Grade 9 Academic Science – Chemistry
Rate of a Reaction
The rate of a reaction is the speed of the reaction. It is not “how much” of a
product is made; instead, it is “how quickly” a reaction takes place.
How can we measure the rate?
Consider a reaction
zinc + hydrochloric acid —> zinc chloride + hydrogen
There are two possible ways of measuring the rate.
1. Measure how quickly one of the products (e.g. the hydrogen) is made. In
this case, we could measure the volume of hydrogen (in milliliters) made
during a time period (e.g., every 10 seconds).
2. Measure how quickly one of the reactants (e.g. the zinc) disappears. So,
we could measure the loss in mass of the zinc and hydrochloric acid (in
grams) as they change into hydrogen gas escaping from a beaker every
time period (e.g., every 10 seconds).
Collision Theory
How different variables change the rate of reaction is called the collision theory.
For a reaction to take place, the particles of the substances that are reacting
have to collide. If they collide with enough energy then they will react. The
minimum amount of kinetic (movement) energy that two particles need if they are
going to react when they collide is called the Activation Energy.
There are two main ways of increasing the rate of a reaction:
1. Increase the number of collisions, and
2. Increase the amount of movement (kinetic) energy so that more collisions
lead to a reaction.
What can change the rate of a reaction?
The four main variables are:
1. Concentration
2. Temperature
3. Size of the particles (i.e., surface area)
4. Catalyst being present
The Effect of Concentration
The concentration of a solution is the solution “strength”.
Calcium carbonate + Hydrochloric acid
—>
Calcium chloride + Water + Carbon dioxide
A stronger acid contains more acid particles and less water particles than a
weaker acid. By increasing the concentration of a solution, more collisions will
occur, and thus, the rate of the reaction goes up.
NOTE: Changing the concentration of the acid does not change how quickly the
particles are moving. That is, it does not increase the amount of energy they
have.
The Effect of Temperature
When the temperature at which a reaction is taking place is increased, the
particles move more quickly. This has two effects. More collisions take place,
and when a collision occurs, there is more chance that the collision will lead to a
reaction because the amount of energy is more likely to be greater than the
minimum amount of energy needed (i.e., activation energy)
The Effect of Particle Size
Solids with a smaller particle size (e.g., powders or small chips) react more
quickly than solids with a larger particle size (e.g., large chips). If the size of the
particles is reduced, the rate of a reaction will increase because the surface area
available for collisions to take place has increased. The increased surface area
increases the number of collisions. NOTE: Surface area has no effect on the
energy of the particles.
Adding a Catalyst
A catalyst is a substance that speeds up a reaction without being used up
itself.
Hydrogen peroxide decomposes to form water and oxygen gas.
Hydrogen peroxide —> Water + Oxygen
This reaction only occurs very slowly. If the compound manganese oxide is
added, it acts as a catalyst for the reaction. When the catalyst is added the
reaction speeds up greatly but the manganese oxide never runs out.
How does a catalyst work?
1. A catalyst provides a surface on which the reaction can take place. This
increases the number of collisions between the particles of the substances
that are reacting.
2. A catalyst lowers the activation energy (the minimum amount of energy
needed for a reaction to take place). Thus, the particles can react with less
energy. If the amount of energy needed for particles to react is lowered,
more particles can react.
Chemical Reactions - Demonstrations
Reactants
Products
Observations


Iron
Sulfur

Iron sulfide

Black, solid mass


Magnesium
Hydrochloric acid


Magnesium chloride
Hydrogen


Vigorous bubbling
Magnesium
disappears


Calcium carbonate
Hydrochloric acid



Calcium chloride
Water
Carbon dioxide


Bubbling
Calcium carbonate
disappears


Sodium
Water


Sodium hydroxide
Hydrogen

Sodium floats and
moves on water
Sodium fizzes


Iron
Copper sulfate


Iron sulfate
Copper



A red-brown colour
appears around the
iron
The blue solution
turns colourless


Sulfur
Oxygen

Sulfur dioxide

In a gas jar, fumes or
pungent gas appear


Magnesium
Oxygen

Magnesium oxide


Blinding white flame
White ash left