Changes in Matter
Chapter 4
1. Physical Changes
-Characteristic properties and the nature of the
substance remain unchanged.
Ex: snow melting, ripping paper, crushing a can
2. Chemical Changes
- Bonds between atoms in molecules are rearranged,
leading to the formation of new substances.
- Characteristic properties are altered.
Ex: burning wood, cooking an egg, cellular respiration, explosions
3. Nuclear Transformations
- Protons and neutrons are rearranged inside an
atom’s nucleus creating new elements.
Ex: Deuterium + tritium  helium
Try This!
Chemical Changes
Signs of a chemical change:
-release of a gas
-release or absorption of heat
-emission of light
-change in colour
-formation of a precipitate
(residue)
Chemical changes are also called chemical reactions.
They can be represented in chemical equations.
The Law of Conservation of Mass
Matter cannot be created or destroyed.
 Mass of the reactants must equal mass of products.
Ex: If you react 16 g of H2 to produce 50 g of H2O, how much O2 did you
use?
2 H2 +O2  2 H2O
***** DO NOT MULTIPLY MASSES BY THE COEFFICIENTS!!!!! *******
Try This!
During a chemical reaction 15 g of hydrochloric acid (HCl) reacts
with 25 g of aluminum (Al) to produce 29 g of aluminum chloride
(AlCl3) and some hydrogen gas (H2) is released. The balanced
equation for this reaction is as follows:
6 HCl + 2 Al → 2 AlCl3 + 3 H2
What mass of hydrogen gas was released?
Conservation of Mass
Atoms cannot be created or destroyed!
Particle Model
Atoms: smallest particle of matter
Molecule: a group of 2 or more atoms
Ex: H2O  this is a molecule composed of 2 hydrogen atoms and 1 oxygen atom.
Subscripts tell us how many atoms of each type are in a molecule.
H2SO4  2 hydrogen, 1 sulfur, 4 oxygen
Coefficients tell us how many molecules there are.
2 CO2  2 carbon dioxide molecules, 2 carbon atoms and 4 oxygen atoms
 See Particle Model Worksheet!
Using Symbols
- Pick a shape to represent each type of atom
- Use a line to show the bond between the atoms.
Examples:
H2O
AlCl3
H3PO4
2 NO2
3 PO4
Balancing Chemical Equations
Rules:
•
•
•
•
•
•
Place coefficients in front of reactants or products.
DO NOT change subscripts.
Coefficients must be whole numbers.
Reduce coefficients to lowest possible form.
Count atoms of each type on both sides of arrow.
Draw pictures if it helps!
Example:
N2 +
CH4
+
O2
H2


NH3
CO2
+
H2O
Stoichiometry
Used to determine the number of moles/mass of
reactants or products in a chemical reaction.
Ex. Calculate the amount of oxygen needed to react
completely with 1.00 g of hydrogen in the following
equation:
Molar Ratio:
Mass:
MM:
mols:
2 H 2 + O 2  2 H 2O
Try This!
Iron oxide, Fe2O3, reacts with carbon, C, to produce
iron, Fe, according to the following balanced equation:
Fe2O3 + 3 C → 2 Fe + 3 CO
What mass of carbon is required for every 6.0 moles of
Fe2O3 reacted?
Endothermic and Exothermic Reactions
Endothermic reaction:
Chemical reaction where energy (heat) is absorbed.
• Requires constant source of energy
• Energy is taken in from the surrounding environment (it
can be written as a reactant in a chemical equation).
Exothermic reaction:
Chemical reaction where energy (heat) is released.
• Requires energy only to be initiated.
• Energy released is absorbed by the surrounding
environment (it can be written as a product in a chemical
reaction)
Chemical Bonds
• Breaking chemical bonds in molecules requires energy.
• Forming a chemical bond releases energy.
Determining whether a reaction is exothermic or endothermic
1. Calculate the amount of energy required to break the
bonds of the reactants.
2. Calculate the amount of energy released by the formation
of the new bonds of the products.
3. Step 1 – Step 2
Try This!
CH4 + 2 O2  CO2 + 2H2O
Step 1: Find the energy needed to break the bonds of the reactants CH4 + 2 O2
(4 x 414 kJ) + (2 x 498 kJ) = 2652 kJ
Step 2: Find the energy released by the formation of the new bonds in the
products CO2 + 2H2O
(2 x 741 kJ) + (2 x 2 x 464 kJ) = 3338 kJ
Step 3: Find out whether the overall reaction has taking in or given off heat.
Step 1 – Step 2
2652 kJ – 3338 kJ = -686 kJ
Because the overall result is negative, this means that the bonds of the products
released more energy than the amount that was taken in to break the bonds of
the reactants.
 A negative result indicated an exothermic reaction.
Try This!
Calculate the reaction energy for the electrolysis of
water. Determine whether it is an endothermic or
exothermic reaction.
2 H2O  2 H2 + O2
Reaction Energy
Endothermic vs. Exothermic Reaction
Energy Diagrams
Calculating Reaction Energy using Stoichiometry
Determine the reaction energy if 32g of methane
react with sufficient oxygen using the equation
below:
CH4 + 2 O2  CO2 + 2 H2O + 686 kJ
1.
•
•
Types of Chemical Change
Acid-Base Neutralization
We can neutralize an acid with a base and vice versa.
A neutralization reaction always produces water and a salt.
Acid (aq) + Base (aq)  Water (l) + Salt (aq)
Reminder: How can you identify an acid, base, salt and water by the
molecular formula?
How does it work?
The H+ ions from the acid and
the OH- ions from the base will
join together to form H2O (water).
The remaining metal from the base
and the non-metal from the acid will
join together to form the salt.
Try This!
How do we recognize a neutralization reaction?
Are the following equations balanced?
a) HCl + NaOH  H2O + NaCl
b) HF + KOH  H2O + KF
c) H2SO4 + Mg(OH)2  2 H2O + MgSO4
d) 2 HBr + Ca(OH)2  2 H2O + CaBr2
2. Combustion
• Reaction requiring oxygen (oxidation reaction) that releases
energy
• Products are always CO2, H2O and Energy
Ex: wood burning, iron rusting, cellular respiration
Fuel + O2  CO2 + H2O + Energy
There are 3 things necessary for combustion to occur:
1. Oxidizing agent (oxygen)
2. Fuel (wood, gas)
3. Ignition Temperature (heat, spark)
Types of Combustion
Type 1: Rapid
• Fast
• Releases a lot of heat and light
Ex: log fire, burning candle, combustion of gasoline in a car engine
Type 2: Spontaneous
• Fast
• No energy from an outside source is required
• Often unpredictable
Ex: forest fire
Type 3: Slow
• Occurs over a very long period of time
• Slow release of energy
Ex: rusting, decomposition, fermentation, cellular respiration
3. Cellular Respiration
• Glucose and oxygen react to form carbon dioxide, water and
energy.
• The energy produced allows your cells to work and keeps
your body temperature stable.
Sugar + Oxygen  Carbon dioxide + Water + Energy
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy
4. Photosynthesis
• Plants use solar energy, carbon dioxide and water to
produce glucose and oxygen.
• Plants produce glucose to feed themselves (producers) and
oxygen that we benefit from.
Carbon dioxide + water + energy  glucose + oxygen
6 CO2 + 6 H2O + Energy  C6H12O6 + 6 O2
Nuclear Transformations
• Reactions that occur in the nucleus of an atom.
• Natural process in which unstable atoms spontaneously
transform into more stable atom(s).
• Energy is released in the form of ionizing radiation.
There are 3 types of radiation:
Type
Charge
Symbol
Penetrating Ability
Alpha
+ 2
α
Stopped by paper
Beta
-1
β
Stopped by foil (at
least 3mm)
Gamma
None
γ
Stopped by concrete
Nuclear Stability
• The nucleus of an atom is composed of protons and neutrons.
• Protons repel each other (like-charges), however the neutrons act
like glue keeping the protons together in the nucleus.
• Most nuclei remain stable because the force of attraction
between neutrons and protons (nuclear force) is greater than the
electrical repulsion between protons.
• If the force of repulsion of protons is greater than the nuclear
force, the atom is unstable and will decay.
The Stability of the Nucleus Depends on:
1. Size of the atom (all atoms with atomic # greater than
83 are unstable)
The larger the atom, the more protons are found in the
nucleus. Because of a larger number of protons, it is very
difficult to offset the repulsion.
2. The number of neutrons in the atom. (The more neutrons,
the more stable the atom as it increases the space
between protons, thus reducing repulsion.)
Reminder: How do we calculate the number of neutrons an
element contains? What is an isotope?
Half-Life
• The time required for half of
the nuclei in a sample of
radioactive material to decay.
• Each substance has a specific
half-life.
• The longer the half-life, the
longer it takes to eliminate that
element from the environment.
Types of Nuclear Transformation
1. Nuclear Fission
-
Neutron hits a nucleus of atom which is split to form two lighter nuclei.
Neutrons are released which bombard other nuclei (chain reaction).
Difficult to control
# protons and mass conserved throughout
the reaction.
2. Nuclear Fusion
• The principal source of energy in stars (sun).
• Occurs when 2 small nuclei collide and join to form a heavier
one and a neutron is released.
• Can only occur at extreme temperatures ( one million
degrees)