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13.1: Chemical Reactions and Equations

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Lesson Objectives

The student will:

  • explain what happens during a chemical reaction.
  • identify the reactants and products in any chemical reaction.
  • convert verbal descriptions of chemical reactions into chemical equations, and vice versa.
  • use the common symbols (s), (l), (g), (aq), and  \rightarrow appropriately.

Vocabulary

  • chemical reaction
  • products
  • reactants

Introduction

In a chemical change, new substances are formed. In order for this to occur, the chemical bonds of the substances break, and the atoms that make up the substances separate and re-arrange themselves into new substances with new chemical bonds. When this process occurs, we call it a chemical reaction. A chemical reaction is the process in which one or more substances are changed into one or more new substances.

In order to describe a chemical reaction, we need to indicate what substances are present at the beginning and what substances are present at the end. The substances that are present at the beginning are called reactants, and the substances present at the end are called products.

Writing Chemical Equations

When sulfur dioxide is added to oxygen, sulfur trioxide is produced. In the chemical equation shown below, sulfur dioxide and oxygen (\text{SO}_2 and \text{O}_2) are reactants, and sulfur trioxide (\text{SO}_3) is the product.

The general equation for a reaction is:

Reactants  \rightarrow Products

There are a few special symbols that we need to know in order to communicate in chemical shorthand. In Table below is a summary of the major symbols used in chemical equations. There are other symbols, but these are the main ones that we need to know.

Common Symbols in Chemical Reactions
Symbol Meaning Example
 \rightarrow separates reactants from products; can be read as “to produce” or “to yield” 2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O}
+ separate reactants from each other or products from each other; can be read as “is added to” \text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3
(s) in the solid state sodium in the solid state = \text{Na}_{(s)}
(l) or (L) in the liquid state water in the liquid state = \text{H}_2\text{O}_{(l)}
(g) in the gaseous state carbon dioxide in the gaseous state = \text{CO}_{2(g)}
(aq) in the aqueous state, dissolved in water sodium chloride solution = \text{NaCl}_{(aq)}

Chemists have a choice of methods for describing a chemical reaction. They could draw a picture of the chemical reaction, like in the image shown below.

Alternatively, they could describe the reaction in words. The image above can be described as two molecules of hydrogen gas reacting with one molecule of oxygen gas to produce two molecules of water vapor.

Chemists could also write the equation in chemical shorthand.

2 \text{H}_{2(g)} + \text{O}_{2(g)} \rightarrow 2 \text{H}_2\text{O}_{(g)}

In the symbolic equation, chemical formulas are used instead of chemical names for reactants and products, and symbols are used to indicate the phase of each substance. It should be apparent that the chemical shorthand method is the quickest and clearest method for writing chemical equations. For example, we could write out that an aqueous solution of calcium nitrate is added to an aqueous solution of sodium hydroxide to produce solid calcium hydroxide and an aqueous solution of sodium nitrate. In shorthand, however, we could simply write:

\text{Ca(NO}_3)_{2(aq)} + 2 \text{NaOH}_{(aq)} \rightarrow \text{Ca(OH)}_{2(s)} + 2 \text{NaNO}_{3(aq)}

How much easier is that to read? Let's try it in reverse. Look at the following reaction in shorthand notation and describe the reaction in words.

\text{Cu}_{(s)} + \text{AgNO}_{3(aq)} \rightarrow \text{Cu(NO}_3)_{2(aq)} + \text{Ag}_{(s)}

The description of this reaction might read something like “solid copper reacts with an aqueous solution of silver nitrate to produce a solution of copper(II) nitrate and solid silver.”

Example:

Transfer the following symbolic equations into verbal descriptions or vice versa.

  1. \text{HCl}_{(aq)} + \text{NaOH}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_2\text{O}_{(l)}
  2. Gaseous propane, \text{C}_3\text{H}_8, burns in oxygen gas to produce gaseous carbon dioxide and liquid water.
  3. Hydrogen fluoride gas reacts with an aqueous solution of potassium carbonate to produce an aqueous solution of potassium fluoride, liquid water, and gaseous carbon dioxide.

Solution:

  1. An aqueous solution of hydrochloric acid reacts with an aqueous solution of sodium hydroxide to produce an aqueous solution of sodium chloride and liquid water.
  2. \text{C}_3\text{H}_{8(g)} + \text{O}_{2(g)} \rightarrow \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)}
  3. \text{HF}_{(g)} + \text{K}_2\text{CO}_{3(aq)} \rightarrow \text{KF}_{(aq)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)}

Lesson Summary

  • A chemical reaction is the process in which one or more substances are changed into one or more new substances.
  • Chemical reactions are represented by chemical equations.
  • Chemical equations have reactants on the left, an arrow that symbolizes “yields,” and the products on the right.

Further Reading / Supplemental Links

This video shows ten amazing chemical reactions that are fun to watch but dangerous to carry out.

Review Questions

  1. Mothballs are commonly used to preserve clothing during off-season. We recognize mothballs by its smell because of a chemical compound known as naphthalene, \text{C}_{10}\text{H}_8. What are the different elements found in naphthalene, and how many atoms of each are found in the formula?
  2. Give the verbal description of the following chemical equations.
    1. \text{H}_2\text{SO}_{4(aq)} + \text{NaCN}_{(aq)} \rightarrow \text{HCN}_{(aq)} + \text{Na}_2\text{SO}_{4(aq)}
    2. \text{Cu}_{(s)} + \text{AgNO}_{3(aq)} \rightarrow \text{Ag}_{(s)} + \text{Cu(NO}_3)_{2(aq)}
    3. \text{Fe}_{(s)} + \text{O}_{2(g)} \rightarrow \text{Fe}_2\text{O}_{3(s)}
  3. Write the chemical equations for the following reactions.
    1. Solid calcium metal is placed in liquid water to produce aqueous calcium hydroxide and hydrogen gas.
    2. Aqueous sodium hydroxide is mixed with gaseous chlorine to produce aqueous solutions of sodium chloride and sodium hypochlorite plus liquid water.
    3. Solid xenon hexafluoride is mixed with liquid water to produce solid xenon trioxide and gaseous hydrogen fluoride.

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