Chemical Equations and Reactions

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

Indications of a Chemical Reaction 1.Evolution of energy as heat and light 2.Production of a gas

Indications of a Chemical Reaction 3.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

Characteristics of Chemical Equations 1.The equation must represent known facts: all reactants and products must be identified 2.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: Cr 2 O 7 2- is Cr = +6 O = -2

Characteristics of Chemical Equations 3.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

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!

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!!) C 2 H 6 (g) + O 2 (g)  CO 2 (g) + H 2 O(l)

Word Word and Formula Word equation Word equation: Aluminum + Hydrochloric acid  Aluminum chloride + Hydrogen Formula equation Formula equation (not balanced): Al(s) + HCl(aq)  AlCl 3 (s) + H 2 (g)

Correct Chemical Equation Begin with formula equation Balance using coefficients  Now it is correctly written!! Wrong Wrong: Al(s) + HCl(aq)  AlCl 3 (s) + H 2 (g) Right Right: 2Al(s) + 6HCl(aq)  2AlCl 3 (s) + 3H 2 (g)

Balancing Chemical Equations 1.Balance the different types of atoms 1 at a time 2.1 st balance the atoms of elements that are combined and that appear only once on each side of the equation 3.Balance polyatomic ions that appear on both sides of the equation as single units 4.Balance H and O atoms after atoms of all other elements have been balanced 5.Check by final count

Balancing from Word Problem 1.1 st figure out the word equation 2.Then write the formula equation from the word equation 3.Balance the equation following the rules from previous slide Practice Practice: Nitrogen dioxide gas reacts with water to form aqueous nitric acid and nitrogen monoxide gas.

Significance of a Chemical Equation 1.The coefficient of a chemical reaction indicate relative, not absolute, amount of reactants and products  Smallest possible amounts 2.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 3.The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction

5 Types of Chemical Reactions 1.Synthesis (or Composition) 2.Decomposition 3.Single-displacement (or Replacement) 4.Double-displacement 5.Combustion

Synthesis Reactions 2 or more substances combine AX AX A(element or compound) + X (element or compound)  AX (Compound)  Elements with oxygen and sulfur (forms oxides and sulfides) H 2 OCa(OH) 2 CaO (s) + H 2 O (l)  Ca(OH) 2(s) S 8 8BaS 8Ba (s) + S 8(s)  8BaS (s)

 Metals with Halogens (usually forming ionic compounds) 2K (s) + Cl 2(g)  2KCl (s)  Reactions with Oxides  Oxides of active metals + water = metal hydroxides  Oxides of nonmetals + water = oxyacids Synthesis Reactions

Reactants: Zn + I 2 Product: Zn I 2 Synthesis Reactions

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

Decomposition Reactions  Binary Compounds  Electrolysis: decomposition by electric current  2H 2 O (l)  2H 2(g) + O 2(g) = LAB!  Carbonates  Forming a metal oxide and CO 2  Hydroxides  Pb(OH) 2(s) ----> PbO (s) + H 2 O (g)

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

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

Double-Displacement Reactions Formation of a Precipitate AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(aq) Formation of a Gas FeS (s) + 2HCl (aq)  H 2 S (g) + FeCl 2(aq) Formation of Water H 2 SO 4(aq) + 2NaOH (aq)  Na 2 SO 4(aq) + 2H 2 O (l)

Double-Displacement Reactions Formation of a Precipitate AgNO 3 (aq) + KCl (aq) �  � AgCl (s) + KNO 3 (aq)

Combustion Reactions Usually: hydrocarbon + oxygen  carbon dioxide and waterExample: CH 4(g) + 2O 2(g)  2H 2 O (g) + CO 2(g)

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

Activity Series Practice 1 st can they occur, 2 nd if so write product, then balance: MgCl 2(aq) + Zn (s)  NO Al (s) + H 2 O (g)  Yes (Al 2 O 3(s) + H 2(g) )