Science 9 Unit B Matter and Chemical Change
Topic 4.0
Substances undergo a chemical change when they interact to produce different
substances.
Chemical Reactions
A chemical reaction takes place when two or more substances combine to form new substances.
Different types of chemical reactions can occur, including combination, decomposition, displacement
and exchange reactions. The substances at the beginning of the reaction are called reactants.
The new materials produced by the reaction are called products.
Reactants produce Products
All chemical reactions can be placed into one of six categories:
1) Combustion: A combustion reaction is when oxygen combines with another compound to form water
and carbon dioxide. These reactions are exothermic, meaning they produce heat.
An example of this kind of reaction is the burning of napthalene:
C10H8 + 12 O2 ---> 10 CO2 + 4 H2O
2)
Synthesis: A synthesis reaction is when two or more simple compounds combine to form a more
complicated one.
These reactions come in the general form of:
A + B ---> AB
One example of a synthesis reaction is the combination of iron and sulfur to form iron II sulfide:
8 Fe + S8 ---> 8 FeS
3)
Decomposition: A decomposition reaction is the opposite of a synthesis reaction - a complex
molecule breaks down to make simpler ones. These reactions come in the general form:
AB ---> A + B
One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:
2 H2O ---> 2 H2 + O2
4)
Single displacement: This is when one element trades places with another element in a compound.
These reactions come in the general form of:
A + BC ---> AC + B
One example of a single displacement reaction is when magnesium replaces hydrogen in water to make
magnesium hydroxide and hydrogen gas:
Mg + 2 H2O ---> Mg(OH)2 + H2
5)
Double displacement: This is when the anions and cations of two different molecules switch
places, forming two entirely different compounds. These reactions are in the general form:
AB + CD ---> AD + CB
One example of a double displacement reaction is the reaction of lead II nitrate with potassium iodide to form
lead II iodide and potassium nitrate:
Pb(NO3)2 + 2 KI ---> PbI2 + 2 K(NO3)
6)
Acid-base: This is a special kind of double displacement reaction that takes place when an acid and
base react with each other. The H+ ion in the acid reacts with the OH- ion in the base, causing the formation
of water. Generally, the product of this reaction is some ionic salt and water:
HA + BOH ---> H2O + BA
One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide:
HBr + NaOH ---> NaBr + H2O
At first it may seem that the launch of a space shuttle and the activation of air bags in a vehicle have
very little in common.
In fact, both of these events require a chemical reaction to work. A chemical reaction takes place when
two or more substances combine to form new substances.
A chemical change in a substance results from a chemical reaction.
The chemical reaction occurring in launching a space shuttle involves almost 1 500 000 L of liquid
hydrogen and 545 000 L of liquid oxygen combining to form water.
During this reaction, enough energy is released to put the shuttle into orbit around Earth.
In a vehicle equipped with air bags, the chemical reaction occurs on a smaller scale, but the results are
also dramatic.
Air bags, packed inside the frame of a vehicle, contain the explosive chemical sodium azide NaN3(s).
When the vehicle is in a collision, the sodium azide reacts and forms large volumes of nitrogen gas
and sodium.
The sodium quickly reacts with another compound in the air bag to make less dangerous compounds.
Fifty grams of sodium azide can produce 30 L of nitrogen gas in milliseconds—a reaction that releases
a burst of energy.
The nitrogen gas inflates the air bags instantly, cushioning the impact of the collision for the driver and
front-seat passenger.
Chemical reactions can be written as word equations which gives the names of all the reactants
(separated by a "plus' sign + ) followed by an arrow which points to the names of all the products
(separated by a 'plus' sign +)
eg. ( iron + oxygen + water → rust )
(Iron plus oxygen plus water produces rust)
A chemical change results from a chemical reaction.
Evidence that a chemical change has occurred
include:
• A change of colour
• The formation of an odour
• The formation of a solid or a gas (bubbles)
• The release or absorption of energy
Reaction 1.
When hydrogen peroxide is left out in the sun, it changes to water and
oxygen gas.
Hydrogen Peroxide produces water and oxygen gas
H2O2 → H2O + O2
2 H2O2 → 2 H2O + O2
Reaction 2
A silver spoon is exposed to air. Over time, it turns a dark brown colour.
Silver plus oxygen produces silver Oxide
Ag + O2 → Ag2O
4 Ag + O2 → 2 Ag2O
Reaction 3.
Sodium and bromine react explosively to produce sodium bromide.
Sodium plus bromine produce sodium bromide
Na + Br2 → NaBr
2Na + Br2 → 2NaBr
A chemical change, which releases energy, is called EXOTHERMIC.
When you burn something in the presence of oxygen energy in the form of heat is given off.
Heat is given off when your body metabolizes food
A chemical change, which absorbs energy, is called ENDOTHERMIC.
The temperature drops in a baking soda vinegar reaction.
Cold Packs absorb heat energy.
CHEMICAL CHANGES INVOLVING OXYGEN (Oxidation Reactions)
Chemical changes occur because some substances react with each other when they come into contact.
Among the most common types of chemical reactions are those involving oxygen.
Three examples of reactions in which oxygen reacts with other substances are:
1) Combustion
2) Corrosion
3) Cellular respiration.
This fire triangle
shows the three factors that
keep a fire going. If any one
of them is missing, the fire
will not continue burning
Combustion is a chemical reaction that occurs when oxygen reacts with a substance to form a
new substance and give off energy. Fire is a common example of a combustion reaction. In burning,
wood reacts with oxygen to give off heat and light and produce carbon dioxide and water.
Combustion could be considered the first chemical reaction used by humans.
Today, it is still one of the most important chemical reactions we use.
Corrosion is a slow chemical change that occurs when oxygen in the air reacts with a metal.
Corrosion is a chemical reaction in which the metal is decomposed (eaten away), when it reacts
with other substances in the environment. Many metals can corrode. The green roofs of the parliament
buildings are an example of corrosion. The red-brown copper color is replaced with the green color
because copper corrodes.
Gold does not corrode. Solid solutions of metals (alloys) resist corrosion. Corrosion protection
involves protecting the metal from contact with the environment and the factors that affect the
reaction rate of this chemical reaction (e.g. painting the metal)
Cellular Respiration is a chemical reaction that takes place in the cells in your body.
C6H1206
+
O2
→
CO2
+
H2O
+ Energy
Identification Tests:
for OXYGEN
Light a wooden splint. Blow out the flame, allowing the splint to continue glowing.
Hold the glowing splint in a small amount of the unknown gas.
If the splint bursts into flame, then the gas being tested is oxygen.
for HYDROGEN
Light a wooden splint. Hold the glowing splint in a small amount of the unknown gas.
If you hear a "pop", then the gas being tested is Hydrogen.
for CARBON DIOXIDE
If you put a burning splint into Carbon Dioxide, the flame will go out and you will know the gas is not
oxygen or hydrogen, but you will not know for sure that it is Carbon Dioxide.
The test for Carbon Dioxide is not a combustion test, but rather uses a liquid called limewater
(a clear colorless solution of calcium hydroxide, or slaked lime) Bubble the unknown gas through the
limewater solution, or add a few drops of the limewater solution to the gas and swirl it around. If the
limewater turns milky, the gas is Carbon Dioxide
Conservation of Mass in Chemical Reactions
The Law of Conservation of Mass
In a chemical reaction, the total mass of the reactants, is always equal to the total mass of the
products.
The law ties in well with the atomic theory, which states that atoms are never created or destroyed.
In a chemical reaction the atoms and molecules are simply rearranged.
In an open system some of the mass seems to disappear, when it is in the form of a gas.
This law of conservation of mass however does not apply to nuclear reactions, because there Is
some loss of mass: the mass is changed into energy.
First suggested by Albert Einstein in his famous equation:
E =MC2
(E Is Energy, M is Mass,
C2 is a large number the square of the speed of light )
A very tiny amount of mass is equal to a very large amount of energy
Factors Affecting the Rate of a Chemical Reaction
The speed of a chemical reaction is called the reaction rate.
•
Temperature of the reactants affects the rate of all reactions
(The higher the temperature the faster the reaction rate)
•
Surface Area of the reactants affects the reaction rate
(The more surface in contact, the faster the reaction rate)
•
Concentration of the reactants affects the reaction rate.
(The higher the concentration, the faster the reaction rate)
•
The presence of a Catalyst affects the reaction rate
(Catalysts are substances that help a reaction proceed faster).
CATALYSTS
Catalysts are substances that help a reaction proceed faster. They are present with the reactants
of a reaction, but they are not consumed during the reaction.
Chemical reactions involving catalysts can be found in both living and non-living things. The most
common example in living things is in your body.
Many reactions, such as the breaking down of food, require a catalyst called an enzyme.
Without enzymes, many reactions would require much higher temperatures- a situation that would
be deadly to the human body.
Enzymes can help get rid of poisons in the body quickly.
For example, one product of reactions in cells is hydrogen peroxide (H2O2).
Hydrogen peroxide is poisonous.
An enzyme called catalase, which is found in many different types of animal and plant cells, speeds up
the breakdown of hydrogen peroxide into harmless oxygen and water.
K AND REFLECT