Ch. 8 – Chemical Reactions Intro to Reactions

Describing a Chemical Reaction A Chemical Reaction is the process by which one or more substances are changed into one or more different substances. It is represented by a Chemical Equation.

Chemical Equations A+B  C+D REACTANTS PRODUCTS

Signs of a Chemical Reaction Evolution of heat and light Formation of a gas Formation of a precipitate (i.e. solid) Color change

Law of Conservation of Mass Mass is neither created nor destroyed in a chemical reaction, simply rearranged. Total mass stays the same Total # atoms stay the same 4 H 2 O 4 H 2 O 36 g 4 g 32 g

2H2(g) + O2(g)  2H2O(g) Writing Equations Identify the substances involved Answer the following questions: How many? – coefficient Of what? – chemical formula In what state? – (s), (l), (g), (aq)

“ #7 makes a 7…and don’t forget Hydrogen” - (makes 7 total) Diatomic Elements Hydrogen Nitrogen Oxygen Flourine Chlorine Bromine Iodine “ #7 makes a 7…and don’t forget Hydrogen” - (makes 7 total) or “HOFBrINCl”

Symbols used in Equations Meaning  “Yields” (s) Solid (l) Liquid (g) Gas (aq) Aqueous (solid dissolved in water) Reactants are heated

Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) Describing Equations Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) How many? Of what? In what state? One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid to produce one unit of aqueous zinc chloride and one molecule of hydrogen gas.

2Al(s) + 3CuCl2(aq)  3Cu(s) + 2AlCl3(aq) Writing Equations Two atoms of solid aluminum react with three units of aqueous copper(II) chloride to produce three atoms of solid copper and two units of aqueous aluminum chloride. What? How many? In what state? 2Al(s) + 3CuCl2(aq)  3Cu(s) + 2AlCl3(aq)

Ch. 8 – Chemical Reactions Balancing Equations

Balancing  

Balancing Example 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1 2 3 Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1 2 3  2  6 3  6   3

Balancing Steps Write the unbalanced equation Count atoms on each side Add coefficients to make #s equal. Coefficient  subscript = # of atoms Reduce coefficients to lowest possible ratio, if necessary. Double check atom balance!!!

Helpful Tips Balance one element at a time Leave Hydrogen and Oxygen for last Update ALL atom counts after adding a coefficient Balance polyatomic ions as a single unit Go to Virtual balancing examples…

Types of Chemical Reactions Ch. 8 – Chemical Reactions Types of Chemical Reactions

Types of Reactions Synthesis reactions Decomposition reactions Single displacement reactions Double displacement reactions Combustion reactions You need to be able to identify each type.

Types of Reactions A + B → AB AB → A + B AB + C → AC + B Synthesis Decomposition Single Replacement Double Replacement Combustion A + B → AB Two or more substances combine to form a new substance A single compound breaks down into two or more simpler substances AB → A + B AB + C → AC + B One element replaces another element in a compound The atoms in two different compounds trade places AB + CD → AC + BD A substance combines with oxygen to yield carbon dioxide and water X + O2  CO2 + H2O

Synthesis (Combination) The combination of 2 or more substances to form a compound Only one product A + B  AB

Synthesis (Combination) H2(g) + Cl2(g)  2 HCl(g)

Practice MgF2(s) AlF3(s) Predict the products… Na(s) + Cl2(g) Mg(s) + F2(g)  Al(s) + F2(g)  NaCl(s) 2 2 MgF2(s) 2 3 2 AlF3(s) Now, balance them…

Decomposition A compound breaks down into 2 or more simpler substances Only one reactant AB  A + B

Decomposition 2 H2O(l)  2 H2(g) + O2(g)

Decomposition 2 2 KBr(l)  K(s) + Br2(l) Products: Binary - break into elements Others - hard to tell (more on this later) 2 2 KBr(l)  K(s) + Br2(l)

A + BC  B + AC Single Replacement One element replaces a similar element in a compound Metal (+) replaces metal (+) Nonmetal (-) replaces nonmetal (-) Free element must be more active (check activity series) A + BC  B + AC

Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s) Single Replacement Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)

Single Replacement Products: ion + compound Metal atom  metal cation(+) Nonmetal atom  nonmetal anion(-) Ni (s) + CuCl2 (aq)  NiCl2 (aq) + Cu (s) 2Al (s) + 3Pb(NO3)2 (aq)  3Pb (s) + 2Al(NO3)3 (aq) Mg (s) + 2HCl (aq)  H2 (g) + MgCl2 (aq) Br2 (l) + KCl (aq)  no reaction Co (s) + 2NaCl (aq)  no reaction

Single Replacement Reactions Write and balance the following single replacement reaction equation: Zn(s) + HCl(aq)  2 ZnCl2 + H2 (g) 2 2 NaF(s) + Cl2 (g) NaCl(s) + F2(g)  Al (s) + Cu(NO3)2 (aq)  2 3 3 Cu (s) + Al(NO3)3 (aq) 2

AB + CD  AD + CB Double Displacement The ions of two compounds change places in a aqueous solution. ions in two compounds “change partners” cation of one compound combines with anion of the other AB + CD  AD + CB

Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq) Double Replacement Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)

Double Displacement Products: Dbl. Displacement reaction will always produce one of the following: A precipitate check solubility table, 2KI (aq) + Pb(NO3)2 (aq)  Pbl2 (s) + 2KNO3 (aq) An insoluble gas that bubbles out of solution, or FeS (s) + 2HCl (aq)  H2S (g) + FeCl2 (aq) A soluble molecular compound that remains dissolved in the solution. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O(l)

CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Combustion A substance reacts with O2, releasing heat and light A hydrocarbon reacts with O2, producing CO2 & H2O A + O2  B CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

Combustion C3H8(g)+ O2(g)  5 3 4 CO2(g)+ H2O(g) Products: ALWAYS form CO2 + H2O Form carbon monoxide and/or carbon if there is limited oxygen C3H8(g)+ O2(g)  5 3 4 CO2(g)+ H2O(g)

Identifying Chemical Reactions Use the codes below to classify each reaction S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement   ____ P + O2 → P4O10 ____ Mg + O2 → MgO ____ HgO → Hg + O2 ____ Al2O3 → Al + O2 ____ Cl2 + NaBr → NaCl + Br2 ____ H2 + N2 → NH3 ____ Na + Br2 → NaBr ____ S8 + F2 → SF6 ____ CuCl2 + H2S → CuS + HCl ____ C + H2 → CH4 ____ HgO + Cl2 → HgCl + O2 ____ KClO3  → KCl  +  O2   ____ BaCl2  +   Na2SO4 →  NaCl   +  BaSO4

Predicting Products From Reactions If given only reactants Figure out what type of reaction it is Write the products. If one of the products is ionic, balance charges. Balance the equation

Activity Series of the Elements Ch. 8 – Chemical Reactions Activity Series of the Elements

Activity Series An activity series is a list that organizes elements according to the ease with which they react. They are used to help predict whether a reaction will occur. A more active element will replace a less active element in single-displacement reactions. If this is not the case, then no reaction occurs. An element may replace any element below it, but not above it. Do all reactions occur? How do you know when a reaction occurs and when it does not?

Activity Series Do all reactions occur? How do you know when a reaction occurs and when it does not?

Practice… Will the following reactions occur? If yes, what are the products? If no, write “no reaction” MgCl2 (aq) + Zn (s) no reaction Al (s) + H2O (g)  Al2O3 + H2 Cd (s) + O2 (g)  CdO (s) Br2 (l) + KCl (aq)  no reaction