Chemical Reactions. Chemical Arithmetic

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1 Chemical Reactions Chapter 6 Chemical Arithmetic Balancing Equations The mole Gram - mole conversions Mole - mole relationships in chemical equations Mass relationships in chemical equations Per cent yeilds 1

2 Chemical Equations Chemical Equation- A shorthand way of representing a chemical reaction Starting substances > Final substances (Reactants) (Products) Physical states of reactants and products may indicated Solid (s) Liquid (l) Gas (g) Aqueous solutions (aq) dissolved in water Properties of Chemical Equations Conservation of mass- matter is neither created or destroyed in chemical reactions Chemical equations must be balanced # of atoms and kinds of atoms must be the same on both sides of the equation (coefficients) Total charge on all species on the left must equal the total charge of all species on the right 2

3 Guidelines for Balancing Equations Subscripts in formulas may not be changed to balance the equation Use coefficients in front of formula to balance equation Balance elements (or chemical groups ) that appear in only one substance on each side of the equation first Go back and forth correcting with each round the effect of your balancing of an element or group Fractions may be used to balance an equation initially. However, all coefficients should be integers at the end Step by Step Procedure for Balancing Chemical Equations 1) 2) 3) 4) Write an unbalanced equation, using correct formulas for all reactants and products Add appropriate coefficients to balance the numbers of atoms of each element Check equation to ensure the #s and types of atoms are the same on both sides of the equation Make sure the coefficients are reduced to their lowest whole number values 3

4 Some Equations to Balance _H 3 PO 4 + _NaOH --> _Na 3 PO 4 + _H 2 O _CH 3 CH 3 + _O 2 --> _CO 2 + _H 2 O _AgNO 3 + _MgCl 2 --> _AgCl + _Mg(NO 3 ) 2 _BaCl 2 + _(NH 4 ) 2 SO 4 --> _BaSO 4 + _NH 4 Cl The Mole Formula Weight (FW) - Sum of the atomic weights of all atoms in a formula Formula unit- all of the atoms in a formula If the substance is composed of molecules, the formula weight is also called the molecular weight Mole (mol) - The amount of a substance equal to the formula weight of the substance in grams Contains Avogadro s number (6.02 x ) of formula units The formula weight in grams is often called the molar mass 4

5 Some Mole Problems Calculate the molecular weight of Ibuprofen, C 13 H 18 O 2. How many molecules of ascorbic acid (vitamin C, C 6 H 8 O 6 ) are in a 500 mg tablet? What is the mass in grams of 5.0 x molecules of aspirin (C 9 H 8 O 4 )? Molar Mass as a Conversion Factor Using the molar mass as a conversion factor allows us to convert moles to grams and grams to moles 5

6 Relationships Involving the Mole Stoichiometry Stoichiometry - Quantitative relationships involving chemical equations and formulas The meaning of a chemical equation The coefficients in a balanced equation indicate the relative number of formula units of each substance that is involved in the reaction 2 C 8 H O 2 --> 16 CO H 2 O When 2 molecules of octane are allowed to react with 25 molecules of oxygen, 16 molecules of carbon dioxide and 18 molecules of water are formed 6

7 2 C 8 H O > 16 CO H 2 O 2 molecules + 25 molecules --> 16 molecules + 18 molecules 12(2) " 12(25) " 12(16) " 12(18) " 24 " 300 " 192 " 216 " 2 dozen " 25 dozen " 16 dozen " 18 dozen " 6.02x10 23 (2) " 6.02x10 23 (25) " 6.02x10 23 (16) " 6.02x10 23 (18) " 2 moles C 8 H moles O 2 16 moles CO moles H 2 O The coefficients in a balanced equation indicate the relative number of moles of each substance that is involved in the reaction. Stoichiometry- continued 2 C 8 H O > 16 CO H 2 O 2 mol C 8 H 18 = 16 mol CO 2 2 mol C 8 H 18 = 18 mol H 2 O 25 mol O 2 = 16 mol CO 2 2 mol C 8 H 18 = 25 mol O 2 16 mol CO 2 = 18 mol H 2 O etc. 7

8 Stoichiometry- continued 2 C 8 H O > 16 CO H 2 O Solving Mole/Gram Conversion Problems 1) 2) 3) 4) Write a balanced chemical equation Choose appropriate molar masses and mole ratios to convert known information into required information Set up factor-label expression and calculate the answer Estimate or check answer using a ballpark solution 8

9 Summary of Mole and Gram Conversions Some Conversion Problems Hydrogen fluoride reacts with glass (silicon dioxide) in the following manner: 4 HF (g) + SiO 2 (s) --> SiF 4 (g) + 2 H 2 O (l) a) How many moles of HF will react completely with 8.90 mol of SiO 2? b) What mass of water (in grams) will be produced by the reaction of 23.0 g of SiO 2? 9

10 Percent Yields Not all reactants are converted to products Losses may occur due to side reactions and handling Percent yield = actual yield X 100% theoretical yield Calculations are based the actual mass of the product for the actual yield and on gram to gram conversions for the theoretical yield Classification of Chemical Reactions Precipitation reactions in which an insoluble precipitate forms upon mixing aqueous reactants Acid-base neutralization reactions in which an aqueous acid reacts with an aqueous base to form an ionic compound (a salt) and water Oxidation-Reduction reactions in which one or more electrons are transferred between reactants 10

11 Solubility Rules Compounds are generally soluble if they contain Group 1 cations (Li +, Na +, K +, Rb +, Cs + ) or ammonium ions (NH4 + ) Compounds are generally soluble if they contain halide (Cl -, Br -, I - ), nitrate (NO 3- ), perchlorate (ClO 4- ), or sulfate (SO 4 2- ) anions. Exceptions: Ag +, Hg 2 2+, & Pb 2+ with halides and Ba 2+, Hg 2 2+ and Pb 2+ with sulfates Acid-Base Neutralization Reactions Neutralization- When equal mole amounts of an acid and a base are mixed together, both acidic and basic properties of each disappear. The resulting solution is said to be neutral. Water and a salt are formed HA(aq) + MOH(aq) ---> H 2 O(l) + MA(aq) acid base water salt Note- salt contains an anion other than OH - ion 11

12 Net Ionic Equations Ionic Equation- an equation in which all ions are explicitly shown Spectator ions- an ion that remains unchanged on both sides of the reaction equation Net Ionic Equation- an equation that excludes the spectator ions and shows only those ions that are involved in the reaction Oxidation-Reduction Reactions Oxidation-Reduction (Redox) Reaction- A chemical reaction in which the oxidation numbers of elements change because of a loss or gain of electrons Oxidation Number- A number that indicates the charge that an atom in a molecule or polyatomic ion would have if all bonds were ionic. Fictitious- No actual charge of this magnitude actually exists when covalent bonds are present Useful for classifying a type of reaction as an oxidation-reduction reaction 12

13 Guidelines for Assigning Oxidation Numbers Assign an oxidation number of zero to any free element, e.g., Na, Mg, Cl 2, S 8, etc. Assign the most electronegative element in a compound or polyatomic ion a negative oxidation number equal to the charge that atom would have if it were a monatomic anion Assign a group 1, 2 or 3 element in a compound or polyatomic ion an oxidation number of +1, +2, and +3, respectively Assign any other element an oxidation number such that the sum of oxidation numbers equals zero for a neutral compound or the overall charge for a polyatomic ion Redox Terminology- Part 1 Oxidation- the loss of electrons Reduction- the gain of electrons - Mnemonic to remember: LEO goes GER The species that contains the element whose oxidation number increased (more positive) is said to be oxidized The species that contains the element whose oxidation number decreased (more negative) is said to be reduced 13

14 Redox Terminology- Part 2 Reducing Agent - species that: Loses electrons Undergoes oxidation Becomes more positive (or less negative) Oxidizing Agent - species that: Gains electrons Undergoes reduction Becomes more negative (or less positive) Redox Examples What species on the left was oxidized? Reduced? Which is the reducing agent? Oxidizing agent? 14

15 Another Redox Reaction What species on the left was oxidized? Reduced? Which is the reducing agent? Oxidizing agent? 15

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