4.1 Introduction to Chemical Reactionspp. 152 - 155

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

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)

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

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)

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

The Conservation of Mass

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

Steps to balancing an equation

Step 1 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

Steps to balancing an equation

Step 3 Count the number of atoms on each side of

the equation

Type of atom Reactants Products

Al 1 1

Br 2 3

Steps to balancing an equation

Step 4 Multiply each of the formulas by the appropriate

coefficients to balance the number of atoms.

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

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

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

More examples on the board

Sodium + chlorine sodium chloride

Nitrogen + hydrogen ammonia

Copper + silver nitrate copper nitrate + silver

Homework

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