Introductory Chemistry Nivaldo Tro Unit 6B Chemical Reactions () Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2006, Prentice Hall Chemical Reactions Reactions involve chemical changes in matter resulting in new substances Reactions involve rearrangement and exchange of atoms to produce new molecules Elements are not transmuted during a reaction Reactants Products 2 Evidence of Chemical Reactions Look for evidence of a new substance Visual clues (permanent) solid that forms when liquid solutions are mixed container becomes very hot or cold emission of light Other clues 3 1
Chemical Equations Shorthand way of describing a reaction Provides information about the reaction Formulas of reactants and products Can be used to determine weights of reactants used and products that can be made Chapter 7 4 Writing Chemical Equations C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O(l) 2H 2 O(l) > H 2 (g) + O 2 physical state l = s = aq = g = Combustion of Methane Methane gas burns to produce carbon dioxide gas and gaseous water whenever something burns it combines with O 2 (g) CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O(g) H H C H H + O O O C + O O H H 6 2
Atom Inventory Practice 1. AgNO 3 2. Sr(OH) 2 3. Al 2 (CO 3 ) 3 4. 2 AgNO 3 5. Ca(HCO 3 ) 2 6. 4 Ca(HCO 3 ) 2 Conservation of Mass and Atoms Law of Conservation of Mass: The mass of all reactants = mass of all products A balanced chemical equation obeys the law of conservation of mass Numbers and kinds of atoms in reactants = number and kinds of atoms in products Writing and 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. 3
Writing and Balancing Chemical Equations 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. Example # 1 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. Example # 2 Under appropriate conditions at 1000 C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water 1. Write the equation in words identify the state of each chemical 2. Write the equation in formulas identify diatomic elements identify polyatomic ions determine formulas 4
Example # 3 C 7 H 14(g) + O 2(g) CO 2(g) + H 2 O (g) Balancing Chemical Equations Balance the following chemical equations: 1. K (s) + H 2 O (l) H 2 (g) + KOH (aq) 2. C 8 H 16 (l) + O 2 (g) CO 2 (g) + H 2 O (l) Patterns of Chemical Reactions 1. Synthesis Reaction (Combination) Two or more substances react to form a single product. X + Y > XY 5
Patterns of Chemical Reactions 2. Decomposition Reaction One substance decomposes to form two or more products. XY > X + Y Decomposition of Water elec 2 H2O(l) 2 H2(g) + O2(g) Patterns of Chemical Reactions 3. Single Replacement Reaction (Displacement) One element reacts with a compound to form a new compound and release a different element. A + XY > AY + X 6
Displacement of Copper by Zinc Zn(s) + CuCl2 (aq) Cu(s) + ZnCl 2 19 Metal Activity Series Element Lithium Potassium Calcium Sodium Magnesium Aluminum Manganese Zinc Chromium Iron Lead Copper Mercury Silver Platinum Gold Metal Ion(s) most Li + Active K + Ca 2+ Na + Mg 2+ Al 3+ Mn 2+ Zn 2+ Cr 3+, Cr 2+ Fe 3+, Fe 2+ Pb 2+ Cu 2+, Cu + Hg 2+ Ag + least Pt 2+ active Au 3+, Au + Processes Used to obtain the Metal Pass electricity through molten mineral (electrometallurgy) Heat mineral with charcoal or carbon monoxide (CO) (pyrometallurgy) Heat mineral in air (pyrometallurgy) or find the element free State of Metal Li(s) K(s) Ca(s) Na(s) Mg(s) Al(s) Mn(s) Zn(s) Cr(s) Fe(s) Pb(s) Cu(s) Hg(s) Ag(s) Pt(s) Au(s) Trends in Metal Activity 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 MgCl 2 + Zn Zn + MgCl 2 N. R. 7
Patterns of Chemical Reactions 4. Double Replacement Reaction (Exchange) There is an interchange of partners between two compounds AB + XY > AY + XB Dissociation When ionic compounds dissolve in water, the anions and cations are separated from each other this is called however not all ionic compounds are soluble in water! when compounds containing polyatomic ions dissociate, Chapter 7 Dissociation Potassium iodide dissociates in water into potassium cations and iodide anions K I K +1 I 1 Copper(II) sulfate dissociates in water into copper(ii) cations and sulfate anions Cu SO 4 Cu +2 SO 4 2 8
When will a Salt Dissolve? A compound is soluble in a liquid if it dissolves in that liquid NaCl is soluble in water, but AgCl is not A compound is insoluble if a significant amount does not dissolve in that liquid AgCl is insoluble in water though there is a very small amount dissolved, but not enough to be significant Using the Solubility Rules to Predict an Ionic Compound s Solubility in Water First check the cation, if it is Li +, Na +, K +, or NH 4 + then the compound will be soluble in water regardless of the anion! If the cation is not Li +, Na +, K +, or NH 4 + then follow the rule for the anion If a rule says the compounds are mostly soluble, then the exceptions are insoluble But if a rule says the compounds are mostly insoluble, then the exceptions are soluble note: slightly soluble insoluble Chapter 7 26 Determine if Each of the Following is Soluble in Water KOH AgBr CaCl 2 Pb(NO 3 ) 2 PbSO 4 9
Precipitation Reactions = Double Replacement Pb(NO 3 ) 2 (aq) + 2 KI(aq) 2 KNO 3 (aq) + PbI 2 (s) 28 No Precipitate Formation = No Reaction KI(aq) + NaCl(aq) KCl(aq) + NaI(aq) all ions still present, no reaction 29 Example: When an aqueous solution of sodium carbonate is added to an aqueous solution of copper(ii) chloride, a white solid forms 1. Write the formulas of the reactants 2. Exchange the Ions 3. Write the formulas of the products cross charges and reduce 4. Determine the solubility of each product 5. Balance the equation 10
Types of Chemical Reactions 5. 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. C x H y + O 2 (g) > CO 2 (g) + H 2 O(g) Practice Classify each reaction as one of the five general types a) Pt (s) + 2F 2(g) PtF 4(l) b) 3Fe (s) + 4H 2 O (g) Fe 3 O 4(s) + 4H 2(g) c) 2CH 3 OH(l) + 3O 2 (g) > 2CO 2 (g) + 4H 2 O(g) d) 2H 3 BO 3(s) B 2 O 3(s) + 3H 2 O (l) e) BaCl 2(aq) + Na 2 SO 4(aq) BaSO 4(s) + 2NaCl (aq) 11