Chapter 7 Chemical Reactions

Homework Assigned Problems (odd numbers only) “Questions and Problems” 7.1 to 7.31 (begins on page 200) “Additional Questions and Problems” 7.41 to 7.49 (page 221) “Challenge Questions” 7.51-7.57 (page 222)

Evidence of a Chemical Reaction Physical changes: Involves no changes in chemical identity of a substance No changes in physical properties (color, physical state, freezing point, boiling point) Chemical changes: A chemical reaction in which one or more substances changes to a different substance Properties that matter exhibits as it undergoes changes in chemical composition

Evidence of a Chemical Reaction As a result of the chemical change, a chemical reaction has occurred which produces at least one new substance Changes that identify chemical reactions (visual clues) color change precipitate formation gas bubbles flames heat release

Evidence of a Chemical Reaction A chemical change is a process that causes a change in the chemical composition of a substance The creation of one or two new substances is characteristic of a chemical change A chemical change occurs when new substances are made Conversion of material(s) into one or more new substances These substances will have different properties from the original material

Evidence of a Chemical Reaction Fe Fe2O3 Li LiOH, H2 HCO3- CO2 Lithium reacts with water to generate lithium hydroxide and hydrogen gas Bicarbonate ion reacting with water to generate carbon dioxide Na NaOH, H2

The Chemical Equation Chemical properties determine whether or not a substance can be changed to another substance Chemical Reactions: are processes that involve chemical changes in matter resulting in new substances involve a rearrangement and exchange of atoms to produce new molecules Elements are not changed during a reaction Chemists have developed a shorthand way of stating chemical changes on paper: The chemical equation Chemical properties determine whether or not a substance can be changed into another substance. The arrow indicates the direction of chemical change.

The Chemical Equation Reactants  Products A chemical equation is a written statement that uses symbols and formulas (or words) to describe the changes during a chemical reaction It shows substances at the beginning of a reaction (reactants) It shows substances formed in the reaction (products) The reactants and products are separated by an arrow It is an abbreviated way of representing a chemical reaction on paper Reactants  Products

The Chemical Equation For a chemical equation to be valid: Only the reactants and products involved in the reaction are shown in the equation The accurate formulas must be used for each of the substances The law of conservation of mass must be obeyed: Equal numbers of the same kind of atom on the reactant and product side of the equation It must be consistent with the experimental facts. Molecular, not empirical formulas always used. Equation must be balanced.

The Chemical Equation The law of conservation of mass is complied with: Matter is not created nor destroyed during a chemical reaction At least one new substance is produced but the quantity of matter does not change Every atom present as a reactant has to be present as a product The sum of the masses of the products is always equal to the sum of the masses of the reactants From the result of a chemical reaction (during the process) bonds in the reactants are broken, atoms rearrange and reform into products

How to Write Balanced Chemical Equations A balanced chemical reaction has the same number of atoms of each element involved on each side of the equation A balanced equation is consistent with the law of conservation of mass Chemical reactions can be written as: Word equations Formula equations Word equations use the chemical names of the reactants and products Formula equations use symbols and formulas of the substances involved reactants products

How to Write Balanced Chemical Equations A balanced chemical reaction has the same number of atoms of each element on both sides of the arrow Every atom must be accounted for (atoms are neither created nor destroyed) Equations are balanced by placing a coefficient in front of one or more of the substances in the equation A chemical reaction obeys the Law of Conservation of Mass If there are 10 carbon atoms in the reactants, there must be 10 carbon atoms in the reactants

How to Write Balanced Chemical Equations Symbols Used in Equations To convey more information than just the type of chemical involved, symbols are used after a chemical formula to indicate its physical state (g) = gas (l) = liquid (s) = solid (aq) = aqueous, dissolved in water

How to Write Balanced Chemical Equations When magnesium metal burns in air it produces a white, powdery compound: magnesium oxide Burning in air means reacting with O2 Write the word equation The reactants are to the left of the arrow The products are to the right of the arrow Two or more reactants or products are separated by a plus sign A word description Reactants on the left and products on the right, separated by an arrow magnesium + oxygen magnesium oxide

How to Write Balanced Chemical Equations Indicate the physical state of each substance Use the correct chemical symbol to indicate liquids and solids Metals are solids, except for Hg which is liquid Use molecular form for gases (H2, O2, N2, all halogens) Identify polyatomic ions Elements in the solid and liquid states are represented in equations by the chemical symbols for the element magnesium(s) + oxygen(g) magnesium oxide(s)

How to Write Balanced Chemical Equations Convert the word equation into a formula equation Use the correct chemical symbol to indicate liquids and solids There must be the same number of each kind of atom on the reactant and product side of the equation Determine if the equation is balanced If not equal, must BALANCE Elements in the solid and liquid states are represented in equations by the chemical symbols for the element The arrow means “to produce” Always use molecular not empirical formulas Use the correct chemical symbol for the elements Elements that are gases at room temperature are identified by their correct molecular form ___Mg (s) +___O2 (g) ___MgO(s)

How to Write Balanced Chemical Equations Unbalanced equations can be adjusted by the use of a coefficient placed to the left of a substance Coefficients will adjust the number of reactant and/or products molecules or formula units NEVER change the subscripts of a compound to balance an element It changes the identity of the compound Can change coefficients but never subscript numbers Adding coefficients to the equation will adjust the number of reactant and or product molecules A coefficient will denote the amount of a substance

How to Write Balanced Chemical Equations Guidelines when writing balancing chemical equations Write the unbalanced chemical equation Pick the element that occurs in only one compound on both sides of the equation The element that occurs as a free element on either side of the equation, balance it last

Balancing a Chemical Equation Example 1 2 2 Coefficient 2 1 Mg 2 1 Mg 2 O 2 1 O The coefficient affects both of the elements

Balancing a Chemical Equation: Example 2 When solid ammonium nitrite is heated it produces nitrogen gas and water vapor Write the formula equation

Balancing a Chemical Equation Example 2 2 x N 2 x N 2 x O 2 1 x O 4 x H 4 2 x H

Balancing a Chemical Equation: Example 3 Nitrogen monoxide gas decomposes to produce dinitrogen monoxide gas and nitrogen dioxide gas Write the formula equation

Balancing a Chemical Equation Example 3 1 x N 3 x N 3 1 x O 3 x O

Balancing a Chemical Equation Example 4 Liquid nitric acid decomposes to reddish-brown nitrogen dioxide gas, liquid water and oxygen gas. Write the formula equation

Balancing a Chemical Equation Example 4 2 2 2 4 2 1 x N 4 2 1 x N 12 6 3 x O 12 7 5 x O 4 2 1 x H 4 2 x H

Aqueous Solutions and Solubility: Compounds Dissolved in Water An aqueous solution: A homogeneous mixture of a substance with water It contains one visible phase, cannot differentiate the various components Same composition throughout with one substance dissolved into another A uniform mixture requires complete interaction of the components

Aqueous Solutions and Solubility: Compounds Dissolved in Water Two parts: Solvent and Solute Solute: Substance being dissolved Solvent: Substance that dissolves the solvent Most solutions are liquid but can be gaseous or solid Can have more than one solute, but only one solvent

Aqueous Solutions and Solubility: Compounds Dissolved in Water Water is an effective solvent for dissolving ionic compounds Water is neutral but the O atom is surrounded by many electrons and gives it a partial negative charge The other end which contains the H atom has a partial positive charge The positive ions are attracted to the O atom and negative ions to the H atom of water As an ionic compound dissolves, it becomes surrounded by water molecules

Aqueous Solutions and Solubility: Compounds Dissolved in Water When sodium chloride crystals are placed in water, they begin to dissolve The attractive forces between the ions and water are stronger than forces between the ions in the crystal Water molecules surround each ion, keeping them apart The ions become uniformly dispersed throughout the solution Cliff page 359, figure 12.2 text

Aqueous Solutions and Solubility: Compounds Dissolved in Water When ionic compounds dissolve in water the ions dissociate and separate into ions floating in water Potassium chloride dissociates in water into potassium cations and chloride anions KCl(aq) = K+ (aq) + Cl- (aq) K+ Cl- K Cl

Aqueous Solutions and Solubility: Compounds Dissolved in Water Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions CuSO4(aq) = Cu+2(aq) + SO42-(aq) Cu+2 SO42- Cu SO4

Aqueous Solutions and Solubility: Compounds Dissolved in Water Potassium sulfate dissociates in water into potassium cations and sulfate anions K2SO4(aq) = 2 K+ (aq) + SO42-(aq) K+ SO42- K SO4

Electrolytes and Nonelectrolytes A substance that exists as ions in solution Formed from an ionic compound that dissociates in water forming a solution with cations and anions Strong electrolytes are ionic compounds that dissociate (nearly) completely in solution and exist as ions Strong electrolyte solutions conduct electricity due to the presence of ions

Electrolytes and Nonelectrolytes Solute is a molecular substance (not ionic) It does not form ions in solution (no dissociation) Substance remains intact, dispersed throughout the solvent as individual molecules Each molecule is separated by molecules of the solvent These substances do not conduct electricity

Electrolytes and Nonelectrolytes Strong electrolyte: Dissociates completely into ions Conducts electricity NaCl, HCl Weak electrolyte: Mainly whole molecules Very few separate (into ions) Conducts electricity less than strong electrolytes Acetic acid, HF Nonelectrolyte: No dissociation into ions Do not conduct electricity Methanol, sucrose

Electrolytes and Nonelectrolytes Strong electrolytes are completely ionized when dissolved in water Sodium chloride dissociates to form Na+ and Cl- Good conductor of electricity Show Timberlake page 380 and Zumdahl pg 135

Electrolytes and Nonelectrolytes Weak electrolytes are only partially ionized when dissolved in water Hydrofluoric acid only partially dissociates to form H+ and F- Poor conductor of electricity Show Timberlake page 380 and Zumdahl pg 135

Electrolytes and Nonelectrolytes Nonelectrolyes are not ionized when dissolved in water e.g. sugar and ethanol do not dissociate into ions in water Do not conduct electricity Molecules remain intact Show Timberlake page 380 and Zumdahl pg 135

Solubility: Soluble and Insoluble Salts Not all ionic compounds are soluble in water If water cannot overcome the ionic forces to separate the ions in solution: insoluble compound The factors to determine solubility are complex which make predictions difficult Through observation a series of statements or rules have developed to guide predictions called the solubility rules (See Table 7.2, page 210 and handout) For all solid in liquid solutions, all ionic compounds do not dissolve in water. Even if it is polar, “like dissolves like” does not always apply. As the size of the charges on the cation and anion become larger, the forces holding the crystal together become so great that they cannot be overcome

Solubility Rules Most compounds that contain NO3- and C2H3O2- ions are soluble in water Most compounds that contain Li+, Na+, K+, or NH4+ ions are soluble in water Most compounds that contain Cl-, Br-, I- ions are soluble, except AgCl, PbCl2, and Hg2Cl2 Most compounds that contain SO42- ions are soluble, except SrSO4, BaSO4, PbSO4, CaSO4 Solubility rules for ionic compounds in water

Solubility Rules Most compounds that contain OH- ions are slightly soluble (will precipitate). Exceptions: NaOH, KOH, are soluble and Ba(OH)2, Ca(OH)2 are moderately soluble Most compounds that contain S2-, CO32-, or PO43- ions are slightly soluble (will precipitate)

Precipitation Reactions Upon mixing two aqueous solutions, if two ions of an insoluble salt come into contact, a solid forms This is a precipitation reaction The solid that forms is called a precipitate Can use the solubility rules to predict whether a solid will form when two ionic solutions are mixed

Precipitation Reactions Two aqueous clear solutions are mixed together: lead (II) nitrate and potassium iodide The reaction between two solutes produces an insoluble product: lead (II) iodide

Predicting Precipitation Reactions There are no simple (basic) rules to predict which ionic compounds will be insoluble Experimental observations have led to the guidelines for predicting solubility (see text: table 7.2, page 210 and handout) These solubility rules will be used to predict the products formed in precipitation reactions

Predicting Precipitation Reactions In solution, the cations of the two reactants exchange anions The chemical formulas are based on the charges on the ions Will the new interactions of cations and anions form an insoluble compound?

Predicting Precipitation Reactions To predict the identities of the products: Write the formulas of the two compounds being mixed as reactants in a chemical equation Separate all soluble ionic compounds into ions AB + CD A+ B- C+ D- Reactants

Predicting Precipitation Reactions Write the formulas of the potentially insoluble products that could form from the new combination of the ions Exchange pairs Charge balance each compound A+ B- C+ D- A+ D- C+ B- Products

Predicting Precipitation Reactions Use the solubility rules to determine whether any of the new products are insoluble If all of the potentially insoluble products are soluble, there will be no precipitate (NO REACTION) If any of the potential products are insoluble, use symbol (s) to indicate a solid forms Balance the equations with coefficients

Writing Equations for Precipitation Reactions Aqueous solutions of calcium chloride and sodium carbonate are mixed Write the formula of the two compounds Write the formulas of the potentially insoluble products that could form from the reactants Ca2+ 2 Cl- 2 Na+ CO32-

Writing Equations for Precipitation Reactions Use the solubility rules to determine whether any of the new products are insoluble NaCl is soluble (Rule 2, Rule 3) CaCO3 is insoluble (Rule 6) Since one of the potential products is insoluble, write the formulas of the products of the reaction

Writing Equations for Precipitation Reactions Include the symbol (s) to indicate solid and (aq) to indicate aqueous products Balance the equation Use coefficients, do not change the subscripts 2

Molecular, Complete Ionic, and Net Ionic Equations Example Two aqueous clear solutions are mixed together: lead (II) nitrate and potassium iodide The reaction between two solutes produces an insoluble product: lead (II) iodide

Molecular, Complete Ionic, and Net Ionic Equations Molecular Equations Equations which show the complete chemical formulas for all reactants and products Uses neutral formulas or symbols without indicating ionic character

Molecular, Complete Ionic, and Net Ionic Equations Complete Ionic Equations Separates the neutral formulas into the ions they will actually form in solution Equations which describe the actual ions and molecules in the solutions as well as the molecules of solid, liquid and gas not dissolved Notice that the potassium ion and the nitrate ion remain unchanged even as product is formed

Molecular, Complete Ionic, and Net Ionic Equations Spectator Ions Ions that appear in identical forms on both sides of the equation Although present, they play no role in the reaction Net Ionic Equation An ionic equation in which the spectator ions are omitted It shows only the ions that are directly involved in a chemical reaction

Predicting Reactions When chemicals (dissolved in water) are mixed and one of these four events can occur, the reaction will generally happen “Forces” that drive a reaction formation of a solid formation of water transfer of electrons formation of a gas

Predicting Reactions Determine if product(s) are: Insoluble (precipitation reaction) Water (acid/base reaction) Electron transfer (redox reaction) Gas (gas-forming reaction)

Acid-Base (Neutralization) Reactions Acids are substances that form H+ ion in water Bases are substances that form OH- ion in water A Neutralization reaction is the reaction between an acid and a hydroxide base to form a salt and water

Acid-Base (Neutralization) Reactions The reaction of hydrochloric acid and sodium hydroxide: A double replacement reaction Ionic Equation (remove spectator ions) Net Ionic Equation (water forms) acid base salt water Heat of formation of water generates heat

Predicting Reactions Determine if product(s) are: Insoluble (precipitation reaction) Water (acid/base reaction) Electron transfer (redox reaction) Gas (gas-forming reaction)

Gas Evolution Reactions Other anions that will react with H+ are sulfide, carbonate, and bicarbonate ion These anions react with acids to form gases Sulfuric acid reacting with aqueous lithium sulfide Molecular equation acid base gas salt

Gas Evolution Reactions Complete ionic equation Net ionic equation (balanced)

Predicting Reactions Determine if product(s) are: Insoluble (precipitation reaction) Water (acid/base reaction) Electron transfer (redox reaction) Gas (gas-forming reaction)

Oxidation-Reduction Reactions When an atom, ion, or molecule becomes more positively charged, a loss of electrons occurs: oxidation When an atom, ion, or molecule becomes more negatively charged, a gain of electrons occurs: reduction When one reactant loses electrons another reactant must gain electrons Reactions in which this electron transfer occurs between reactants is called an oxidation-reduction (redox) reaction

Oxidation-Reduction Reactions A substance reacting with elemental oxygen A metal reacting with a nonmetal Electron transfer between two reactants Be able to identify these types of redox reactions

Combustion Reactions Combustion occurs when a hydrocarbon (or other combines with oxygen to produce carbon dioxide, water heat (flame) The reaction of oxygen with any substance If a combustion reaction is possible then the substance will burn Commonly known as burning, the reaction of oxygen with any substance

Combustion Reactions heat heat heat Examples The combustion reaction of hydrocarbons produces carbon dioxide, water, and heat The combustion of propane The combustion of glucose The combustion of sulfur Also a combination reaction, also produces heat heat hydrocarbon heat The combustion of sulfur also produces sulfur trioxide hydrocarbon heat

Classifying Chemical Reactions Chemical reactions can be separated into groups of similar reactions There are several ways to group reactions but most can be placed into one of four classes Synthesis Decomposition Single-displacement Double-displacement

Classifying Chemical Reactions Synthesis Reactions Reactions in which two or more substances combine to form a third substance One new product forms General form of equation: Can have Elements combine An element and a compound Two compounds A + B AB

Synthesis Reactions The combinations can include Examples: Two elements An element and a compound Two compounds Examples:

Classifying Chemical Reactions Decomposition Decomposition Reactions Reactions in which one reactant breaks down into simpler (smaller) substances Generally initiated by addition of light energy, electric current, or heating substances to high temperature Opposite of a Synthesis Reaction General Form of Equation AB A + B

Decomposition Reactions Can be broken down to: Smaller compounds Elements Both Examples Symbol for heat is the delta triangle Light is symbolized by hV

Classifying Chemical Reactions Single-Displacement Reactions One element displaces another element Forms a new compound which frees the replaced element Most reactions occur in an aqueous solution General Form of Equation A + BC AC + B

Single-Displacement Reactions In this type of single-displacement reaction there are two types Metal displaces a metal Metal displaces hydrogen Examples: fi metal displaces metal metal displaces hydrogen

Classifying Chemical Reactions Double-Replacement Reactions Double-Displacement Reactions Two compounds exchange ions or atoms to form new compounds Also called exchange reactions Shows the exchange of “associates” when comparing the reactants and products General Form of Equation AB + CD AD + BC

Double Replacement Reactions Most of these reactions occur in aqueous solution Most involve acids, bases, and ionic compounds Products formed Precipitate (a solid that is insoluble) A gas Water

Double Replacement Reactions Examples: precipitate gas water

Summary of Reaction Types

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