6.2
Collision Theory
In science a theory explains a general pattern or principle. Theories draw upon established
principles and knowledge with the aim of extending them in a logical and consistent way that
enables scientists to make useful predictions. All scientific theories are tentative and subject to
being tested and modified. As theories become more developed, they grow into more organized
bodies of knowledge that enable us to understand and predict a wider range of phenomena.
Scientific theories fall into two categories:
1. Theories that have been shown to be incorrect and are now obsolete, usually because they
are were consistent with new observations;
2. All other theories
Theories cannot be proven to be absolutely correct. There is always the possibility that further
observations will disprove it. A theory that cannot be refuted or falsified is not a scientific theory.
Collision theory is a well established theory used to explain the general conditions that must be met
in order for a chemical reaction to occur. Collision theory states that the following three conditions
generally must be met in order for the successful formation of products in a chemical reaction.
1.
2.
3.
There must be sufficient frequency of collisions between the reactants.
The reactants must collide with the proper orientation or geometry so their reactive parts
come together.
The reactants must have energy that is equal to or greater the activation energy. The
activation energy (Ea) is defined as the minimum energy required in order for the reactants
to collide with a sufficient energy to break the bonds in the reactants so the products can
form (E ≥ Ea).
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We have already learned that kinetic theory is a model used to describe the characteristics of
matter. In kinetic theory the average K.E of a gas is described as being proportional to its absolute
temperature (K). Kinetic theory is also explains the energy distribution of the reacting molecules in
a chemical reaction using what is known as a Maxwell-Boltzman energy distribution curve for a
fixed amount of a gas. The curve shows the probability of gas molecules having a particular energy
at a certain point in time
The highest point in the curve represents the most probable energy for the greatest number of
molecules at that point in time. Notice that over 60% of the reactants have energy that is less than
the activation energy and only a small proportion have energy that is greater than or equal to the
activation energy. Only a small proportion of the reactant molecules as indicated by the area under
the curve have energy that is greater than or equal to the activation energy. The curve also does
not touch the energy (x) axis. It will only do this at energy of infinity. This is important to remember
when you are asked to draw the curve.
Since the average kinetic energy of molecules is proportional to the absolute temperature in Kelvin,
so when the temperature of a gas increases the average kinetic energy of the molecules increases.
Notice how the shape of the curve changes with increasing temperature. The table below
compares them.
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Comparison
T2 peak is lower
than T1
Explanation
because the range of different kinetic energies has
increased in T2 but the total number of molecules remains the
same.
The area under
both graphs is the
same
When the temperature increases the number of molecules does
not change
T2 curve is more to
the left than T1
When the temperature increases a greater proportion of the
molecules have higher kinetic energy. T2 has a greater
proportion of the molecules with have E > Ea
The rate of a reaction is defined as the decrease in the concentration of reactants per unit time or
the increase in the concentration of product per unit time. It is measured in moldm-3s-1.
The concentration of reactants decreases with time and gives a negative rate.
Rate = - ∆R ÷ ∆t
The concentration of products increases with time and give a positive rate.
Rate = ∆P ÷ ∆t
Temperature, pressure, concentration and the addition of a catalyst all effect the rate of reaction.
Qualitative effects of particle size, temperature, concentration, pressure and catalysts on the rate
of a reaction.
1. Addition of a catalyst
A catalyst is defined a substance that provides the reactants with an alternative pathway/route that
has lower activation energy. The lower activation energy will increase the proportion of molecules
having an energy, E ≥ activation energy, Ea, increasing the rate of the reaction. When the
activation energy is large, fewer molecules have E ≥ Ea so the rate of conversion of reactants to
products is slow.
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Catalysts are big business because 80% of processes in the chemical industry use catalysts. Catalyst
sales are worth billions of dollars a year and is a growth industry. Catalysts have such an enormous
impact on the chemical industry because they:
• enable reactions to take place more quickly
• make processes more efficient
• make chemical processes environmentally friendly.
Many transition metal compounds and elements act as catalysts in industry. For example:
The catalyst vanadium(V)pentoxide, V2O5 is used in the production of the sulfur trioxide which is
used to produce sulfuric acid in the contact process. This process is of economic importance
because sulfuric acid is consumed in large amount in the manufacture of many different substances
like fertilizers, dyes and detergents.
V2O5
2SO2(g) + O2(g) ⇔ 2 SO3(g)
The catalyst Iron, Fe is used in the manufacture of ammonia, NH3 using the Haber process.
Ammonia is another economically significant substance because it is used in the manufacture of
nitrogen based fertilizers. Nitrogen is an important nutrient needed for plant growth and is applied
extensively in the form of fertilizer to increase the yield of commercial plant crops grown for food.
Fe
N2(g) + H2(g) ⇔ 2 NH3(g)
The catalyst Nickel, Ni is used in the food industry to solidify vegetable oils in the manufacture of
margarine. Vegetable oils are liquids at room temperature and contain large numbers of carboncarbon double bonds. When they are reacted with hydrogen gas in the presence of a nickel
catalyst, the double bonds are converted to single bonds. Increasing the number of carbon-carbon
single bonds increases the melting point of the oil, so that it becomes a solid at room temperature.
Ni
CH2=CH2(g) + H2(g) → CH3-CH3
ethane
ethane
Platinum, Pt and Palladium, Pd are used in catalytic converters attached to car exhausts. They
convert the smog causing pollutants nitrogen oxide and carbon monoxide produced from the
incomplete combustion of gasoline into more environmentally friendly nitrogen and carbon dioxide
gas.
Pt
2CO(g) + NO(g) → 2 CO2(g) + N2(g)
Check out http://resources.schoolscience.co.uk/JohnsonMatthey/index.htm for information on
catalysts.
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2. Pressure
Affects gases only. Increasing the pressure (by decreasing the volume) of gaseous reactants will
increase the concentration of the reactants, increasing the frequency of collisions, and increasing
the rate of the reaction.
3. Temperature
The average kinetic energy of molecules is proportional to their absolute temperature in
Kelvin and to the square of its velocity. When the temperature of a reaction
increases the average kinetic energy of the molecules also increases resulting in
more frequent collisions and more energetic collisions.
Increasing the temperature increases both the frequency of the collisions and the energy
of the collisions because a greater proportion of the molecules now have energy, E ≥
activation energy, Ea. The rate of the reaction increases. An increase in temperature does
not however change magnitude (size) of the activation energy it just increases the proportion of
molecules with this activation energy.
4. Concentration
Increasing the concentration of a reactant that is in aqueous solution will increase the number of
moles of reactant. This will increase the frequency of collisions between the reactants increasing
the rate of the reaction. Concentration does not affect the energy of the collisions. Increasing the
amount of a solid reactant does not
affect the concentration because
concentration affects aqueous solutions
only.
NOTE: In a reaction increasing the
temperature has a greater effect on the
rate of a reaction than increasing the
concentration. This concentration affects
only the collision frequency.
Temperature affects both the frequency
and the number of collisions with energy,
E ≥ activation energy, Ea
5. Particle size (surface area)
Particle size affects solid reactants only.
If the surface area of the reactant is
increased, commonly by grinding into a
fine powder, more of the solid reactant
will be exposed. This will increase the
frequency of collisions between reactants
increasing the rate of the reaction.
Adding more solid reactant of the same
particle size will not affect the rate of the reaction if it is the excess reagent.
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Questions
1. (N04) When excess calcium carbonate is added to dilute hydrochloric acid the following
reaction takes place.
CaCO3(s) + HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
State and explain at a molecular level using collision theory two ways in which the rate of
this reaction could be increased and two different ways that the rate of the reaction could
be decreased. [8]
2. Define the following terms:
a) activation energy
b) rate of a reaction
3. (N02/S) Under what conditions is the rate of the reaction of magnesium metal and
hydrochloric acid solution the fastest.
A.
10cm3 of 1.0 moldm-3 HCl(aq) at 25°C
B.
10cm3 of 2.0 moldm-3 HCl(aq) at 25°C
C.
10cm3 of 2.0 moldm-3 HCl(aq) at 35°C
D.
10cm3 of 1.0 moldm-3 HCl(aq) at 35°C
4. (N04/S) For a given reaction, why does the rate of the reaction increase when the
concentration of the reactants are increased.
A.
the frequency of molecular collisions increases
B.
the activation energy increases
C.
The average kinetic energy of the molecules increases
D.
The rate constant increases
5. (N03/S) The rate of reaction between two gases increases when the temperature is
increased and a catalyst is added. Which statements are both correct for the effect of these
changes on the reaction?
A.
B.
C.
D.
Increasing the temperature
Collision frequency increases
Activation energy increases
Activation energy does not change
Activation energy increases
Adding a catalyst
Activation energy increases
Activation energy does not change
Activation energy decreases
Collision frequency increases
6. Answer the section 7.2 exercises on pages 235-236 of Derry, Lanna, Maria Connor and Carol
Jordan. Chemistry for use for the IB Diploma Standard level.
Please turn over for HL questions
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HL Only
7. The catalyst vanadium (V)pentoxide, V2O5 is used in the production of the sulfur trioxide
which is used to produce sulfuric acid in the contact process. V5+(aq) is colored and can
behave as a reducing agent, whereas Zn2+(aq) is not colored and does not behave as a
reducing agent. (6)
8. Elements with atomic number 21 to 30 are d-block elements. Identify which of these
elements are not considered to be typical transition elements. (1)
9. Identify two transition elements used as catalysts in industrial processes, stating the process
in each case. (2)
10. Apart from the formation of complex ions and their use as catalysts, state two other
properties of transition elements. (2)
11. State two possible oxidation states for iron and explain these in terms of electron
arrangements. Explain why many compounds of d-block (transition) elements are colored.
(5)
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Bibliography
Clark, Jim. Chem Guide. 2008. <http://www.chemguide.co.uk/>.
Clugston, Michael and Rosalind Flemming. Advanced Chemistry. Oxford: Oxford University Press,
2000.
Derry, Lanna, Maria Connor and Carol Jordan. Chemistry for use for the IB Diploma Standard level.
Melbourne: Pearson Education, 2008.
Green, John and Sadru Damji. Chemistry for use with the International Baccalaureate Programme.
Melbourne: IBID Press, 2007.
Neuss, Geoffrey. IB Diploma Programme Chemistry Course Companion. Oxford: Oxford University
Press, 2007.
—. IB Study Guides, Chemistry for the IB Diploma. Oxford: Oxford University Press, 2007.
Organisation, International Baccalaureate. Online Curriculum Centre.
<http://occ.ibo.org/ibis/occ/guest/home.cfm>.
—. "Chemistry Data Booklet." International Baccalaureate Organisation, March 2007.
—. "Chemistry Guide." International Baccalaureate Organisation, March 2007.
—. "IB Chemistry Examination Papers ." Cardiff: International Baccalaureate Organisation, 19992008.
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