Writing Chemical Equations

Building Science Champions

Objectives

Describe the information conveyed in a chemical equation.

Apply the principle of conservation of mass to chemical reactions.

Identify and describe three classes of chemical reactions.

Key Terms

Chemical equation Subscript Reactant Product Conservation of mass Coefficient Synthesis Decomposition Replacement

Chemical Equations

Chemical Equations are short, easy ways to show a chemical reaction by using elements’ symbols instead of words.

Symbols are easy to understand.

Writing Chemical Equations The formula will show the ratio of

elements in a compound. C12H22O11 is the chemical formula for sugar. The letter corresponds to a particular

element on the periodic table. The subscript corresponds to the element

that precedes the subscript. If there is no subscript after a letter then

there is one (1) of that particular element. CO2

Structure of an Equation

A chemical equation summarizes a chemical reaction.

The material you begin with is the REACTANT.

The completed reaction is the PRODUCT.

Reactant + Reactant Product + Product

= YIELDS

Structure of an Equation

When there are two or more reactants they are separated by a plus sign.

When there are two or more products they are separated by a plus sign.

The ratio of products to reactant can change.

Reactant + Reactant ProductReactant Product + ProductReactant + Reactant + Reactant

Product

Conservation of Mass

Regardless of the total number of reactants and products involved, all the atoms at the start of the reaction have to be present at the end of the reaction.

The amount of matter involved in a chemical reaction does not change. The total mass of the reactants must equal the total mass of the product.

The LAW of CONSERVATION

The law of conservation states:

▪“Matter cannot be created nor destroyed during a chemical reaction.”

Review Chemical equations are short hand

for chemical reactions. Subscripts go to the element before

them. To the left of the yield signs are

reactants and to the right are products.

There can be multiple products and reactants.

Matter cannot be created nor destroyed.

Checkpoint

How do masses of the atoms in the reactants of a chemical reaction compare with the atoms in the products?

The number of atoms remain equal and the total of mass is unchanged.

Balancing Chemical Equations

A chemical reaction must show the same number of each type of atom on both sides of the equation.

When this happens the equation is BALANCED.

Balancing Equation

H2 + O2 H2O Does the left side equal the right

side?

2 Hydrogen and 2 Oxygen on the left 2 Hydrogen and 1 Oxygen on the

right

Not equal. So what can be done to balance the equation?

H2 + O2 H2O

Add a coefficient of 2 in front of the H2O Coefficient is the number placed in front of

chemical formulas in an equation. A coefficient tells how many atoms or

molecules there are. The coefficient is factored out until the next

sign. H2 + O2 2 H2O Left side – 2 Hydrogen and 2 Oxygen Right side – 4 hydrogen and 2 Oxygen How can we balance them out?

H2 + O2 2 H2O

Place a 2 in front of the H2 in the reactant.

2H2 + O2 2H2O Left side – 4 Hydrogen and 2 Oxygen Right side – 4 Hydrogen and 2

Oxygen

We are balanced!!!!!!!!!!!!!!!!!

Mg + O2 MgO

Left side – 1 Magnesium and 2 Oxygen

Right Side – 1 Magnesium and 1 Oxygen

Is the equation balanced? Now what? Add 2 in front of Magnesium in the

reactant and Magnesium in the product.

2Mg + O2 2MgO

Are we balanced?

You Try C + Cl2 CCl4 C + 2 Cl2 CCl4

Al2O3 Al + O2 2 Al2O3 4 Al + 3 O2

***You can only change coefficients you CANNOT change subscripts. ***

Classifying Chemical Reactions

There are 3 major classifications of chemical reactions: Synthesis Decomposition Replacement

Synthesis

When two or more substances combine to make a more complex substance the process is called synthesis.

Put things together. Simple + Simple Complex More reactants than products

2 SO2 + O2 + 2 H2O 2 H2SO4

Sulfur dioxide + Oxygen + water yields sulfuric acid

Decomposition

Decomposition breaks down reactants into smaller products.

Complex Simple + Simple More products than reactants

2 H2O2 2 H2O + O2Hydrogen peroxide yields water and oxygen gas

Replacement

When one element replaces another in a compound, or when 2 elements in different compound trade places, this is replacement.

Reactant + Reactant Product + Product

CaCO3 + 2 HCl CaCl2 + H2O3

2 CuO + C 2 Cu + CO2

Review

Chemical equations show chemical reactions

Matter cannot be created nor destroyed

Three types of chemical reactions Synthesis Decomposition Replacement

References

Anderson, M. et all (2012) Physical Science. McGraw-Hill: Columbus

Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey