CHAPTER 4 TYPES OF CHEMICAL EQUATIONS AND SOLUTION STOICHIOMETRY

CHAPTER 4 TYPES OF CHEMICAL EQUATIONS AND SOLUTION STOICHIOMETRY

Water, the common solvent Solution is a homogeneous mixture Solvent is the substance that does the dissolving Solute is the substance that gets dissolved Aqueous Solutions (aq) form when water is the dissolving medium Water is known as the universal solvent Most valuable property of water is its ability to dissolve different substances Due to its bent/v-shape O-H covalent bonds Electrons shared by O-H but not equally Oxygen has a greater attraction for the electrons than hydrogen causing water to be a polar molecule This polarity gives water its great ability to dissolve compounds

Dissolving ionic solids in water (+) end of the water molecule is attracted to the (-) charged anion (-) end of the water molecule is attracted to the (+) charged cation This process of hydration is what causes a salt (ionic compound) to dissolve in water Ionic substances (salts) dissolve in water, they break up into the individual cations and anions. Try #11 (a-d) on page 180 Remember aqueous (aq) means dissolved in!!!

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Solubility-depends on the strength of attractive forces within the molecule and the attraction to the water molecule. Ionics dissolve Water dissolves many non-ionic substances Ethanol C 2 H 5 OH-is a polar molecule so will dissolve in water Oil-is a non-polar molecule-not attracted to a water molecule so won t dissolve RULE Like dissolves like Solvent water will dissolve ionic or polar solutes Water will not dissolve non-polar molecules

The nature of aqueous solutions: Strong and Weak electrolytes Electrolyte-property of a solution Solute-material that is dissolved Solvent-dissolving medium Electrical conductivity-ability to conduct electricity Strong-lots of current Weak-little current Non-no current Conductivity due to the presence of ions

Strong Electrolyte Completely ionizes in water 1. Soluble salts examples include NaCl and BaCl 2 2. Strong acids H 2 SO 4 /HNO 3 /HCl HNO 3 H + + NO - 3 HCl H + + Cl - H 2 SO 4 H + + HSO 4 100 molecules 100 ions + 100 ions They all completely dissolve into ions! 3. Strong bases contain hydroxide NaOH Na + + OH - KOH K + + OH -

Weak Electrolyte Small degree of ionization HC 2 H 3 O 2 H + + C 2 H 3 O 2 - Weak acid- only dissociates 1% If you have 100 molecules that means that 99 molecules 1 ion + 1 ion Weak Base NH 3 + H 2 O NH 4 + + OH -

Non electrolyte Dissolves but does not produce ions Ethanol (C 2 H 5 OH), Sucrose(C 12 H 22 O 11 ) Molecules remain intact

Electrolyte Demo NaCl solution Distilled water Tap water 1 M HCl 1 M NaOH Sucrose solution 6 M Acetic Acid 6 M NH 4 OH Hydrogen Peroxide Ethanol

The composition of Solutions Concentration of solutions are important in order to use stoichiometry Molarity = moles of solute liters of solution 1.0 M NaCl means that the solution is made by dissolving 1.0 mol of NaCl in enough water to make one liter. The solution contains 1.0 mol of Na + and 1.0 mol of Cl - From this equation you can also calculate moles of solute of liters of solution. Liters x Molarity = moles of solute Liters of solution = moles of solute Try problems 15, 17, 19, 21 Molarity on page 180.

Standard solution-concentration is accurately known Dilution moles before dilution = moles after dilution M 1 x V 1 = M 2 x V 2 Volumetric Flasks Try 23, and 25

Answer the following questions: 1. A salt solution has a volume of 250 ml and contains 0.70 mol of salt. What is the molarity of the solution? 2. How would prepare a 500 ml solution of 0.050 M sodium bromide? 3. You need 250 ml of 0.20 M NaCl solution, but the stock solution you have is a 1.0 M NaCl. How would you prepare the required solution? 4. How would you prepare a solution of magnesium chloride that contains.3833 moles of magnesium chloride in 500 ml of water. Calculate the molarity of the final solution. 5. How would you prepare a 350 ml solution of 2.5 M Nitric Acid if the stock solution was 12M?

Types of solution reactions Precipitate Acid-base Oxidation-reduction There are other types of reactions including: Single replacement Double replacement Combustion Decomposition Synthesis (combination) We will cover these throughout the rest of the year. Not directly covered in the book.

4.5 Precipitation Reactions When two solutions are combined and an insoluble substance forms. Solid that forms is called a precipitate (ppt.) K 2 CrO 4 + Ba(NO 3 ) 2? How do you predict the products? (Double Replacement rxn) Predicting the identity of the solid product in a ppt. rxn requires knowledge of the solubilities of common ionic substances. Solubility Rules-need to memorize Slightly soluble-tiny amount dissolves (not noticeable) Soluble-dissolves Insoluble-doesn t dissolve Focus on the actual components of the solution before any rxn occurs. Page 181 Try 29 a-d.

Assignment Due on Friday October 25th Pages 180-181 11 e-i,12, 16, 18, 20, 22, 24 a & b, 26, 28, 30, 32, 34, 36, & 38

4.6 Describing Reactions in Solution 3 Types of Equations Molecular equation-shows the reactant and products of the rxn. Complete Ionic equation-all reactants and products that are strong electrolytes are represented as ions. Net ionic equation Includes only those solution components (ions) that undergo a change. Spectator ions not included Spectator ions-do not participate in the reaction These are the ions that are present before and after the reaction BASICALLY NET IONIC IS WHAT IONS FORM THE SOLID IN THE RXN Page 181 Try 31b

4.7 Stoichiometry of precipitate reactions Steps needed to solve the problem 1. Determine the species (ions) in the solution and determine what rxn will occur. 2. Write out the balanced equation. (sometimes may need net ionic equation) 3. Determine limiting reactant if needed. 4. Solve for moles or grams of what your trying to solve. Lets do 39 and 41 In notes do 40, 42, and 43 for tomorrow

4.8 Acid-Base Reactions Brønsted / Lowry definition of acids and bases An acid is a proton donor A base is a proton acceptor Proton is H + Acid-Base Rxns-neutralization reaction- enough base is added to react exactly with the acid in the solution General Acid + Base Water + Salt Strong acids completely dissociate- break up into ions Weak acids don t break up into ions-remember weak electrolyte Strong acids-hcl, HBr, HI, HNO 3, HClO 4, and H 2 SO 4 All other acids considered weak

General reactions-exceptions when a solid forms Strong Acid (aq) + Strong Base (aq) Water (l) + Salt (aq) Net ionic: H + + OH - H 2 O Strong Acid (aq) + Strong Base (s) Water (l) + Salt (aq) Net Ionic: H + + Strong Base Water + Cation from Base Weak Acid (aq) + Strong Base (aq) Water (l) + Salt (aq) Net Ionic: Weak Acid + OH - Water + Anion from acid OH - is such a strong base that for stoichiometric calculations we assume it reacts completely with any weak acid. Remember net ionic includes only those components that undergo a change Spectator ions are not included!!! Try 45 and 47

Acid-Base reaction calculations 1. List the species present in the combined solution before any reaction occurs and decide what reaction will occur. 2. Write the balanced net ionic equation for this reaction. 3. Calculate the moles of the reactants. For reactions in solution, use the volume of the original solutions and their molarities. 4. Determine the limiting reactant when needed. 5. Calculate the moles of the required reactant or product. 6. Convert to grams or volume of the solution what ever is asked. Let s do 49 and 51 together.

Acid/Base Titrations Show buret w/example Titration-delivering from a buret a measured volume of solution of a known concentration (the titrant) into a solution containing the substance being analyzed (the analyte) Equivalence point=stoichiometric point the point in a titration when enough titrant has been added to react exactly with the analyte. *Neutralization has occurred* This point is often marked by an indicator Changes color @ or near the equivalence point. Endpoint-the point when the indicator changes color You want to choose an indicator such that the endpoint occurs exactly @ the equivalence point.

Common indicator for strong acid/base titrations is phenolphthalein. Colorless when acidic Pink when basic Stays colorless until all the acid is consumed and turns pink when the 1 st drop of excess base is added Goal always is a light pink color-dark pink means you have gone too far. Show titration using HCl-in flask-with phenolphthalein NaOH-in buret Lets try some calculations involving titrations-53 and 55. Assignment in your notes 46, 48, 50, 52, 54, 56 due on Monday.

4.9 Oxidation-Reduction Reactions Reactions in which one or more electrons are transferred. Redox Reactions Oxidation States(Oxidation # s) A way to keep track of electrons in a redox rxn Remember the # is written after the plus or minus sign +n or n (because these are not actual charges like on ions that are written 2+ or 1-) Ionic compounds Ions charge=oxidation state Element and diatomic molecules Oxidation state= 0 Covalent Compounds Imagine the charges of the atoms if the electrons were not shared but divided between the atoms Sum of the oxidation states must be zero for an electrically neutral compound

A non-integer value for an oxidation state does occur-rarely. Oxidation states to commit to memory Oxygen (-2) Fluorine (-1) Hydrogen (+1) Determine the others using simple math and rules from the sheet. Try #57

Redox Rxns Must have a gain and loss of electrons (transfer of electrons) Oxidation is an increase in oxidation state (a loss of electrons) OIL-Oxidation involves loss LEO-Loss Electrons Oxidation Reduction is a decrease in oxidation state (gain of electrons) RIG-reduction involves gaining GER-gain electrons reduction OIL RIG LEO GER Oxidizing Agent-Electron acceptor Reducing Agent-Electron Donor When naming oxidizing and reducing agents name the whole compound Try #61 Do 58 & 62 in notes

4.10 Balancing Redox Rxns Half-rxn method 2 half-rxns One is an oxidation rxn One is a reduction rxn Easiest to separate these two half rxns

Steps for balancing a redox rxn when occurring in an acidic solution. 1. Write separate equations for the oxidation and reduction half reactions 2. For each half-rxn a) Balance all elements except H and O b) Balance Oxygen with H 2 O c) Balance Hydrogen using H + d) Balance charges using electrons (e - ) 3. If necessary multiply one or both balanced half-rxns by an integer to equalize the number of electrons 4. Add half-rxns-cancel identical species 5. Check that the elements and charges are balanced Make sure both sides of the rxn are equal Try 63.

Steps for Balancing a redox rxn in a basic solution. 1. Same steps for in a acidic solution 2. Except add step e.) to #2 e.) add OH - to each side to combine w/ H + to create water-you don t want H + in a basic solution. Must add the OH - to both sides of the rxn!!!! 3. Again at the end make sure all the elements and charges are equal on both sides of the rxn. Try 65. Assignment 64,66 & 67 in notes