science world 2 – chapter 1
investigating reactions
REACTION RATE
Chemical reactions take place at different speeds. For example,
fireworks react quickly once lit, but the rusting of a car will take
years. The speed of a chemical reaction is called its reaction
rate. You can speed up or slow down a reaction by changing
the conditions under which they occur. These include:
 the temperature at which the reaction occurs. Higher
temperatures cause faster reactions.
 the concentration of the reacting substances. More
concentrated reactants cause faster reactions.
 the size of the reacting particles. Smaller particles react faster
than big pieces.
 the presence of a catalyst. Catalysts are substances that
speed up the reaction, but are not used up in the reaction.
EXPLAINING REACTION RATES
Chemical reactions can be explained in terms of the particle
theory of matter. This theory says that:
1.
All matter is made up of microscopic particles.
2.
There are spaces between particles.
3.
There are attractive forces between particles, which
become weaker the further apart the particles are.
4.
The particles are always moving.
5.
At high temperatures the particles move faster than at low
temperatures.
A chemical reaction can only occur when the particles of the
reacting substances come into contact with each other (i.e. they
collide). According to the particle theory, higher temperatures
cause particles to collide more often and more violently (more
energetically) than at lower temperatures. For these two
reasons, reaction rate increases with temperature.
1
Concentration is a measure of how much solute is dissolved in
a solution, compared to the amount of solvent. A higher
concentration of a reactant usually increases the reaction rate.
This is because the reacting particles are closer together and
collide more often.
The rate of a reaction is also affected by the size of the reacting
particles. Smaller particles have a greater surface area
available to react and therefore react more quickly than big
pieces that have a small surface area. For example, kindling
(wood chips) burns a lot quicker than a big log.
CATALYSTS & ENZYMES
Catalysts are chemicals that can increase the rate of a reaction
but are not actually used up in the reaction. They are used in
industry to control the speed of reactions and to cut down the
costs of producing chemicals. Though catalysts are mostly used
to speed up reactions, they can also be used to slow them down.
A catalyst that slows down the rate of a reaction is called an
inhibitor.
There are many chemical reactions that occur in living things.
These reactions need biological catalysts called enzymes to
control them, otherwise they would occur much too slowly to
keep the organism alive. Your body contains thousands of
enzymes, each one controlling the speed of one particular
reaction.
EXOTHERMIC & ENDOTHERMIC
Whenever there is a chemical reaction an energy change occurs.
Energy is produced in some reactions, and is needed to start
others.
An exothermic reaction is one that releases energy. This
energy is usually in the form of heat, but other forms of energy
can also be produced. One of the most important exothermic
reactions is combustion, or burning.
2
An endothermic reaction is where some form of energy must
be supplied before the reaction will occur. They often result in
the temperature of the reaction decreasing (i.e. it gets colder).
An important endothermic reaction is photosynthesis, where
plants need energy from the sun in order to make food for
themselves.
LAW OF CONSERVATION OF MASS
The Law of Conservation of Mass states that “the total mass of
the reactants is always equal to the total mass of the
products”.
What this means is that the number of atoms that take part in a
chemical reaction is equal to the number of atoms in the
products. No atoms are lost in the reaction – they are simply
rearranged.
For example when hydrogen peroxide decomposes into water
and oxygen the following reaction occurs:
2H2O2
hydrogen peroxide

2H2O
water
+
O2
oxygen
There are 4 hydrogen atoms and 4 oxygen atoms on both sides
of the equation, indicating that the law of conservation of mass
has held true.
SPELLING WORDS
EASY
1
2
3
4
5
6
7
8
9
10
rate
rusting
burning
product
matter
spaces
forces
faster
contact
energy
EASY
11
12
13
14
15
16
17
18
19
20
area
piece
powder
speed
reactant
change
increase
surface
mass
equal
1
2
3
4
5
6
7
8
9
10
3
HARD
investigating
reaction
explosion
temperature
photosynthesis
particles
theory
attraction
collision
energetically
11
12
13
14
15
16
17
18
19
20
HARD
concentration
combustion
dilute
catalyst
inhibitor
industry
enzyme
exothermic
endothermic
conservation