1. Chemical equations and reactions Chapter 8.1

2. Objectives: List three observations that suggest that a chemical reaction has taken place. List three requirements for a correctly written chemical equation. Write a word equation and a formula equation for a given chemical reaction. Balance a formula equation by inspection.

4. Defined as represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.

5. Example: this chemical equation shows the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide, and water(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)

6. Indications of Chemical Reactions Evolution of heat and light Production of gas Formation of a precipitate Color change Chemistry comes alive!

8. Coefficients are added where necessary

9. Specifies relative number of moles of that substanceEx: Hydrogen gas reacts with oxygen gas to yield water H2 + O2 H2O 2 2

12. formula equation - 2ndstep in writing chemical equation

13. Represents the reactants and products of a chemical reaction by their symbol

14. Example:

15. (g) represents that it is in gaseous stateCH4 (g) + O2 (g) CO2 (g) + H2O (g)

16. balance the equation– last step in writing a chemical equation Satisfy law of conservation of mass by inserting coefficients # of atoms of reactant must = # of atoms of products 2 2 Reactant Product Step 1: Type & number C C 1 1 Step 3: When reactants = products its balanced H H 4 2 4 Step 2: Add coefficients till reactants = products O O 2 3 4 4 CH4 (g) + O2 (g) CO2 (g) + H2O (g)

17. Additional symbols used in chemical reactions Examples: Yields Reversible reactions (s) or solid state (g) or gaseous state (l) liquid state (aq) aqueous (dissolved in water) reactants are heated 2 atm Pressure 0⁰ C Temperature MnO2 Catalyst 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) 2HgO(s) 2Hg(l) + O2(g) C2H2(s) + H2(g) pressure C2H6(g) Pt

19. Reverse reaction for a chemical reaction has the same relative amounts of substances as the forward reaction Balancing Chemical Equations Identify the names of the reactants and the products, and write a word equation. water hydrogen + oxygen Write a formula equation by substituting correct formulas for the names of the reactants and the products. H2O(g) H2(g) + O2(g)

21. First balance the atoms of elements that are combined and that appear only once on each side of the equation

22. Balance polyatomic ions that appear on both sides of the equation as single units.

23. Balance H atoms and O atoms after atoms of all other elements have been balancedH2O(g) H2(g) + O2(g) 2 2 Product Reactant H H 2 2 4 4 O O 1 2 2 Count atoms to be sure that the equation is balanced.

25. Step 2: convert to formula equation Step 3: add all symbols 2 Na2O + H2O NaOH (s) (l) (aq) Step 4: balance Reactant Product Na 2 1 Na 2 2 H 1 2 H 2 O 1 2 O Practice 2 Translate the following chemical reaction PbCl2(aq) + Na2CrO4(aq) PbCrO4(s) + 2 NaCl(aq) Aqueous lead(II) chloride reacts with aqueous sodium chromate to produce a precipitate of lead(II) chromate and aqueous sodium chloride.

27. Practice 3 The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write the balanced chemical equation for the reaction. zinc + hydrochloric acid zinc chloride + hydrogen Zn(s) + HCl(aq) ZnCl2(aq) + H2(g) 2 Reactant Product Zn 1 1 Zn 1 H 2 2 H 2 1 Cl 2 Cl

28. Practice 4 Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction. Aluminum carbide + water + aluminum hydroxide methane 3 4 Al4C3(s) + H2O(l) + Al(OH)3(s) CH4(g) 12 Reactant Product 4 4 1 Al Al 3 3 1 C C 24 24 7 15 2 H H 12 1 3 12 O O

29. Practice 5 Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate. These products settle out, taking suspended solid impurities with them. Write a balanced chemical equation for the reaction. Aluminum sulfate + Calcium hydroxide + calcium sulfate aluminum hydroxide Al2(SO4)3 (aq) + 2 3 3 Ca(OH)2(aq) + CaSO4(s) Al(OH)3(s) Reactant Product 2 1 2 Al Al 3 1 3 SO4 SO4 3 3 1 1 Ca Ca 6 2 3 6 OH OH