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.