Chapter 19 chemistry grades 5 8
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Chapter 19 chemistry grades 5 8

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Chapter 19 chemistry grades 5 8 Chapter 19 chemistry grades 5 8 Presentation Transcript

  • Chemical Reactions Chapter 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, ace tic, 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.