Chapter 23: 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 * When chemical reactions occur, new compounds form when bonds between atoms in the reactants break and new bonds form. * An atomic nucleus changes only when nuclear decay or a nuclear reaction, such as nuclear fission or fusion, occurs. * The energy released by a nuclear reaction is millions of times greater than the energy released by a chemical reaction. * Different parts of the atom are used in chemical and nuclear reactions.
Conservation of Mass * When Lavoisier demonstrated the law of conservation of mass, he set the field of chemistry on its modern path. * Lavoisier also pioneered early experimentation on the biological phenomena of respiration and metabolism that contributed early milestones in the study of biochemistry, medicine, and even sports medicine.' * Antoine Lavoisier’s work led him to the conclusion that language terminology would be critical to communicate novel scientific ideas. * One of the questions that motivated Lavoisier was the mystery of exactly what happened when substances changed form. * When he determined the mass of the liquid mercury and gas, their combined masses were exactly the same as the mass of the red powder he had started with. * Lavoisier also established that the gas produced by heating mercury(II) oxide, which we call oxygen, was a component of air. He did this by heating mercury metal with air and saw that a portion of the air combined to give red mercury(II) oxide.
Writing Equations * Many words are needed to state all the important information. * Chemical Equation: a way to describe a chemical reaction using chemical formulas and other symbols. * On the left side of the equation are the reactants. On the right side of the equation are the products.
Unit Managers * Knowing the number of units of reactants enables chemists to add the correct amounts of reactants to a reaction. * Also, these units, or coefficients, tell them exactly how much product will form.

Section 2: Chemical Equations

Balanced Equations * Mercury metal forms when mercury oxide is heated. Because mercury is poisonous, this reaction is never performed in a classroom laboratory. * The formulas in a chemical equation must accurately represent the compounds that react. * Balanced Chemical Equation: The balancing process involves changing coefficients in a reaction * Finding out which coefficients to use to balance an equation is often a trial-and-error process

Section 3: Classifying Chemical Reactions

Types of Reactions * If you have ever observed something burning, you have observed a combustion reaction. * 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. * Many combustion reactions also will fit into other categories of reactions. * Synthesis Reaction: two or more substances combine to form another substance. * The generalized formula for this reaction type is as follows: A + B → AB. * Decomposition Reaction: occurs when one substance breaks down, or decomposes, into two or more substances. * The general formula for this type of reaction can be expressed as follows: AB → A + B. * Most decomposition reactions require the use of heat, light, or electricity. * Water decomposes into hydrogen and oxygen when an electric current is passed through it. A small amount of sulfuric acid is added to increase conductivity. * Single-Displacement Reaction: When one element replaces another element in a compound * Single-displacement reactions are described by the general equation A + BC → AC + B. * Copper in a wire replaces silver in silver nitrate, forming a blue-tinted solution of copper(II) nitrate. * Sometimes single-displacement reactions can cause problems. * Solid barium sulfate is formed from the reaction of two solutions. * A metal can replace any metal below it on the list but not above it. * 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. * A precipitate is an insoluble compound that comes out of solution during this type of reaction. * The generalized formula for this type of reaction is as follows: AB + CD → AD + CB. * One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons. * Chemists use the term oxidation to describe the loss of electrons and the term reduction to describe the gain of electrons. * The cause and effect of oxidation and reduction can be taken one step further by describing the substances after the electron transfer. * The substance that gains an electron or electrons obviously becomes more negative, so we say it is reduced.

Section 4: Reaction Rates and Energy

Chemical Reactions and Energy * When its usefulness is over, a building is sometimes demolished using dynamite. Dynamite charges must be placed carefully so that the building collapses inward, where it cannot harm people or property. * All chemical reactions release or absorb energy. This energy can take many forms, such as heat, light, sound, or electricity. * According to the law of conservation of energy, energy cannot be created or destroyed, but can only change form. * In compounds, chemical potential energy is stored in chemical bonds between atoms.
Activation Energy: The minimum amount of energy needed to start a reaction. * In order to form new bonds, atoms have to be close together. * If there is not enough energy, the reaction will not start. * Activation energy, which differs from reaction to reaction, is required for both exothermic and endothermic reactions.
Endergonic Reactions * Sometimes a chemical reaction requires more energy to break bonds than is released when new ones are formed. These reactions are called endergonic reactions. * The energy absorbed can be in the form of light, heat, or electricity. * Endothermic Reaction: When the energy needed is in the form of heat * It also can describe physical changes. * Some reactions are so endothermic that they can cause water to freeze. * As an endothermic reaction happens, such as the reaction of barium hydroxide and ammonium chloride, energy from the surrounding environment is absorbed, causing a cooling effect. Here, the reaction absorbs so much heat that a drop of water freezes and the beaker holding the reaction sticks to the wood. * With an endothermic reaction, the chemical reaction will not take place unless energy is added. * With an endothermic reaction, the reactants have a lower energy level than the products. The reactants must overcome the activation energy barrier in order to form new products.
Exergonic Reactions * Chemical reactions that release energy are called exergonic reactions * In these reactions, less energy is required to break the original bonds than is released when new bonds are formed. * Glow sticks contain three different chemicals—an ester and a dye in the outer section and hydrogen peroxide in a center glass tube. Bending the stick breaks the tube and mixes the three components. The energy released is in the form of 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. * The energy diagram for an exothermic reaction is the reverse of an endothermic reaction. With an exothermic reaction, the products have less stored energy than the reactants. * In an exothermic reaction, molecules have enough energy to overcome the activation energy barrier. Energy is released with the formation of new products.
Chemical Reaction Rates * According to the kinetic theory of matter, atoms and molecules are always moving. * Rate of Reaction: the speed at which reactants are consumed and products are produced in a given reaction. * Reaction rate is important in the manufacturing industry because the faster the product can be made, the less it usually costs. * Energy is needed by atoms and molecules to break old bonds and to form new ones. * One way to increase the activation energy is to add heat or increase the temperature. * A chemical reaction will go faster at higher temperature and slower at lower temperature * The closer atoms and molecules are to each other, the greater the chance of collision. The amount of substance present in a certain volume is called its concentration. * Increasing the concentration of a substance increases the reaction rate. * Only atoms or molecules in the outer layer of a substance can collide with other reactants. * When a substance is finely divided, it has a larger surface area than when it was whole * Increasing the surface area increases the chance for collisions, which will increase the reaction rate. * Agitation or stirring is a physical process that allows reactants to mix. * A low stirring rate will slow the reaction due to fewer collisions. * Chemical reactions can be controlled by agitation. * Another way to influence the reaction rate is with pressure * By increasing the pressure of gases, molecules have less room to move about and the concentration of the reactants increases. * Catalyst: 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. * A catalyst lowers the activation energy of the reaction. * Inhibitors: Substances that are 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. Catalysts increase the rate and inhibitors decease the rate.