Rates of Reactions
1
Temperature
Concentration
Particle size
Factors affecting rate of reactions
Pressure
Catalyst
Synthesizing factors affecting the rate of
reaction
-Design experiments to investigate factors affecting the rate of
reaction.
-Give examples of reactions that are affected by size of reactant,
concentration, temperature and catalyst.
-Explain how each factor affects the rate of reaction.
-Describe how factors affecting the rate of reaction are applied in
daily life and in industrial processes.
Describe the effect of concentration,
temperature, particle size, pressure
and catalyst on the rate of reaction .
Before learning this topic, you must be
aware of the kinetic theory and collision
theory
Factors affecting rate of reactions
What is the rate of reaction ?
It’s the amount of reactants consumed or products
formed per unit time
How do chemical reaction happen?
For a reaction to occur the particles must collide.
The colliding particles must have a minimum
energy called " Activation Energy "
Factors affecting rate of reactions
As particles collide they will be chemically
bonded, and as a result products are formed .
Factors affecting rate of reactions
Different chemical reactions could occur at significantly
different speeds .
Fireworks occur in a fraction of a second.
Factors affecting rate of reactions
Iron rusting would take weeks
Forming of petroleum could take thousands of years.
Factors affecting rate of reactions
So, What controls the reaction's speed ???
That's exactly what we are going to find
out in the next set of slides
1- Amount of reactants (concentration) .
2- Temperature .
3- Particles size .
4- The presence of a catalyst .
5- Pressure (for the reactions of gases) .
Effect of concentration on reaction rates
Does reaction speed depend on the amount of reactants?
Sure it does !!
Effect of concentration on reaction rates
If the concentration of reactants is high then there will
be more particles around and therefore a higher
probability of pumping into each other's (colliding)
For reactants to form; collision must be at a particular
orientation
Only successful collisions lead
to yielding products
Effect of concentration on reaction rates
Greater numbers
Higher
of reacting
concentration particles
Faster
Reaction Smaller distances
among particles
Rate
successful
collisions High
probability of
collisions
Effect of concentration on reaction rates
Increasing concentration increases the rate by
increasing the number of collisions
Effect of concentration on reaction rates
Consider the following reaction :
2HCl + Na2S2O3 2NaCl + SO2 +H2O + S
The formation of a pale yellow precipitate of
sulphur can be used to monitor the progress of
the reaction
Effect of concentration on reaction rates
Experiment 1 :-
- Draw an ( X ) on a white paper and
place a conical flask on top of it .
- Add HCl to Na2S2O3 in the flask .
- Record the time taken for the ( X ) to
disappear.
- Repeat the same process with different concentrations of Na2S2O3 .
- Inverse the time to get the rate and plot the rate versus
concentration .
Effect of concentration on reaction rates
Conclusion ?
RATE
=
1
Time Rate is directly
proportional to
concentration
CONCENTRATION
Effect of concentration on reaction rates
Conclusion
The higher the concentrations of
reactants, the faster the reaction
would proceed
Effect of temperature on reaction rates
How could reactions be affected by heat?
- Particles in a reaction mixture have a certain amount of kinetic energy
So , What happens if the reaction temperature was raised?
Effect of temperature on reaction rates
Kinetic energy of particles will be increased
which means they would move much faster
Faster particles movement enhances the
probability of successful collisions and
therefore increasing the reaction rate .
Effect of temperature on reaction rates
Experiment 2 :-
- Conducting the procedure of experiment 1 at different
temperatures, but at constant concentrations of
hydrochloric acid and sodium thiosulfate :
Effect of temperature on reaction rates
The rates of sulfur precipitation at different temperatures are
compared:
Effect of temperature on reaction rates
Conclusion ?
Rate increases
with increasing
temperature
TEMPERATURE
Effect of temperature on reaction rates
Conclusion
The higher
temperature, the
faster the reaction rate
Effect of temperature on reaction rates
Real life applications for the effect of heat
in reactions rate :
Why does the refrigerator keep the food fresh?
Food is always exposed to biological reactions
performed by microorganisms
At the cold fridge temperature the rate of food
decomposition is slowed down because of the
decrease in biological reaction rates due to the
drop of temperature
Therefore the food would last longer
Effect of particle size on reaction rates
Lumpy
Solid particles
Finely crushed
Effect of particle size on reaction rates
How could particle size affect the reaction rate ?
Finely crushed particles are found to be more reactive
than lumpy aggregates .
How come?!
Powdered particles have a relatively
greater surface area than lumpy solid
Effect of particle size on reaction rates
How does the surface area of a particle affect
it's reactivity?
Fine particles with large surface area
would have greater accessibility to
reagents as compared with lumpy solid
Effect of particle size on reaction rates
Experiment 3 :-
As the reaction below is performed :
2HCl + CaCO3 CaCl2 + H2O + CO2
The rate of release of CO2 gas can be used to
monitor the progress of the reaction
Effect of particle size on reaction rates
Add the HCl to a large chips of CaCO3 in the flask
As CO2 is released the mass of the flask drops
Effect of particle size on reaction rates
-Record the time taken for the release of CO2 until
no further mass loss .
-Repeat the reaction using the same mass of
powdered CaCO3 under the same reaction
conditions ( the same volume and concentration of
HCl and the same temperature ).
- Compare the time taken by both reactions to be
completed
Effect of particle size on reaction rates
Result:
Finely divided particles react faster than large particles
Effect of particle size on reaction rates
Conclusion
Particles of smaller size react much
more rapidly compared with
lumpy aggregates
Effect of catalyst on reaction rates
Consider the decomposition of hydrogen peroxide :
2H2O2 2H2O + O2
This reaction occur spontaneously however it may take
years to be completed
Effect of catalyst on reaction rates
How could this reaction be accelerated?
A catalyst will do the job
The reaction above could be accelerated by using
manganese dioxide MnO2 , it will make the reaction
occur within few seconds rather than several years
MnO2 is The Catalyst of the reaction
So , A catalyst is a substance that would speed up the reaction rate
without being consumed during the process
Effect of catalyst on reaction rates
How does the catalyst work?
Catalyst serve two functions:
Firstly : it provides a surface upon which reactants are
oriented appropriately (no random collisions)
Secondly : it lowers the activation energy of the reaction
Effect of catalyst on reaction rates
Enzymes are biological catalysts
All living organisms require catalysts for biological reactions,
these catalyst are called "Enzymes"
Enzymes are portions
Living organisms contain tens of thousands of different enzymes
that catalyze all biochemical activities of the cells including
metabolism and growth
Effect of catalyst on reaction rates
Enzymes make the reactions possible
under mild physiological conditions
which would otherwise require
vigorous procedures if conducted in a
lab
Effect of catalyst on reaction rates
How do enzymes do it?
Enzymes perform catalysis by bringing
reagents onto it’s surface in a way
similar to lock and key, so the
reaction is not left for probability of
collision
Effect of pressure on reaction rates
Increasing the pressure on a reaction involving
reacting gases increases the rate of reaction.
Changing the pressure on a reaction which involves
only solids or liquids has no effect on the rate.
Effect of pressure on reaction rates
Pressure only affects gaseous reactions
As reacting gases are compressed their
volume decreases and the number of
particles per unit volume will be increased
Probability of effective collision will
increase and therefore reaction rate will as
a result be faster
Effect of pressure on reaction rates
An example:
In the manufacture of ammonia by the Haber Process, the
rate of reaction between the hydrogen and the nitrogen is
increased by the use of very high pressures.
In fact, the main reason for using high pressures is to
improve the yield of ammonia.
Summary
So far we have learned the following concepts :
1- Reaction rate is influenced by temperature,
concentrations, particles size , presence of a catalyst and
pressure.
2- The reaction rate is directly proportional to
concentration, temperature and pressure .
3- However, it’s inversely proportional to the particles size .
4-The reaction rate is also enhanced by presence of
a catalyst .
Q1
Vinegar reacts with baking soda if :
A - The vinegar is cold .
B - The baking soda is finely crushed .
C - Vinegar is heated .
D – Both ( B and C ) .
Answer:
Q2
• Catalysts speed up reactions by :
• A – Increasing activation energy of reactants .
• B – Decreasing activation energy of reactants .
• C – Reducing the reactions pressure .
• D – Increasing the temperature of reaction .
• Answer:
Q3
• Hydrogen peroxide decomposition will be
enhanced by :
• A – A catalyst .
• B – Addition of an acid .
• C – Adding ice .
• D – Adding sodium chloride .
• Answer:
Q4
• Heating a chemical reaction will result in :
• A – Increase kinetic energy of reactants .
• B – Increase the number of successful collisions .
• C – Increase the reaction rate .
• D – All of above .
• Answer:
Q5
• If gases are compressed :
• A – Distance among them will increase .
• B – Their collision will be reduced .
• C – Number of gas particles per unit volume will
be greater .
• D – They will react at a lower rate .
• Answer:
Q6
• The function of enzymes is :
• A – To increase the activation energy of reactants
species .
• B – To act as a biological catalyst .
• C – To neutralize acids.
• D – Noun of the above .
• Answer:
Q7
• A reaction rate is defined as :
• A – Amount of reactants consumed per unit time .
• B – Amount of products formed per unit time.
• C – Amount of activation energy of the reaction .
• D – Both ( A and B ) .
• Answer:
Q8
• Speed of the reaction could be slowed down by :
• A – Heat.
• B – Increase of reactant amounts .
• C – Use of a catalyst .
• D – Noun of the above .
• Answer:
Q9
• Small particles would :
• A – Have a small surface area .
• B – Have a large surface area .
• C – React faster than large particles .
• D – Will need small activation energy .
• Answer:
Q10
• In a chemical reaction a catalyst will :
• A – Be consumed .
• B – Slow down the reaction .
• C – Increase the reactants concentrations .
• D – Noun of the above .
• Answer:
Q11
• A reaction speed will be accelerated by :
• A – Decreasing temperature .
• B – Increasing the amount of reactants .
• C – Decreasing the pressure .
• D – Increasing it’s activation energy .
• Answer:
