Writing Chemical Reactions

Objectives

  • Determine if a process is exothermic or endothermic.
  • Write chemical formulas from chemical names.
  • Write and interpret balanced chemical reactions.
  • Identify types of chemical reactions.
  • Predict the products of a chemical reaction.
  • Draw conclusions based on experimental data.

Introduction

The purpose of this lab is to provide students with an opportunity to conduct chemical reactions and then predict and write the full, balanced chemical equation for the reaction.

Classifying Types of Chemical Reactions

There are 5 basic types of chemical reactions. Being able to recognize when each of these reactions occurs aids in predicting the products that form.

Combination Reaction

These reactions occur when two substances combine to make a single new substance. Combination reactions follow the general formula:

A + B → AB

Decomposition Reaction

Decomposition reactions occur when a compound breaks into two new substances. Decomposition reactions follow the general formula:

AB → A + B

Decomposition and combination reactions are the opposite of each other.

Single-Replacement Reaction

Single-replacement reactions occur when an element reacts with a compound. One of the elements within the compound is replaced by the element it is reacting with. Single replacement reactions follow the general formula:

A + BC → B + AC

Double-Replacement Reaction

Double-Replacement Reactions (aka the Do-si-do Reaction) involve the reaction of two compounds. The cations in each compound exchange anion partners, forming two new compounds. Double-replacement reactions follow the following formula:

AB + CD → AD + CB

Combustion

Combustion reactions occur when a compound reacts with oxygen gas to produce heat energy.  Typically, hydrocarbons (compounds made only of carbon and hydrogen) are involved in these reactions; however, combustion can also occur with other compounds. The complete combustion of hydrocarbons always produces carbon dioxide and water. Below is the basic formula for a hydrocarbon reaction:

Hydrocarbon + O₂ → CO₂ + H₂O

Below is a generic reaction of a combustion reaction that does not involve a hydrocarbon, where A is the element reacting with oxygen.

A + O₂ → AO

Notice how the above combustion reaction also resembles a combination reaction. Simply replace the O with B, and they follow the same formula. This means that a combustion reaction may also be a combination reaction

Combination Reaction: A + B → AB