Tro's "Introductory Chemistry", Chapter 7 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. Atoms of different elements can combine to make new compounds. Molecules can combine to make bigger molecules. Molecules can decompose into smaller molecules or atoms. Atoms can be exchanged between molecules or transferred to another molecule. Atoms can gain or lose electrons, turning them into ions. Or changing the charge on ions that are already there. Tro's "Introductory Chemistry", Chapter 7

Evidence of Chemical Reactions Look for evidence of a new substance. Visual clues (permanent). Color change. Precipitate formation. Solid that forms when liquid solutions are mixed. Gas bubbles. Large energy changes. Container becomes very hot or cold. Emission of light. Other clues. New odor. Whooshing sound from a tube. Permanent new state. Tro's "Introductory Chemistry", Chapter 7

Evidence of Chemical Change Emission of Light Release or Absorption of Heat Color Change Formation of a Gas Formation of Solid Precipitate Tro's "Introductory Chemistry", Chapter 7

Evidence of Chemical Change, Continued In order to be absolutely sure that a chemical reaction has taken place, you need to go down to the molecular level and analyze the structures of the molecules at the beginning and end. Is boiling water a chemical change? Tro's "Introductory Chemistry", Chapter 7

Tro's "Introductory Chemistry", Chapter 7 Practice—Decide Whether Each of the Following Involve a Chemical Reaction. Photosynthesis Heating sugar until it turns black Heating ice until it turns liquid Digestion of food Dissolving sugar in water Burning of alcohol in a flambé dessert Yes, CO2 and H2O combine into carbohydrates Yes, sugar decomposing No, molecules still same Yes, food decomposing and combining with stomach acid No, molecules still same Yes, alcohol combining with O2 to make CO2 and H2O Tro's "Introductory Chemistry", Chapter 7

Tro's "Introductory Chemistry", Chapter 7 Chemical Equations Short-hand 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 masses of reactants used and products that can be made. Tro's "Introductory Chemistry", Chapter 7

Tro's "Introductory Chemistry", Chapter 7 Conservation of Mass Matter cannot be created or destroyed. Therefore, the total mass cannot change. And the total mass of the reactants will be the same as the total mass of the products. In a chemical reaction, all the atoms present at the beginning are still present at the end. If all the atoms are still there, then the mass will not change. Tro's "Introductory Chemistry", Chapter 7

Symbols Used in Equations Symbols used to indicate state after chemical. (g) = gas; (l) = liquid; (s) = solid. (aq) = aqueous = dissolved in water. Energy symbols used above the arrow for decomposition reactions. D = heat. hn = light. shock = mechanical. elec = electrical. Tro's "Introductory Chemistry", Chapter 7