Describing Chemical ReactionsOn May 6, 1937, the huge airship Hindenburg erupted into a fireball.

Within a short time, 210,000 cubic meters of hydrogen had burned and the airship was destroyed.

The chemical reaction that

occurred is

“hydrogen combines with

oxygen to produce water.”

Basic chemical equation format

Reactants Products

yields

Word Equations use chemical names

bleach + ammonia poisonous gas

write the formulas of the reactants to the left of the yields sign (arrow) and the formulas of the products to the right.

To write a chemical equation

H2 + O2 H2O

Reactants Product

An unbalanced(skeleton) equation does not indicate the relative amounts of the reactants and products.

Here is the unbalanced equation for rusting:

Fe + O2 Fe2O3

Why is it unbalanced?

Methane + Oxygen Carbon dioxide + Water

CH4 + O2 CO2 + H2O

A catalyst is a substance that speeds up the reaction but is not used up in the reaction.

Without Catalyst With Catalyst

Special conditions, including use of a catalyst, are written above the arrow

H2O2 H2O + O2

KI

Writing Chemical Equations

Complete chemical equations show the state of matter (in parenthesis) after each formula.

NaCl (s )

H2O (l )

O2 (g )

iron + oxygen iron(III) oxide

Fe(s) + O2(g) Fe2O3(s)

Hydrogen Peroxide Water and Oxygen

H2O2(aq) H2O(l) + O2(g)

Chemical equations are not complete unless they are BALANCED.

To write a balanced equation add the coefficients that will show the reaction obeying the law of conservation of mass.

PRODUCTSREACTANTS

Coefficients are small whole numbers that are placed in front of the formulas in an equation in order to balance it.

subscripts coefficient

2H2 + O2 2H2O

NEVER change subscripts

subscriptscoefficients

NEVER change subscripts

In a balanced equation each side of the equation has the same number of atoms of each element and mass is conserved.

1. Propane gas reacts with oxygen to produce water vapor and carbon dioxide. Choose the correct word equation for this reaction.

a. propane + carbon dioxide water + oxygen

b. propane + oxygen + water carbon dioxide

c. propane + oxygen + water + carbon dioxide

d. propane + oxygen water + carbon dioxide

2. What coefficient for H2SO4 is required to balance the following equation?

Ca3(PO4)2 + ____ H2SO4 3CaSO4 + 2H3PO4

a. 1

b. 2

c. 3

d. 4