Chapter 9 CHEMICAL REACTIONS Honors Chemistry

Section 9.1 Reactions and Equations Chemical Reactions The process by which one or more substances are rearranged to form different substances is called a chemical reaction.chemical reaction All chemical reactions are the result of chemical changes.

Evidence of a chemical reaction Change in temperature Ex: wood burning Change in color Ex: leaves changing colors Odor forms Ex: rotting garbage Gas or precipitate forms Ex: sour milk

Representing Chemical Reactions Chemists use statements called equations to represent chemical reactions. Reactants are the starting substances.Reactants Products are the substances formed in the reaction.Products

A chemical equation uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.chemical equation

Balancing Chemical Equations Coefficients are used to balance chemical equations. A coefficient in a chemical equation is the number written in front of a reactant or product.coefficient 2 H 2 (g) + O 2 (g)  2 H 2 O (l)

Hints for Balancing Chemical Equations Count atoms of reactants & products Never change or alter subscripts Add coefficients to balance # of atoms Balance H and O last!!! Check that coefficients are in lowest ratio!!!

Why Balance Chemical Equations??? The most fundamental law in chemistry is the law of conservation of mass. Balanced equations show this law!!!

Section 9.2 Classifying Chemical Reactions Types of Chemical Reactions - Chemists classify reactions in order to organize the many types. 1. A synthesis reaction is a reaction in which two or more substances react to produce a single product.synthesis reaction A + B  AB

When two elements react, the reaction is always a synthesis reaction.

2. In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light.combustion reaction

3. A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds.decomposition reaction AB  A + B Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur.

4. A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction.single replacement reaction A + BX → AX + B A metal will replace a less active metal. A nonmetal will replace a less active nonmetal.

Replacement Reactions A metal will not always replace a metal in a compound dissolved in water because of differing reactivities. An activity series can be used to predict if reactions will occur. Halogens also have different reactivities and do not always replace each other.

5. Double replacement reactions occur when ions exchange between two compounds.Double replacement reactions The cations trade places to form new compounds.

The solid product produced during a chemical reaction in a solution is called a precipitate.precipitate All double replacement reactions produce either water, a precipitate, or a gas.

This table shows the steps to write double replacement reactions.

Section 9.3 Reactions in Aqueous Solutions Aqueous Solutions An aqueous solution contains one or more dissolved substances in water.aqueous solution The solvent is the most plentiful substance in a solution.solvent The solute is the substance being dissovled.

Water is always the solvent in an aqueous solution. Compounds that produce hydrogen ions, H +, in aqueous solutions are acids. Ionic compounds can also be solutes in aqueous solutions. When ionic compounds dissolve in water, their ions separate in a process called dissociation. NaCl (s) - water  Na + (aq) + Cl - (aq) CaBr 2(s) - water  Ca 2+ (aq) + 2 Br – (aq)

When two solutions that contain ions as solutes are combined, the ions might react. If they react, it is always a double replacement reaction. Three products can form: precipitates, water, or gases. Types of Reactions in Aqueous Solutions

Aqueous solutions of sodium hydroxide and copper(II) chloride react in a double replacement reaction. Use your solubility rules to determine if a precipitate forms. NaOH(aq) + CuCl 2 (aq) → ???????

Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations. complete ionic equations Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations.spectator ions Formulas that include only the particles that participate in reactions are called net ionic equations.net ionic equations

Some reactions produce more water molecules. No evidence of a chemical reaction is observable. HBr (aq) + NaOH (aq) → Without spectator ions H + (aq) + OH – (aq) → H 2 O (l)

Gases that are commonly produced are carbon dioxide (CO 2 ), hydrogen cyanide (HCN), and hydrogen sulfide (H 2 S). 2HI (aq) + Li 2 S (aq) → H 2 S (g) + 2LiI (aq)