1. Unit 2 – Chemical Reactions
Chemical Equations

2. Before we begin….
Do you remember the signs that a chemical change has taken place?
Chemical Change = any change in which a new substance is formed
Evidence of Chemical Change:
Change in colour
Change in odour
Formation of gas/solid
Release/absorption of heat

3. Collision-Reaction Theory
A theory stating that chemical reactions involve collisions and rearrangements of atoms or groups of atoms and that the outcome of collisions depends on the energy and orientation of the collisions
No reaction occurs if:
Molecules don’t have enough energy
Molecules don’t collide in the right orientation

4. Chemical Equations
A short form method used to represent chemical reactions, using symbols and formulas.
Indicates the: chemical formulas, relative number of entities and states of matter of the reactants and products.
In general:
REACTANTS PRODUCTS
Ex:
NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)
reacts with
yields
and

5. Word Equation This equation is used to identify substances by name.
Ex: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)
An aqueous solution of sodium hydroxide reacts with hydrochloric acid to yield an aqueous solution of sodium chloride and water.

6. Chemical Equations
Skeleton equation lists the chemical formulas of the substances.
Ex: Fe(s) + O2(g)  Fe2O3(s)
Is this equation balanced?
Law of Conservation of Mass
In any chemical reaction mass of reactants = mass of products

7. Relative # of Entities: Balanced Chemical Equation
use coefficients to balance
Coefficient = a whole number indicating the ratio of molecules of each substance involved in a chemical reaction
The large number on the left side of a molecule’s formula
Example: Mg Cl  MgCl2
Example: 6 K + N2  2 K3N

8. Ex: Fe(s) + O2(g)  Fe2O3(s)
Example: Mg Cl  MgCl2
Example: 6 K + N2  2 K3N
Notice…there are equal numbers of each type of atom on both sides of the equation
But you cannot change any of the chemical formulas
Follow the “trial and error” method. (see handout provided)
Ex: Fe(s) O2(g)  Fe2O3(s)
Ex: Fe(s) O2(g)  2 Fe2O3(s)

9. State of Matter Example: 6 K(s) + N2(g)  2 K3N(s) Solid = (s)
Liquid = (l)
Gas = (g)
Solution = (aq)
Example: 6 K(s) + N2(g)  2 K3N(s)

10. Now Try This Worksheet “Balancing Chemical Equations” # 1,3,5 ...
Worksheet “Word Equations” page 1 all

11. Homework
p121 textbook: #1, 2, 4, 5, 6, & 10