1. Unit 3: Writing Chemical Reactions
SCH3U/4C
Ms. Richardson

2. Chemical Reactions
A chemical reaction is a process that occurs in which substances combine and interact and result in the formation of a new product.
Recall the evidence we use to indicate if a chemical change has occurred…

3. Evidence of a Chemical Change
Light is produced
Precipitate is formed
Gas is produced
Change in colour
Change in odour

4. Describing Chemical Reactions
Starting substances involved in a chemical reaction are known as reactants
The new substances formed from the interaction of the reactants are known as products

5. Chemical Equations
To make describing chemical reactions simpler, we use a condensed statement that represents the change occurring.
These ‘statements’ are known as a chemical equations.

6. Types of Chemical Equations
Chemical equations can be represented three ways:
Word equations
Skeletal equations
Balanced Chemical Equations
Symbols used in chemical reactions:
+ indicates more than 1 reactant/product
reaction arrow
indicates a reversible reaction

7. Word Equations Reactants and products are represented using only words
A reaction arrow separates reactants from products.
We read the reaction arrow as “reacts to produce”
iron + chlorine  iron (III) chloride
“iron combined with chlorine, reacts to produce iron (III) chloride”

8. Skeletal Equations
Chemical formulas are used to represent reactants and products.
Usually, the physical state of the substances involved are indicated.
Symbol
State
(s)
Solid
(l)
Liquid
(g)
Gas
(aq)
aqueous

9. Problems with Skeletal Equations
Skeletal equations are preferred over word equations as they provide more information.
However often times, the skeletal equations do not satisfy the Law of Conservation of Mass

10. Problems with Skeletal Equations
Example:
hydrogen + oxygen  water
H2(g) + O2(g)  H2O(l)
If we count the atoms involve:
Reactants: 2 H atoms + 2 O atoms
Products: 2 H atoms + 1 O atom

11. Satisfying the Law of Conservation of Mass
Recall that the subscript refers to how many atoms of that element.
All chemical reactions should have equal atoms going in and equal atoms being produced.
But, we cannot change subscripts in a compound (that changes the chemical composition!)

12. Balancing Chemical Equations
We can balance each side of the equation by adding coefficients.
This increases the number of each molecule we have, so that the number of atoms of each element will be equal.

13. Rules for Balancing Equations
Write the skeletal equation
Count the atoms of each element in the reactants and products
Add coefficients so that you have equal atoms on both sides
H2 + I2  HI
Reactants
Products
H = 2
H = 1
I = 2
I = 1
H2 + I2  2HI
Reactants
Products
H = 2
I = 2

14. Balancing Chemical Equations
If you have common multiples as your coefficients, you must reduce to the lowest ratio.
See pages 118 – 120 in your text (MGH 11) for extra tips on balancing.
Balancing takes practice, don’t let it frustrate you!!!!