Chapter 9 Chemical Reactions

Section 1 Reactions & Equations

Chemical Reaction The process by which the atoms of one or more substances are rearranged to form different substances.

How Can You Tell If A Chemical Reaction Has Taken Place? Temperature change Color change Odor Gas formation (Bubbles) Formation of a solid

Representing Chemical Reactions Chemists use statements called equations to represent chemical reactions. These equations have two parts Reactants – The starting materials Products – The substances that are formed during a chemical reaction.

Word Equations Example

Skeleton Equation Example

Chemical Equations A statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.

Balancing Chemical Equations Recall that matter is neither created nor destroyed in a chemical reaction. In order to follow the law of conservation of matter we must balance our chemical equations so that the reactant and product sides are equal to one another.

Coefficient – In a chemical equation is the number written in front of a reactant or product. Example Coefficients are usually whole numbers; the number 1 is usually omitted.

Steps for Balancing Equations Write the skeleton for the reaction. Count the atoms of the elements in the reactants. Count the atoms of the elements in the product. Change the coefficients to make the number of atoms of each element equal on both sides. Write the coefficients in their lowest possible ratios. Check your work!

Example: Balancing Chemical Equations

Classifying Chemical Reactions Section 2 Classifying Chemical Reactions

Types of Chemical Reactions We will look at five types of chemical reactions. Each type of reaction will have a general formula that will be helpful in recognizing the type of reaction present.

Synthesis Reaction A chemical reaction in which two or more substances (A and B) react to produce a single product (AB). General Form Example

Combustion Reaction In a combustion reaction oxygen combines with a substance and releases energy in the form of heat and light. In a combustion reaction 𝑂 2 is always present Usual products are 𝐶𝑂 2 & 𝐻 2 𝑂

Decomposition Reaction A reaction in which a single compound breaks down into two or more elements or new compounds. General Form Example

Single-Replacement Reaction A reaction in which the atoms of one element replace the atoms of another element in a compound. General Form Example

Double-Replacement Reaction A reaction in which ions between two compounds are exchanged. General Form Example

Helpful Hints… Nonmetals replace nonmetals Metals will replace Hydrogen or another metal. More active metals will replace less active metals (see Figure 13 p.293). A less active metal will not replace a more active metal.

Looking At Double-Replacement Reactions Only occurs if one of the products is a solid precipitate. Precipitate – an ionic compound formed by a double-replacement reaction Which is not soluble in water It forms a cloudy appearance in the solution; if allowed to sit, the precipitate will settle to the bottom of the container

Solubility Rules for Ionic Compounds Always Soluble (aqueous) Name Symbol Group I Metals Li+1 Na+1 K+1 Ammonium NH4+1 Acetate C2H3O2-1 Nitrate NO3-1

Generally Insoluble (form precipitates) Name Symbol Exceptions carbonate CO3-2 Group I Metals, ammonium chromate CrO4-2 phosphate PO4-3 sulfide S-2 Group I & II Metals, ammonium hydroxide OH-1 Group I Metals, Ca+2, Sr+2, Ba+2, ammonium

Precipitates with (solid) Mostly Soluble (usually aqueous) Name Symbol Precipitates with (solid) halide ions (X) Cl-1, Br-1, I-1 AgX, HgX2, PbX2 sulfate SO4-2 SrSO4, BaSO4, PbSO4, Hg2SO4 Use the Solubility Table on p.974

Reactions in Aqueous Solutions

As we have talked about previously a solution is a homogeneous mixture. Many of the reactions discussed in the previous section involve substances dissolved in water. When a substance is dissolved in water, a solution forms. An aqueous solution contains one or more substances called solutes dissolved in water and water in this case would be the solvent.

What Happens In an Aqueous Solution? Ions form Example p.974 Solubility Table

Ionic Equations To show the details of reactants that involve ions in aqueous solutions, chemists use ionic equations. Ionic equations differ from chemical equations in that substances that are ions in solution are written as ions in the equation.

Complete Ionic Equation An ionic equation that shows all of the particles in a solution as they exist. Example

Spectator Ion Ions that do not participate in a reaction. Example

Net Ionic Equation Ionic equations that include only the particles that participate in the reaction. Spectator ions are not included Example