haber Process
  • 2. Contact Process
  • 3. Manufacture of Margarine
  • 4. Cat cracking
  • 5. Biological washing powder
  • 6. Enzyme technology

7
Collision Theory states that for a reaction to
take place particles must collide with
sufficient energy to overcome the Activation
energy for that particular reaction
8
METHODS USED IN KINETICS TO FOLLOW A REACTION
  • Method 1 Loss in mass as the reaction
    progresses. e.g in a reaction which produces CO2
  • Method 2 Volume of gas evolved as a reaction
    progresses. e.g in a gas syringe or by
    displacement of water in a graduated cylinder.
  • Method 3 Using a pH probe if there is a change
    in acidity as the reaction progresses.

9
Method 2 Using a gas syringe
10
More methods for following the course of a
reaction
  • Method 4 Use a conductivity stick if there is a
    change in the number of ions in solution.
  • Method 5 Use a colorimeter if there is a change
    in colour of one of the species.
  • There are several more methods, but I am sure you
    now understand that we can use any suitable
    method which measures changes as the reaction
    progresses.

11
What does a real reaction look like ?
When is the reaction finished?
What is happening here
What is happening to the rate of the reaction
with time? How can we find the rate at any instant
12
What is going on during the reaction?
  • At the start The reaction is fast because here
    we have the highest concentration of reactants,
    therefore the greatest number of successful
    collisions.
  • As reaction progresses the rate declines as
    there are fewer particles to collide.
  • At the end the slope is zero (flat) no more
    particles to react

13
When is the reaction HALF completed?
  • The rate at any instant is the gradient (slope)
    at any instant.
  • Clearly, if the rate changes all the time, the
    time for ½ the reaction is not ½ the time it
    takes for the complete reaction.

Lets say 48cm3 of CO2 was given off on
completion. Half reaction when 24cm3, Read off
graph time to give 24cm3
14
Factors affecting rate
  • Concentration in terms of Collision Theory
  • Particle size of solids
  • clearly if the same of solid is used but the
    particles are smaller (bigger surface area) will
    provide more places for reaction
  • Temperature
  • clearly particles will have a higher collision
    energy at higher temp. and more successful
    collisions.

BUT
15
Fact 1 An increase in the concentration of a
solution, or the pressure of a gas, results in an
increase in the reaction rate
Explanation If the concentration of a solution,
or the pressure of a gas, is increased, then
there are more particles in a given volume.
Therefore there will be more collisions in a set
amount of time and the probability of more
successful collisions becomes higher. As a
result the reaction rate will increase.
16
Fact 2 An increase in surface area of a solid
results in a increase in the rate of a reaction.
Explanation If the surface area of a solid is
increased, there are more particles exposed to
the other reactant. Therefore there will be more
successful collisions in a set amount of time and
the rate increases
17
Temperature effect and collisions
  • The increased Kinetic Energy of collisions at
    higher temperatures only accounts for a small
    proportion of the increased rate.
  • On average a 10oC rise doubles the rate of a
    reaction.
  • The major reason for this is the proportion of
    particles, at a higher temperature, which now
    have an energy greater than the Activation energy.

18
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19
Fact 3 An increase in temperature results in an
increase in reaction rate.
Explanation If the temperature is increased, the
average kinetic energy of the particles
increases. There is therefore a greater chance
of more collisions having an energy greater than
the activation energy and reaction rate
increases. Also (less importantly) because
particles are moving faster, there will be more
collisions in a given time, therby increasing the
rate
20
What the syllabus says.
  • 6.2.5 State and explain qualitatively the
    Maxwell-Boltzmann energy distribution curve for a
    fixed amount of gas at different temperatures and
    its consequences for changes in reaction rate.
  • Students should be able to explain why the area
    under the curve is constant and does not change
    with temperature.

21
Catalyts (Enzymes)
  • Grade 10 A catalyst is a substance that
    increases the rate of a reaction but is not used
    up by the reaction.
  • SL and HL -level Catalysts affect the rate of a
    reaction by providing an alternative reaction
    pathway with a lower activation energy

22
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24
Fact 4 Addition of a catalyst can lead to an
increase in reaction rate
Explanation A catalyst acts by enabling a
reaction to proceed via a route of lower
activation energy. There will therefore be more
collisions of sufficient energy to cause a
reaction to occur, and rate will increase
25
SUMMARY
26
Orders, rate equations and the rate constant
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29
Mechanisms Train Station Analogy
  • People can
  • Exit a train at 20 per second
  • Ascend the escalator at 10 per second
  • Pass the ticked barrier at 50 per second
  • People will only pass the ticket barrier at only
    10 per second because of the limiting escalator.
  • This is the rate determining step which is
    indicated by the experimentally determined rate
    equation

30
Equation Example
  • The reaction
  • NO2(g) CO(g) ? NO(g) CO2(g)
  • occurs in two steps
  • NO2 NO2 ? NO NO3 (slow step)
  • NO3 CO ? NO2 CO2 (fast step)
  • The slow step is the rate determining step and
    the rate equation should be
  • Rate kNO22

31
Orders and Reaction mechanism
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33
Rate k A
Rate k (CH3)3CCl
34
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35
Going further Mr Arrhenius
36
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