1 Chapter 8 Chemical Equations and Reactions
2 Section 8 1 Describing Chemical Reactions
3 Chemical Reaction the process by which one or more substances are changed into one more different substances. Reactants original substances (substances started with) Products resulting substances (substances ended with)
4 Reactants Products
5 Indications of a Chemical Reaction Changes that indicate a chemical reaction has occurred: 1. Evolution of Heat and Light 2. Production of a Gas 3. Formation of a Precipitate - Precipitate a solid that is produced as a result of a chemical reaction in a solution and that separates from the solution 4. Color Change
6 Characteristics of a Chemical Reaction 1. The equation must represent the known facts All reactants and products must be identified 2. The equation must contain the correct formulas for the reactants and products Diatomic molecules (H, N, O, F, Cl, Br, I) enter an equation in their molecular state i.e. H 2, I 2
7 3. The Law of Conservation of Mass must be satisfied Atoms are neither created nor destroyed in a chemical reaction Same number of atoms must appear before and after the arrow To equalize the reaction, coefficients are added Coeffiecient a small whole number that appears in front of a formula in a chemical reaction
9 Word and Formula Equations First step Write a word Equation Does not give the quantity (coefficients) Ex: Methane + Oxygen Carbon Dioxide + Water The arrow reads as react to yield or yield Second Step Replace the words with the appropriate symbols and formulas Formula Equation represents the reactants and products of a chemical reaction by their symbols or formulas CH 4 + O 2 CO 2 + H 2 (Not Balanced)
10 Third Step Balance Chemical Equation Must follow the Law of Conservation of Mass A coefficient multiplies the number of each element indicated in a chemical formula CH 4(g) + O 2(g) CO 2(g) + H 2(g)
11 Additional Symbols Used in Chemical Equations (pg. 246) Symbol Explanation Yields Reversible Reaction Solid State Used only to indicate a precipitate Liquid State Aqueous (dissolved in water) Gaseous State Used only to indicate a gaseous product Reactants are heated Pressure in which reaction is carried out Temperature Formula of Catalyst
12 How would the following read?
13 Write word and formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in the water). Include symbols for physical states in the formula equation. Then, balance the formula equation to five a balanced Chemical equation.
14 Write word and balanced chemical equations for the following reactions. Include symbols for physical states when indicated. a. Solid calcium reacts with solid sulfur to produce solid calcium sulfide. b. Hydrogen gas reacts with fluorine gas to produce hydrogen fluoride gas. c. Solid Aluminum metal reacts with aqueous zinc chloride to produce solid zinc metal and aqueous aluminum chloride.
15 Significance of a Chemical Equation 1. The coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products. Usually shows the smallest number of atoms, molecules, or ions that will satisfy the Law of Conservation of Mass H 2 (g) + Cl 2 (g) 2HCl (g)
16 2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction s coefficients. 3. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.
17 Balancing Chemical Equations 1. Identify the names of the reactants and products, and write a word equation 2. Write a formula equation by substituting correct formulas for the names of the reactants and products 3. Balance the formula equation according to the law of conservation of mass. 4. Count atoms to be sure that the equation is balanced.
18 The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write the balanced chemical equation for the following.
19 Write the word, formula, and balanced chemical equations for each of the following reactions: a. Magnesium and hydrochloric acid react to produce magnesium chloride and hydrogen. b. Aqueous nitric acid reacts with solid magnesium hydroxide to produce aqueous magnesium nitrate and water.
20 Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate.
21 Section 8 2 Types of Chemical Reactions
22 5 Basic Types of Reactions 1. Synthesis Reactions 2. Decomposition Reactions 3. Single-Replacement Reactions 4. Double-Replacement Reactions 5. Combustion Reactions
23 Synthesis Reactions Synthesis Reaction (composition) two or more substances combine to form a new compound A + X AX A and X can be elements or compounds AX is a compound
24 Reactions with Oxygen and Sulfur Almost all elements react with Oxygen to form oxides Ex: 2Mg (s) (g) 2MgO (s) Group 2 Elements react similarly with sulfur to for sulfides Ex: 16Rb (s) + S 8 (s) 8Rb 2 S (s)
25 2Na (s) + Cl 2 (g) Mg (s) + F 2 (g) U (s) + 3F 2 (g)
26 Synthesis Reactions with Oxides Active metals are highly reactive metals. Oxides react with metals to produce metal hydroxides CaO (s) + H 2 O (l) Ca(OH) 2 (s) Many oxides in the upper right side of the table react with water to produce oxyacids. SO 2 (g) + H 2 0 (l) H 2 SO 3 (aq)
27 CaO (s) + SO 2 (g)
28 Decomposition Reactions Decomposition Reaction a single compound undergoes a reaction that produces two or more simpler substances AX A + X Take place when energy in the form of electricity or heat is added
29 Decomposition of Binary Compounds Electrolysis the decomposition of a substance by an electric current 2H 2 O (l) electricity 2H 2 (g) + O 2 (g) Reactions involving heat 2HgO (s) 2Hg (l) + O 2 (g)
30 Metal carbonate breaks down to produce a metal oxide and carbon dioxide gas. CaCO 3 (s) CaO (s) + CO 2 (g) Metal Hydroxides decompose to yield metal oxides and water. Ca(OH) 2 (s) CaO (s) + H 2 O (g)
31 Metal chlorate decomposes to produce a metal chloride and oxygen 2KClO 3 (s) 2KCl (s) + 3O 2 (g) Certain acids decompose into nonmetal oxides and water H 2 CO 3 (aq) CO 2 (g) + H 2 0 (l)
32 Single-Replacement Reactions Single-replacement reaction (displacement) one element replaces a similar element in a compound A + BX AX + B Or Y + BX BY + X
33 Replacement of a metal in a compound by another metal 2 Al (s) + 2 Pb(NO 3 ) 2 (aq) 3Pb (s) + 2Al(NO 3 ) 3 (aq) Replacement of Hydrogen in water by metal 2Na (s) + 2H 2 0 (l) 2NaOH (aq) + H 2 (g)
34 Replacement of Hydrogen in an acid by a metal Mg (s) + 2HCl (aq) H 2 (g) + MgCl 2 (aq) Replacement of Halogens Cl 2 (g) + 2KBr (aq) 2KCl (aq) + Br 2 (l)
35 Double-Replacement Reactions Double-Replacement Reaction the ions of two compounds exchange places in an aqueous solution to form two new compounds AX + BY AY + BX
36 Formation of a precipitate 2KI (aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) Formation of a Gas FeS (s) + 2HCl (aq) H 2 S (g) + FeCl 2 (aq) Formation of Water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 0 (l)
37 Combustion Reactions Combustion Reaction a substance combines with oxygen, releasing a large amount of energy in the form of light and heat 2H 2 (g) + O 2 (g) 2H 2 0 (g) The burning or propane: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 0 (g)
38 Section 8 3 Activity Series of the Elements
39 The ability of an element to react is known as an element s activity Activity Series a list of elements organized according to the ease with which the elements undergo certain chemical reaction - Metals greater loss of e- - Nonmetals greater gain of e- The order in which elements are listed is determined by single-replacement reactions
40 The most-active element, placed at the top in the series, can replace each element in the series below it An element further down can replace any element below it, but not any above Halogens are written in their series the way they appear on the periodic table
41 Just because an chemical equation can be written does not mean that the reaction will actually take place Can happen: 2Al (s) + 3ZnCl 2 (aq) 3Zn (s) + 2AlCl 3 (aq) Cannot happen: *Al is above Zn* Co (s) + 2NaCl (aq) No reaction *Na is above Co*
42 Examples Use the activity series to explain whether each of the possible reactions listed below will occur. For those reactions that will occur, predict what the products will be: 1. Zn (s) + H 2 O (l) 50 degrees 2. Sn (s) + O 2 (g)
43 Au(s) + HCl (aq) Cd(s) + HCl (aq) Mg(s) + Co(NO 3 ) 2 (aq)