Presentation on theme: "Types of Chemical Reactions & Solutions"— Presentation transcript:
1 Types of Chemical Reactions & Solutions Chapter 4
2 What is a Solution?Soluble – a substance that can dissolve in a given solventMiscible: two liquids that can dissolve in each otherExample: water and antifreezeInsoluble – substance cannot dissolveImmiscible: two liquids that cannot dissolve in each otherExample: oil & water
3 Why Do Some Substances Dissolve and not Others? To dissolve, solute particles must dissociate from each other and mix with solvent particlesAttractive forces between solute and solvent must be greater than attractive forces within the soluteProcess of surrounding solute particles with solvent particles is called SOLVATIONIn water, it is also called HYDRATION
5 Aqueous Solutions of Molecular Compounds Water is also a good solvent for many molecular compounds (Example: sugar)Sugar has many O-H bonds (polar)When water is added, the O-H bond becomes a site for hydrogen bonding with waterWater’s hydrogen bonds pulls the sugar molecules apartOil is not a good solute because it has many C-H bonds (not polar) and few or no O-H (polar) bonds
6 Factors that Affect Solvation Rate Increase Solvation Rate (Dissolve Faster) by:Agitation (stirring)Increase surface area (make particles smaller)Temperature (make it hotter)All these increase the number of collision between water and the solute
7 The Nature of Aqueous Solutions Composition of a solution can vary by changing amount dissolved:Electrical ConductivityPure Water is a poor conductorGood conductors have strong electrolytesWeak conductors have weak electrolytesNon-conductors contain non-electrolytes
8 The Nature of Solutions Originally identified by ArrheniusConductivity arises from the presence of IONS in the solutionIons are charged particlesIonic theory starts to make senseArrhenius postulates:The extent to which a solution can conduct electricity depends DIRECTLY on the number of ions present.
9 Strong Electrolytes Strong electrolytes COMPLETELY ionize in solution Example: NaCl Na+ & Cl-Arrhenius first associated acidity to the presence of H+ ions (acidus = sour)Acids ionize to form H+ ionsHCl, HNO3, H2SO4 are strong acidsStrong Acids Completely IonizeStrong Bases also completely ionizeOH- compounds completely ionize
10 Weak ElectrolytesWeak Electrolytes exhibit a small degree of ionization in waterWeak Acids, Weak BasesExample: Acetic Acid is a weak acid (~1/100 molecules dissociates)Example Ammonia is a weak base (~1/100 molecules dissociates)Non-Electrolytes dissolve in water, but do not dissociate into ionsExamples: sugar, alcoholDo not conduct electricity in solution
11 Composition of Solutions The nature of the chemical reaction frequently depends on the amounts of chemicals present:Molarity = moles solute/1 liter solutionConcentration is determined BEFORE it dissolves1.0M NaCl is made by measuring 1.0 Moles of NaCl and adding enough water to make 1 L of solutions1.0M does not mean it contains 1.0 mole of NaCl unitsIt contains 1.0 mole of Na+ ions and 1.0 mole of Cl- ionsFinding moles from molarity:Liters solution x molarity = moles of solute
12 ExampleWhat is the concentration of each type of ion in the solution 0.50 M Co(NO3)2Solid compound dissolves into Co2+ ions and NO3- ionsCo(NO3)2 (s) - Co2+ (aq) + 2 NO3- (aq)Solution contains 0.50 moles Co2+ ionsSolution contains 1.0 moles NO3- (2*0.50)This is a CRITICAL conceptual idea to remember for future problems
13 How to Make a Solution of Known Concentration Example: Make 1.00 L of M K2Cr2O7. How do you do this?Determine moles of K2Cr2O7 needed:1.00L solution x mol K2Cr2O7/L solution = mol K2Cr2O7Convert moles K2Cr2O7 grams0.200 mol K2Cr2O7 x g K2Cr2O7/mol K2Cr2O7 = 58.8g K2Cr2O7Measure out 58.8g K2Cr2O7. Transfer it to a 1.00L volumetric flask. Add distilled water to the mark on the flask.
14 DilutionDilution: Adding water to a prepared (or stock) solution in order to achieve a desired molarity.Key: Moles of solute after dilution = moles of solute before solutionM1V1 = M2V2Proper Procedure:Use measuring or volumetric pipettes to accurately measure soluteMeasuring pipette – has graduated linesVolumetric pipette – has ONE measurement to fill to
15 Dilution ProcedureHow to make 500mL or 1.00M acetic acid from a 17.4M stock solutionCalculate volume of stock solution needed:Figure out moles of acetic acid:500mL solution x 1L solution/1000mL Solution x 1.00 mole HC2H3O2 = mole HC2H3O2V = mol HC2H3O2 /17.4mol HC2H3O2 /1L solution = L or 28.7mL of solution500 mL of 1M solution – 28.7mL HC2H3O2 = 471.3mL H2O measured into flaskAdd 28.7mL HC2H3O2 to H2O
16 Types of Chemical Reactions Last Year, you learned:Single replacementDouble replacementcombustion,Acid/BaseNo Longer a Sufficient ConceptWe must expand upon what you know to better understand what is happening in a reaction
17 New Types of Reactions New Categories of Reactions PrecipitationAcid/BaseOxidation-ReductionVirtually all reactions can fit into these classifications
18 Precipitation Reactions A precipitation reaction forms a solid that falls (precipitates) from the solution.Example:K2CrO4 (aq) + Ba(NO3)2 (aq) products including a yellow solidActually looks more like:2K+ (aq) + CrO42- (aq) + Ba2+ (aq) + 2 NO3 (aq) -- productsHow can they form a yellow solid?
19 Precipitation Reactions Predicting products is very hardActual reaction products must be confirmed experimentally before you can really conclude the reactionPredicting from what we know:Compound must be electrically neutralMust contain both cations and anionsWhat possible combinations exist?K2CrO4, KNO3, BaCrO4, Ba(NO3)2Can’t be K2CrO4 or Ba(NO3)2 – these are reactantsKNO3 will always be soluble so precipitate must be BaCrO4
20 Precipitation Reactions How Do We Know That?Based on Simple Solubility RulesTerms:Soluble – the salt will dissolve in water to a great extentSlightly Soluble = Insoluble – only a tiny, insignificant portion of the salt dissolves in water
21 Simple Solubility Rules Text page 150 – Memorize them!Most nitrates (NO3) are solubleMost salts of alkali metals and ammonium ions are solubleMost Chloride, Bromide, and Iodide salts are soluble, EXCEPT Silver, Lead, MercuryMost Sulfates are soluble, EXCEPT Barium, Lead, Mercury, and CalciumMost Hydroxide salts are slightly soluble, EXCEPT Sodium and Potassium which are highly soluble. Barium, Tin, and Calcium are marginally solubleMost Sulfide, carbonates, chromates, and phosphates are only slightly soluble
22 Describing Solution Reactions Convert the Formula Equation to Complete Ionic EquationList all the ions on both sidesSolids (precipitates) are not ionsAll strong electrolytes are shown as ions in (aq)This will reveal that some ions do not participate in the reaction and are spectator ionsBe Able to Identify spectator IonsCreate a Net Ionic EquationRe-write the complete ionic equation, but remove the spectator ions from both sides
24 Predict Products of Reactions: CaCl2(aq) + 2Ag2SO4(aq) ??H2SO4 + Na2CO3 ??Na2CrO4 + AgNO3 ??Write the total ionic equation for the reaction of hydrofluoric acid with potassium hydroxide.When aqueous solutions of iron(III) sulfate (Fe2(SO4)3) and sodium hydroxide were mixed, a precipitate formed. What is the precipitate?
25 Stoichiometry for Solution Reactions Identify all the species (ions or compounds) present in the reaction and determine what reaction occursWrite the balance NET IONIC EquationCalculate Moles of ReactantsDetermine Limiting ReactantCalculate Moles of Product or productsConvert to grams or other units
26 Acid/Base Reactions Arrhenius Acids: H+ ions = AcidOH- ions = baseRefinement of Concept by Bronsted and Lowry:Acid is a proton donorBase is a proton acceptor
27 Acid/Base Reactions What’s the Difference? Example: What does a H+ ion look like?A bare protonBut you can have bases that are not OH-Example:KOH (aq) + HC2H3O2 (aq) ??K+ (aq) + OH- (aq) + HC2H3O2 (aq) are the species present before any reaction occursA precipitation reaction could occur between K+ and OH- but KOH is solubleOr is there another possible proton donor to OH-?Weak Acid – does not ionize
28 Acid/Base Reactions YES: HC2H3O2 molecules Hydroxide ion is such a strong base that for purposes of stoichiometric equations, it can be assumed to react completely with any weak acid encounteredActual net ionic equation is:OH- + HC2H3O2 H2O + C2H3O2-Acid/Base reactions are called neutralization reactions
29 Acid/Base Reaction Calculations List species present in combined solution BEFORE any reaction occursWrite a balance NET ionic equationCalculate moles of reactants using volumes and molaritiesDetermine limiting reactant where appropriateCalculate moles of required reactant or productConvert to grams or volume as required
30 Acid/Base Titrations Titration is a volumetric analysis: Uses a buretUses volume of a KNOWN solution (titrant)Delivered into an unknown solution (analyte)When titrant added is exactly reacted with analyte you have the equivalence point or stoichiometric pointEquivalence point is marked with an indicatorWhen the indicator changes color, you have reached the endpoint of the titration
31 Acid/Base TitrationsOnce you have reached the ENDPOINT, it is a stoichiometry problem.How much standard solution (titrant) was used?How many molesWhat was the reaction?How many moles of analyte was neutralized?What volume of analyte was neutralized?Calculate molarity of the analyte.Review Sample Exercise 4.15
32 ReDox in Acidic Solutions Write separate equations for the half-reactionsFor each half-reaction:Balance all elements except H and OBalance O with waterBalance H using H+Balance charge using electronsMultiply half-reaction by integer to equalize electron totalsAdd half-reactions and cancel identical speciesCheck that elements & Charges are balanced
33 ReDox in Basic Solutions Write separate equations for the half-reactionsFor each half-reaction:Balance all elements except H and OBalance O with waterBalance H using H+Balance charge using electronsTo both sides of the equation, add OH- ions to equal H+ ionsForm H2O and eliminate from both sidesMultiply half-reaction by integer to equalize electron totalsAdd half-reactions and cancel identical speciesCheck that elements & Charges are balanced
34 ReDox in Basic Solutions Book: Page 177, Example 4.20As(s) + CN-(aq) + O2(g) Ag(CN)-2(aq)Balance oxidation ½ reaction firstBalance as if H+ ions were present. Balance C and N first2CN-(aq) + Ag(s) Ag(CN)-2(aq)Balance the charge2CN-(aq) + Ag(s) Ag(CN)-2(aq) + e-Balance the reduction ½ reaction (O2)O2(g) 2H2O(l)
35 ReDox in Basic Solutions O2(g) 2H2O(l)Balance the Hydrogen with H+O2(g) + 4H+(aq) 2H2O(l)Balance the Charge4e- + O2(g) + 4H+(aq) 2H2O(l)Multiply Balanced oxidation ½ reaction by 44(2CN-(aq) + Ag(s) Ag(CN)-2(aq) + e-)8CN-(aq) + 4Ag(s) 4Ag(CN)-2(aq) + 4e-
36 ReDox in Basic Solutions Add the ½ reactions and cancel identical species:8CN-(aq) + 4Ag(s) 4Ag(CN)-2(aq) + 4e-4e- + O2(g) + 4H+(aq) 2H2O(l)4e- + O2(g) + 4H+(aq) 8CN-(aq) + 4Ag(s) 4Ag(CN)-2(aq) + 4e- + 2H2O(l)
37 ReDox in Basic Solutions Add OH- to both sides to cancel the H+ ions:O2(g) + 4H+(aq) 8CN-(aq) + 4Ag(s) 4Ag(CN)-2(aq) + 2H2O(l)+ 4OH OH-Eliminate water molecules formed:O2(g) + 4H2O(l) 8CN-(aq) + 4Ag(s) 4Ag(CN)-2(aq) + 2H2O(l) +4OH--2H2O H2OFinal Balanced ReDox Reaction:O2(g) + 2H2O(l) 8CN-(aq) + 4Ag(s) 4Ag(CN)-2(aq) +4OH-Double-Check to see that elements balance, charges balance.