1 Chemical Reactions: An Introduction Chapter 6

2 6.1Evidence of Chemical Reactions Chemical reactions involve the rearrangement and exchange of atoms to produce new compounds. Evidence: (fig 6.1, 6.2, 6.3) –color change, precipitate formation, gas bubbles, flames, heat release, cooling, light –new odor, permanent new state

3 6.2Chemical Equations A chemical reaction is a chemical change involving a rearrangement of the ways in which atoms are grouped. A chemical equation is a shorthand way of describing a reaction. It is written in the following form: Reactants  Products Ex: CH 4 + O 2  CO 2 + H 2 O The arrow indicates the direction of the change and is read as “yields” or “produces”. A reactant is a starting substance in a chemical reaction, whereas a product is a substance resulting from a chemical reaction.

4 Chemical equations provide information about the reaction: –The formulas of reactants and products. –The states of reactants and products. Physical states are shown by symbols used after a chemical formula: ( g) = gas; (l) = liquid; (s) = solid (aq) = aqueous, dissolved in water –The numbers of reactant and product molecules that are required (when balanced). –Used to predict the amount of product produced in a chemical reaction based on amount of reactants.

5 Law of Conservation of Mass –In chemical reactions matter cannot be created or destroyed. –The number and type of atoms in the reactants are still present in the products, they’ve only been rearranged. –The total mass of the reactants will equal the total mass of the products. –Elements are not transmuted (changed) in a reaction. –Equations are balanced to ensure compliance with the law.

6 6.3Balancing Chemical Equations Use proper formulas for each reactant and product. A balanced chemical equation: –obeys the Law of Conservation of Mass. –shows equal numbers of atoms of each element on reactant side as on product side. –shows the relationship between the relative numbers of molecules/formula units/atoms of reactants and products. The formulas of reactants or products can never be changed in balancing a chemical equation. Equations are balanced by changing the number of molecules/formula units/atoms in the reaction. See page 162 in text on how to write and balance equations. See specific examples in text and class

Writing chemical equations. You must be able to read a description of a chemical reaction and write a balanced chemical equation from the names of the compounds. For example: try writing balanced chemical equation for problems 17, 19 and 21 on page 167 of your text. You can, of course, practice writing chemical equations for the even- numbered problems and checking your answers in the Appendix (green tab). It is expected that you can write chemical equations from their descriptions on the test. 7