1. 4.1 Introduction to Chemical Reactionspp

2. Chemical Reaction
A processes in which one or more substances changes into one or more new substances
Evidence of Chemical Reactions:
Unexpected change in colour
Energy is released or absorbed
A gas is produced
A precipitate (solid) forms when mixing two solutions

3. Describing Chemical Reactions
A word equation is a way of representing a chemical reaction. It tells you what reacts and what is produced.
Specific conditions of the reaction may be included (e.g. catalyst – a substance that makes a reaction occur faster)

4. Writing word equations
They are written in a particular order.
Reactants (what you start with) are always on the left side of the arrow
Products (what you make) are always on the right side of the arrow
Multiple reactants or products are separated by a + sign

5. Skeleton Equations
A representation of a chemical reaction where the formulae of the reactants are connected to the formulae of the products by an arrow.
e.g. CH4(g) + O2(g)  H2O(l) + CO2(g)
This provides more detail than a word equation (e.g. state of reactants / products)
(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (i.e. dissolved in water)

6. The Conservation of Mass
In a chemical reaction, the total mass of the reactants is always equal to the total mass of the products
Atoms do not change in a reaction. The molecules that they form can be changed but the atoms themselves are not changed

7. The Conservation of Mass

8. Balanced Chemical Equations
Provide even more information than the skeleton equations (i.e. coefficients are used to show the ratio of chemicals which follows the law of conservation of mass)
This equation is balanced because the total number of atoms of each type are the same on both the reactant and product sides

9. Steps to balancing an equation
Write the word equation of the reaction
e.g. aluminum + bromine  aluminum bromide
Step 2
Write the skeleton equation by replacing each name with a correct formula.
e.g. Al +Br2  AlBr3

10. Steps to balancing an equation
Count the number of atoms on each side of the equation
Type of atom
Reactants
Products Al 1 Br 2 3

11. Steps to balancing an equation
Multiply each of the formulas by the appropriate coefficients to balance the number of atoms.

12. Start out by picking the element with the most number of atoms and try to balance it first
Start with Bromine.
The 2 and 3 will be balanced if we multiply the reactant side by 3 which would give it 6 Br, and multiply the product side by 2 to give us 6 Br.
Now we have 2 Al products which need to be balanced so we add a 2 to the Al on the reactant side.
2Al + 3Br2  2AlBr3

13. Tips for Balancing Chemical Equations
Remember that H, O, F, Br, I, N, and Cl exist as diatomic elements (i.e. H2, O2, etc.)
Balance compounds first and elements last
Balance hydrogen and oxygen last
If a polyatomic ion appears in both a reactant and a product, think of it as a single unit

14. Tips for Balancing Chemical Equations
Once you think the chemical equation is balanced, do a final check by counting the atoms of each element one more time
If you go back and forth between two substances, using higher and higher coefficients, double-check each chemical formula

15. More examples on the board
Sodium + chlorine  sodium chloride
Nitrogen + hydrogen  ammonia
Copper + silver nitrate  copper nitrate + silver

16. Homework
Read pp. 152 – 155
Answer # 1, 3 – 5 on p. 155