This document discusses chemical reactions and how to write balanced chemical equations. It defines physical and chemical changes, and notes that chemical changes result in new substances being formed. A chemical reaction is described as the rearrangement of atoms when bonds break and form. Signs that a chemical reaction occurred include changes in properties, energy, color, odor, state of matter. Chemical equations use symbols to represent elements and formulas for compounds. They show the reactants on the left and products on the right. For an equation to be balanced, the number of atoms of each element must be the same on both sides. The document explains how to identify the reactants and products and balance chemical equations by adding coefficients in front of the formulas and symbols.

1. Understanding Chemical
Reactions
Chapter 9.1
p 300 - 309

2. Changes is Matter
• Physical Changes • Chemical Changes
– Does not produce
new substances.
– Might have
different physical
properties
– Changes in states
of matter: solid,
liquid, gas
– One or more
substances change
into new substances
– Have different
physical and
chemical properties.
– Cooking is an
example of chemical
change.

3. Chemical Reactions
• Chemical changes are called chemical
reactions
• A chemical reaction is a process in which
atoms of one or more substances
rearrange to form one or more new
substances.

4. Signs of a Chemical Reaction
• Changes in the physical properties of
color, state of matter, and odor are all
signs that a chemical reaction might have
occurred
• A change is energy is another sign.

5. Signs of a Chemical Reaction
• Change in
Properties
• Change in Energy
– Warming or cooling
– Release of light
– Color
– Odor
– Formation of bubbles
– Formation of a
precipitate
* A precipitate is a solid that forms from a reaction between two liquids

6. What happens in a chemical
reaction?
• Atoms rearrange to form new substances
– There are 2 types of substances: Elements and
Compounds
– Substances have a fixed arrangement of atoms
– A drop of water each water molecule will have one
oxygen atom bonded to two hydrogen atoms
– If this arrangement changes the substance is no
longer water and a different substance has formed
– The new substance will have different chemical and
physical properties
– This is what happens during a chemical reaction

7. Chemical Reactions
• Bonds break and bonds form
– Atoms rearrange when chemical bonds
between the atoms break
– Particles are moving in all substances, even
solids
– These particles collide with one another.
– If particles collide with enough energy, the
bonds between the atoms can break,
separate, rearrange, and form new bonds.

9. Chemical Equations
• A chemical equation is a description of a
reaction using element symbols and
chemical formulas
• Element symbols represent elements
• Chemical formulas represent compounds

10. Elemental Symbols
• Each element can exist as just one atom
– C = carbon
Cu = Copper
• Some elements exist in nature as diatomic
molecules – two atoms of the same
element bonded together.
– This is shown by a subscript 2
– H2 = hydrogen
O2 = oxygen

11. Chemical Formulas
• A chemical formulas uses elements’
symbols and subscripts to describe the
number of atoms in a compound
• If an element does not have a subscript,
the compound contains only one atom of
that element
Balanced: 2Na + Cl2 => 2NaCl

12. Writing Chemical Equations
• A chemical equation included both the
substances that react and the substances that
are formed in a chemical equation.
• The starting substances in a chemical reaction
are the reactants
• The substances produced by the chemical
reaction are products
• The arrow shows the direction of the reaction
• Reactants are written left of the arrow and
products are written to the right

13. Conservation of Mass
• French chemist Antoine Lavoisier found
that the total mass of the reactants always
equaled the total mass of the products.
• This is the law of conservation of mass,
which states that the total mass of
reactants before the reaction must equal
the total mass of products after the
chemical equation.

14. Atoms are conserved
• Mass is conserved because atoms are
conserved
• Bonds break and new bonds form during a
chemical reaction
• However, atoms are neither created, nor
destroyed

15. Is an equation balanced?
• An equation is written so that the number
of atoms of each element is the same, or
balanced, on each side of the arrow
Reactants
Conservation of mass
Products
Conservation of atoms

16. Make it balanced!
Reactants
Elements
Products
4
2
H
4
2
2
O
2
1
4H
2
2
2O
4 H, 2 O
• Add Coefficients!
• A coefficient is a number placed in front of an element
symbol or chemical formula in an equation.
• Only coefficients can be changed when balancing an
equation.
• Changing subscripts changes the identities of the
substances that are in the reaction.

18. Balancing a Chemical Equation
1. Write the unbalanced equation.
2. Count atom of each element in the
reactants and in the products.
3. Add coefficients to balance the atoms.
4. Write the balanced chemical equation.

19. Balance
•
____ N2 + ____ H2 → ____ NH3
•
____ Al + ____ HCl → ____ AlCl3 + ____ H2 (g)

20. Lighting a Candle
• Lab