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.