MK

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.