Chapter 9 Chemical Reactions

l Section 1: Objectives –Identify the parts of a chemical equation –Learn how to write a chemical equation –Learn how to balance a chemical equation.

A chemical reaction is the process by which one or more substances are changed into one or more different substances. Chemical reactions are described by chemical equations. A chemical equations represents, with symbols and formulas, the identities and relative amounts of the starting material and products.

Indications of a Chemical Reaction 1)Evolution of heat and light. 2)Production of a gas. 3)Formation of a precipitate (solid). (a solid appears after two liquids are mixed) 4) Color change

All chemical reactions… l have two parts: 1.Reactants = the substances you start with (left side of arrow) 2.Products = the substances you end up with (right side of arrow) l The reactants will turn into the products. Reactants  Products

Symbols in equations l the arrow (→) separates the reactants from the products (arrow points to products) –Read as: “reacts to form” or yields l (s) after the formula = solid: Fe (s) l (g) after the formula = gas: CO 2(g) l (l) after the formula = liquid: H 2 O (l)

l (aq) after the formula = dissolved in water, an aqueous solution: NaCl (aq) is a salt water solution. l Symbols if a reaction is heated. Symbols in equations

Formula Equations: Represents the reactants and products by symbols and formulas: Fe (s) + O 2(g)  Fe 2 O 3(s) Cu (s) + AgNO 3(aq)  Ag (s) + Cu(NO 3 ) 2(aq) NO 2(g)  N 2(g) + O 2(g)

Balanced Chemical Equations l Atoms can’t be created or destroyed in an ordinary reaction: –All the atoms we start with we must end up with (meaning: balanced!) l A balanced equation has the same number of each element on both sides of the equation.

1)Metals 2)Non-metals 3)save balancing the H and O until LAST! (hint: I prefer to save O until the very last) Rules for balancing:

l Never change a subscript to balance an equation (You can only change coefficients) –If you change the subscript (formula) you are describing a different chemical. –H 2 O is a different compound than H 2 O 2 l Never put a coefficient in the middle of a formula; they must go only in the front 2 NaCl is okay, but Na 2 Cl is not. Rules

Example 1 Count atoms. –Reactants: 2 atoms N and 2 atoms H –Products: 1 atom N and 3 atoms of NH 3 N 2 + H 2 NH 3 Nitrogen + hydrogen ammonia

Balancing Equations Nothing is balanced. Balance the nitrogen first by placing a coefficient of 2 in front of the NH 3. N 2 + H 2 2NH 3

Balancing Equations Hydrogen is not balanced. Place a 3 in front of H 2. Reactant side: 2 atoms N, 6 atoms H Product side: 2 atoms N, 6 atoms H It’s balanced! N 2 + 3H 2 2NH 3

Example 2 Count Atoms: Reactants: Cu – 1, H – 2, S – 1, O – 4 Products: Cu – 1, H – 2, S - 2, O - 7 Cu + H 2 SO 4 CuSO 4 + H 2 O + SO 2

Balancing Equations Sulfur is not balanced. Place a two in front of sulfuric acid. Count atoms: 2 H 2 SO 4 H – 4, S – 2, O - 8 Cu + 2H 2 SO 4 CuSO 4 + H 2 O + SO 2

Balancing Equations Hydrogen needs to be balanced so place a 2 in front of the H 2 O. Count the number of atoms. Cu + 2H 2 SO 4 CuSO 4 + 2H 2 O + SO 2

Balancing Equations Reactants: Cu – 1, H – 4, S – 2, O – 8 Products: Cu – 1, H – 4, S – 2, O – 8 It’s balanced! Cu + 2H 2 SO 4 CuSO 4 + 2H 2 O + SO 2

Practice Balancing Examples _AgNO 3 + _Cu  _Cu(NO 3 ) 2 + _Ag _Mg + _N 2  _Mg 3 N 2 _P + _O 2  _P 4 O 10 _Na + _H 2 O  _H 2 + _NaOH _CH 4 + _O 2  _CO 2 + _H 2 O

Section 2 Types of Chemical Reactions l OBJECTIVES: –Describe the five general types of reactions. –Predict the products of the five general types of reactions.

Type of Reactions Chemical reactions are classified into five general types Combination (synthesis) Decomposition Single Replacement Double Replacement Combustion

Combination (Synthesis) Two or more elements or simple compounds combine to form (synthesize) one product A + B AB 2Mg + O 2 2MgO 2Na + Cl 2 2NaCl S O 2 8 SO 2

Combination (Synthesis) 1) Reactions of elements with oxygen and sulfur. The groups 1 and 2 metals react with oxygen to form oxides. 2 Mg + O 2 2 MgO (group 2) 4 Li + O 2 2 Li 2 O (group 1)

Combination (Synthesis) 1) Reactions with oxygen and sulfur. Sulfur which is right below oxygen reacts in a similar manner 8 Ba + S 8 8 BaS 16 Rb + S 8 8 Rb 2 S

Combination (Synthesis) 2) Reactions of metals with halogens 2 Na + Cl 2 2 NaCl 2 K + I 2 2 KI Mg + F 2 MgF 2

Decomposition One substance is broken down (split) into two or more simpler substances. These reactions usually require heat ABA + B Examples: 2 HgO2 Hg + O 2 2 KClO 3 2 KCl + 3 O 2

Decomposition 1) Decomposition of binary compounds 2 H 2 O2 H 2 (g) + O 2 (g) (electrolysis) electricity 2 HgO2 Hg + O 2

Decomposition 2) Decomposition of metal carbonates CaCO 3 CaO + CO 2 3) Decomposition of metal hydroxides Ca(OH) 2 CaO + H 2 O

Decomposition 4) Decomposition of metal chlorates 2 KClO 3 2 KCl + 3 O 2

Learning Check Classify the following reactions as 1) combination or 2) decomposition: ___A. H 2 + Br 2 2HBr ___B. Al 2 (CO 3 ) 3 Al 2 O 3 + 3CO 2 ___C. 4 Al + 3C Al 4 C 3

Solution Classify the following reactions as 1) combination or 2) decomposition: _1_A. H 2 + Br 2 2HBr _2_B. Al 2 (CO 3 ) 3 Al 2 O 3 + 3CO 2 _1_C. 4 Al + 3C Al 4 C 3

Single Replacement One element replaces a similar element in a compound. A + BX AX + B Zn + 2HCl ZnCl 2 + H 2 Fe + CuSO 4 FeSO 4 +Cu Many single replacement reactions take place in an aqueous solution.

Single Replacement 1) Replacement of a metal by another metal. Refer to activity series table to determine if the reaction will occur. Al + Pb(NO 3 ) 2 Pb + Al(NO 3 ) 3 Reaction is not balanced. Try to balance.

Single Replacement 1) Replacement of a metal by another metal. Refer to activity series table to determine if the reaction will occur. 2 Al + 3 Pb(NO 3 ) 2 3 Pb + 2 Al(NO 3 ) 3 Balanced!

Single Replacement 2) Replacement of hydrogen in water by a metal. Na + H 2 O NaOH + H 2 (g) Unbalanced! Group 1 metals react vigorously with water.

Single Replacement 2) Replacement of hydrogen in water by a metal. 2 Na + 2 H 2 O 2 NaOH + H 2 (g) Balanced!

Single Replacement 3) Replacement of hydrogen in an acid by a metal. Mg + 2 HCl MgCl 2 + H 2 (g) balanced! Group 1 and 2 metals can react with acids to produce hydrogen gas.

Single Replacement 4) Replacements of halogens. Cl 2 + KBr KCl + Br 2 unbalanced! Refer to activity series table. Order: F > Cl > Br > I

Single Replacement 4) Replacements of halogens. Cl KBr 2 KCl + Br 2 balanced! Refer to activity series table.

Single Replacement 4) Replacements of halogens. F NaCl 2 NaF + Cl 2 balanced! Refer to activity series table. Order: F > Cl > Br > I

Single Replacement 4) Replacements of halogens. Br 2 + KCl products? Refer to activity series table. Order: F > Cl > Br > I

Double Replacement Two elements in reactants exchange places. AX + BY AY + BX AgNO 3 + NaCl AgCl + NaNO 3 ZnS + 2HCl ZnCl 2 + H 2 S One new product is usually a precipitate or gas

Double Replacement 1) Formation of a precipitate. KI + Pb(NO 3 ) 2 KNO 3 + PbI 2 (s) unbalanced Exchange of metals in the reaction. PbI 2 is a solid that precipitates out of the reaction.

Double Replacement 1) Formation of a precipitate. 2 KI + Pb(NO 3 ) 2 2 KNO 3 + PbI 2 (s) balanced Exchange of metals in the reaction.

Double Replacement 2) Formation of a gas. Fe(II)S + 2 HCl H 2 S (gas) + FeCl 2 balanced Iron(II) sulfide reacts to form hydrogen sulfide gas.

Double Replacement 3) Formation of a water. HCl + NaOH NaCl + H 2 O (liquid) balanced Usually an acid-base reaction

Learning Check Classify the following reactions as 1) single replacement 2) double replacement __ A) 2Al + 3H 2 SO 4 Al 2 (SO 4 ) 3 + 3H 2 __B) Na 2 SO 4 + 2AgNO 3 Ag 2 SO 4 + 2NaNO 3 __C) 3C + Fe 2 O 3 2Fe + 3CO

Solution Classify the following reactions as 1) single replacement 2) double replacement 1_ A) 2Al + 3H 2 SO 4 Al 2 (SO 4 ) 3 + 3H 2 2_B) Na 2 SO 4 + 2AgNO 3 Ag 2 SO 4 + 2NaNO 3 1_C) 3C + Fe 2 O 3 2Fe + 3CO

Combustion A reaction in which a compound (often carbon) reacts with oxygen. C + O 2 CO 2 CH O 2 CO 2 + 2H 2 O C 3 H O 2 3 CO H 2 O C 6 H 12 O O 2 6 CO H 2 O Reaction releases energy in the form of heat or light.

Learning Check Balance the combustion equation ___C 5 H 12 + ___O 2 ___CO 2 + ___H 2 O

Solution Balance the combustion equation 1 C 5 H O 2 5 CO H 2 O

End

Oxidation and Reduction  Reactions that involve a loss or gain of electrons  Occurs in many of the 4 types of reactions and combustion  Important in food metabolism, batteries, rusting of metals

Requirements for Oxidization- Reduction Electrons are transferred Two processes occur Oxidation = Loss of electrons (LEO) ZnZn e - Reduction = Gain of electrons (GER) Cu e - Cu

Balanced Red-Ox Equations Combine the oxidation and reduction reactions to make Loss of electrons = Gain of electrons Zn + Cu e - Zn e - + Cu Zn + Cu 2+ Zn 2+ + Cu

Gain/Loss of Hydrogen In organic and biological reactions oxidation = Loss of H reduction = Gain of H

Learning Check R3 Identify the following as an 1) oxidation or a reduction process: __A. SnSn e- __B. Fe e - Fe 2+ __C. Cl 2 + 2e - 2Cl -

Solution R3 Identify the following as an 1) oxidation or a reduction process: 1_ A. SnSn e- 2_ B. Fe e - Fe 2+ 2_ C. Cl 2 + 2e - 2Cl -

Learning Check R4 In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction Ag + + Cl - Ag + Cl A. Which reactant is oxidized 1) Ag + 2) Cl - 3) Ag B. Which reactant is reduced? 1) Ag + 2) Cl - 3) Cl

Solution R4 In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction Ag + + Cl - Ag + Cl A. Which reactant is oxidized 2) Cl - Cl - Cl + e - B. Which reactant is reduced? 1) Ag + Ag + + e- Ag