Chapter Seven: Chemical Reactions

Chemical Equations

Chemical Equations

A chemical reaction is the process by which one or more substances are changed into one or more different substances.

A chemical reaction can be represented by a chemical equation.

Chemical EquationsA correct chemical equation shows what changes take place. It also shows the amount of chemicals and compounds in a reaction.

2C2H2 + 5O2 4CO2 + 2H2O

(Reactants) (Yields) (Products)

Chemical Equations

Reactants = Starting substancesProducts = Substances Formed

Writing Balanced Equations

Determine the reactants and products

Write the reactants on the left side of the equation and the products on the right

Show an equal number of atoms for each element on both sides of the equation.

Example

When propane gas burns in the air, the reactants are propane (C3H8)and oxygen (O2). The products formed are carbon dioxide (CO2)and water (H2O).

Step 1: Find the reactants and products.

Example

Step 2: Write a chemical equation

C3H8 + O2 CO2 + H2O

Example

Step 3: Balance the chemical equation

*In balancing a chemical equation, change only the coefficients. Never change the subscripts.

C3H8 + 5O2 3CO2 + 4H2O

Physical StatesBesides specifying the compounds

involved in the reaction, we often indicate the physical states of the reactants and products by the following symbols: (s)Solid (l)Liquid (g)Gas (aq)Dissolved in water

Physical States

Homework Assignment

Page 235: 33-36 Write the Q’s

Writing Reactions Identify the reactants and products and

write the equation for each of the following chemical reactions.

The key to getting this right is to balance each of the pieces correctly (Write the balanced formula. Remember from chapter 4?)

After writing the reactions balance them.

Writing ReactionsThere are a few elements that need to be

treated in a special way because of how they bond with each other.

Example: Elemental oxygen, is never found by itself. It is always found as O2. Oxygen is one of the diatomic elements. This means they are always paired up.

There are 7 diatomic elements: H, I, Br, O, N, Cl, F. (Remember HI BrONClF!) These elements are all gases!

Writing ReactionsWhen writing the balanced formula for

elemental metals they are always written by themselves.

Example: Elemental Copper is just Cu. Elemental Aluminum is just Al.

This will be the case for all elemental metals.

Writing Reactions

Example 1:

Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas.

Step 1: Find the reactants and products

Writing Reactions

The reactants are Mg metal and Water, the products are Mg hydroxide and H gas.

Step 2: Write the balanced formula for each piece.

Mg (s) = Magnesium Metal H2O (l) = Water

Mg(OH)2 (s) = Magnesium Hydroxide

H2 (g)= Hydrogen gas

Writing Reactions

Step 3: Put the reactants and products into a chemical reaction.

Mg(s) + H2O(l) Mg(OH)2 (s) + H2(g)

Step 4: Balance the reaction

2

Writing Reactions

Write the following reactions and then balance them:

1. Solid Ammonium dichromate decomposes to solid chromium (III) oxide, gaseous nitrogen, and gaseous water.

2. Gaseous ammonia (NH3) reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

AssignmentPage 234 #’s 9-11, 13, 18, 21, 25-28I want you to write the question, circle

the reactants, underline the products. Then write out the reaction in symbol form and balance it.