1. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 1 Chemical Reactions Reactions involve chemical changes in matter that result in new substances. Reactions involve rearrangement and exchange of atoms to produce new molecules. Reactants  Products CH 4 + 2O 2 → CO 2 + 2H 2 O CH 4 O2O2 CO 2 H2OH2O

2. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 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

3. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 3 Evidence of Chemical Reactions (cont.)

4. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 4 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 –CH 4 (g)+ 2O 2 (g) → CO 2 (g) + 2H 2 O(l) –Can be used to determine weights of reactants used and of products that can be made

5. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 5 Symbols Used in Equations Symbols used after chemical formula to indicate state: –(g) = gas; (l) = liquid; (s) = solid –(aq) = aqueous, dissolved in water –e. g. NH 3 (aq) indicates ammonia dissolved in water

6. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 6 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.

7. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 7 Combustion of Methane Methane gas burns to produce carbon dioxide gas and liquid water 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

8. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 8 Combustion of Methane Balanced To show a reaction obeys the Law of Conservation of Mass, it must be balanced. 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

9. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 9 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 equations show the relationship between the relative numbers of molecules of reactants and products. –Can be used to determine mass relationships

10. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 10 Balancing Chemical Equations If the equation is in words, write the formulas for the reactants and products (include physical states). Balance the equation by counting the atoms on both sides of the equation and changing the coefficients as needed. Never change the subscripts! This is done by trial and error. Start with the most complicated compound first. The best balanced equation is the one with the smallest integer coefficients (not fractions).

11. Copyright © Houghton Mifflin Company. All rights reserved. 6 | 11 Balance the following equations: Solid potassium reacts with gaseous nitrogen to form solid potassium nitride. C 3 H 8 (g) + O 2 (g) → CO 2 (g) + H 2 O(l) CuSO 4 (aq) + KI(s) → CuI(s) + I 2 (s) + K 2 SO 4 (aq)