Copyright©2004 by Houghton Mifflin Company. All rights reserved. 1 Introductory Chemistry: A Foundation FIFTH EDITION by Steven S. Zumdahl University of Illinois

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 2 Chemical Reactions: An Introduction Chapter 6

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 3 Lightning over the town of Tamworth in New South Wales, Australia. Lightning “catalyzes” many reactions in the atmosphere Source: Gordon Garradd/SPL/Photo Researchers, Inc.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 4 Chemical Reactions Reactions involve chemical changes in matter resulting in new substances (but no new elements) Reactions involve rearrangement and exchange of atoms to produce new molecules –Elements are not transmuted during a reaction Reactants  Products

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 5 Evidence of Chemical Reactions A chemical change occurs when new substances are made visual clues (permanent) –color change, precipitate formation, gas bubbles, flames, heat release, cooling, light other clues –new odor, permanent new state

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 6

Figure 6.1: Bubbles of hydrogen and oxygen gas form when an electric current is used to decompose water.

An injured girl wearing a cold pack to help prevent swelling. The pack is activated by breaking an ampule; this initiates a chemical reaction that absorbs heat rapidly, lowering the temperature of the area to which the pack is applied.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 9 A hot pack used to warm hands and feet in winter. When the package is opened, oxygen from the air penetrates a bag containing solid chemicals. The resulting reaction produces heat for several hours.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 10 When colorless hydrochloric acid is added to a red solution of cobalt(II) nitrate, the solution turns blue, a sign that a chemical reaction has taken place.

Figure 6.3b: A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate.

Bubbles of hydrogen gas form when calcium metal reacts with water.

Methane gas reacts with oxygen to produce a flame in a bunsen burner.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 14 Chemical Equations Shorthand way of describing a reaction Provides information about the reaction –Formulas of reactants and products –States of reactants and products –Relative numbers of reactant and product molecules that are required –Can be used to determine weights of reactants used and of products that can be made

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 15 Conservation of Mass Matter cannot be created or destroyed In a chemical reaction, all the atoms present at the beginning are still present at the end Therefore the total mass cannot change Therefore the total mass of the reactants will be the same as the total mass of the products

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 16 Types of Chemical Reactions There are 5 basic types of chemical reactions –Synthesis—two or more elements or compounds combine to form one compound A + B  C –Decomposition—a compound is “broken” into two or more components C  A + B

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 17 Types of Chemical Reactions –Single Replacement—one ion is substituted in a compound AB + C  AC + B –Double Replacement—two ionic compounds “swap” partners AB + CD  AD + CB –(Complete) Combustion --reaction with oxygen to produce ONLY water and carbon dioxide

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 18 Word/Skeleton/Balanced Chemical Equations Word equation describes the reaction with words Skeleton equation describes the reactants and products involved in the reaction, but not relative amount Balanced equation describes reaction fully, telling compounds/elements present, state of reactants and products, and relative amounts of each.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 19 Symbols Used in Equations symbols used after chemical formula to indicate state –(g) = gas; (l) = liquid; (s) = solid –(aq) = aqueous, dissolved in water

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 20 Synthesis Examples…..Word equation first, then balanced chemical equation –Magnesium plus oxygen gas produces magnesium oxide 2Mg (s) + O 2(g)  2MgO (s) –Sodium metal reacts with chlorine gas to produce sodium chloride 2Na (s) + Cl 2(g)  2NaCl (s)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 21 Decomposition Examples— –Hydrogen peroxide decomposes into water and oxygen gas H 2 O 2(l)  H 2 0 (l) + O 2(g) –Electrolysis of water produces hydrogen gas and oxygen gas 2H 2 O (l)  2H 2(g) + O 2(g)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 22 Single Replacement Magnesium plus water produces magnesium hydroxide and hydrogen gas –Mg (s) + 2H 2 O (l)  Mg(OH) 2(s) + H 2(g) Iron and Copper (II) chloride produce copper and iron (III) chloride –2Fe (s) + 3CuCl 2(au)  3Cu (s) + 2FeCl 3(aq)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 23 Double Replacement Aqueous lead (II) nitrate plus aqueous potassium iodide produce solid lead (II) iodide and aqueous potassium nitrate –Pb(NO 3 ) 2(aq) + 2KI (aq)  PbI 2(s) + 2KNO 3(aq) Sodium bromide plus hydrochloric acid produce sodium chloride and hydrobromic acid –NaBr (s) + HCl (aq)  NaCl (s) + HBr (aq)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 24 (Complete) Combustion Methane plus oxygen gas produces water and carbon dioxide –CH 4(g) + 2O 2(g)  2H 2 O (l) + CO 2(g)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 25 Combustion of Methane methane gas burns to produce carbon dioxide gas and liquid water –whenever something burns it combines with O 2 (g) CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(l) H H C H H OO + O O C + O HH 1 C + 4 H + 2 O1 C + 2 O + 2 H + O 1 C + 2 H + 3 O Amounts not equal Equation not balanced (Skeleton equation)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 26 Balancing Equations Balancing a skeleton equation involves a process called inspection, and uses COEFFICIENTS (large numbers in front of compounds or elements) to balance. Use Tips for balancing as presented on handout……Check for diatomic elements, balance metals, balance non-metals, balance oxygen, balance hydrogen, recount.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 27 Combustion of Methane Balanced to show the reaction obeys the Law of Conservation of Mass it must be balanced, use coefficients CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(l) H H C H H OO + O O C + O HH OO + O HH + 1 C + 4 H + 4 O coefficients

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 28 Writing Equations Use proper formulas for each reactant and product proper equation should be balanced –obey Law of Conservation of Mass –all elements on reactants side also on product side –equal numbers of atoms of each element on reactant side as on product side balanced equation shows the relationship between the relative numbers of molecules of reactants and products

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 29 The reactants (a) potassium metal and (b) water. (c) The reaction of potassium with water.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 30 Sample – Recognizing Reactants and Products when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 –Metals are solids, except for Hg which is liquid ¬write the equation in words –identify the state of each chemical magnesium(s) + oxygen(g)  magnesium oxide(s) ­write the equation in formulas –identify diatomic elements –identify polyatomic ions –determine formulas Mg(s) + O 2 (g)  MgO(s)

Zinc metal reacts with hydrochloric acid to produce bubbles of hydrogen gas.

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 32 Balancing by Inspection ¬Count atoms of each element apolyatomic ions may be counted as one “element” if it does not change in the reaction Al + FeSO 4  Al 2 (SO 4 ) 3 + Fe 1 SO 4 3 bif an element appears in more than one compound on the same side, count each separately and add CO + O 2  CO O 2

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 33 Balancing by Inspection ®Find Least Common Multiple and factors needed to make both sides equal ¯Use factors as coefficients in equation aif already a coefficient then multiply by new factor °Recount and Repeat until balanced

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 34 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 ¬write the equation in words –identify the state of each chemical magnesium(s) + oxygen(g)  magnesium oxide(s) ­write the equation in formulas –identify diatomic elements –identify polyatomic ions –determine formulas Mg(s) + O 2 (g)  MgO(s)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 35 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 ®count the number of atoms of on each side –count polyatomic groups as one “element” if on both sides –split count of element if in more than one compound on one side Mg(s) + O 2 (g)  MgO(s) 1  Mg  1 2  O  1

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 36 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 ¯pick an element to balance –avoid element in multiple compounds °find least common multiple of both sides & multiply each side by factor so it equals LCM Mg(s) + O 2 (g)  MgO(s) 1  Mg  1 1 x 2  O  1 x 2

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 37 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 ±use factors as coefficients in front of compound containing the element 3if coefficient already there, multiply them together Mg(s) + O 2 (g)  2 MgO(s) 1  Mg  1 1 x 2  O  1 x 2

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 38 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 ²Recount Mg(s) + O 2 (g)  2 MgO(s) 1  Mg  2 2  O  2 ³Repeat 2 Mg(s) + O 2 (g)  2 MgO(s) 2 x 1  Mg  2 2  O  2

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 39 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water write the equation in words –identify the state of each chemical ammonia(g) + oxygen(g)  nitrogen monoxide(g) + water(g) ­write the equation in formulas –identify diatomic elements –identify polyatomic ions –determine formulas NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g)

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 40 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water ®count the number of atoms of on each side –count polyatomic groups as one “element” if on both sides –split count of element if in more than one compound on one side NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g) 1  N  1 3  H  2 2  O  1 + 1

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 41 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water ¯pick an element to balance –avoid element in multiple compounds °find least common multiple of both sides & multiply each side by factor so it equals LCM NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g) 1  N  1 2 x 3  H  2 x 3 2  O  1 + 1

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 42 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water ±use factors as coefficients in front of compound containing the element 2 NH 3 (g) + O 2 (g)  NO(g) + 3 H 2 O(g) 1  N  1 2 x 3  H  2 x 3 2  O  1 + 1

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 43 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water ²Recount 2 NH 3 (g) + O 2 (g)  NO(g) + 3 H 2 O(g) 2  N  1 6  H  6 2  O  ³Repeat 2 NH 3 (g) + O 2 (g)  2 NO(g) + 3 H 2 O(g) 2  N  1 x 2 6  H  6 2  O  1 + 3

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 44 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water ´Recount 2 NH 3 (g) + O 2 (g)  2 NO(g) + 3 H 2 O(g) 2  N  2 6  H  6 2  O  2 + 3

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 45 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water µRepeat –A trick of the trade, when you are forced to attack an element that is in 3 or more compounds – find where it is uncombined. You can find a factor to make it any amount you want, even if that factor is a fraction! –We want to make the O on the left equal 5, therefore we will multiply it by NH 3 (g) O 2 (g)  2 NO(g) + 3 H 2 O(g) 2  N  2 6  H  x 2  O  2 + 3

Copyright©2004 by Houghton Mifflin Company. All rights reserved. 46 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water ³Multiply all the coefficients by a number to eliminate fractions –x.5  2, x.33  3, x.25  4, x.67  3 2 x [2 NH 3 (g) O 2 (g)  2 NO(g) + 3 H 2 O(g)] 4 NH 3 (g) + 5 O 2 (g)  4 NO(g) + 6 H 2 O(g) 4  N  4 12  H   O  10