chapter 19 chemistry grades 5 8

Chemical ReactionsChapter 19

19-1 How Matter Reacts

Objectives:

Explain what a chemical reaction is.

Distinguish between exothermic and endothermic reactions

Explain chemical equations

How Matter Works

• In a chemical reaction, the initial substances are called the reactants: the new substances produced are called the products.

How Matter Reacts

• Exothermic reactions• React → Products +

Energy (heat) • Chemical reactions

that give off energy

• Endothermic Reaction• Reactants + Energy

(heat) → Products• Chemical reactions

that need energy

How Matter Works

• It is impossible to actually see what goes on in chemical reactions. It is possible, however, to tell whether or nt a reaction has taken place.

• Changes in appearance, temperature, and odor all indicate that a chemical reaction has occurred.

• Scientists describe what happens in a reaction by using chemical equations that represent with symbols and formulas the reactants and products in a reaction.

19-2 Types of Chemical Reactions

• Objectives• Identify the basic types of reactions• Explain how types of reactions differ from one another

Types of Chemical Reactions

• Five different kinds of chemical reactions:• Composition• Decomposition• Replacement• Ionic• Combustion

Chemical Reactions

• Composition reactions occur when two or more atoms combine

• Composition reaction pattern: A+B→ AB• Examples: exploding fireworks, a burning match,

catalytic converters in automobiles

Decomposition Reactions

• Decomposition reactions occur when a compound breaks down into simpler substances

• Decomposition reaction pattern: AB→A+B• Examples: production of lime from limestone, digestion

of food, carbonic acid

Combustion Reactions

• Combustion reactions occur when substances are combined with oxygen, releasing large amounts or light and heat energy

• Usually involve the burning of hydrocarbons, compounds made up of carbon and hydrogen

• Examples: burning of natural gas, oil, coal; burning of wood

Replacement Reactions

• Replacement reactions occur when one element is replaced by a similar element

• Replacement reaction patterns: A +BC → AC + B, AB + CD → AC + BD.

Examples: copper plating, silver plating, chrome plating

Ionic Reactions

• Ionic reactions are similar to replacement reactions except ions are involved.

• Form precipitates, solids that settle out of the solution.• Ionic reaction pattern: A+B + C+D →AD + CB• Examples: pigments used in paints

19-3 Factors That Affect Reactions

• Objective• Identify and analyze factors that influence the speed of

reactions

Factors

• Surface area• Amount of material• Temperature• Catalysts

Surface Area

• The more surface area of the reactants that is exposed, the more atoms can be involved in the reaction, and the faster the reaction happens.

Amount of Material

• The amount of reactants can sometimes affect how quickly the reaction occurs. Usually, if the amount of reactants is increased, the speed of the reaction will also increase.

Temperature

• Increasing the temperature speeds up most chemical reactions because the atoms or molecules move faster.

• Decreasing the temperature slows them down.

Catalysts

• Catalysts are substances that speed up the rate of chemical reactions but are not permanently altered in the reaction.

• Catalysts do not appear in the final products of the reactions they affect.

Enzymes

• Enzymes are catalysts in your body. They bring about or speed up chemical reactions.

• Examples: • salivary gland breaks down starch• Gastric glands breaks down proteins• Pancreas breaks down starch• Intestine breaks down cane sugar, milk sugar

19-4 Acids and Bases

• Objectives• Describe the characteristics of acids• Describe the characteristics of bases• Explain pH

Acids and Bases

• The word acid comes from Latin and means “ sour.” • Scientists describe acids as substances that produce

hydronium ions when they react with water.• Bases are used to make soap, detergent, and drain cleaner• A base is substance that releases hydroxide ions, which

give bases their special chemical characteristics.

Properties of acids and bases

Acids

• Formulas begin with H.• Sour taste• Turn litmus paper red• Release hydrogen gas

when they react with metals

• Examples: hydrochloric, sulfuric, acetic, citric

Bases• Formulas end with OH• Bitter taste• Turn litmus paper blue• Slippery feel• Examples: • sodium hydroxide, • calcium hydroxide, magnesium

hydroxide, aluminum hydroxide

Acids and Bases

• You can test for the presence of acids and bases by using special substances called indicators.

• The strength or weakness of an acid or a base is measured on the pH scale.

• The pH scale measure how many hydronium ions are in the solution.• Acids have pH values less than 7, while bases have values greater

than 7. • A measure of 7 on the pH scale indicates a neutral solution of pure

water, meaning its neither acid or base.• Neutralization occurs when acids and bases are mixed together.• The acid and base react chemically to form natural salt and water.