Chapter 21: Chemical Reactions

Section 1: Chemical Changes

• Describing Chemical Reactions
  • Chemical Reaction: a change in which one or more substances are converted into new substances.
  • Reactants: The substances that react.
  • Products: The new substances produced.
• Conservation of Mass
  • The total mass of the products always equals the total mass of the reactants.
• Writing Equations
  • Chemical Equation: a way to describe a chemical reaction using chemical formulas and other symbols.
  • Chemical equations make it easier to calculate the quantities of reactants that are needed and the quantities of products that are formed.
• Unit Managers
  • Atoms are rearranged but never lost or destroyed.
  • Coefficients: represent the number of units of each substance taking part in a reaction.
  • Knowing the number of units of reactants enables chemists to add the correct amounts of reactants to a reaction.
  • Rust can seriously damage iron structures because it crumbles and exposes more iron to the air.
  • Unlike rust, aluminum oxide adheres to the aluminum surface, forming an extremely thin layer that protects the aluminum from further attack.
  • Copper is another metal that corrodes when it is exposed to air, forming a blue-green coating called a patina.

Section 2: Chemical Equations

• Balanced Equations
  • The formulas in a chemical equation must accurately represent the compounds that react.
  • Balancing an equation doesn’t change what happens in a reaction—it simply changes the way the reaction is represented.
  • Balanced Chemical Equation: has the same number of atoms of each element on both sides of the equation.

Section 3: Classifying Chemical Reactions

• Types of Reactions
  • Chemists have defined five main categories of chemical reactions: combustion, synthesis, decomposition, single displacement, and double displacement.
  • Combustion Reaction: occurs when a substance reacts with oxygen to produce energy in the form of heat and light.
  • Combustion reactions also produce one or more products that contain the elements in the reactants.
  • Synthesis Reaction: two or more substances combine to form another substance.
  • A decomposition reaction is just the reverse of a synthesis.
  • Decomposition Reaction: occurs when one substance breaks down, or decomposes, into two or more substances.
    • Most decomposition reactions require the use of heat, light, or electricity.
  • Single-Displacement Reaction: When one element replaces another element in a compound.
  • A metal will replace any less active metal.
  • Double-Displacement Reaction: the positive ion of one compound replaces the positive ion of the other to form two new compounds.
  • A double-displacement reaction takes place if a precipitate, water, or a gas forms when two ionic compounds in solution are combined.
  • Precipitate: an insoluble compound that comes out of solution during this type of reaction.
  • One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons.
  • Oxidation: describes the loss of electrons
  • Reduction: describes the gain of electrons.
  • Chemical reactions involving electron transfer of this sort often involve oxygen, which is very reactive, pulling electrons from metallic elements.
  • The cause and effect of oxidation and reduction can be taken one step further by describing the substances after the electron transfer.

Section 4: Chemical Reactions and Energy

• Chemical Reactions—Energy Exchanges
  • All chemical reactions release or absorb energy.
  • When most chemical reactions take place, some chemical bonds in the reactants are broken, which requires energy.
  • Bond formation releases energy.
• More Energy Out
  • Exergonic Reactions: Chemical reactions that release energy.
  • Exergonic reaction produces visible light.
  • Exothermic Reaction: When the energy given off in a reaction is primarily in the form of heat.
  • Exothermic reactions provide most of the power used in homes and industries.
• More Energy In
  • Endergonic Reactions: A chemical reaction that requires more energy to break bonds than is released when new ones are formed.
  • Electricity is often used to supply energy to endergonic reactions.
  • Endothermic Reaction: When the energy needed is in the form of heat
  • Catalyst: a substance that speeds up a chemical reaction without being permanently changed itself.
  • When you add a catalyst to a reaction, the mass of the product that is formed remains the same, but it will form more rapidly. The catalyst remains unchanged and often is recovered and reused.
  • Inhibitors: used to slow down a chemical reaction.
  • One thing to remember when thinking about catalysts and inhibitors is that they do not change the amount of product produced. They only change the rate of production.