MODULE 6: CHEMICAL KINETICS
Most Essential Learning Competencies
1. Describe how various factors influence the rate of a reaction. (STEM_GC11CKIIIi-j-130)
2. Differentiate zero, first and second-order reactions. (STEM_GC11CKIIIi-j-132
Discussion of the Lesson
In a chemical reaction, materials are consumed
to form a new one. The substances that go away over a
period of time in a chemical reaction are called
reactants. On the other hand, the substances that are
newly formed with a different set of properties from
their original material are known as products.
A very common chemical reaction is
combustion, like the burning of a candle. The reaction of wax materials with oxygen from the air
is accompanied by the giving off of the energy in the form of heat and/or light as it produced
water vapor and carbon dioxide. Burning that we witness in our daily life takes place with air as
its source of oxygen. These materials would actually burn faster or explosively in pure oxygen.
But some burning takes very slowly like paper “burns” as it is turning yellow over time.
The area of chemistry concerned with the speed or rate at which chemical reactions occur
is called chemical kinetics. This module explains why some reactions occur faster than others and
why the same reaction takes place at a slower or faster rate depending on the conditions. So get
ready and enjoy learning your new lesson!
Natural chemical processes like aging, fruit ripening, and rusting of nails occur at different
reaction rates. What factors influence how fast or slow chemical reactions occur?
Various factors determine the rate of a given chemical reaction. In this module, five (5)
factors are presented. These are:
1. surface area of reactants 4. temperature
2. concentration of reactants 5. presence of a catalyst
3. nature or reactivity of reactants
A. Surface Area of Reactants
Burning a piece of wood can be done in many ways. If you burn it as one big block or log of
wood, it will take a few hours before the whole block gets totally burned. But if you use wood
shavings, chips, or sawdust, it would take only a few minutes to burn it all up. Particle size
explains it all.
Let us look at this block of wood.
A block of wood has 6 sides. Each of the surfaces comes in contact with oxygen in the air.
If you make the particle size of the wood smaller, it increases the number of surface areas exposed.
This consequently results in an increase in the contact area between the wood and oxygen. Thus
the rate of burning increases.
Did you know that a spark can cause coal dust in a mine to explode and yet not cause an
explosion with charcoal in a barbecue grill? The surface area of reactants can explain this
phenomenon.
B. Concentration of Reactants
The Collision Theory suggests that reacting particles must bump properly with each other with a
sufficient amount of energy so that a chemical reaction can occur. An increase in the concentration
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of the reactants means that there will be more particles colliding with each other in a given amount
of time, thus increasing the possibilities that a reaction takes place.
Therefore, the reaction between aluminum and hydrochloric acid in Dexter’s experiment occurs
faster as the concentration of HCl increases.
C. Nature or Reactivity of Reactants
The reaction rate varies with the nature and reactivity of the substances involved. Generally,
fast reactions include acid reactions, the formation of salts, and ion exchange. Reactions tend to
be slow when covalent bond formation takes place between the molecules and when large
molecules are formed.
In the reaction between the magnesium and the acid, both reactants are gradually used up.
However, the acid is in excess, so it is mainly the
loss of magnesium as the surface area becomes
smaller that causes the change in the rate. The
reaction is fastest at the start. But as the
magnesium is used up, the rate falls until the
reaction has stopped and no more gas is produced,
as evidence.
Magnesium reacts faster in hydrochloric acid compared to less reactive metal like
aluminum. Thus replacing magnesium with aluminum will require a longer period of time for the
reaction to complete.
Similarly, when you replace hydrochloric acid with weaker acid like acetic acid, the reaction
takes place slower. Hence, reacting equal amounts of magnesium ribbon in equal amounts of
different types and strengths of acids will change the production rate of hydrogen gas.
Over a period of time, canned goods spoil inside the metal tin cans, as the acid in food
reacts with the metal coating and corrodes it producing gas and lumps up inside. So the next time
you go with your mother to the grocery store, inspect the canned goods for any signs of bulging
metal containers.
D. Temperature
The effect of temperature can also be explained by Collision Theory. Temperature is directly
proportional to kinetic energy. This means that at higher temperatures, molecules are more in
motion. The frequent the motion will be, the greater is the chance for colliding molecules to result
in a faster reaction.
This explains why ripe fruits are placed inside the refrigerator to slow down the ripening
process. However, meat is cooked faster in a pressure cooker because of the higher temperature
inside the cooking vessel. Did you know that the normal body temperature is about 37˚C? The
chemical reactions inside our body take place at this temperature. When we get injured, the
biochemical reactions that cause inflammation of tissues can be slowed down by applying a cold
compress on the affected area.
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E. Presence of a Catalyst
A catalyst is usually a substance that, when added to reacting mixture, increases the rate
of the reaction without being consumed in the process and can be recovered after the reaction is
completed. Catalysts are the opposite of inhibitors, which slow down reactions. How catalysts
work will be discussed in detail in the next module.
Ethylene is a natural plant hormone that promotes ripening. The natural ripening of fruits
takes a long time. However, the process can be hastened to meet the demands of the market. That
is why artificial ripening is practiced. This process is mimicked using chemicals. One of the
artificial ripening agents used is ethephon (2-chloroethyl phosphonic acid). It penetrates the fruit
and decomposes ethylene. Another compound that is used regularly is calcium carbide locally
known as kalburo, which produces acetylene, which is an analogue of ethylene. However, it has
traces of arsenic and phosphorus that are considered as carcinogenic.
Can you spot the artificially ripened bananas?
During a chemical reaction, the amount of reactants decreases with time while
simultaneously the amount of products increases. Hence, reaction rate can be expressed as the
change in the concentration of any of the reactants or products in a specific span of time.
Mathematically, it can be determined as:
The negative sign indicates a decrease in concentration with time. However, rate is always
expressed as a positive quantity. The concentration of all substances is expressed in molar units.
Therefore, units of reaction rates are generally given as mol/(L • sec).
Activity 1: Appear or Disappear?
Directions: Consider the graph on concentration versus time. Answer the questions briefly.
Sulfur is a non-metallic element which burns but does not ignite until heated to about
232˚C. The combustion reaction is: S (s) + O2 (g) → SO2 (g)
1. What happens to the concentration of sulfur and oxygen in a certain period of time?
2. What happens to the amount of sulfur dioxide over a given time?
3. What is the correct expression for the appearance of sulfur dioxide gas? Encircle the letter only.
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Sometimes a particular reactant will have a greater effect on the reaction rate. To show the
relationship between reactant concentrations and the rate of reaction, the rate law is used. This
mathematical equation takes the general expression of:
Rate = k [A]m [B] n
where k is the rate constant that is determined experimentally and changes with temperature
A and B are the reactants; and
the powers of m and n are the reaction order.
For example, the following reaction: 2 NO(g) + 2 H2(g) → N2(g) + 2 H2O(g)
By experimental results, it follows the rate law of: Rate = k [NO] 2 [H2]. Stoichiometric
coefficients have nothing to do with rate law.
Note that doubling the concentration of hydrogen, H2 will only double the rate since its
concentration is only taken to the 1st power. However, doubling the concentration of nitric oxide,
NO, will cause a fourfold increase in the rate since its concentration in the rate law is squared.
Consequently, the reaction is said to have second-order dependence with respect to NO but
only a first order dependence with respect to H2.The overall order of the reaction is 3. This is
because the value of m is 2, and the value of n is 1, and 2+1=3. Therefore, the power to which a
reactant appears in its rate law expression determines its reaction order. The over-all order of
reaction is determined as the sum of the powers of the concentration factors in the rate equation.
The overall order of reaction gives an indication of how changing the concentration of the
reactants will change the speed of the reaction. That is why, for higher orders of reaction, changing
the concentration of the reactants results in large changes in the rate of reaction.
Did you know that nitric oxide (NO) is a major pollutant produced during the combustion
of fossil fuels and has many deleterious effects? To limit its release to the atmosphere, strict
environmental regulations have been passed in, and scientists work harder toward the
development of more efficient methods to decrease NO emitted from combustion processes.
Activity 2. Take my Order
Directions: Consider the given graphs below. Mix and match the graphs with the correct
descriptions. Use the following options: X- zero order; Y- first order; Z- second order.
__1. Changing the concentration of the reactant will have no effect on its rate of reaction.
__2. It depends on the concentration of one reactant.
__3. The rate law is rate = k [A].
__4. The rate is expressed as rate = k.
__5. May be expressed in any of the following equations: rate = k [A]2 , rate = k [B]2, rate = k [A][B].
Did you know that the decomposition of ammonia (NH 3) in the presence of molybdenum or
tungsten is a zero-order reaction?
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The surface of the catalyst is almost completely covered by NH 3 molecules. The adsorption
of gas on the surface cannot change by increasing the pressure or concentration of NH3. Thus, the
concentration of gas phase remains constant although the product is formed.
Activity 3. It’s a Wrap
Directions: Write the letter of the word or phrase that best completes the statement or answers
the question on the space provided.
A. catalysts E. temperature I. first order
B. concentration F. rate of reaction J. reaction order
C. rate law G. reactivity
D. surface area H. reactants
_______ 1. It refers to the amount of substance per unit volume.
_______ 2. Some reactions are slow and some are fast.
_______ 3. It tells the concentration dependence of one reactant in a reaction.
_______ 4. It is characterized as the tendency of a material to undergo a chemical reaction.
_______ 5. It shows the relationship between concentrations of reactants and reaction rate.
_______ 6. It describes the powers of the concentration factors in the rate equation.
_______ 7. It is a property of a material which increases as particle size decreases.
_______ 8. These are substances that go away with time in a reaction.
_______ 9. It increases the kinetic energy present in a material.
_______ 10. These are materials that do not change their amount throughout the chemical reaction.
Activity 4: You Complete Me
Directions: Examine the given factors and decide on their effect on the rate at which certain
reactions may occur. Write INCREASE if it makes the reaction go faster. Write DECREASE if it
slows down a chemical process.
Factors Effect on Reaction Rate
1. Lower temperature
2. Greater frequency of collision
3. Increasing the concentration of reactants
4. Increasing the particle size of reacting molecules
5. Using a catalyst
Activity 5: Law and Order
Directions: Consider the given chemical reactions and their corresponding rate law equation.
Determine the over-all reaction order of each process.
Reaction Rate Law Equation Reaction Order
1. CH3Cl + OH- → CH3OH + Cl- Rate = k [CH3Cl] [OH-]
2. 2N2O → 2N2 + O2 Rate = k [N2O]
3. H2(g) + I2(g) → 2HI(g) Rate = k[H2] [I2]
4. N2(g) + H2(g) → NH3(g) Rate = k[N2]0 [H2]0
5. CH4 + Cl2 → CH3Cl + HCl Rate = k [CH4] [Cl2]1/2
ASSESSMENT
Directions: Write only the letter that corresponds to your answer.
1. Which situation appears to violate the collision theory?
A. The reaction rate decreases as the reaction mixture is cooled.
B. The reaction rate increases as the surface area of the reactants increases.
C. The reaction rate increases with reactivity of materials.
D. A reaction takes place more rapidly in less concentrated reactants.
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2. Which statement is true about catalysts?
A. They can slow down the reaction by consuming the reactants.
B. They are unrecovered at the end of the reaction.
C. The amount of catalysts decreases as the reaction continues.
D. They speed up the reaction.
3. How does temperature affect the rate of a chemical reaction?
A. Increases B. Decreases C. Keeps it constant D. Makes it fluctuate
4. Knowing that a more active metal will replace a less active metal in a compound,
which of the following statements is true?
A. Gold would replace silver C. Silver would replace copper
B. Mercury would replace lead D. Lead would replace copper
5. Which of these is a reason why surface area of reactants is increased?
A. To stabilize the reaction C. To prevent a compound from forming
B. To speed up the reaction D. To produce lesser amount of products
6. Assuming that each of the following graphs has the same concentration (y) and time (x) axes,
which has the greatest initial rate of disappearance of reactant?
7. Which is the correct expression for the reaction rate of this reaction?
8. What is the reaction order for the reaction: CHCl3(g) + Cl2(g) → CCl4(g) + HCl(g)
if experimentally the rate law is: Rate = k[CHCl3][Cl2]1/2?
A. First order with respect to Cl2
B. Zero order with respect to CHCl3
C. Second order with respect to CCl4 and HCl
D. 1.5 over-all order of reaction
9. How do exponents (orders) in rate laws compare to coefficients in the balanced equations?
A. The exponents do not have any relation to the coefficients.
B. They are directly proportional to each other.
C. The exponents and coefficients are the same.
D. They are ratio to one another.
10. Which factor affects the rate constant k?
A. Surface area B. Temperature C. Concentration D. Reactivity
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