Transcript Ch. 3 - Chemical Reactions

Chemical Reactions
Intro to Reactions
I
II III IV V
What is a Chemical Reaction
A chemical reaction is a change in
which one or more reactants change
into one or more products.
 Bonds are broken and reformed

What makes up a Chemical
Reaction

A chemical reaction is made up of chemical
compounds, represented as chemical
formula
- Empirical Formula
- Molecular Formula

These compounds begin as reactants that
then chemically change to become
products.
What are reactants & products

A reactant is a substance present at
the start of a reaction.

A product is a substance present at
the end of a reaction.

Reactants and products are written
as chemical equations for us to read
and use.
Chemical Equations
A+B  C+D
REACTANTS
PRODUCTS
Chemical Equations (symbols & meanings)
C. Johannesson
Chemical Equations
Coefficients and Subscripts
Which is What
•Subscript : the number located slightly below
any element in a formula. Represents the
ratio of numbers present in a molecule.
H20
Subscript
•Coefficient : Numbers placed in front of a
formula to help balance an equation (related to
moles)
2H20
Coefficient
Signs of a Chemical Reaction
Creation of heat and light
 Formation of a gas
 Formation of a precipitate
 Color change

Writing Equations
2H2(g) + O2(g)  2H2O(g)

Identify the substances involved.

Use symbols to show:
 How many? - coefficient
 Of what? - chemical formula
 In what state? - physical state
Practice Writing Equations
Two atoms of aluminum react with
three units of aqueous copper(II)
chloride to produce three atoms of
copper and two units of aqueous
aluminum chloride.
• How many?
• Of what?
• In what state?
2Al(s) + 3CuCl2(aq)  3Cu(s) + 2AlCl3(aq)
Special Words Describing
Equations

Describing Coefficients:
 individual atom = “atom”
 covalent substance = “molecule”
 ionic substance = “unit”
3CO2  3 molecules of carbon dioxide
2Mg
 2 atoms of magnesium
4MgO  4 unitsC. of
magnesium
oxide
Johannesson
Describing Equations
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
• How many?
• Of what?
• In what state?
One atom of solid zinc reacts with
two molecules of aqueous
hydrochloric acid to produce one unit
of aqueous zinc chloride and one
C. Johannesson gas.
molecule of hydrogen
Law of Conservation of Mass

mass is neither created nor destroyed
in a chemical reaction
total mass stays the same
 atoms can only rearrange

4H
36 g
2O
4H
2O
4g
32 g
C. Johannesson
Law of Conservation of Mass

Because of this law the number of
atoms on the left side of the equation
must ALWAYS equal the number of
atoms on the right side of the
equation.
4H
36 g
2O
4H
2O
4g
32 g
C. Johannesson
Balancing Equation Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest
possible ratio, if necessary.
5. Double check atom balance!!!
Helpful Tips
Balance one element at a time.
 Update ALL atom counts after adding
a coefficient.
 If an element appears more than
once per side, balance it last.
 Balance polyatomic ions as single
units.
 “1 SO4” instead of “1 S” and “4 O”

Balancing Example
Aluminum and copper(II) chloride react
to form copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
2 1
Al
1 2
3 1
Cu
1 3
6 2
Cl
3 6