Chemical Equations and Chemical Reactions Chapter 8.1
Objectives List observations that suggest that a chemical reaction has taken place List the requirements for a correctly written chemical equation. Write a word equation and a formula equation for a given chemical reaction Balance a formula equation by inspection
Chemical Reaction - process by which one or more substances are changed into one or more different substances Reactants - the original substances Products - the resulting substances Law of Conservation of Mass - the total mass of reactants must equal the total mass of products for any given chemical reaction
Chemical Equation - represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction
Indications of a Chemical Reaction 1. Release of energy as heat and/or light 2. Production of a gas 3. Formation of a precipitate 4. Color change
Characteristics of Chemical Equations 1. Must represent known facts 2. Must contain the correct formulas for reactants and products 3. Law of Conservation of Mass must be satisfied
Word Equations An equation where the parts of a reaction are represented with text Hydrogen mixed with oxygen will produce water
Skeleton Equation Utilizes symbols and formulas to represent the reactants and products in a reaction H 2 + O 2 H 2 O
Elements That Normally Exist as Diatomic Molecules Occur as two atoms when written by themselves in an equation Hydrogen Nitrogen Oxygen Halogens
Signifcance of a Chemical Equation Coefficients indicative relative, not absolute, amounts of reactants and products 2H 2 + O 2 2H 2 O Ratio should always show the smallest possible relative amounts of each substance
Signifcance of a Chemical Equation We can determine the relative masses of each substance with the knowledge we gained in chapter 7
Signifcance of a Chemical Equation The reverse reaction has the same relative amounts as the forward reaction 2H 2 + O 2 2H 2 O 2H 2 O 2H 2 + O 2
What a Chemical Equation Does NOT tell us An equation gives no indication of whether a reaction will actually occur Chemical equations give no information about the speed at which reactions occur Equations do not give any information about how the bonding between atoms or ions changes during the reaction
Assignment Write an equation for each of the following: Solid calcium reacts with solid sulfur to produce solid calcium sulfide Hydrogen gas and fluorine gas react to form hydrogen fluoride gas Solid aluminum reacts with aqueous zinc chloride to produce zinc and aluminum chloride Write a sentence for each of the following: CS 2 + 3O 2 CO 2 + 2SO 2 NaCl + AgNO 3 NaNO 3 + AgCl
Balancing Chemical Equations 1. Identify the names of the reactants and the products, and write a word equation. water hydrogen + oxygen
Balancing Chemical Equations 2. Write a formula equation by substituting correct formulas for the names of the reactants and the products. H 2 O(l) H 2 (g) + O 2 (g) (not balanced)
Balancing Chemical Equations 3. Balance the formula equation according to the law of conservation of mass. Balance the different types of atoms one at a time. First balance the atoms of elements that are combined and that appear only once on each side of the equation. Balance polyatomic ions that appear on both sides of the equation as single units. Balance H atoms and O atoms after atoms of all other elements have been balanced.
Balancing Chemical Equations 4. Count atoms to be sure that the equation is balanced. 2H 2 O(l) 2H 2 (g) + O 2 (g) (4H + 2O) = (4H) + (2O) If the coefficients do not represent the smallest possible whole-number ratio of reactants and products, divide the coefficients by their greatest common factor in order to obtain the smallest possible whole-number coefficients.
Helpful Balancing Hints 1. Never touch subscripts when balancing equations.that will change the composition and therefore the substance itself. 2. Check to be sure that you have included all sources of a particular element that you are balancing on a particular side. There may be two or more compounds that contain the same element on a given side of an equation.
Helpful Balancing Hints 3. Adjust the coefficient of lone elements near the end of the balancing act since any change in their coefficient will not affect the balance of other elements 4. Diatomic elements (H,N,O, & Group 17) always appear as 2 atoms i.e O 2
Assignment Na 2 O + H 2 O NaOH Aluminum sulfate, an ingredient in antiperspirants, is made by the reaction of solid aluminum oxide with aqueous sulfuric acid. In addition to aqueous aluminum sulfate, water is also produced. Write a balanced chemical equation for the reaction. NH 3 (g) + O 2 (g) NO(g) + H 2 O(g) Pb 3 O 4 (s) + HNO 3 (aq) Pb(NO 3 ) 2 (aq) + PbO 2 (s) + H 2 O(l) One of the most important uses of sulfuric acid is in the production of phosphoric acid for use in the making of fertilizers. Solid calcium phosphate is reacted with aqueous sulfuric acid to form phosphoric acid and solid calcium sulfate. Write a balanced chemical equation for the reaction. When calcium carbonate, CaCO 3, is heated strongly, it decomposes to form calcium oxide, CaO, and carbon dioxide, CO 2. Write a balanced chemical equation for this reaction.
Types of Chemical Reactions Chapter 8.2
Types of Chemical Reactions Objectives: Define and give general equations for synthesis, decomposition, single-displacement, and doubledisplacement reactions. Classify a reaction as a synthesis, decomposition, single-displacement, double-displacement, or combustion reaction. Predict the products of simple reactions given the reactants.
Types of Chemical Reactions The classification scheme described in this section provides an introduction to five basic types of reactions: synthesis decomposition single-displacement double-displacement combustion reactions
Synthesis Reactions Two or more substances combine to form a new substance E + E C A + B AB 8 Fe + S 8 8 FeS Not a practical method for creating compounds
Synthesis Reactions Oxides of active metals react with water to produce metal hydroxides. example: Calcium oxide reacts with water to form calcium hydroxide. CaO(s) + H 2 O(l) Ca(OH) 2 (s)
Synthesis Reactions Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids. example: SO 2 (g) + H 2 O(l) H 2 SO 3 (aq) Certain metal oxides and nonmetal oxides react with each other in synthesis reactions to form salts. example: CaO(s) + SO 2 (g) CaSO 3 (s)
Decomposition Reactions Breaking a compound into its component parts Energy must be added C E + E AB A + B 2 H 2 O 2 H 2 + O 2
Decomposition Reactions Compounds break down as follows: Binary Compound 2 elements Acids Acidic Oxide + Water Metallic Hydroxide Metallic Oxide + Water Metallic Carbonate Metallic Oxide + Carbon Dioxide
Decomposition Reactions Electrolysis - The decomposition of a substance by an electric current example: 2H 2 O(l) electricity 2H 2 (g) + O 2 (g) Oxides of the less-active metals, which are located in the lower center of the periodic table, decompose into their elements when heated. example: 2HgO(s) 2Hg(l) + O 2 (g)
Single Displacement Reactions One element displaces another element E + C E + C A + BC B + AC Mg + 2H 2 O Mg(OH) 2 + H 2
Single Displacement Reactions A metal will not always replace another metal in an aqueous compound Metal must be more reactive than the metal it replaces NR indicates no reaction takes place Halogens only replace elements below them on the periodic table
Activity Series of the Elements Chapter 8.3
Activity Series Activity - the ability of an element to react The more readily an element reacts with other substances, the greater its activity is Activity Series - a list of elements organized according to the ease with which the elements undergo certain chemical reactions For metals, greater activity means a greater ease of loss of electrons, to form positive ions. For nonmetals, greater activity means a greater ease of gain of electrons, to form negative ions.
Chapter 8 Section 3 Activity Series of the Elements The order in which the elements are listed is usually determined by single-displacement reactions. The most-active element is placed at the top in the series. It can replace each of the elements below it from a compound in a single-displacement reaction. Activity series are used to help predict whether certain chemical reactions will occur. Activity series are based on experiment.
Activity Series
Single Displacement Reactions Displacement of a Metal in a Compound by Another Metal To replace a metal, the replacing metal must be more active than the metal it is replacing Aluminum is more active than lead 2Al(s) + 3Pb(NO 3 ) 2 (aq) 3Pb(s) + 2Al(NO 3 ) 3 (aq)
Single Displacement Reactions Displacement of Hydrogen in Water by a Metal The most-active metals, such as those in Group 1, react vigorously with water to produce metal hydroxides and hydrogen. 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Less-active metals, such as iron, react with steam to form a metal oxide and hydrogen gas. 3Fe(s) + 4H 2 O(g) Fe 3 O 4 (s) + 4H 2 (g)
Single Displacement Reactions Displacement of Hydrogen in an Acid by a Metal The more-active metals react with certain acidic solutions, such as hydrochloric acid and dilute sulfuric acid, replacing the hydrogen in the acid The reaction products are a metal compound (a salt) and hydrogen gas Mg(s) + 2HCl(aq) H 2 (g) + MgCl 2 (aq)
Single Displacement Reactions Displacement of Halogens In Group 17 each element can replace any element below it, but not any element above it. Cl 2 (g) + 2KBr(aq) F 2 (g) + 2NaCl(aq) Br 2 (l) + KCl(aq) 2KCl(aq) + Br 2 (l) 2NaF(aq) + Cl 2 (g) no reaction
Double Displacement Reactions One element replaces the like charged element in a compound C + C C + C AB + CD AD + CB Pb(NO3) 2 + 2KI PbI 2 + 2KNO 3 Always produce a precipitate, gas, and/or water
Double-Displacement Reactions Formation of a Precipitate The formation of a precipitate occurs when the cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound example: 2KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) The precipitate forms as a result of the very strong attractive forces between the Pb 2+ cations and the I anions.
Combustion Reactions Combustion Reaction - a substance combines with oxygen, releasing a large amount of energy in the form of light and heat example: combustion of hydrogen 2H 2 (g) + O 2 (g) 2H 2 O(g) example: combustion of propane C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g)
Assignment Page 284, 1-5