Chapter 10 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. Some ways to know that a chemical reaction has occurred include absorption of heat and release of energy in the form of light or heat, color change, production of a gas, and the appearance of gas bubbles or a solid.

Chemical Equations Statements called equations are used to represent chemical reactions. Reactants, or starting substances, and the products, or substances formed during a reaction, are shown in a chemical reaction. The arrow in the reaction means “yields”. Plus signs separate different reactants and products.

Examples REACTANT + REACTANT “yields” PRODUCT + PRODUCT

State Symbols Sometimes state symbols will be used to indicate the state of matter in the reaction. (s) Solid (l) Liquid (g) Gas (aq) Aqueous [Water solution]

Equations Equations can be written in words, in which the reactants and products are indicated by their names. Word equations can be replaced by skeleton equations which use chemical formulas rather than words. Skeleton equations are more informative because they identify the atoms that make up each reactant and product.

Example Write a word equation and skeleton equation for the reaction in which hydrogen gas reacts with oxygen gas to form liquid water. hydrogen(g) + Oxygen(g) Water (l) H2 (g) + O2 (g) H2O (l)

Practice Write a word equation and a skeleton equation for each of the following descriptions of chemical reactions. Solid lithium reacts with chlorine gas to produce solid lithium chloride. Nitrogen gas reacts with oxygen gas to produce nitrogen dioxide gas. Solid iron reacts with aqueous copper (II) nitrate to produce solid copper and aqueous iron (II) nitrate.

Balanced Chemical Equations Skeleton equations do not reflect the fact that matter is conserved during a reaction. An equation must reflect that the same number of each kind of atom on both sides of the arrow. This is called a balanced chemical equation. To balance a chemical equation, use coefficients, numbers placed in front of the compound. When no coefficient appears, the number is understood to be 1.

Procedure Write the skeleton equation. Count the atoms of the elements in the reactants. Polyatomic atoms that remain unchanged can be counted as elements. Count the atoms of the elements in the products. Change coefficients to make numbers equal. DO NOT CHANGE SUBSCRIPTS. Write coefficients in their lowest possible ratios. Check your work.

Classifying Chemical Reactions

Types of Reactions There are five basic types of chemical reactions. The first of these types is the Synthesis reaction. In the synthesis reaction, there are more than one reactants and only one product. Synthesis means combine.

Combustion Reactions A combustion reaction is a reaction that has as one reactant oxygen gas. Combustion reactions produce lots of heat and light and the product is usually carbon dioxide and water. CH4 (g)+ 2O2 (g) CO2 (g) + 2H2O (g)

Decomposition A decomposition reaction is one where a single compound breaks down into two or more elements or new compounds. 2Al2O3 4 Al + 3O2

Single-Replacement Reactions In a single-replacement reaction, atoms of an element replace the atoms of another element in a compound. For example, in the replacement of silver in silver nitrate by zinc metal. Zn (s) + 2AgNO3 (aq) Zn(NO3)2 (aq) + 2 Ag (s)

Activity Series Not all metals will replace one another. Only a metal that is more chemically reactive will replace one that is less reactive. The order of such reactivity is called an activity series and you have a copy of one in your reference tables.

Reactivity Any metal in an activity series will replace the metals below it in their compounds. With elemental halogens, one replaces the other if the second lies below it in the periodic table. Fluorine is more reactive than, chlorine, which is more active than bromine, which is more active than iodine.

Example Use an activity series to predict the single-replacement reaction that will occur between aluminum metal and lead (II) nitrate (Pb(NO3)2) in a water solution and write its balanced chemical equation. Answer: 2Al (s) + 3Pb(NO3)2 (aq) 2Al(NO3)3 (aq) + 3 Pb(s)

Practice Predict whether a single-replacement reaction will occur between the following pairs of possible reactants. If so, write the balanced chemical equation for the reaction. Chlorine gas and aqueous potassium iodide Magnesium metal and aqueous copper (II) sulfate Copper metal and iron (III) chloride in water solution Lead metal and aqueous silver nitrate

Double Replacement Reactions In this reaction type, there is an exchange of the positive ions between two compounds, often in water solutions. Often a solid, of low solubility is produced during the reaction and settles out of the solution. This solid is called a precipitate. For example, Pb(NO3)2 (aq) + 2NaCl (aq) 2NaNO3 (aq) +PbCl2 (s)

Practice Write a balanced chemical equation for each of the following double-replacement reactions. A water solution of sinc bromide (ZnBr2) and a water solution of potassium hydroxide form a water solution of potassium bromide and a precipitate of zinc hydroxide. A water solution of copper (II) sulfate and a water solution of barium chloride produce a water solution of copper (II) chloride and solid barium sulfate. A precipitate of iron (III) carbonate and a water solution of sodium nitrate are formed when a water solution of iron (III) nitrate and a water solution of sodium carbonate are mixed.

Reactions in Aqueous Solutions

Solutions When a substance dissolves in water, a solution forms. A solution is a homogeneous mixture because it has a constant composition throughout. A solution contains one or more substances called solutes dissolved in the solvent.

Solutions (cont’d) The solvent is the most plentiful substance in the solution. An aqueous solution is a solution in which the solvent is water. When dissolved to form aqueous solutions, the ions of ionic compounds separate.

Ionic Equations To show all of the particles in solution as they really exist, a complete ionic equation can be written. Na+ (aq) + Cl- (aq) + Ag+ (aq) + NO3- (aq) Na+ (aq)+ NO3- (aq) + AgCl (s) Notice that the sodium and nitrate ions are present on both sides of the equation. Such ions that do not participate in the reaction are called spectator ions.

Net Ionic Equations An ionic equation that does not show spectator ions but only the particles that participate in the reaction is called a net ionic equation.

Example Write the balanced chemical equation for the reaction between aqueous solutions of strontium nitrate and potassium sulfate, which forms the precipitate strontium sulfate. Then write the complete ionic and net ionic equations.

Practice Write the balanced chemical, complete ionic and net ionic equations for each of the following reactions. Aqueous solutions of lead (II) nitrate and ammonium chloride are mixed, forming a precipitate of lead (II) chloride. Aqueous solutions of aluminum chloride and sodium carbonate are combined, producing solid aluminum carbonate.

Production of Water or a Gas Some double-displacement reactions in aqueous solution produce water or a gas (or both) rather than a precipitate. When this happens, show the water or gas as a product in the net ionic equation. DO NOT IONIZE!!!

Example When hydrochloric acid and potassium hydroxide solutions are mixed, water results, together with an aqueous solution of potassium chloride. Write the balanced chemical equation, a complete ionic equation and a net ionic equation for this reaction.

Practice Write the balanced chemical, complete ionic and net ionic equations for the reactions between the following substances, which produce water. Nitric acid (HNO3) and aqueous barium hydroxide Sulfuric acid (H2SO4) and aqueous sodium hydroxide Phosphoric acid (H3PO4) and aqueous lithium hydroxide

Lab Tomorrow