Chemical Equations and Reactions

1. Chemical Equations and Reactions

2. Some Definitions Chemical reaction = The process by which one or more substances are changed into one or more different substances Chemical equation = Represents a reaction (with symbols and formulas) the identities and relative molecular or molar amounts of the reactants and products in a reaction

3. Indications of a Chemical Reaction • Evolution of energy as heat and light • Production of a gas

4. Indications of a Chemical Reaction • Formation of a precipitate A solid that is produced as a result of a chemical reaction in solution and that separates from the solution 4. Color Change

5. Characteristics of Chemical Equations • The equation must represent known facts: all reactants and products must be identified • The equation must contain the correct formulas for the reactants and products: use knowledge of writing formulas with oxidation states Assigning unknown oxidation numbers to multi atom elements For example: Cr2O72- is Cr = +6 O = -2

6. Characteristics of Chemical Equations • The law of the conservation of mass must be satisfied • Balancing equations --- YAY!! • Same # of atoms of each element must appear on each side of a correct chemical equation *Memorize symbols on page 266

7. Word Equation An equation which the reactants and products in a chemical reaction are represented by words ONLY Qualitative Information Given Ethane + Oxygen  Carbon dioxide + Water Time to practice writing word equations from a word problem!

8. Formula Equation An equation representing the reactants and products of a chemical reaction by their symbols or formulas (also including state symbols) ONLY Qualitative Information Given (Not usually balanced!!) C2H6 (g) + O2(g)   CO2(g)   +  H2O(l)

9. Word and Formula Word equation:                Aluminum + Hydrochloric acid  Aluminum chloride + Hydrogen Formula equation (not balanced):         Al(s)  +  HCl(aq)    AlCl3(s) +  H2(g)

10. Correct Chemical Equation • Begin with formula equation • Balance using coefficients • Now it is correctly written!! • Wrong: • Al(s)  +  HCl(aq)    AlCl3(s) +  H2(g) • Right: • 2Al(s)  +  6HCl(aq)    2AlCl3(s) +  3H2(g)

11. Balancing Chemical Equations • Balance the different types of atoms 1 at a time • 1st balance the atoms of elements that are combined and that appear only once on each side of the equation • Balance polyatomic ions that appear on both sides of the equation as single units • Balance H and O atoms after atoms of all other elements have been balanced • Check by final count

12. Balancing from Word Problem • 1st figure out the word equation • Then write the formula equation from the word equation • Balance the equation following the rules from previous slide Practice: Nitrogen dioxide gas reacts with water to form aqueous nitric acid and nitrogen monoxide gas.

13. Significance of a Chemical Equation • The coefficient of a chemical reaction indicate relative, not absolute, amount of reactants and products • Smallest possible amounts • The relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients • Figure out grams from moles to get total of product • The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction

14. 5 Types of Chemical Reactions • Synthesis (or Composition) • Decomposition • Single-displacement (or Replacement) • Double-displacement • Combustion

15. Synthesis Reactions 2 or more substances combine A(element or compound) + X (element or compound) AX (Compound) • Elements with oxygen and sulfur (forms oxides and sulfides) • CaO(s) + H2O(l)Ca(OH)2(s) • 8Ba(s) + S8(s) 8BaS(s)

16. SynthesisReactions • Metals with Halogens (usually forming ionic compounds) 2K(s) + Cl2(g)  2KCl(s) • Reactions with Oxides • Oxides of active metals + water = metal hydroxides • Oxides of nonmetals + water = oxyacids

17. SynthesisReactions Reactants: Zn + I2 Product: Zn I2

18. Decomposition Reactions Single compound produces 2 or more simpler substances AX (Compound)  A(element or compound) + X (element or compound)

19. Decomposition Reactions • Binary Compounds • Electrolysis: decomposition by electric current • 2H2O(l) 2H2(g) + O2(g) = LAB! • Carbonates • Forming a metal oxide and CO2 • Hydroxides • Pb(OH)2(s) ----> PbO(s) + H2O(g)

20. Single-Displacement Reactions In the case of a positive ion being replaced:  A+ BC B +ACOR In the case of a negative ion being replaced:  A +BCC+BA For either case we have: element + compound  element + compound

21. Double-Displacement Reactions AB + CDCB + AD Exchange Partners! Basically:Compound + Compound  Compound + Compound

22. Double-Displacement Reactions Formation of a Precipitate AgNO3(aq) + NaCl(aq)AgCl(s) + NaNO3(aq) Formation of a Gas FeS(s) + 2HCl(aq)  H2S(g) + FeCl2(aq) Formation of Water H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l)

23. Double-Displacement Reactions Formation of a Precipitate AgNO3(aq) + KCl(aq)� � AgCl(s) + KNO3(aq)

24. Combustion Reactions Usually: hydrocarbon + oxygen  carbon dioxide and water Example: CH4(g) + 2O2(g) 2H2O(g) + CO2(g)

25. Activity Series Whether or not a chemical reaction will really occur! See Handout and/or Page 286 in your textbook (same thing!)

26. Activity Series Practice 1st can they occur, 2nd if so write product, then balance: MgCl2(aq) + Zn(s)  NO Al(s) + H2O(g)  Yes (Al2O3(s) + H2(g))