Rates of reaction

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Unit 1

• Rates of reaction

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Contents

• Fast and slow reactions
• Measuring the rate
• Experiments
• Factors influencing the rates of reaction
• Collision theory
• Testing and assessment

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Reactions at different rates

• There are chemical reactions that occur very
  slowly and others that occur very quickly.
• Rust formation
  Fireworks
• Exercise 1
  

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Control rates

• Being able to control the speed of chemical
  reactions is important both in everyday life (for
  example in cooking) and when making new materials
  on an industrial scale.

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Rates and time

• If a reaction takes longer, its rate decreases.
• Rate of reaction ? 1/time

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Measuring rates

• The only way we can find out about the rate of a
  particular reaction is to carry out an
  experiment.A balanced equation for the reaction
  tells us nothing about its rate.
• We need to have some ways of measuring
• either the rate at which reactants are used up
  in a reaction
• or the rate at which products are formed in a
  reaction.
• The practical methods which can be used are
  various

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Mass loss method

• The rate of a reaction that produces a gas can be
  measured by following the mass loss as the gas
  escapes from the reaction flask.
• For example we can measure the loss of mass in
  this reaction
• CaCO3(s) 2 HCl(aq) ?CaCl2(aq) H2O(l)
  CO2(g)
• A gas is produced when calcium carbonate reacts
  with hydrochloric acid.

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other methods

• Measuring
• colour change
• volume of gas evolved
• formation of precipitate

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Experiments

• Tasks
• First phase
• You will work in groups.
• Each group will carry out an experiment.
• In each group there will be workers, observers
  and one speaker.
• The workers have to manage the experiment as
  described in the guide.
• The observers have to observe and write down the
  experimental data.
• All the components contribute to complete the
  exercises and the guided report.
• Second phase
• The speaker of each group reads the completed
  guided report to the whole class.
• The other students, while listening to the
  speaker, complete the guided reports.
• After that there will be few minutes for the
  checking.

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Equipment

Glass tubing
Test tube stand
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Safety

• Wear chemical splash goggles for the acidic
  solutions.
• Normal laboratory care is needed.

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Experiments

• Measuring rate
• Nature of reactants
• Concentration
• Surface area
• Temperature
• Catalysts

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And now..

• Everybody in the chemistry lab

have fun!!!!!
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Back from the lab..

• What have we learned through the experiments?

Summarizing...
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Factors affecting rate of reaction
Stirring
Particle size
Catalysts
Temperature
Nature of reactants
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Results

• The speed (rate) of chemical reactions
• increases
• if the temperature increases
• if the reactants are in ionic form
• if the concentration of dissolved reactants or
  the pressure of gases increases
• if solid reactants are in smaller pieces (greater
  surface area)
• if a catalyst is used.
• Exercise 2

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Theory of collisions

• In order to react molecules and atoms must touch
  each other.
• In a gas or liquid phase, this generally happens
  when the reactant molecules collide.
• They must hit each other hard enough to react.
• If they collide and are in the right position,
  they can form a chemical bond.

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Kineastetic exercise

• Two students are bonded with two ribbons as in
  an O2 molecule.
• Two students are bonded with three ribbons as in
  an N2 molecule .
• Each pair must meet the other to change partner
  breaking the old bonds and forming new ones (NO
  molecule)

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Kineastetic exercise

• Breaking the old bonds requires energy
• 180 KJ / mol
• Forming the new bonds releases energy
• -112 KJ/ mol
• The energy ?H is taken in 68 KJ/ mol
• The reaction is endothermic.

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example 1 ..
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.. its energetic profile
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example 2 ..
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its energetic profile

• The exothermic reaction is represented by the
  following graph.

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Example 3.water formation
Reactants
Energy
Products
Reaction coordinate
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Theory of collisions and factors affecting a
reaction

• Collision frequency more collisions mean more
  chances to form bonds.
• Collision energy higher energy makes bonds
  break the chance of new bond forming increases.
• Collision geometry molecules must face the right
  direction.
• Exercise 4

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Concentration

• If there is more concentration of a substance in
  a system the chance that molecules will collide
  is higher and the rate of the reaction speeds up.
• If there is less concentration of reactants
  there will be fewer collisions and the reaction
  will probably happen at a slower speed.

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Pressure

• In reactions with gases , the number of
  particles is increased by pressing them.
• Then increasing the pressure, the rate of
  reaction increases.

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Surface area

• Smaller particles ? faster reaction.
• Smallest particles possible are molecules or
  ions.
• Dissolving reactants speed up reactions.
• When two solids react with each other the
  reaction is slow.
• Molecules can only collide at the surface.
• Smaller particles ?bigger surface area.

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Effects of stirring

• Stirring the reactants is an important rate
  factor
• if the reacting mixture is not stirred the
  reactant concentration in solution becomes much
  less near the solid as it tends to settle out.

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Effects of temperature

• Chemical reactions can only occur when reacting
  particles collide with each other and with
  sufficient energy.
• Activation energy is the minimum amount of energy
  particles must have to react.
• Increasing the temperature increases the speed of
  the reacting particles so that they collide more
  frequently and more energetically.This increases
  the rate of reaction.

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Activation energy

• The minimum amount of energy needed for
  initiating a chemical reaction is the activation
  energy (Ea).
• This is different according to the type of bonds
  of the reactants.
• Colliding reactant molecules possess kinetic
  energy gt the activation energy or Ea.

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Activation energy - example
Activated Complex or Transition State
Activation Energy
Reactants
Energy
Products
Reaction coordinate
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Activated Complex or Transition State
Reactants
Energy
Products
Reaction coordinate
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Activated complex
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Activated complex

• The species temporarily formed by the reactant
  molecules are called activated complex.The
  concentration of the activated complex is
  extremely small.

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Activation energy decrease

• Suppose you have a mountain between two valleys
  so that the only way for people to get from one
  valley to the other is over the mountain.
• Now suppose a tunnel is cut through the mountain.
  Many more people will now manage to get from one
  valley to the other by this easier route.
• You could say that the tunnel route has a lower
  activation energy than going over the mountain.

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Catalysts

• Substances that
• speed up a reaction without being used up.
• Speed up a reaction by giving the reaction a new
  path.
• The new path has a lower
• activation energy and more
• molecules have this energy.
• The reaction goes faster.
• Inhibitors substances that block
  catalysts.Exercise 5 (only pc)

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Use of catalysts

• Increasing the rates of chemical reactions is
  important in industry because it helps to reduce
  costs.
• Catalysts increase the rate of a reaction by
  helping break chemical bonds in reactant
  molecules.

Catalyst silencer of a car
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Concentration
Reactants
Energy
Products
Reaction coordinate
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Enzymes

• Enzymes are catalysts. Most are proteins.
  Enzymes bind temporarily to one or more of the
  reactants of the reaction they catalyze. In doing
  so, they lower the amount of activation energy
  needed and thus speed up the reaction.
• Examples
• Catalase. It catalyzes the decomposition of
  hydrogen peroxide into water and oxygen. 2H2O2 ?
  2H2O O2
• One molecule of catalase can break 40 million
  molecules of hydrogen peroxide each second.

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Example

• Enzymes are used to make some soft-centred
  chocolates.
• An enzyme called invertase will convert a sugar
  called sucrose into smaller sugar molecules
  called glucose and fructose.
• The breaking down of some sucrose
  moleculescauses the chocolate to become softer
  and sweeter (fructose is particularly sweet).

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Revise the whole topic and don't forget the
glossary !

Glossary
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Finally ....testing...