Chemical Reactions Chapter 11 General Chemistry

Objectives Balance chemical equations by applying the law of conservation of mass. Recognize synthesis, decomposition, single displacement, double displacement, and neutralization reactions. Write chemical equations from words. Predict the products of chemical reactions. Write net ionic equations for reactions.

Chemical Reactions Basics Environmental Chemistry unit 2 Sect C

Chemical Reactions A chemical reactions are chemical changes. Recall: chemical changes are when one or more substances are changed into different substances Examples: wood burning, metal rusting

Indications of a Chemical Reaction Certain easily observed changes indicate that a chemical reaction has occurred: evolution of energy by heat and/or light color change production of gas production of precipitate A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate My.hrw.com

Law of Conservation of Matter Chemical reactions obey the Law of Conservation of Matter in all chemical and physical changes, matter is neither created or destroyed The total mass in a chemical reaction remains constant Environmental Chemistry unit 2 Sect C

Chemical Equations A chemical equation represents, with symbols and formulas, the identities and the amounts of the reactants and products in a chemical reaction. Has two parts: Reactants: the substances you start with Products: the substances you end up with The reactants turn into the products 4 Fe + 3 O2  2 Fe2O3 Reactants  products Environmental Chemistry unit 2 Sect C

Ways to Express a Chemical Reaction The way atoms are joined is changed in a chemical reaction. Can be described several ways: 1. In a sentence Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation Copper + chlorine ® copper (II) chloride 3. In formula equation: Cu + Cl2  CuCl2 Environmental Chemistry unit 2 Sect C

Symbols Used in Chemical Equations

Additional Symbols Used in Equations shows that heat is supplied to the reaction is used to indicate a catalyst is supplied, in this case, platinum. Environmental Chemistry unit 2 Sect C

What is a catalyst? A catalyst is a substance that speeds up a reaction, without being changed or used up by the reaction. Enzymes are biological or protein catalysts. Environmental Chemistry unit 2 Sect C

Study Buddy Review What are four things that indicate a chemical reaction has taken place? What is the Law of Conservation of mass? What does the symbol (l) mean? What does (aq) mean? Environmental Chemistry unit 2 Sect C

Balancing Chemical Equations Environmental Chemistry unit 2 Sect C

Skeleton and Balanced Equations Skeleton equations: do not indicate how many of each element/compound Balanced equations: the number of atoms of each element is the same on both sides of the reaction. Environmental Chemistry unit 2 Sect C

Numbers in Balanced Equations Use coefficients to balance equation Coefficients: large whole numbers placed in front of formula which represents number of units of that compound Subscript: small whole number placed in chemical formula to represent number of atoms of an element in a compound Environmental Chemistry unit 2 Sect C

4 Fe + 3 O2  2 Fe2O3 subscript Coefficient Environmental Chemistry unit 2 Sect C

Balanced Equation A balanced equation has the same number of atoms of each element on both sides of the equation Atoms can’t be created or destroyed All the atoms at the beginning must appear in the end Environmental Chemistry unit 2 Sect C

® O + C C O O C + O2 ® CO We need one more oxygen in the products. Can’t change the formula, because it describes what it is (carbon monoxide in this example) Environmental Chemistry unit 2 Sect C

® Must be used to make another CO But where did the other C come from? + C O C O Must be used to make another CO But where did the other C come from? Environmental Chemistry unit 2 Sect C

® Must have started with two C 2 C + O2 ® 2 CO C C O O + O C C O Environmental Chemistry unit 2 Sect C

Rules for balancing: Assemble, write the correct formulas for all the reactants and products Count the number of atoms of each type of element appearing on both sides Balance the atoms of an element one at a time by adding coefficients (the numbers in front) - save H and O until LAST! Check to make sure it is balanced. Environmental Chemistry unit 2 Sect C

Never Never change a subscript to balance an equation. If you change the formula you are describing a different reaction. H2O is a different compound than H2O2 Never put a coefficient in the middle of a formula 2 NaCl is okay, Na2Cl is not. Environmental Chemistry unit 2 Sect C

Balancing Equations Examples H2 (g) + O2 (g)  H2O (l) Zn + HCl  H2 + ZnCl2 Pb (NO3)2 + K2S  PbS + KNO3 Environmental Chemistry unit 2 Sect C

Balancing Equations Answers 2 H2 (g) + O2 (g)  2 H2O (l) Zn + 2 HCl  H2 + ZnCl2 Pb (NO3)2 + K2S  2PbS + 2KNO3 Environmental Chemistry unit 2 Sect C

Word Equation Examples Write a balanced chemical equation for the following: Aqueous solutions of sulfuric acid and sodium hydroxide react to form aqueous sodium sulfate and water.

Five General Types of Chemical Reactions

Five General Types of Chemical Reactions Synthesis (Composition) Decomposition Single-Replacement Double-Replacement Combustion By knowing the type of reaction that is occurring, you can predict the products that will be formed.

General Types of Chemical Reactions Combination: A + B  AB Decomposition: AB  A + B Single Replacement : AX +B  A + BX Double Replacement: AX + BY  AY + BX Combustion: CxHy + O2  CO2 + H2O

Synthesis (Combination) General form: A + B  AB (two reactants make a single product) A, B = elements or compounds AB = compound consisting of A and B This is the only type of chemical reaction in which there is a single product formed. This single product is always more complex than the reactants.

Examples of Synthesis Reactions a. calcium + oxygen yields calcium oxide 2Ca + O2  2CaO b. carbon dioxide + water yields carbonic acid CO2 + H2O  H2CO3 Notice: All equations show two (or more) reactants, but only one product. http://www.ric.edu/ptiskus/reactions/Index.htm

Decomposition Reactions (Decomposing = breaking down into smaller parts; microorganisms) A reaction in which a single compound is broken into 2 or more smaller compounds or elements. The opposite of direct combination/synthesis. You can identify this reaction because there is only one reactant. General form: AB  A + B AB = compound A, B = elements or simpler compounds

Examples of Decomposition Reactions Water yields hydrogen and oxygen 2H2O  2H2 + O2 marble (rock) yields (with heating) calcium oxide and carbon dioxide CaCO3  CaO + CO2 Notice: single compound decomposes into two (or more) products. http://www.ric.edu/ptiskus/reactions/Index.htm

Single-Replacement Reactions One element and one compound recombine (switch partners) General Form: A + BX  AX + B AX, BX = ionic compounds A, B = elements X = ion that switches partners

Examples of Single-Replacement Reactions Magnesium metal and copper (II) sulfate Mg + CuSO4  MgSO4 + Cu Iron metal and copper (II) sulfate) Fe + CuSO4  FeSO4 + Cu www.ric.edu/ptiskus/reactions/Index.htm

Double-Replacement Reactions Positive ions in two compounds are exchanged. Look for: 2 compounds as reactants and 2 compounds as products. Reactants are usually ionic compounds. Specific example: neutralization reactions between acid and base General form: AX + BY  AY + BX (Notice: X and Y “change partners”)

Examples of Double Replacement Reactions Calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid. CaCO3 + 2HCl  CaCl2 + H2CO3 www.ric.edu/ptiskus/reactions/Index.htm

Rules of Double-Displacement Reactions Reactants must be dissolved in water (releasing the ions). Will occur if: One of the products is a molecule (covalent), A precipitate (solid comes out of solution), or An insoluble gas.

C2H5OH (gasohol) + 3O2  3H2O + 2CO2 Combustion Reactions Reactants: a fuel (usually a hydrocarbon) and oxygen. Products: water, carbon dioxide, and lots of energy! (heat, light) Examples: CH4 (methane) + 2O2  2H2O + CO2 C2H5OH (gasohol) + 3O2  3H2O + 2CO2 www.ric.edu/ptiskus/reactions/Index.htm

Study Buddy Review Describe each of the following to your study buddy: Synthesis (combination) Decomposition Single-Replacement Double-Replacement Combustion

Activity Series

Activity Series and Single Replacement Reactions Al + CuCl2  ??? Cu + AlCl3  ??? Only one of the following reactions will occur How do you know which one?

Activity Series of Metals http://wps.prenhall.com/wps/media/objects/165/169060/tool0403.gif

Activity Series Rules Elements at top of series are most active Elements at bottom of series are least active (coinage metals) Elements at the top of the series will replace elements below it in a chemical reaction. Example: Al + CuCl2  will occur Cu + AlCl3  will NOT occur

Activity Series Examples Using the activity series, predict whether each of the possible reactions will occur: Cr + H2O  Pt + O2  Cd + 2HBr 

Activity Series Examples Using the activity series, predict whether each of the possible reactions will occur: Cr + H2O  NR Pt + O2  NR Cd + 2HBr  CdBr2 + H2

References Dr. Stephen L. Cotton Charles Page High School Mrs. Lijek