Chemical Reactions

Describing Chemical Reactions 11.1

Introduction to Chemical Equations In a chemical reaction, one or more reactants change into one or more products Chemical equations are used to show what happens in a chemical reaction

Word Equation Word equations are shorthand methods used for writing a description of a chemical reaction The reactants are written on the left and the products on the right, and an arrow separates them The arrow means to give, to yield, to produce, and in some causes to decompose Reactants are separated from each other by a plus sign The plus sign here means “reacts with” Products are separated from each other by a plus sign The plus sign here means “and” Na(s) + H 2 O (l)  NaOH (aq) + H 2 (g) Solid sodium reacts with water to form sodium hydroxide and hydrogen.

Chemical Equations A chemical equation is a representation of a chemical reaction A skeleton equation is a chemical equation that does not indicate the relative amounts (coefficients) of the reactants and products Sometimes skeleton equations are already balanced Key Question How do you write a skeleton equation? Write the chemical formulas for the reactants to the left of the arrow and the formulas for the products to the right of the arrow. You must give the states of matter as (s), (l), (g), or (aq) (aq) is used when a substance is dissolved in water A catalyst is a substance that speeds up the reaction but is not used up in the reaction A catalyst is neither a reactant nor a product, so its formula is written above the arrow See Table 11.1

Balancing Chemical Equations Coefficients are small whole numbers that are placed in front of the formulas in an equation in order to balance it A balanced equation is obtained when each side of the equation has the same number of atoms of each element and mass is conserved Key Question What are the steps for writing and balancing a chemical equation? See Next Slide

Rules for Writing and Balancing Equations (Graphic Organizer) 1. Determine the correct formulas for all the reactants and products 2. Write the skeleton equation 3. Determine the number of atoms of each element in the reactants and products Count polyatomic ions as a single unit if it appears on both sides of the equation 4. Balance the elements one at a time by using coefficients, which will show the law of conservation on mass 5. Make sure all coefficients are in the lowest possible ratio

Types of Chemical Reactions 11.2

Classifying Reactions Key Question What are the five general types of reactions? Combination, decomposition, single-replacement, double-replacement, combustion

Combination Reactions A combination reaction is a chemical change in which two or more substances react to form a single new substance Mg (s) + O 2  MgO (s)

Decomposition Reactions A decomposition reaction is a chemical change in which a single compound breaks down into two or more simpler products NaN 3 (s)  Na (s) + N 2 (g)

Model Writing and Balancing Combination Reactions and Decomposition Reactions Page 359, numbers (Blue Book) Pages 331 and 332, numbers (Red book)

Single-Replacement Reaction A single replacement reaction is a chemical change in which one element replace a second element in a compound Zn (s) + AgNO 3 (aq)  Zn(NO 3 ) 2 (aq) + Ag(s) Whether one metal will displace another metal from a compound depends upon the relative reactivities of the two metals Activity series list metals in order of decreasing reactivity *Attention to removing H in acids and bases* A reactive metal will replace any metal listed below it in the activity series A halogen can also replace another halogen from a compound the activity of a halogen decreases as you go down group 7

Double-Replacement Reactions A double-replacement reaction is a chemical change involving an exchange of positive ions between two compounds They generally take place in aqueous solution and often produce a precipitate, a gas, or a molecular compound (H 2 O) See examples on page 362

Combustion Reactions A combustion reaction is a chemical change in which an element or a compound reacts with oxygen, often producing energy in the form of heat and light A combustion reaction always involves O 2 as a reactant, and most of the time the other reactant is a hydrocarbon The complete combustion of a hydrocarbon produces CO 2 and H 2 O C 8 H 18 (l) + O 2 (g)  CO 2 (g) + H 2 O An example of combustion of a metal Mg (s) + O 2 (g)  MgO (s) A metal oxide is always produced!

Reactions in Aqueous Solutions 11.3

Net Ionic Equations Complete ionic equation is an equation that shows dissolved ionic compounds as dissociated free ions An ion that appears on both sides of an equation and is not directly involved in the reaction is called a spectator ion The net ionic equation is an equation for a reaction in solution that shows only those particles that are directly involved in the chemical change In writing net ionic equations you must make sure that the ionic charge is balanced

Key Question What does a net ionic equation show? Only those particles involved in the reaction and is balanced with respect to both mass and charge. *Both single and double displacement reactions can be written as net ionic equations!

Predicting the Formation of a Precipitate Mixing solutions of two ionic compounds can sometimes result in the formation of an insoluble salt called a precipitate Whether or not a precipitate forms depends upon the solubility of the new compounds that form Key Question How can you predict the formation of a precipitate in a double-replacement reaction? By using the general rules for solubility of ionic compounds