Rates of Reactions
• States that particles are constantly moving
• Molecules must be moving fast and hit hard for a bond to form
(i.e. a chemical reaction occurs)
• As particles of matter are moving, they collide or hit each other.
• Slow moving particles just bounce off each other.
• If the particles are fast and they hit hard enough, the atoms can rearrange
themselves to form NEW compounds!
• Collisions happen all the time, but most of them are not strong enough to
cause a reaction. If we want a reaction to happen faster, we need to increase
the total number of collisions.
Factors Affecting Rates
1. Temperature
Note:
The higher the temperature, the faster the rate of reaction.
low temperature
high temperature
When particles are moving faster, they collide with other particles more
often, and with more energy. An increase in these collisions will result in
chemical reactions.
2. Surface Area
Note:
The higher the surface area, the faster the rate of reaction.
low surface area
high surface area
The particles on the outside of the reactants react first, so if there are more
particles on the surface, then they can react faster. We can increase the
surface area by breaking the reactants into smaller pieces.
3. CONCENTRATION
Note:
The higher the concentration, the faster the rate of reaction.
low concentration
high concentration
An increase in concentration means there are more particles in a certain
space. When more particles are packed together, they will collide more
often with each other so more reactions occur.
4. CATALYSTS
Note:
The addition of a catalyst increases the rate of reaction.
A catalyst is a substance that increases the rate of a chemical reaction
without being used up in the reaction. The catalyst works by reducing the
amount of energy needed for a reaction to occur. This means that more
collisions can occur thus more reactions occur.
no catalyst
with catalyst
EXAMPLE:
MnO2
H2O2
H2O + O2
(Managanese dioxide is the catalyst)
Think of a catalyst like a
helicopter that helps you up a
mountain so you don't have to
climb! It makes it easier to go up
the mountain, so more people
can do it!
Factors that affect the rates of reaction
Questions!
Read p.255, 260-264 and answer the questions below.
1. Write a definition for “rate of reaction”.
2. On p.260, the textbook discusses the collision model. Explain how figure 3 is an appropriate
diagram for this discussion.
3. Look at figure 4 and summarize the effect of temperature on the rate of reaction.
4. Look at figure 6 and summarize the effect of surface area on the rate of reaction. Do you think this
figure is an effective one for this factor? Explain.
5. (a) Write a definition for catalyst.
(b) How does figure 9 relate to your definition?
(c) How does figure 9 explain the effect of a catalyst on the rate of a reaction?
6. (a) Write a definition for concentration.
(b) How does concentration affect the rate of a reaction?
(c) How does figure 5 relate the effect of concentration on the reaction of an acid with
a metal?