Chemical Equations Chemical Reaction = process whereby one or more reactants are changed into products. Unit 6. Balancing Chemical Equations

Transcription

1 Chemical Equations Chemical Reaction = process whereby one or more reactants are changed into products Unit 6 Chemical Nomenclature and Chemical Reactions indicate physical states: (s), (l), (g), (aq) equation must be balanced matter must be conserved (same number of each kind of atom on reactant and product sides) Charge must be conserved (same overall charge) only change coefficients; never change subscripts to balance an equation Balancing Chemical Equations Balancing Chemical Equations Step 1 Write an unbalanced equation containing the correct formulas of the reactants and products. Step 2 Write the symbol for each element underneath the yield symbol. Step 3 Write the number of each element present on the reactant side of the equation in front of the element symbol. Write the number of each element present on the product side of the equation behind the element symbol. Step 4 Balance each element. Leave the elements that are present in multiple compounds until the very end; they might take care of themselves. Step 5 Verify that the number of atoms of each element is balanced. 1

2 Examples NH3(g) + O2(g) à NO(g) + H2O(g) Examples C7H14(g) + O2(g) à CO2(g) + H2O(g) Examples Balancing Chemical Equations Glass is sometimes decorated by etching patterns on its surface. Etching occurs when hydrofluoric acid reacts with silicon dioxide in the glass to form gaseous silicon tetrafluoride and liquid water. Write and balance the equation for this reaction. 2

3 Combination Reaction Combination Reactions (Synthesis) - Two or more substances react to form a single product X + Y --> XY 2. 2Na(s) + Cl2(g) --> 2NaCl (s) 2. Decomposition Reactions - One substance decomposes to form two or more products. XY --> X + Y 2H2O (l) --> 2H2 (g) + O2(g) Decomposition Reaction 3

4 Combustion Reaction 3. Combustion Reaction A combustion reaction is a chemical change in which an element or a compound reacts with oxygen, often producing energy in the form of heat and light. CxHy + O2 -> CO2 + H2O +ENERGY! 4. Single Replacement n Metal Activity Series One element displaces another in a compound element + compound element + compound Type 1: element is a metal n M + M A M + MA n Cu(s) + 2 AgCl(aq) 2 Ag(s) + CuCl2 Type 2: element is a halogen n X2 + 2 BX X 2 + BX n Cl2 + 2 KBr Br2 + 2 KCl Use activity series to predict whether reaction goes Metal Ion(s) Processes Used to obtain the Metal State of Metal Li+ K+ Ca2+ Na+ Mg2+ Al3+ Pass electricity through molten mineral (electrometallurgy) Li(s) K(s) Ca(s) Na(s) Mg(s) Al(s) Manganese Zinc Chromium Iron Mn2+ Zn2+ Cr3+, Cr2+ Fe3+, Fe2+ Heat mineral with charcoal or carbon monoxide (CO) (pyrometallurgy) Mn(s) Zn(s) Cr(s) Fe(s) Lead Copper Mercury Silver Platinum Gold Pb2+ Cu2+, Cu+ Hg2+ Ag+ Pt2+ Au3+, Au+ Element Lithium Potassium Calcium Sodium Magnesium Aluminum most Active least active Heat mineral in air (pyrometallurgy) or find the element free Pb(s) Cu(s) Hg(s) Ag(s) Pt(s) Au(s) 4

5 Trends in Metal Activity l A more reactive metal (higher in the activity series) will cause ions of a less reactive metal (lower in the activity series) to change to its corresponding metal Mg + ZnCl 2 à Mg + ZnCl 2 à MgCl 2 + Zn What are the charges on the metals? Details: Zn + MgCl 2 à Oxidation of Cu(s) occurs with reduction of Ag + (aq) Mnemonics for Oxidation/Reduction Cu(s) + 2 Ag + (aq) à Cu 2+ (aq) + 2 Ag(s) OIL RIG: Cu: + Ag + + Ag + à Cu 2+ + Ag + Ag LEO (the lion) says GER: Cu is oxidized: it loses 2 electrons; Cu is the Ag + is reduced: it gains 1 electrons; Ag + is the 5

6 Terminology - Redox Reactions OXIDATION REDUCTION OXIDIZING AGENT electron acceptor; species is reduced. REDUCING AGENT electron donor; species is oxidized. Oxidation Numbers An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. As a general rule, a bonded atom s oxidation number is the charge that it would have if the electrons in the bond were assigned to the more electronegative element. Assign an oxidation number to each Cr atom. K2CrO4 Cr2O3 6

7 Single Replacement 5. Double Replacement - There is an interchange of partners between two compounds AB + XY --> AY + XB Pb(NO3)2 (aq) + K2CrO4 (aq) --> PbCrO4 (s) +2KNO3 (aq) The reaction proceeds only if one of the products is insoluble in the reaction mixture ZnBr2(aq) + 2 Ag(NO3)2(aq) Zn(NO3)2(aq) LiCl(aq) + Na2SO4(aq) 2 NaCl(aq) + Li2SO4(aq) + 2 AgBr(s) 7

8 Double Replacement Practice using your Solubility Rules chart. Predict whether or not the following ionic compounds are soluble in water. Write S for soluble, or I for insoluble Fe(OH)3 Mg(NO3)2 BaS K2CO3 AgI (NH4)2S NiCrO4 SrSO4 Predicting Whether a Reaction Will Occur Keys to understanding spontaneous chemical processes: n 1. Energy spread. Reactions occur so that more stable products are formed n 2. Matter spread. The most common driving forces that pull reactants toward products are n Formation of n Formation of n Transfer of electrons n Formation of Practice Classify each reaction as one of the four general types a) Pt(s) + 2F2(g) à PtF4(l) b) 3Fe(s) + 4H2O(g) à Fe3O4(s) + 4H2(g) c) 2H3BO3(s) à B2O3(s) + 3H2O(l) d) BaCl2(aq) + Na2SO4(aq) à BaSO4(s) + 2NaCl (aq) 8