Transcript Science 9: Unit B – Matter and Chemical Change

Science 9: Unit B –
Matter and Chemical
Change
Topic 8: Reaction Rate and
Common types of Reactions
Reaction Rate
 Different reactions occur at different
speeds. However, techniques can be
used and chemicals added to alter the
natural rate of a reaction. For example
heating a chemical mixture, stirring,
swirling, and using smaller particles can
all speed up a reaction-rate. On the
other hand, cooling a reaction or mixture,
and not moving it slows the reaction rate.
Catalysts and Inhibitors
 Catalyst – A chemical that is added to a
reaction mixture, but is NOT used up in the
reaction. Its purpose is to help speed up a
reaction. A catalyst in your saliva helps speed
up the breakdown of potatoes into starch.
 Inhibitor – A chemical that is added to a
reaction mixture, but is NOT used up in the
reaction. Its purpose is to help slow down or
delay the reaction, usually because the reaction
is unwanted. An example of an inhibitor are ‘clot
busters’ which are used in blood donor clinics to
slow down the clotting process in the donated
blood.
Corrosion Reaction
 Corrosion occurs when a metal reacts
with oxygen to form a(n) ionic compound
known as a metal oxide. The most
famous example of corrosion occurs as
a car rusting. Cars are usually made of
iron. Iron will react with oxygen when
water is present. That’s why cars in
Alberta will outlast cars in Ontario and
Quebec. There’s more humidity in the air
in the East. The chemical equation for
the formation of rust is:
 4Fe(s) + 3O2(g) → 2Fe2O3 (s)
How to Prevent Rust
 To protect against rust forming, car
makers add several protective layers of
paint. In similar cases, a non-corrosive
metal (does not react with oxygen) is
added on top of the iron, this process is
called electro-plating as it’s done
electrically. Chromium is often used in
car bumpers.
 If zinc is used in electroplating, the
process is called galvanization.
 The process where a metal is
electroplated with another metal is called
electrolysis.
COMBUSTION
 Combustion reactions involve the same
three elements reacting to form the
same two products, but in different
amounts.
 Combustion involves a hydrocarbon
reacting with oxygen. A hydrocarbon is a
compound made up of hydrogen and
carbon. Examples of hydrocarbons
include methane (natural gas) CH4,
propane C3H8, butane C4H10, and
octane (gasoline) C8H18.
Combusting a hydrocarbon
 With any hydrocarbon, the reaction
works out the same. An initial source of
heat provides the energy needed to let
the hydrocarbon react with surrounding
oxygen to produce carbon dioxide,
water, and a large amount of energy.
Combustion reactions are very
exothermic.
Two Examples of Combustion
Reactions
 For example, with a regular butane lighter, the reaction
occurs as follows: When you press down on the button,
the butane is released into the surrounding air. At the
same time, the spark provides the initial energy needed
to ignite the butane and oxygen producing a larger
amount of energy (flame) and carbon dioxide and water
vapour.
 2C4H10(l) + 13O2(g) → 8CO2(g) + 10H2O(g) + Energy
 Another common example involves barbecuing. Propane
gas is released from the tank and comes into contact with
oxygen in the bbq. A match or spark provides the initial
energy) releasing a large amount of heat energy.
 C3H8(l) + 5O2(g) → 3CO2(g) + 4H2O(g) + Energy
Combination Reactions
 Two or more reactants are combined
together to form a single product. Note
that the products formed are usually
more stable than the reactants and so
energy and catalysts are needed only to
start the reaction.
 Eg. H2(g) + Cl2(g) → 2HCl(l)
Dissociation Reactions
 The opposite of a combination reaction.
Here a single compound is broken up
into its individual elements. Because
compounds tend to be more stable than
elements by themselves these reactions
tend to be easily reversible.
 Eg. 2HCl(l) → H2(g) + Cl2(g)