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.