BILL NYE THE SCIENCE GUY
7.1: DESCRIBING REACTIONS
The most useful way of describing reactions (or a
chemical change) is by stating what was present
before and after the chemical change.
process in which one or more substances are changed into new
substance that reacts
the new substance that is formed
CONSERVATION OF MASS
The French chemist Antoine Lavoisier
established that the total mass of the products
always equals the total mass of the reactants.
This is called the Law of Conservation of Mass (mass is
not created nor destroyed in a chemical reaction).
shorthand method to describe a chemical
reaction using chemical formulas and
Hg(l) + O2(g)
(aq) – aqueous (substance dissolved in water)
(l) – liquid
(g) – gas
coefficients – the numbers to the left of the formulas used to help
balance the equation
If you notice that the number of atoms on the left side does
not equal the number of atoms on the right side then we
must balance the equation.
Since mass is conserved before and after a chemical reaction, an equation MUST BE
balanced in order for it to be true.
BALANCING EQUATION RULES
1. You should NEVER change the subscripts in a
2. Start by, counting the number of atoms of each
element on each side of the equation.
3. Change one or more coefficients until the
equation is balanced
Balance the following equation
NiCl2(aq) + NaOH(aq) Ni(OH)2(s) + NaCl(aq)
HgO(s) Hg(l) + O2(g)
7.2: TYPES OF REACTIONS
are classified by the type of reactant or
the number of reactants and products.
There are 4 different types of reactions that we
A synthesis reaction is a reaction in which two or
more substances react to form a single substance.
The product synthesized is always a compound
A + B AB
2Na + Cl2 2NaCl
The opposite of synthesis
A decomposition reaction is a reaction in which a
compound breaks down into two or more simpler
The reactant MUST BE a compound.
AB A + B
2H2O 2H2 + O2
A single replacement reaction is a reaction in
which one element takes the place of another
element in a compound.
A + BC B + AC
Cu + 2AgNO3 2Ag + Cu(NO3)2
A double replacement reaction is one in which
two different compounds exchange positive ions
and form two new compounds.
AB + CD AD + CB
Pb(NO3)2 + 2KI PbI2 + 2KNO3
7.3: ENERGY CHANGES IN REACTIONS
Chemical Energy – the energy stored in the
chemical bonds of a substance.
Chemical Reactions involve the breaking of
chemical bonds in the reactants and the formation
of chemical bonds in the products.
Breaking Bonds REQUIRES energy.
This means we need to ADD energy in order to break
the bonds of reacting molecules in order to get the
The formation of chemical bonds RELEASES
When new chemical bonds are formed, a bit of energy is
released usually in the form of heat or light.
ENERGY IN REACTIONS
During a chemical reaction, energy is either
ABSORBED or RELEASED.
We describe these reactions in two different ways either
Exothermic or Endothermic.
A chemical reaction that RELEASES energy is called
an exothermic reaction.
The energy released as the products form is greater
than the energy required to break the bonds in the
Think of it as energy is EXITING the reaction
EXiting _ Exothermic
A chemical reaction that absorbs energy from its
surroundings is called an endothermic reaction.
This means that there is more energy require to break
the bonds of the reactants than is released by the
formation of the products.
CONSERVATION OF ENERGY
The total amount of energy BEFORE a reaction is
EQUAL to the total amount of energy AFTER a
This is called the Conservation of Energy.
8.1: FORMATION OF SOLUTIONS
A solution is a mixture that forms when
substances dissolve and form a homogeneous
In order for a solution to form, one substance must
dissolve in another.
Every solution has two components
A solute is a substance who particles are dissolved in a
A solvent is the substance in which the solute dissolves
There are three ways that substances can dissolve
into water: dissociation, dispersion, ionization
In order for a solution to form, the attractions that hold the solute
together and the solvent together must be overcome.
The process in which an ionic compound separates
into ions as it dissolves is called dissociation.
Example: Sodium Chloride & Water
Sugar dissolves into water by dispersion, or
breaking into small pieces that spread throughout
Example: Sugar & Water
The process in which molecules gain or lose
electrons is known as ionization.
Example: Ions are formed by the reaction of the solute
and solvent particles.
PROPERTIES OF LIQUID SOLUTIONS
Conductivity: ability to conduct electric current
Boiling Point: temperature needed for solution to
change from liquid phase to gas phase
Freezing Point: temperature needed for solution to
turn from liquid phase to solid phase.
Solutions can also be described as endothermic or
exothermic depending upon whether energy is
released or absorbed.
8.2: SOLUBILITY & CONCENTRATION
The maximum amount of solute that dissolves in a
given amount of solvent at a constant temperature
is. called solubility
Depending upon the amount of solute in a solution,
solutions can be described as either saturated,
unsaturated or supersaturated.
A saturated solution is one that contains as much
solute as the solvent can hold at a given
A solution that has less than the maximum amount
of solute that can be dissolved is called an
A supersaturated solution is one that contains
more solute than it can normally hold at a given
Usually very unstable.
FACTORS AFFECTING SOLUBILITY
Polarity of the solvent
“like dissolves like”
Solution formation is more likely to happen when the solute
and solvent are either both polar or both nonpolar.
The solubility of a solids increases as the solvent
Increasing pressure on a gas increases solubility in a liquid.
The concentration of a solution is the amount
solute dissolved in a specified amount of solution.
Can be expressed as percent by volume, percent by
mass and molarity.
Each line on the graph is
called a solubility curve for a
You can use a solubility
curve to figure out how
much solute will dissolve at
any temperature given on
left uranium salt in a drawer with a photographic
when he developed the plate, he found an outline
of the clumps of the uranium salt
he hypothesized that the uranium salt emitted some
sort of energy
Marie and Pierre Curie
students of Becquerel
2 years later, they discovered Po and Ra while
studying uranium ore “pitch blende”
THEY DISCOVERED …
release of matter and energy from nucleus
Light energy (electromagnetic spectrum)
all forms of radiation
Protons are held together by strong
short range force
as the distance increases, the force weakens
causes protons and neutrons to be attracted to
to hold a nucleus together tightly, the
nucleus can decay and give off matter and
stays together permanently
nucleus does not stay together; emits matter and energy
the process of nuclear decay
elements after #83 are radioactive
all elements after #92 are synthetic and
decay soon after they are created
Mass number = # protons + # neutrons
Atomic number = # of protons
(12 = mass #)
(6= atomic #)
most elements have at least one radioactive
same element with a different number of neutrons
Example: Carbon-12 (stable)
the process of changing one element into another through
occurs until a stable nucleus is formed
alpha decay (α)
releases alpha particle (a helium nucleus)
a helium nucleus consists of 2 protons and 2 neutrons
atomic mass is 4
atomic number is 2
Beta Decay (β)
release beta particle it occurs when a neutron breaks down
into 1 electron and 1 proton
the result is an atom with 1 more proton
PRACTICE: ALPHA & BETA DECAY
process of splitting a nucleus into several smaller nuclei
Two nuclei with low masses are combined to form one
nucleus of larger mass
the amount of time it takes for
half the nuclei in a sample of the
isotope to decay
the nucleus left after the
isotope decays is called the
some half-lives are seconds,
others are millions of years
EXAMPLE: HALF LIFE
Assume a 20g sample of Ba-139 has a
half-life of 86 minutes.
how much Ba-139 remains after 86 minutes?
after 172 minutes?
how many ½ lives leave 1.25g of Ba-139?