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4.02 Chemical Reactions The Law of Conservation of Mass Dr. Fred Omega Garces Chemistry 111 Miramar College 1 Chemical Reactions January 10 Making Substances Chemical Reactions; the heart of chemistry is the chemical reaction which is responsible for making new substances. If the reaction is new, then it is similar to inventing a new product. Synthesis of: Nylon: Aspirin: Salicylic acid + acetic anhydride O OH C O H H 3C + C O O C H3 O C g Aspirin (acetylsalicylic acid) OH C O C O O O C H3 H O C C H3 Acetylsalicyclic acid (Aspirin) Glucose: Air Bag: 2Na3N (s) 2 + 3O(s) _ CO2 + H2O + hν g C6H12O6 3Na2O (s) + N2 (g) Chemical Reactions January 10 The Law of Conservation of Mass Chemical reactions are based on the "Law of conservation of mass". Σ Atoms reacting Before reaction i) 12 H2 + 6 N2 = g i) 24 H’s + 12 N’s g ii) C3H8 g + 5 O2 + 3 Σ Atoms produced After reaction 8 NH3 + 2N2 (excess) 24 H’s + 12 N’s 3 CO2 + 4H2O + Chemical Reactions January 10 Reaction Process 9 H2(g) + 6 N2 (g) g 6 NH3 (g) + 3 N 2 Atoms in reactant = Atoms in product (excess N2) N Note: 18 H and 6 N on each side of the chemical equation !! The net molecular equation is: 2 N2 H2 H2 H2 H2 N2 H2 H2 H2 N2 H2 H2 N N2 2 NH 3 2 NH 3 2 2 NH 3 9 H2 (g) + 3 N2 (g) g 6 NH3 (g) Symbol Meaning + and or plus a reacts to form or yields (s) Solid (l) liquid (g) gas (aq) aqueous (water) solution Δ Heat added Pt Catalyst (in this case, platinum) ""→ !! !→ 4 (g) Chemical Reactions January 10 Evidence of a Chemical Change Chemical change requires that reactant bonds are broken and product bonds form. The chemical identity of the starting material is altered. This can be detected by making physical observations during the reaction. Evidence: 5 1. Gas formation: Combustion of ethanol. 2. Precipitation or solid formation: BaCl2 and Potassium chromate. 3. Temperature change: NaOH in water. 4. Color change: Pigments or reactions. Chemical Reactions January 10 Chemical Reactions Reactants P4 (s) + 6 Cl2 (g) → Products 4 PCl3 (s) 1 molecule 6 molecules → 4 molecules 1.00 mole 6.00 moles → 4.00 moles Δ (g) 6 (g) Chemical Reactions (g) (l) January 10 Generic Form aA + bB → c C + dD Where A, B, C and D are specie (compounds) that are involve in the reaction. a, b, c and d are coefficient that tell how many of each species react. # of atoms (reacted) = # of atoms (produced) + C3H8 (g) Species: + + 5 O2 (g) g 3 CO2 (g) + 4 H2O (g) C3H8 (propane), O2 (oxygen), CO2 (carbon dioxide), H2O (water) Number of species: 1 (propane), 5 O2 (oxygen), 3 (carbon dioxide) and 4 (water) 7 Chemical Reactions January 10 Balancing Process Consider the following reaction: Mg magnesium To balance this equation: Mg Mg 2 Mg + O2 g MgO2 + O2 g 2 MgO + O2 g 2 MgO + O2 Oxygen from air g MgO Product i) No!! (never since MgO2 is different from MgO) ii) Oxygen balanced, but Mg is not balanced ii) Correct, all species are balanced Note: Once the formula for the substance is established, (i.e., the chemicals involved in the reaction are determined) the subscript can never be changed. Only the coefficient can be altered. * MgO cannot be changed to MgO2 just to balance the equation. Furthermore, MgO2 is not chemically sound. 8 Chemical Reactions January 10 Type of Reactions 1. Combination Reaction A + B g AB Example: i) __ SO3 + __ H2O ii) __ P + __ S g __ H2SO4 __ P2S5 g iii) __ N2 + __ H2 4a. Double Displacement Reaction: AB + CD g AD + CB Example: i) __ Ba(NO3)2 + __ H2S g __ BaS + __ HNO3 ii) __ MgCl2 + __ Na3PO4 g __ Mg3(PO4)2 + __ NaCl g __ NH3 2. Decomposition Reaction AB g A + B Example: i) __ H2CO3 g __ H2O + 4b. Acid Base (Double Displacement) Reaction: HA + MOH g H2O + MA Example: i) __ HNO3 + __ Ba(OH)2 g __ H2O + __ Ba(NO3)2 __ CO2 ii) __ HCl + __ NH4OH g ii) __ KClO3 g __ KCl + __ O2 3. Single Displacement Reaction: A + BC g AC + B Example: i) __ Zn + __ Cu(NO3)2 g __ Zn(NO3)2 + __ Cu ii) __ Ca + __ H2O g __ Ca(OH)2 + __ H2 9 __ H2O + __ NH4Cl 5. Combustion Reaction CxHy + O2 g CO2 + H2O Example: i) __ C4H10 + __ O2 g __ CO2 + __ H2O ii) __ C6H12O6 + __ O2 g Chemical Reactions __ CO2 + __ H2O January 10 Balancing (Guidelines) 1 H2 + 2H 2 H2 O2 H2O 2O 2H 1O H2 O2 ? + O2 2 H 2O 2H's, 2 O's 3 2 H2 + 4H's, 2O ' s O2 2 4H's, 2 O's 4 2 H2 + H2 O 4H's, 2O ' s O2 2 H2 O Chemical identified oxygen is not balance Cannot since identity will be change d Change coefficient of product to two waters (H2O) Oxygen is balanced but hydrogen is not. Coefficient must be adjust e d Two placed in front of H2, to change H atom to four 2 O's and 4 H's are on each side of the balanced equation The equation is now balanc e d Summary : Balancing chemical reaction • • • • Box the chemical and do not change the formula. Assign coefficient of 1 to the most complicated molecule. Balance elemental chemicals last. i.e., H2, N2, O2 Use fraction coefficient to balance odd number of atoms in the equation. • Balance polyatomic cations / anions as a single unit if these ions occur in reactant and product unchanged (i.e., PO43- and SO42- ) • Convert fraction to integer. 10 Chemical Reactions January 10 Summary : Balancing chemical reaction 1• Box the chemicals so that you don’t change the formula. 2• Assign a coefficient of one to the most complicated molecular specie. Ex#1 - 5 3• Balance elemental chemicals last. i.e., H2 N2 O2 Ex#2 & 3 4• Use fractional coefficients to balance an odd number of atoms in the equation Ex#2 & 3 5• Convert the fraction to integer by multiplying through whole equation by the denominator of the fraction Ex#2 & 3 6• Balance polyatomic cation / anion as a single unit if these ions occurs in reactant and product unchanged (i.e., PO43- and SO42- ). Ex#4 & 5 11 Chemical Reactions January 10 Example: Balancing Reactions #1 Sodium oxide and water forms sodium hydroxide #2 Ferric(III) Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide #3 C5H12 #4 (g) + O2 g (g) H2SO4 (aq) + Ca(OH)2 #5 Ca3(PO4)2 (s) + Na2O + H2O g Fe2S3 + 9/2 O2 (aq) g H2O(l) + CaSO4 H2SO4 (aq) g CaSO4 (s) + (s) H3PO4 (aq) 2NaOH g Fe2O3 + 3SO2 C7H14 (g) + 21/2 O2 (g) 12 H2O(g) + CO2 (g) g convert to integer by multiplying by 2 7H2O(g) + 7CO2 (g) Chemical Reactions January 10 Example: Balancing Reactions #1 Sodium oxide and water forms sodium hydroxide __Na2O + __H2O 1 + 1 g g 2 __NaOH Sum Coef. a) 3 b) 4 c) 5 d) 6 #2 Ferric(III) Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide __Fe2S3 + __O2 Fe2S3 + 9/2 O2 1 + 13 9/2 g 1 g g __Fe2O3 + __SO2 Fe2O3 + 3SO2 + 3 Chemical Reactions Sum Coef. a) 10 b) 14 c) 19 d) None January 10 Example: Balancing Reactions #3 C5H12 (g) + O2 __C5H12 (g) + __O2 (g) 6 g 1 + 8 #4 H2SO4 (aq) + Ca(OH)2 g + H2O(g) + CO2 (g) g (g) __H2O(g) 5 Sum Coef. a) 13 b) 20 c) 18 d) None (aq) g __H2SO4 (aq) + __Ca(OH)2 (aq) g 1 + 14 1 g 2 + __CO2 (g) H2O(l) + CaSO4 (s) __H2O(l) + __CaSO4 (s) Sum Coef. a) 4 b) 5 c) 10 d) None 1 Chemical Reactions January 10 Example: Balancing Reactions #1 Sodium oxide and water forms sodium hydroxide #2 Ferric(III) Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide #3 C7H14 #4 (g) + O2 H2SO4 (aq) + Ca(OH)2 #5 Ca3(PO4)2 (s) + Na2O + H2O g Fe2S3 + 9/2 O2 H2O(g) + CO2 (g) (aq) g H2O(l) + CaSO4 H2SO4 (aq) g CaSO4 (s) + (s) H3PO4 (aq) 2NaOH g Fe2O3 + 3SO2 C7H14 (g) + 21/2 O2 (g) 15 g (g) g convert to integer by multiplying by 2 7H2O(g) + 7CO2 (g) Chemical Reactions January 10 Type of Reactions (1): Combination 1. Combination Reaction -A Reaction in which two substances chemically combine to form a third. A + B g Example: i) __ SO3 (g) + __ H2O 1:1:1 ii) __ P4 16:5:8 (s) + __ S8 (s) iii) __ Al(s) + __ Br2 (l) AB (l) g __ H2SO4 (aq) g __ P2S5 (s) g __ Al2Br6 (s) 2:3:1 16 Chemical Reactions January 10 Type of Reactions (2): Decomposition 2. Decomposition Reaction -A Reaction in which a single compound breaks up into two or more other substances. AB g A + B Example: i) __ H2CO3 (aq) g __ H2O(l) + __ CO2 (g) 1:1:1 ii) __ KClO3 (s) g __ KCl(s) + __ O2 (g) iii)2:2:3__ HgO(s) g __ Hg (l) + __ O2 (g) 2:2:1 17 Chemical Reactions January 10 Type of Reactions (3): Single displacement C. Single Displacement Reaction -A Reaction in which one element reacts by replacing another element in a compound. A + BC g AC + B Example: i) __ Zn (s) + __ Cu(NO3)2 (aq) ii) __ Ca (s) + __ H2O (l) g __ Zn(NO3)2 (aq) + __ Cu g __ Ca(OH)2 (aq) + __ H2 (aq) iii) __ Cu(s) + __ AgNO3 (aq) g __ Cu(NO3)2 (aq) + __ Ag(s) 18 Chemical Reactions (s) 1:1:1:1 1:2:1:1 January 10 Type of Reactions (4a) Double Displacement (Chp8) 4a. Double Displacement Reaction: - A Reaction in which two compounds exchange partners to form two new compounds. AB + CD g AD + Example: i) __ Ba(NO3)2 (aq) + __ H2S (aq) g CB __ BaS(s) + __ HNO3 (aq) ii) __ MgCl2 (aq) + __ Na3PO4 (aq) g __ Mg3(PO4)2 (s) + __ NaCl (aq) iii) __ Ba(NO3)2 19 (aq)+ __ K2CrO4 (aq) g __ BaCrO4 (s) + __ KNO3 (aq) Chemical Reactions 1:1:1:2 3:2:1:6 January 10 Type of Reactions (4) Driving Force for Reactivity Factors Towards Reactivity 1. Gas formation (Low molar mass covalent molecule) __ZnS(s) + __ HCl (aq) g __ZnCl2 (aq) + __H2S (g) __HNO3(aq) + __NaCN(aq) g __ NaNO3 (aq) + __HCN (g) 2. Weak Electrolyte or water formation (Neutralization) __HI(aq) + __Al(OH)3 (s) g __H2O (l) + __AlI3 (aq) __LiOH(aq) + __HNO3 (aq) g __LiNO3(aq) + __H2O (l) 3. Precipitation formation (Solubility Table) __Pb(NO3)2 (aq) + __NaCl(aq) g __ NaNO3 (aq) + __PbCl2(s) __MnS(s) + __ H2SO4 20 (aq) g ???? Chemical Reactions January 10 Type of Reactions (4b): Neutralization, a type of double displacement 4b. Acid-Base (Neutralization Reaction): -A Reaction that occurs between an acid and a base with the formation of an ionic compound and water. HA + MOH g H2 O + MA Example: i) __ HNO3 (aq) + __ NaOH (aq) ii) __ H3PO4 (aq) + __ NH3 21 (aq) g __ H2O g __ H2O(l) + Chemical Reactions (l) + __ NaNO3 (aq) 1:1:1:1 __ (NH4)3PO4 (aq) 1:3:1:1 January 10 Type of Reactions (5) Combustion 5. Combustion Reaction -A Reaction of a substance with either pure oxygen or oxygen in the air with the rapid release of heat and the appearance of a flame. CxHy + O2 g Example: i) __ C6H12O6 (l) + __ O2 (g) CO2 + H2O g __ CO2 (g) + __ H2O(l) 1:6:6:6 ii) __ C4H10 (l) + __ O2 (g) g __ CO2 (g) + __ H2O 2:13:8:10 22 Chemical Reactions (l) 2:13:8:10 January 10 Summary Understand the Basic idea of a Chemical Reaction Write and Balance a Chemical Reaction Recognize the different chemical reactions 1. Combination 2. Decomposition 3. Single Displacement 4. Double Displacement 5. Combustion 23 Chemical Reactions January 10