UNIT 2: Chemical Reactions

Chapter 3: Chemical Names, Formulas, and Equations

Chapter 4: Classifying Compounds and Chemical Reactions

Chapter 4: Classifying Compounds and Chemical Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions

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What are some of the properties that the reactants must have to be useful in airbags?

Airbags on motorcycles are a fairly recent safety innovation, but airbags have been in cars since the 1980s. Airbags inflate when a chemical reaction occurs, and a gas is produced.

4.1 Types of Chemical Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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Chemists have developed a classification system for chemical reactions using the following categories:

•combination reactions

•decomposition reactions

•single replacement reactions

•double replacement reactions

•combustion reactions

Potassium reacts with water to produce potassium hydroxide and hydrogen gas.

Combination Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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• A combination reaction is a chemical reaction in which two or more reactants combine to form one new product.

• A combination reaction can be expressed pictorially as shown below.

• The reactants can be elements or compounds, but the product is always a compound.

The combination reaction between liquid hydrogen and liquid oxygen propelled space shuttles into orbit.

2H2(ℓ) + O2(ℓ) → 2H2O(g)

Decomposition Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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• A decomposition reaction is a chemical reaction in which a single compound breaks down into two or more products.

• A decomposition reaction can be expressed pictorially as shown below.

• The products may be a combination of elements and compounds, but the reactant is always a compound.

In hydrolysis, energy is used to decompose water into hydrogen gas and oxygen gas.

2H2O(ℓ) → 2H2(g) + O2(g)

Reviewing Combination and Decomposition Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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In the electrolysis of water, what is the purpose of the battery and what are the products of the reaction?

What are the seven diatomic molecules that you need to know when writing chemical equations for combination reactions?

Single Replacement Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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• A single replacement reaction is a chemical reaction in which one element takes the place of another element in a compound.

• A single replacement reaction can be expressed pictorially as shown below.

Chlorine gas in the flask on the left bubbles through an aqueous solution of sodium bromide. In a single replacement reaction, the chlorine replaces the bromine in the sodium compound.

2NaBr(aq) + Cl2(g) → 2NaCl(aq) + Br2(ℓ)

Double Replacement Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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• A double replacement reaction is a chemical reaction in which the positive ions of two different compounds exchange places, resulting in the formation of two new compounds.

• A double replacement reaction can be expressed pictorially as shown below.

A red precipitate, Ag2CrO4(s), forms when two ionic solutions, AgNO3(aq) and K2CrO4(aq), undergo a double replacement reaction.

Reviewing Single and Double Replacement Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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Do metals or non-metals undergo single replacement reactions?

Why would it be more efficient for double replacement reactions to occur in aqueous solution rather than the reaction occurring between two solid compounds?

Combustion Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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• A combustion reaction is a chemical reaction in which a compound or element reacts with oxygen to form compounds called oxides.

• The general equation of a combustion reaction is

CxHy + O2 CO2 + H2O

• Heat and light are often produced in a combustion reaction.

• What are two examples of combustion reactions?

• What is incomplete combustion, and why is it hazardous when it occurs in homes?

Summary of Reaction Types

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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Section 4.1 Review

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.1

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• Chemical reactions can be classified as combination, decomposition, single replacement, double replacement, or combustion reactions.

• Combustion reactions may be complete or incomplete. Incomplete combustion of organic compounds such as hydrocarbons produces the toxic compound carbon monoxide.

4.2 Acids and Bases

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Some compounds are classified as an acid or a base.

• Citrus fruits, vinegar, soaps, and baking soda are examples of acids and bases.

• What does this whip scorpion have to do with acids and bases?

Acids

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• An acid is a chemical that produces hydrogen ions when dissolved in water. An acidic solution has a pH of less than 7.

• Acids taste sour. Many acids are corrosive, and they will react with many substances.

How are hydrofluoric acid and this glass globe related?

Bases

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• A base is a chemical that produces hydroxide ions when dissolved in water. A basic solution has a pH greater than 7.

• Bases taste bitter, and they have a slippery feel. Many bases are corrosive, and they are often used in cleaning products.

Which two elements make up a hydroxide ion?

Consumer products such as cleaners often contain bases.

The pH Scale

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• The pH scale is a numerical scale that is used for identifying a solution as acidic, basic, or neutral.

• Acid solutions have a pH of less than 7. The lower the pH number, the higher the concentration of hydrogen ions.

• Basic solutions have a pH of greater than 7. The higher the pH number, the greater the concentration of hydroxide ions.

• A neutral solution has a pH of 7 and approximately the same number of hydrogen ions and hydroxide ions.

Is a tomato that has a pH of about 4 acidic or basic?

The pH Scale

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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Differences in pH Values

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• The concentration of hydrogen ions associated with a value on the pH scale differs from the value above it or below it by a power of 10.

• A solution that has a pH of 4 has a concentration of hydrogen ions that is 10 times greater than a solution that is pH 5.

• The pH of a solution can be measured using pH paper or a pH meter.

• If grapes have a pH of 3 and a banana has a pH of 5, how much greater is the concentration of hydrogen ions in grapes than in the banana?

Identifying Acids and Bases

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• An acid-base indicator is a chemical that has a characteristic colour when in the presence of an acid or a base.

• Red and blue litmus papers can be used to determine if a solution is an acid or a base. Red litmus paper turns blue in a basic solution, and blue litmus paper turns red in an acidic solution.

The pH paper on the left and the pH meter on the right both determine the pH of a solution.

Are the solutions that were tested acidic or basic?

A Summary of Acid and Base Properties

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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Names of Acids

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Two categories of acids are binary acids and oxoacids.

• Binary acids are composed of two elements—hydrogen and a non-metal.

• Oxoacids are composed of hydrogen, oxygen, and another element.

HF(aq)hydrofluoric acid

What are the rules for naming a binary acid and an oxoacid using the classical method?

H2SO4(aq)sulfuric acid

Chemical Formulas of Acids

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• The chemical formula of an acid can be considered composed of a certain number of hydrogen ions and the negative ion.

• The negative ion is identified by the name of the acid.

Names and Chemical Formulas of Bases

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Bases are ionic compounds composed of metal ions and hydroxide ions. Their names are written following the IUPAC guidelines for ionic compounds.

• The chemical formula for a base includes enough hydroxide ions in the formula to make the total charge of the compound zero.

Acids in the Atmosphere and Waterways

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Rainwater is naturally acidic and normally has a pH of between 5.6 to 5.8.

• Various human activities release compounds into the air that lower the pH of precipitation.

What human

activities addto acid precipitation?

Why is acid precipitation a problem?

Neutralization Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Neutralization is a chemical reaction between an acid and a base that results in production of a salt and water.

• A salt, such as sodium chloride, is an ionic compound.

• The increased pH of waterways due to acid precipitation can be temporarily reduced by adding calcium carbonate (lime) to the water.

Why is liming waterways a temporary solution to the problem of the effects of acid precipitation?

Salts Formed From Neutralization Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Different salts formed in the neutralization reactions between acids and bases have a variety of uses.

• Commercially important salts include sodium chloride, magnesium acetate, and calcium acetate.

What might be the original source of the underground salt deposits in Canada?

Section 4.2 Review

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Acids and bases are two important classes of compounds with their own characteristic properties.

• The pH scale tells you how acidic or basic a substance is. Acids have pH values below 7. Bases have pH values above 7. Neutral solutions have a pH of 7.

• Universal indicator, pH paper, and a pH meter are all tools for determining the pH of a solution.

• Acids have both IUPAC and classical names. There are special rules for naming binary acids and oxoacids.

Continued…

Section 4.2 Review (continued)

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.2

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• Bases are named using IUPAC guidelines for ionic compounds.

• Rainwater is naturally acidic, but human activities can lower the pH of rainwater further. This is called acid precipitation, and it can severely damage ecosystems.

• When most acids and bases react, they form a salt and water. The reaction between an acid and a base is called neutralization.

• Antacids and the liming of lakes are examples of applications of neutralization reactions.

4.3 Rates of Chemical Reactions

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.3

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• Reaction rate is a measure of how fast a reaction occurs.

• Some reactions, such as the process of rusting, occur slowly and some reactions, such as the deployment of air bags, occur quickly.

Is the reaction shown in this photo occurring slowly or quickly?

Factors That Affect the Rate of a Reaction

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.3

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There are four main factors that affect reaction rates.

•Higher temperatures result in an increase in frequency and energy of collisions among particles.

•Increased surface area results in more collisions that result in a reaction.

Continued…

Glow sticks are powered by a chemical reaction. Why might the glow stick on the left be brighter?

Factors That Affect the Rate of a Reaction

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.3

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• A greater concentration of particles increases the chances that a collision will result and a reaction will occur.

• Catalysts lower the energy required for a reaction to occur.

How could you increase the reaction rate between the wood and oxygen in the air?

Section 4.3 Review

UNIT 2 Chapter 4: Classifying Compounds and Chemical Reactions Section 4.3

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• Chemical reactions occur at different rates. For example, rusting is a slow reaction. The combustion of propane is a fast reaction.

• Rates of reaction are affected by a number of factors, including temperature, surface area, concentration, and the presence of a catalyst.