Lecture 19&20 For Print

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  • 1. 12/2/2008 Formula – Week -10: Lecture 19 & 20 Subscripts vs. Coefficients Chemical Formulas • The subscripts • We know a Chemical Symbol identifies an tell you how element. many atoms of • A Formula is a combination of symbols that a particular identifies a compound, an ion, or a molecule of element are in an element. a compound. Ex: • The formula of compounds = HCl (NH4)3PO4 , H2O HCl, coefficient formula of ions = OH- , (NH4)+ tells you about formula of molecule = H2, N2, O2, F2, Cl2, the quantity, Br2, I2, P4, S8, O3 or number, of • A Formula also indicates the relative quantities molecules of of elements contained in the compound or ion and implies some kind of chemical bonding the between the atoms compound. Give the example of (NH4)3PO4 and some others. Ref: Read page-128 to 132 of Goldberg – fundamental of Chemistry Chemical Reaction Equation Chemical Reaction – Chemical equations show the conversion A process in which at least one new of reactants (the molecules shown on the substance is produced as a result of left of the arrow) into products (the chemical change. molecules shown on the right of the arrow). – Chemical reactions occur when bonds • + sign separates molecules on the same between the outermost parts of atoms are side formed or broken • The arrow is read as “yields” – Chemical reactions involve changes in matter, the making of new materials with new • Example properties, and energy changes. C + O2  CO2 – Symbols represent elements, formulas • This reads “carbon plus oxygen react to describe compounds, chemical equations yield carbon dioxide” describe a chemical reaction 1
  • 2. 12/2/2008 Reading& writing A Chemical Equation 4 NH3 + 5 O2 4 NO + 6 H2O Symbols Used in Equations Four molecules of NH3 react with five molecules O2 to produce four molecules NO and six molecules of • Solid (s) H2O • Liquid (l) or • Gas (g) Four moles NH3 react with 5 moles O2 to produce four moles NO and six moles H2O • Aqueous solution (aq) H SO Chemical symbols give a “before-and-after” • Catalyst 2 4 picture of a chemical reaction • Escaping gas () Reactants Products • Precipitate solid ( ) MgO + C CO + Mg • Change of temperature () magnesium oxide carbon-monoxide Ex: to form reacts with carbon and magnesium 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) ---> A Balanced Chemical Equation Balance Equations with Same numbers of each type of atom on each Coefficients side of the equation Coefficients in front of formulas balance Al + S Al2S3 Not Balanced each type of atom 2Al + 3S Al2S3 Balanced 4NH3 + 5O2 4NO + 6H2O Chemical Equations 4N = 4N Because of the principle of the 12 H = 12 H conservation of matter, 10 O = 10 O an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 2
  • 3. 12/2/2008 Steps to Balancing Equations Balancing Equations There are four basic steps to balancing a chemical – When balancing a chemical reaction equation. 1. Write the correct formula for the reactants and the you may add coefficients in front of the products. DO NOT TRY TO BALANCE IT YET! compounds to balance the reaction, but You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! you may not change the subscripts. 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. • Changing the subscripts changes the 3. Determine where to place coefficients in front of compound. Subscripts are formulas so that the left side has the same number of atoms as the right side for EACH element in order to determined by the valence electrons balance the equation. (charges for ionic or sharing for 4. Check your answer to see if: – The numbers of atoms on both sides of the covalent) equation are now balanced. – The coefficients are in the lowest possible whole number ratios. (reduced) Some Suggestions to Help You Ex: Steps in Balancing An Some of Helpful Hints for balancing equations: Equation • Take one element at a time, working left to right except for H and O. Save H for next Fe3O4 + H2 Fe + H2O to last, and O until last. • IF everything balances except for O, and Fe: Fe3O4 + H2 3 Fe + H2O there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an O: Fe3O4 + H2 3 Fe + 4 H2O element) • (Shortcut) Polyatomic ions that appear on H: Fe3O4 + 4 H2 3 Fe + 4 H2O both sides of the equation should be balanced as independent units Read from Page 207 to Page 213 of Goldberg- Fundamentals of Chemistry 3
  • 4. 12/2/2008 Learning Check E5 Balancing Balance each equation. The coefficients for Equations each equation are read from left to right A) ____C3H8(g) + _____ O2(g) ----> ____C ----> A. Mg + N2 Mg3N2 1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1 _____CO2(g) + _____ H2O(g) B. Al + Cl2 AlCl3 B) ____B4H10(g) + _____ O2(g) ---> ____B ---> 1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2 C. Fe2O3 + C Fe + CO2 ___ B2O3(g) + _____H2O(g) O(g) 1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3 D. Al + FeO Fe + Al2O3 C) Na3PO4 + Fe2O3 ----> ----> 1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1 E. Al + H2SO4 Al2(SO4)3 + H2 Na2O + FePO4 1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3 1. Synthesis reactions Types of Reactions • There are five types of chemical • Synthesis reactions occur when two reactions we will talk about: substances (generally elements) 1. Synthesis reactions combine and form a compound. 2. Decomposition reactions (Sometimes these are called 3. Combustion reactions combination or addition reactions.) 4. Single displacement reactions reactant + reactant  1 product 5. Double displacement reactions • Basically: A + B  AB • You need to be able to identify the type of reaction and predict the product(s) • Example: 2H2 + O2  2H2O • Example: C + O2  CO2 4
  • 5. 12/2/2008 Combination (Synthesis) Reaction Practice oxidation of metals- rust, burning burning of Mg metal • Predict the products. Write and balance 2Mg(s) + O2(g) 2MgO(s) the following synthesis reaction rusting of iron equations. 4Fe(s) + 3O2(g) 2Fe2O3(s) • Sodium metal reacts with chlorine gas burning non-metals C(s) + O2(g) CO2(g) Na(s) + Cl2(g)  • A compound may combine with free • Solid Magnesium reacts with fluorine gas element to from another compound Mg(s) + F2(g)  2CrCl2(s) + Cl2(g) 2CrCl3(s) • Aluminum metal reacts with fluorine gas • Two compound may combine together to form a single compound. Al(s) + F2(g)  MgO(s) + CO2(g) MgCO3(s) CaO(s) + H2O(l) Ca(OH)2 (s) 2. Decomposition Reaction Decomposition Exceptions a compound is broken down into simpler compounds or constituent elements 1 Reactant  Product + Product • Carbonates and chlorates are special case • In general: decomposition reactions that do not go XY  X + Y requires energy to the elements. Example: • Carbonates (CO32-) decompose to carbon dioxide and a metal oxide HYDROLYSIS - decompose water • Example: CaCO3  CO2 + CaO with electricity - hydrogen fuel cells • Chlorates (ClO3-) decompose to oxygen gas 2H2O(l) 2H2(g) + O2(g) and a metal chloride electricity • Example: 2 Al(ClO3)3  2 AlCl3 + 9 O2 MnO2 2KClO3(s) Heat 2KCl(s) + 3O2(g) decomposition of mercury(II) oxide 2HgO(s) 2Hg(s) + O2(g) 5
  • 6. 12/2/2008 Practice Identify the type of reaction for each of the 3. Combustion Reactions following synthesis or decomposition reactions, and write the balanced equation: • Combustion reactions occur when a Solid Lead (IV) oxide decomposes hydrocarbon reacts with PbO2(s)  oxygen gas. Aluminum nitride decomposes • This is also called burning!!! In order to AlN(s)  burn something you N2(g) + O2(g)  Nitrogen monoxide need the 3 things in the BaCO3(s)  “fire triangle”: 1) A Fuel (hydrocarbon) Co(s)+ S(s)  (make Co be +3) 2) Oxygen to burn it with NH3(g) + H2CO3(aq)  3) Something to ignite NI3(s)  the reaction (spark) Combustion Reactions 4. Single Replacement Reactions • In general: CxHy + O2  CO2 + H2O • Single Replacement Reactions occur when • Products in combustion are ALWAYS carbon one element replaces another in a dioxide and water. (although incomplete burning compound. does cause some by-products like carbon • A metal can replace a metal (+) OR monoxide) a nonmetal can replace a nonmetal (-). • Combustion is used to heat homes and run • element + compound product + product automobiles (octane, as in gasoline, is C8H18) A + BC  AC + B (if A is a metal) OR • Example A + BC  BA + C (if A is a nonmetal) C5H12 + 8O2  5CO2 + 6H2O Ex: • Write the products and balance the Zn(s) + CuSO4(aq) ZnSO4 (aq) + Cu(s) following combustion reaction: Zinc (metal) displaces copper (metal) C10H22 + O2  F2(g) + CuCl2(aq) CuF2(aq) + Cl2(g) Fluorine (non metal) displaces chlorine (nonmetal) 6
  • 7. 12/2/2008 Single Replacement Reactions Metals Nonmetals Relative reactivity of uncombined Most Active Li F2 Most Active • Write and balance the following single K O2 replacement reaction equation: Ba Cl2 • Zinc metal reacts with aqueous hydrochloric acid Ca Br2 Na I2 Least Active Zn(s) + HCl(aq)  ZnCl2 + H2(g) Mg Al Note: Zinc replaces the hydrogen ion in the elements Mn reaction Zn Cr • Sodium chloride solid reacts with fluorine gas Fe NaCl(s) + F2(g)  NaF(s) + Cl2(g) Co Ni Note that fluorine replaces chlorine in the compound Sn • Aluminum metal reacts with aqueous copper Pb (II) nitrate H2 Cu Al(s)+ Cu(NO3)2(aq) Hg Ag Pt Practice Example 8.8: (from Goldberg- Page 218) Least Active Au 5. Double Replacement Reactions Practice • Reaction that has the interchanging of two ions from • Predict the products. Balance the equation two different compounds. 1. HCl(aq) + AgNO3(aq)  • Compound + compound  product + product 2. CaCl2(aq) + Na3PO4(aq)  • general form: 3. Pb(NO3)2(aq) + BaCl2(aq)  AB + CD----> AD + CB CD----> 4. FeCl3(aq) + NaOH(aq)  • Example: Example: 5. H2SO4(aq) + NaOH(aq)  Pb(NO3)2 Pb(NO3)2 + 2 KI ----> PbI2 ----> + 2 KNO3 6. KOH(aq) + CuSO4(aq)  • During a reaction the cations (or anions) switch • State the type, predict the products, and places. balance the following reactions: • The products usually consist of a precipitate. 1. BaCl2 + H2SO4  • Example: 2. C6H12 + O2  AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq) 3. Zn + CuSO4  • Another example: 4. Cs + Br2  K2SO4(aq) + Ba(NO3)2(aq)  KNO3(aq) + BaSO4(s) 5. FeCO3  7