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Chapter 11

Matter and Change

11.1 Describing Chemical Reactions

11.2 Types of Chemical Reactions

11.3 Reactions in Aqueous Solution

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Fuel cells produce electricity through a chemical reaction without any of the combustion that you find in typical gasoline engines.

CHEMISTRY & YOU

How is a chemical reaction going to change the way you drive?

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Introduction to Chemical Equations

How do you write a skeleton equation?

Introduction to Chemical Equations

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Every minute of the day, chemical reactions take place—both inside you and around you.

After a meal, a series of chemical reactions take place as your body digests food.

Plants use sunlight to drive the photosynthetic processes needed to produce plant growth.

Introduction to Chemical Equations

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Introduction to Chemical Equations

All chemical reactions involve changing substances.

In a chemical reaction, one or more reactants change into one or more products.

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Introduction to Chemical Equations

Cooking food always involves a chemical reaction.

The recipe tells you which ingredients to mix together and how much of each to use.

Chemical reactions take place when the ingredients or reactants are mixed together and heated in the oven.
The product, in this case, is a batch of muffins.

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Introduction to Chemical Equations

Chemists use a chemical equation—a quick, shorthand notation—to convey as much information as possible about what happens in a chemical reaction.

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Introduction to Chemical Equations

How do you describe what happens in a chemical reaction?

The reactants are written on the left and the products on the right.
An arrow separates them.
You read the arrow as yields, gives, or reacts to produce.

Reactants → products

Word Equations

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Introduction to Chemical Equations

How could you describe the rusting of iron?

You could say, “Iron reacts with oxygen to produce iron(III) oxide (rust).”
It is quicker to identify the reactants and product by means of a word equation.

Word Equations

Iron + oxygen → iron(III) oxide

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Introduction to Chemical Equations

In a word equation, write the names of the reactants to the left of the arrow, separated by plus signs.
Write the names of the products to the right of the arrow, also separated by plus signs.

Notice that no plus sign is needed on the product side of the equation on the previous slide because iron(III) oxide is the only product.

Word Equations

Iron + oxygen → iron(III) oxide

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Introduction to Chemical Equations

The production of a new substance, a gas, is evidence of a chemical change.

Two new substances are produced in this reaction, oxygen gas and liquid water.

Word Equations

Hydrogen peroxide → water + oxygen

You could describe this reaction by saying “hydrogen peroxide decomposes to form water and oxygen gas.”

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Introduction to Chemical Equations

The burning of methane is a chemical reaction.

Word Equations

Methane + oxygen → carbon dioxide + water

Burning a substance typically requires oxygen, so methane and oxygen are the reactants.
The products are water and carbon dioxide.

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Chemical Equations

Introduction to Chemical Equations

A chemical equation is a representation of a chemical reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right).

Here is a chemical equation for rusting:

Fe + O₂ → Fe₂O₃

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Chemical Equations

Introduction to Chemical Equations

Equations that show just the formulas of the reactants and products are called skeleton equations.

A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products.
The first step in writing a complete chemical equation is to write the skeleton equation.

Fe + O₂ → Fe₂O₃

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Chemical Equations

Introduction to Chemical Equations

To write a skeleton equation, write the chemical formulas for the reactants to the left of the yields sign (arrow) and the formulas for the products to the right.

Fe + O₂ → Fe₂O₃

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Chemical Equations

Introduction to Chemical Equations

To add more information to the equation, you can indicate the physical states of substances by putting a symbol after each formula.

Use (s) for a solid, (l) for a liquid, (g) for a gas, and (aq) for a substance in an aqueous solution (a substance dissolved in water).

Fe(s) + O₂(g) → Fe₂O₃(s)

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Chemical Equations

Introduction to Chemical Equations

In many chemical reactions, a catalyst is added to the reaction mixture.

A catalyst is a substance that speeds up the reaction but is not used up in the reaction.
A catalyst is neither a reactant nor a product, so its formula is written above the arrow in a chemical equation.

H₂O₂(aq) H₂O(l) + O₂(g)

MnO₂

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Interpret Data

Symbols Used in Chemical Equations
Symbol	Explanation
+	Separates two reactants or two products
→	“Yields,” separates reactants from products
	Use in place of → for reversible reactions
(s), (l), (g)	Designates a reactant or product in the solid state, liquid state, or gaseous state; placed after the formula
(aq)	Designates an aqueous solution; the substance is dissolved in water; placed after the formula
	Indicates that heat is supplied to the reaction
	A formula written above or below the yields sign indicates its use as a catalyst (in this example, platinum).

Δ

heat

Pt

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Hydrochloric acid reacts with solid sodium hydrogen carbonate. The products formed are aqueous sodium chloride, water, and carbon dioxide gas. Write a skeleton equation for this chemical reaction.

Sample Problem 11.1

Writing a Skeleton Equation

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Write the correct formula for each substance in the reaction.
Indicate the state of each substance.
Separate the reactants from the products with an arrow.
Use plus signs to separate the two reactants and each of the three products.

Sample Problem 11.1

Analyze Identify the relevant concepts.

1

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Start with the names of reactants and products. Include their physical states.

Reactants

sodium hydrogen carbonate (solid)

hydrochloric acid (aqueous)

Products

sodium chloride (aqueous)

water (liquid)

carbon dioxide (gas)

Sample Problem 11.1

Solve Apply concepts to this situation.

2

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Write the correct formula for each reactant and product.

Reactants

NaHCO₃(s)

HCl(aq)

Products

NaCl(aq)

H₂O(l)

CO₂(g)

Sample Problem 11.1

Solve Apply concepts to this situation.

2

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Separate the reactants from the products with an arrow. Use plus signs to separate the reactants and the products.

NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

Sample Problem 11.1

Solve Apply concepts to this situation.

2

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We can use everyday words to describe chemical reactions. What is the advantage of using an equation?

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We can use everyday words to describe chemical reactions. What is the advantage of using an equation?

A chemical equation for a reaction is easier to read quickly. It shows all of the relevant information, including quantities of reactants and products, the direction of the reaction, and any catalysts in a set form that can be quickly read.

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Balancing Chemical Equations

What are the steps for writing and balancing a chemical equation?

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Balancing Chemical Equations

How would you write a word equation for the manufacture of bicycles?

Simplify your task by limiting yourself to four major components: frames, wheels, handlebars, and pedals.

Frame + wheel + handlebar + pedal → bicycle

Reactants

Product

Your word equation shows the reactants (the kinds of parts) and the product (a bicycle).

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Balancing Chemical Equations

But if you were responsible for ordering parts to make a bicycle, this word equation would be inadequate.

It does not indicate the quantity of each part needed to make one bicycle.

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Balancing Chemical Equations

A standard bicycle is composed of one frame (F), two wheels (W), one handlebar (H), and two pedals (P).

The formula for a bicycle would be FW₂HP₂.
This equation is unbalanced.

F + 1W + H + 1P → FW₂HP₂

An unbalanced equation does not indicate the quantity of reactants needed to make the product.

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Balancing Chemical Equations

A complete description of the reaction must include not only the kinds of parts involved, but also the quantities of parts required.

This equation for making a bicycle is balanced.

F + 2W + H + 2P → FW₂HP₂

To balance the equation, the number 2 was placed before wheels and pedals.
The number 1 is understood to be in front of frame, handlebar, and bicycle.

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Balancing Chemical Equations

These numbers are called coefficients— small whole numbers that are placed in front of the formulas in an equation in order to balance it.

The number of each bicycle part on the reactant side is the same as the number of those parts on the product side.

F + 2W + H + 2P → FW₂HP₂

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A chemical reaction is also described by a balanced equation in which each side of the equation has the same number of atoms of each element and mass is conserved.

Balancing Chemical Equations

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A chemical reaction is also described by a balanced equation in which each side of the equation has the same number of atoms of each element and mass is conserved.

Balancing Chemical Equations

As reactants are converted to products, the bonds holding the atoms together are broken, and new bonds are formed.
The atoms themselves are neither created nor destroyed; they are merely rearranged.
In any chemical change, mass is conserved.

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Balancing Chemical Equations

To write a balanced chemical equation, first write the skeleton equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass.

In every balanced equation, each side of the equation has the same number of atoms of each element.

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C(s)�Carbon

+

O₂(g)�Oxygen

CO₂(g)�Carbon dioxide

Reactants�1 carbon atom, 2 oxygen atoms

Product�1 carbon atom, 2 oxygen atoms

Balancing Chemical Equations

Carbon burns in the presence of oxygen to produce carbon dioxide.

This equation is balanced.
You do not need to change the coefficients.

They are all understood to be 1.

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Balancing Chemical Equations

When hydrogen and oxygen are mixed, the product is water.

The formulas for all the reactants and the product are correct, but this equation is not balanced.

As written, the equation does not obey the law of conservation of mass.

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Balancing Chemical Equations

When hydrogen and oxygen are mixed, the product is water.

If you put the coefficient 2 in front of H₂O, oxygen will be balanced.

Now twice as many hydrogen atoms are in the product as are in the reactants.

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Balancing Chemical Equations

When hydrogen and oxygen are mixed, the product is water.

To correct this equation, put the coefficient 2 in front of H₂.

The equation is now balanced.

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The reaction between oxygen and hydrogen in fuel cells produces the energy to power a car. What are the products of the reaction in a fuel cell that make the fuel-cell car a zero-emission car?

CHEMISTRY & YOU

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The reaction between oxygen and hydrogen in fuel cells produces the energy to power a car. What are the products of the reaction in a fuel cell that make the fuel-cell car a zero-emission car?

Water is the product of the reaction between oxygen and hydrogen in a fuel cell. Water is not a pollutant, and so fuel-cell cars are considered “zero-emission” cars.

CHEMISTRY & YOU

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Interpret Data

Rules for Writing and Balancing Equations
1. Determine the correct formulas for all the reactants and products. 2. Write the skeleton equation by placing the formulas for the reactants on the left and the formulas for the products on the right with a yields sign (→) in between. If two or more reactants or products are involved, separate their formulas with plus signs. 3. Determine the number of atoms of each element in the reactants and products. Count a polyatomic ion as a single unit if it appears unchanged on both sides of the equation.	4. Balance the elements one at a time by using coefficients. When no coefficient is written, it is assumed to be 1. Begin by balancing elements that appear only once on each side of the equation. Never balance an equation by changing the subscripts in a chemical formula. Each substance only has one correct formula. 5. Check each atom or polyatomic ion to be sure that the number is equal on both sides of the equation. 6. Make sure all the coefficients are in the lowest possible ratio.

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Students suspended copper wire in an aqueous solution of silver nitrate. They noticed a deposit of silver crystals on the copper wire when the copper reacted with the silver nitrate. They recorded the equation for this reaction but didn’t balance it. Balance their equation:

AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + Ag(s)

Sample Problem 11.2

Balancing a Chemical Equation

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Because the nitrate polyatomic ion appears as a reactant and a product, this ion can be balanced as a unit.

Sample Problem 11.2

Analyze Identify the relevant concepts.

1

Apply the rules for balancing equations.

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Balance the nitrate ion.

Remember that a coefficient must always go in front of a compound’s formula, not in the middle of it.

Sample Problem 11.2

Solve Apply concepts to this situation.

2

2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + Ag(s)

Put a coefficient 2 in front of AgNO₃(aq).

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Balance the silver.

Sample Problem 11.2

Solve Apply concepts to this situation.

2

2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2Ag(s)

Put a coefficient 2 in front of Ag(s).

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Aluminum is a good choice for outdoor furniture because it reacts with oxygen in the air to form a thin protective coat of aluminum oxide. Balance the equation for this reaction.

Al(s) + O₂(s) → Al₂O₃(s)

Sample Problem 11.3

Balancing a Chemical Equation

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Notice the odd number of oxygen atoms in the product.

Sample Problem 11.3

Analyze Identify the relevant concepts.

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Apply the rules for balancing equations.

Al(s) + O₂(s) → Al₂O₃(s)

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Sample Problem 11.3

First balance the aluminum by placing the coefficient 2 in front of Al(s).

2Al(s) + O₂(s) → Al₂O₃(s)

Solve Apply concepts to this situation.

2

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Sample Problem 11.3

Multiply the formula with the odd number of oxygen atoms (on the right) by 2 to get an even number of oxygen atoms on the right.

2Al(s) + O₂(s) → 2Al₂O₃(s)

Solve Apply concepts to this situation.

2

Any whole-number coefficient placed in front of O₂will always give an even number of oxygen atoms on the left.

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Sample Problem 11.3

Balance the oxygens on the left by placing a 3 in front of O₂.

2Al(s) + 3O₂(s) → 2Al₂O₃(s)

Solve Apply concepts to this situation.

2

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Sample Problem 11.3

Then rebalance the aluminum by changing the coefficient of Al(s) from 2 to 4.

4Al(s) + 3O₂(s) → 2Al₂O₃(s)

Solve Apply concepts to this situation.

2

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Balance the following equation.

C₃H₈(g) + O₂(g) → CO₂(g) + H₂O(l)

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Balance the following equation.

C₃H₈(g) + O₂(g) → CO₂(g) + H₂O(l)

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)

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Key Concepts

To write a skeleton equation, write the formulas for the reactants to the left of the yields sign and the formulas for the products to the right.

After writing the skeleton equation, use coefficients to balance the equation so that it obeys the law of conservation of mass.

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Glossary Terms

chemical equation: an expression representing a chemical reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas for the products (on the right)
skeleton equation: a chemical equation that does not indicate the relative amounts of reactants and products

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Glossary Terms

catalyst: a substance that increases the rate of reaction by lowering the activation-energy barrier; the catalyst is not used up in the reaction
coefficient: a small whole number that appears in front of a formula in a balanced chemical equation
balanced equation: a chemical equation in which mass is conserved; each side of the equation has the same number of atoms of each element

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BIG IDEA

The law of conservation of mass states that mass is neither created nor destroyed.
In order to show that mass is conserved during a reaction, a chemical equation must be balanced.

Reactions

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