Chapter 9: Chemical Reactions Section 1: Reactions and Equations

Chemical Reactions The process by which atoms of one or more substances are rearranged to form different substances is called a chemical reaction. Ways to tell a chemical reaction has occurred: Temperature change Color change Odor Gas formation Formation of a solid

Representing Chemical Reactions Chemists use statements called equations to represent chemical reactions. Equations show a reactions reactants, which are the starting substances. Equations show a reactions products, which are the ending substances. Example: HCl + NaOH NaCl + H2O

Representing Chemical Reactions Arrows are used in between reactants and products. The direction of the arrow indicates the direction in which the reaction proceeds. The equilibrium arrow indicates that a reaction can proceed in either direction.

Balancing Chemical Equations To accurately represent a chemical reaction by an equation, the equation must show equal numbers of atoms of each reactant and each product on both sides of the arrow. This is known as a balanced chemical equation. A balanced chemical equation uses chemical formulas to show the identities and relative amounts of substances used in a chemical reaction.

Balancing Chemical Reactions To balance a chemical equation, you must find the correct coefficients. A coefficient in a chemical equation is the number written in front of a reactant or product. Coefficients are usually whole numbers and are not written if the value is one.

Classifying Chemical Reactions Section 2 Classifying Chemical Reactions

Chemical Reactions The process by which atoms of one or more substances are rearranged to form different substances is called a chemical reaction. Ways to tell a chemical reaction has occurred: Temperature change Color change Odor Gas formation Formation of a solid

Representing Chemical Reactions Chemists use statements called equations to represent chemical reactions. Equations show a reactions reactants, which are the starting substances. Equations show a reactions products, which are the ending substances. Example: HCl + NaOH NaCl + H2O

Types of Chemical Reactions Chemists classify reactions in order to recognize patterns and predicts the products of chemical reactions. There are five major types of reactions: Synthesis Combustion Decomposition Replacement (single and double) Redox Reactions

Synthesis Reactions A chemical reaction in which two or more substances (A and B) react to produce a single product (AB). A + B AB Examples CaO(s) + H2O(l) Ca(OH)2(s) 2SO2(g) + O2(g) 2SO3(g)

Combustion Reactions In a combustion reaction, oxygen combines with a substance and release energy in the form of light and heat. Combustion reactions are very common. Examples C(s) + O2(g) CO2(g) CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Decomposition Reactions Essentially the opposite of synthesis reactions. In a decomposition reaction, a single compound breaks down into two or more elements or new compounds. AB A + B

Decomposition Reactions Examples NH4NO3(s) N2O(g) + 2H2O(g) 2NaN3(s) 2Na(s) + 3N2(g)

Replacement Reactions There are two types of replacement reactions: single replacement and double replacement. A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction. A + BX AX + B

Single Replacement Reactions Examples Cu(s) + 2AgNO3(aq) 2Ag(s) + Cu(NO3)2(aq) F2(g) + 2NaBr(aq) 2NaF + Br2(l)

Double Replacement Reactions Involves the exchange of ions between two compounds. AX + BY AY + BX Examples Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H2O(l) 2NaOH(aq) + CuCl2(aq) 2NaCl(aq) + Cu(OH)2(s)

Redox Reaction- A brief intro Oxidation-Reduction reactions involve a transfer of electrons. Oxidation involves losing electrons, whereas reduction involves gaining electrons

Reactions in Aqueous Solutions Section 3 Reactions in Aqueous Solutions

Aqueous Solutions An aqueous solution contains one or more substances called solutes dissolved in water. In this case, water is the solvent – the most plentiful substance in the solution.

Molecular & Ionic Compounds in Solution Certain molecular and ionic compounds can be solutes in aqueous solutions. Compounds like HCl and NaCl break down into ions when placed in water. HCl(aq) H+(aq) + Cl-(aq) NaCl(aq) Na+(aq) + Cl-(aq)

Types of Reactions in Aqueous Solutions Some reactions that occur in aqueous solutions produce precipitates (solids) Example 2NaOH(aq) + CuCl2(aq) 2NaCl(aq) + Cu(OH)2(s) This equation does not show some details of this reaction. To do this, we must write ionic equations.

Ionic Equations To show the details of reactions 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.

Ionic Equations 2NaOH(aq) + CuCl2(aq) 2NaCl(aq) + Cu(OH)2(s) 2Na+(aq) + 2OH-(aq) + Cu+2(aq) + 2Cl-(aq) 2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s) An ionic equation that shows all of the particles in a solution as they exist is called a complete ionic equation. Note that the sodium and chloride ions are both reactants and products. Because they are both reactants and products, they do not participate in the reaction and are called spectator ions and are usually not shown in reactions.

Ionic Equations Net ionic equations are ionic equations that include only the particles that participate in the reaction. 2Na+(aq) + 2OH-(aq) + Cu+2(aq) + 2Cl-(aq) 2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s) 2OH-(aq) + Cu+2(aq) Cu(OH)2(s)