Chemical Reactions: An Introduction Chapter 6 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 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 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2

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©2000 by Houghton Mifflin Company. All rights reserved. 3

Figure 6.2: Hot and cold pack reactions Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Figure 6.5(b)&(c): The reaction of potassium with water Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Figure 6.3 (a): Chemical reactions Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Figure 6.3 (b): Chemical reactions Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Figure 6.3 (c): Chemical reactions Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 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©2000 by Houghton Mifflin Company. All rights reserved. 4

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 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©2000 by Houghton Mifflin Company. All rights reserved. 5

Combustion of Methane + methane gas burns to produce carbon dioxide gas and liquid water whenever something burns it combines with O2(g) CH4(g) + O2(g)  CO2(g) + H2O(l) H C O + 1 C + 4 H + 2 O 1 C + 2 O + 2 H + O 1 C + 2 H + 3 O Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6

Combustion of Methane Balanced to show the reaction obeys the Law of Conservation of Mass it must be balanced CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l) H C O + 1 C + 4 H + 4 O 1 C + 4 H + 4 O Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 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 can be used to determine mass relationships Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8

Symbols Used in Equations symbols used after chemical formula to indicate state (g) = gas; (l) = liquid; (s) = solid (aq) = aqueous, dissolved in water Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9

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 O2 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) + O2(g)  MgO(s) Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10

Balancing by Inspection Count atoms of each element polyatomic ions may be counted as one “element” if it does not change in the reaction Al + FeSO4 Al2(SO4)3 + Fe 1 SO4 3 if an element appears in more than one compound on the same side, count each separately and add CO + O2  CO2 1 + 2 O 2 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Figure 7.6: The thermite reaction gives off so much heat that the iron formed is molten Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Balancing by Inspection Pick an element to balance avoid elements from 1b Find Least Common Multiple and factors needed to make both sides equal Use factors as coefficients in equation if already a coefficient then multiply by new factor Recount and Repeat until balanced Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12

Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide burning in air means reacting with O2 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) + O2(g)  MgO(s) Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide burning in air means reacting with O2 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) + O2(g)  MgO(s) 1  Mg 1 2  O  1 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide burning in air means reacting with O2 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) + O2(g)  MgO(s) 1  Mg 1 1 x 2  O  1 x 2 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide burning in air means reacting with O2 use factors as coefficients in front of compound containing the element if coefficient already there, multiply them together Mg(s) + O2(g)  2 MgO(s) 1  Mg 1 1 x 2  O  1 x 2 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide burning in air means reacting with O2 Recount Mg(s) + O2(g)  2 MgO(s) 1  Mg 2 2  O  2 Repeat 2 Mg(s) + O2(g)  2 MgO(s) 2 x 1  Mg 2 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17

Examples NH3(g) + O2(g)  NO(g) + H2O(g) 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 NH3(g) + O2(g)  NO(g) + H2O(g) Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18

Examples NH3(g) + O2(g)  NO(g) + H2O(g) 1  N 1 3  H  2 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 NH3(g) + O2(g)  NO(g) + H2O(g) 1  N 1 3  H  2 2  O  1 + 1 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19

Examples NH3(g) + O2(g)  NO(g) + H2O(g) 1  N 1 2 x 3  H  2 x 3 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 NH3(g) + O2(g)  NO(g) + H2O(g) 1  N 1 2 x 3  H  2 x 3 2  O  1 + 1 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20

Examples 2 NH3(g) + O2(g)  NO(g) + 3 H2O(g) 1  N 1 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 NH3(g) + O2(g)  NO(g) + 3 H2O(g) 1  N 1 2 x 3  H  2 x 3 2  O  1 + 1 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21

Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water Recount 2 NH3(g) + O2(g)  NO(g) + 3 H2O(g) 2  N 1 6  H  6 2  O  1 + 3 Repeat 2 NH3(g) + O2(g) 2 NO(g) + 3 H2O(g) 2  N 1 x 2 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22

Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water Recount 2 NH3(g) + O2(g)  2 NO(g) + 3 H2O(g) 2  N 2 6  H  6 2  O  2 + 3 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23

Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water Repeat 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 2.5 2 NH3(g) + 2.5 O2(g) 2 NO(g) + 3 H2O(g) 2  N 2 6  H  6 2.5 x 2  O  2 + 3 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24

Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water Multiply all the coefficients by a number to eliminate fractions 2 x [2 NH3(g) + 2.5 O2(g) 2 NO(g) + 3 H2O(g)] 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) 4  N 4 12  H  12 10  O  10 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25