Factors Affecting the Rate of a Chemical Reaction

Factors Affecting the Rate of a Chemical Reaction
The speed at which a chemical reaction occurs is called the rate of reaction.
For example, a log in a campfire undergoing combustion (burning)
can either smolder slowly for several hours or rapidly burn down to
ash. What controls the rate of this chemical reaction?
The Kinetic Molecular Theory
The particle theory of matter tells us that as molecules are heated,
they move more rapidly. In solids, particles do not move freely since they are
held together by attractive charges. In liquids, molecules move freely and slide
over each other but remain at the bottom of a container. In a gas, the particles
are far apart and can move freely in a straight line.
The collision model of chemical reactions states that the rate of a chemical
reaction is affected by the number of collisions between reactant molecules. If a
molecule is moving with enough speed (kinetic energy) and collides with another
molecule, the reaction can take place. Therefore, the rate of reaction is
controlled by:
1) the number of collisions.
2) the fraction of effective collisions.
1. Temperature
As the temperature of a substance is raised, the speed or kinetic energy of the
molecules within that substance increases. This increases results in1) more
collisions and 2) a larger fraction of successful collisions. Therefore as we raise
the temperature of a substance undergoing a chemical reaction, we increase the
reaction rate.
E.g. For example, compare the reaction of iodide with starch.
Temperature = __________ oC
Temperature = ___________ oC
Reaction time = _________ sec.
Reaction time = __________ sec.
2. Concentration
As we increase the concentration of reactants in a chemical reaction, we pack
more molecules within the same volume (e.g. grams/L). Putting more molecules
in the same volume increases the chance of collisions and therefore increases
the rate of reaction.
Reactant concentration = 1%
Reactant concentration = 2%
Reaction time = _________ sec.
Reaction time = __________ sec.
3. Surface Area
Surface area is the area of sample of matter. When reactants of different phases
(e.g. solid & gas, liquid & gas, solid & liquid) are in contact, the surface area
affects the number of possible collision sites between reactants. For example,
grinding a solid into a fine powder can greatly increase the surface area and
speed up a chemical reaction.
Large pieces of starch
SA = 1 cm2 x 6 = 6 cm2
Smaller grains of starch
SA = 0.25 cm2 x 6 x 8 =12 cm2
Longer reaction time
(i.e. slower)
Shorter reaction time
(i.e. faster)
4. Catalysts
A catalyst is a substance that increases the rate of reaction without being
consumed in the reaction. Catalysts can lower the collision energy required for a
chemical reaction to occur. This increases the fraction of reactant collisions that
lead to a reaction.
Catalysts are very important in industry and pollution control. A “catalytic
converter” is required on all automobiles to complete the combustion reaction
and prevent the release of many smog-forming molecules.
Blast Off!
In your final chemistry experiment, you will be manipulating the first three
variables (Temperature, Concentration and Surface Area) to optimize the
reaction of Alka-Seltzer tablets. The goal is to blast the lid off a film canister in
precisely 15 seconds.
In groups of 3, complete Design steps a) to d) on p.276. Identify your
independent and dependent variables. Have your design approved before you
leave class.
Homework: Questions p264 #1-5, 7-10