7.01 Chemical Reactions

Transcription

1 7.01 Chemical Reactions The Law of Conservation of Mass Dr. Fred Omega Garces Chemistry 100 Miramar College 1 Chemical Reactions

2 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: acid) Aspirin: Salicylic acid + acetic anhydride g Aspirin (acetylsalicylic O C OH O H H 3 C + CH O 3 O C C O C CH O 3 C H O O O O C CH 3 OH Acetylsalicyclic acid (Aspirin) Glucose: CO 2 + H 2 O + hn g C 6 H 12 O 6 Air Bag: 2Na3N (s) + 3O(s) _ 3Na2O (s) + N2 (g) 2 Chemical Reactions

3 The Law of Conservation of Mass Chemical reactions are based on the "Law of conservation of mass". S Atoms reacting = S Atoms produced Before reaction After reaction i) 12 H N 2 g 8 NH 3 + 2N 2 (excess) i) 24 H s + 12 N s g 24 H s + 12 N s ii) C 3 H O 2 g 3 CO 2 + 4H 2 O Chemical Reactions

4 Reaction Process 9 H 2(g) + 6 N 2 (g) g 6 NH 3 (g) + 3 N 2 (g) (excess N 2 ) Atoms in reactant = Atoms in product N 2 N 2 H 2 H 2 H 2 H N 2 2 H 2 H 2 H 2 N 2 N 2 H H 2 2 N 2 2 NH 3 2 NH 3 2 NH 3 Note: 18 H and 6 N on each side of the chemical equation!! The net molecular equation is: 9 H 2 (g) + 3 N 2 (g) g 6 NH 3 (g) Symbol Meaning + and or plus a (s) (l) (g) (aq) Δ "" Pt!! reacts to form or yields Solid liquid gas aqueous (water) solution Heat added Catalyst (in this case, platinum) 4 Chemical Reactions

5 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: 1. Gas formation: Combustion of ethanol. 2. Precipitation or solid formation: BaCl 2 and Potassium chromate. 3. Temperature change: NaOH in water. 4. Color change: Pigments or reactions. 5 Chemical Reactions

6 Chemical Reactions Reactants P 4 (s) + 6 Cl 2 (g) Products 4 PCl 3 (s) 1 molecule 6 molecules 4 molecules 1.00 mole 6.00 moles 4.00 moles D (g) (g) (g) (l) 6 Chemical Reactions

7 Where Generic Form aa + bb c C + dd 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) + + C 3 H 8 (g) + 5 O 2 (g) g 3 CO 2 (g) + 4 H 2 O (g) Species: C 3 H 8 (propane), O 2 (oxygen), CO 2 (carbon dioxide), H 2 O (water) Number of species: 1 (propane), 5 O 2 (oxygen), 3 (carbon dioxide) and 4 (water) 7 Chemical Reactions

8 Balancing Process Consider the following reaction: To balance this equation: Mg + O 2 g MgO magnesium Oxygen from air Product Mg + O 2 g MgO 2 i) No!! (never since MgO 2 is different from MgO) Mg + O 2 g 2 MgO ii) Oxygen balanced, but Mg is not balanced 2 Mg + O 2 g 2 MgO 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 MgO 2 just to balance the equation. Furthermore, MgO 2 is not chemically sound. 8 Chemical Reactions

9 1. Combination Reaction A + B g AB Example: i) SO 3 + H 2 O g H 2 SO 4 Type of Reactions 4a. Double Displacement Reaction: AB + CD g AD + Example: CB i) Ba(NO 3 ) 2 + H 2 S g BaS + HNO 3 ii) P + S g P 2 S 5 iii) N 2 + H 2 g NH 3 2. Decomposition Reaction AB g A + B Example: i) H 2 CO 3 g H 2 O + CO 2 ii) KClO 3 g KCl + O 2 3. Single Displacement Reaction: A + BC g AC + B Example: i) Zn + Cu(NO 3 ) 2 g Zn(NO 3 ) 2 + Cu ii) Ca + H 2 O g Ca(OH) 2 + H 2 ii) MgCl 2 + Na 3 PO 4 g Mg 3 (PO 4 ) 2 + NaCl 4b. Acid Base (Double Displacement) Reaction: HA + MOH g H 2 O + MA Example: i) HNO 3 + Ba(OH) 2 g H 2 O + Ba(NO 3 ) 2 ii) HCl + NH 4 OH g H 2 O + NH 4 Cl 5. Combustion Reaction CxHy + O 2 g CO 2 + H 2 O Example: i) C 4 H 10 + O 2 g CO 2 + H 2 O ii) C 6 H 12 O 6 + O 2 g CO 2 + H 2 O 9 Chemical Reactions

10 Balancing (Guidelines) 1 H2 + O2 H2O 2H 2O 2H 1O H 2 O 2? Chemical identified oxygen is not balance Cannot since identity will be changed 2 H2 + O 2 2 H 2 O Change coefficient of product to two waters (H 2 O) 2H's, 2 O's 4H's, 2O's Oxygen is balanced but hydrogen is not. Coefficient must be adjusted 3 2 H 2 + O 2 2 H 2 O Two placed in front of H 2, to change H 4H's, 2 O's 4H's, 2O's atom to four 2 O's and 4 H's are on each side of the balanced equation 4 2 H 2 + O 2 2 H 2 O The equation is now balanced 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., H 2, N 2, O 2 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., PO 4 3- and SO 4 2- ) Convert fraction to integer. 10 Chemical Reactions

11 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., H 2 N 2 O 2 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., PO 4 3- and SO 4 2- ). Ex#4 & 5 11 Chemical Reactions

12 Example: Balancing Reactions #1 Sodium oxide and water forms sodium hydroxide #2 Ferric(III) Sulfide & molecular oxygen yields ferric(iii) oxide & sulfur dioxide #3 C 5 H 12 (g) + O 2 (g) g H 2 O (g) + CO 2 (g) #4 H 2 SO 4 (aq) + Ca(OH) 2 (aq) g H 2 O (l) + CaSO 4 (s) #5 Ca 3 (PO 4 ) 2 (s) + H 2 SO 4 (aq) g CaSO 4 (s) + H 3 PO 4 (aq) #6 Cl 2 (g) + NaOH (aq) g NaCl (aq) + NaClO 3 (aq) + H 2 O (l) 3:6:5:1:3 #7 KOH (aq) + O 2 (g) g KO 2 (s) + H 2 O (l) Na 2 O + H 2 O g 2NaOH Fe 2 S 3 + 9/2 O 2 g Fe 2 O 3 + 3SO 2 convert to integer by multiplying by 2 C 7 H 14 (g) + 21/2 O 2 (g) g 7H 2 O (g) + 7CO 2 (g) 12 Chemical Reactions

13 Example: Balancing Reactions #1 Sodium oxide and water forms sodium hydroxide Na 2 O g 2 + H 2 O g NaOH Sum Coef. a) 3 b) 4 c) 5 d) 6 #2 Ferric(III) Sulfide & molecular oxygen yields ferric(iii) oxide & sulfur dioxide Fe 2 S 3 + O 2 g Fe 2 O 3 + SO 2 Fe 2 S 3 + 9/2 O 2 g Fe 2 O 3 + 3SO /2 g Sum Coef. a) 10 b) 14 c) 19 d) None 13 Chemical Reactions

14 Example: Balancing Reactions #3 C 5 H 12 (g) + O 2 (g) g H 2 O (g) + CO 2 (g) C 5 H 12 (g) + O 2 (g) g H 2 O (g) CO 2 (g) g Sum Coef. a) 13 b) 20 c) 18 d) None #4 H 2 SO 4 (aq) + Ca(OH) 2 (aq) g H 2 O (l) + CaSO 4 (s) H 2 SO 4 (aq) + Ca(OH) 2 (aq) g H 2 O (l) + CaSO 4 (s) g Sum Coef. a) 4 b) 5 c) 10 d) None 14 Chemical Reactions

15 Example: Balancing Reactions #6 Cl 2 (g) + NaOH (aq) g NaCl (aq) + NaClO 3 (aq) + H 2 O (l) #7 KOH (aq) + O 2 (g) g KO 2 (s) + H 2 O (l) g /2 g Chemical Reactions

16 Example: Balancing Reactions #1 Sodium oxide and water forms sodium hydroxide #2 Ferric(III) Sulfide & molecular oxygen yields ferric(iii) oxide & sulfur dioxide #3 C 7 H 14 (g) + O 2 (g) g H 2 O (g) + CO 2 (g) #4 H 2 SO 4 (aq) + Ca(OH) 2 (aq) g H 2 O (l) + CaSO 4 (s) #5 Ca 3 (PO 4 ) 2 (s) + H 2 SO 4 (aq) g CaSO 4 (s) + H 3 PO 4 (aq) #6 Cl 2 (g) + NaOH (aq) g NaCl (aq) + NaClO 3 (aq) + H 2 O (l) 3:6:5:1:3 Na 2 O + H 2 O g 2NaOH Fe 2 S 3 + 9/2 O 2 g Fe 2 O 3 + 3SO 2 convert to integer by multiplying by 2 C 7 H 14 (g) + 21/2 O 2 (g) g 7H 2 O (g) + 7CO 2 (g) 16 Chemical Reactions

17 Type of Reactions (1): Combination 1. Combination Reaction -A Reaction in which two substances chemically combine to form a third. A + B g AB Example: i) SO 3 (g) + H 2 O (l) g H 2 SO 4 (aq) 1:1:1 ii) P 4 (s) + S 8 (s) g P 2 S 5 (s) 16:5:8 iii) Al (s) + Br 2 (l) g Al 2 Br 6 (s) 2:3:1 17 Chemical Reactions

18 Type of Reactions (2): Decomposition 2. Decomposition Reaction -A Reaction in which a single compound breaks up into two or more other substances. Example: AB g A + B i) H 2 CO 3 (aq) g H 2 O (l) + CO 2 (g) 1:1:1 ii) KClO 3 (s) g KCl (s) + O 2 (g) 2:2:3 iii) HgO (s) g Hg (l) + O 2 (g) 2:2:1 18 Chemical Reactions

19 Type of Reactions (3): Single displacement C. Single Displacement Reaction -A Reaction in which one element reacts by replacing another element in a compound. Example: A + BC g AC + B i) Zn (s) + Cu(NO 3 ) 2 (aq) g Zn(NO 3 ) 2 (aq) + Cu (s) ii) Ca (s) + H 2 O (l) g Ca(OH) 2 (aq) + H 2 (aq) 1:1:1:1 iii) Cu (s) + AgNO 3 (aq) g Cu(NO 3 ) 2 (aq) + Ag (s) 1:2:1:1 19 Chemical Reactions

20 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 + CB Example: i) Ba(NO 3 ) 2 (aq) + H 2 S (aq) g BaS (s) + HNO 3 (aq) ii) MgCl 2 (aq) + Na 3 PO 4 (aq) g Mg 3 (PO 4 ) 2 (s) + NaCl (aq) iii) Ba(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) g BaCrO 4 (s) + KNO 3 (aq) 1:1:1:2 3:2:1:6 20 Chemical Reactions

21 Type of Reactions (4) Driving Force for Reactivity Factors Towards Reactivity 1. Gas formation (Low molar mass covalent molecule) ZnS (s) + HCl (aq) g ZnCl 2 (aq) + H 2 S (g) HNO 3(aq) + NaCN (aq) g NaNO 3 (aq) + HCN (g) 2. Weak Electrolyte or water formation (Neutralization) HI (aq) + Al(OH) 3 (s) g H 2 O (l) + AlI 3 (aq) LiOH (aq) + HNO 3 (aq) g LiNO 3(aq) + H 2 O (l) 3. Precipitation formation (Solubility Table) Pb(NO 3 ) 2 (aq) + NaCl (aq) g NaNO 3 (aq) + PbCl 2(s) MnS (s) + H 2 SO 4 (aq) g???? 21 Chemical Reactions

22 Solubility Rules (chp8.3) Solubility Table: Guideline on precipitate formation. Soluble Substance Insoluble Substances Containing - Exceptions Containing - Exceptions Nitrates, (NO 3- ) Perchlorates (ClO 4- ) Acetates (CH 3 CO 2- ) None Carbonate (CO 3 2- ) Chromate (CrO 4 2- ) Phosphate (PO 4 3- ) Sulfides (S 2- ) Slightly soluble (for our purpose consider these insoluble) Halogens (X - ) Ca, Sr, Ba Alkali, X - = Cl -, Br -,I - Ag, Hg, Pb Hydroxides (OH-) NH + 4 Sulfates (SO 4 2- ) Alkali Grp1A, & NH 4 + Ca, Ba, Hg, Pb None Soluble - dissolve, no precipitate (aq -phase) insoluble (or slightly sol.) - does not dissolve, precipitate forms. (s-phase) 22 Chemical Reactions

23 Type of Reactions (4a) Double Displacement A) Molecular equation Mg(NO3)2 (aq) + 2 KOH (aq) 2KNO3 (aq) + Mg(OH) 2(s) Mg+2 2 NO 3-2 K + 2 OH- 2 K+ 2 NO3 - Mg+2 2 OH - B) Complete ionic equaiton C) Net ionic equaiton Summary: a) Molecular equation: Mg(NO 3 ) 2(aq) + 2KOH (aq) D 2KNO 3(aq) + Mg(OH) 2 (s) b) Complete ionic equation: Mg 2+ (aq) + 2 NO 3 - (aq) + 2 K + (aq) + 2 OH - (aq) D 2 K + (aq) + 2 NO 3 - (aq) + Mg(OH) 2 (s) c) Net ionic equation: Mg 2+ (aq) + 2 OH - (aq) D Mg(OH) 2 (s) 23 Chemical Reactions

24 Type of Reactions (4a) Double Displacement A) Molecular equation Mg(NO Mg(NO3)2 3 ) 2(aq) + KOH 2 (aq) D KNO2 KNO3 3(aq) + (aq) Mg(OH) + 2 (s) 2(s) (aq) Mg+2 2 NO 3-2 K + 2 OH- 2 K+ 2 NO3 - Mg+2 2 OH - B) Complete ionic equaiton C) Net ionic equaiton b) Complete ionic equation: Mg 2+ (aq) + 2 NO 3 - (aq) + 2 K + (aq) + 2 OH - (aq) D c) Net ionic equation: 2 K + (aq) + 2 NO (aq) Mg(OH) 2 (s) 24 Chemical Reactions

25 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. Example: HA + MOH g H 2 O + MA i) HNO 3 (aq) + NaOH (aq) g H 2 O (l) + NaNO 3 (aq) 1:1:1:1 ii) H 3 PO 4 (aq) + NH 3 (aq) g H 2 O (l) + (NH 4 ) 3 PO 4 (aq) 1:3:1:1 25 Chemical Reactions

26 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. Example: CxHy + O 2 g CO 2 + H 2 O i) C 6 H 12 O 6 (l) + O 2 (g) g CO 2 (g) + H 2 O (l) 1:6:6:6 ii) C 4 H 10 (l) + O 2 (g) g CO 2 (g) + H 2 O (l) 2:13:8:10 2:13:8:10 26 Chemical Reactions

27 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 Driving force: i) weak electrolyte, ii) gas iii) precipitation 5. Combustion 27 Chemical Reactions