Presentation on theme: "Ch. 4 Patterns of Chemical Reactivity"— Presentation transcript:
1Ch. 4 Patterns of Chemical Reactivity Demo: Pour together two clear colorless liquidsDid a chemical reaction occur? How do you know?Demo: AlkaSeltzer in water, or calcium in waterDid a chemical reaction occur?04m13vd1
2Observing and Predicting Reactions How do we know whether a reaction occurs? What clues does nature offer? Make a list.Review photos of reactionsGo to next topic
3What clues does nature offer that a chemical reaction occurred? pptcrystalcolorgasfumessmoketemperatureflamesmagneticsoundlightsolid decompexplosionsolid dissol.odorelect. cond.pH changedensityelectrictyyGo to next topic
22Generate ElectricityChemical reaction in the batteryreturn
234.4 Observing and Predicting Reaction Patterns Predictions:do an experimentuse periodicityuse classifications of reactionsexample: combustion reactions involve the reaction of an element or a compound with oxygen, usually with the evolution of heat
24Reaction Classifications In the following particulate representations, a circle represents an atom and different circles represent different elements. Using these representations, draw pictures of all the different types of atomic/molecular changes these substances could undergo.DFCBBEA
29Reactions in Solution Precipitation Reactions compound 1 + compound 2 compound 3 + compound 4Also called double replacement or metathesis reactions.exchange of ionic partners AB + CD AD + CBPb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)And other related reactions04m10vd1
31Precipitation Reactions Precipitation reactions: (An example of a "double replacement" or "metathesis" reaction).Precipitation Reactions form a solid when two solutions are combined.An example is the combining aqueous potassium chromate with aqueous lead nitrate to form the precipitate lead chromate (still used in school bus paint!!)
32Describing Reactions in Solution To identify the precipitate or predict the formation of a precipitate the solubilities of compounds can be used. These rules should already be memorized!Table 4.1, pg 144
33Solubility Principles Most nitrate and acetate salts are soluble.Most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+) are soluble.Most chloride, bromide and iodide salts are soluble. Notable exceptions are salts containing the ions Ag+, Pb2+ and Hg22+.
34Solubility Principles Most sulfate salts are soluble. Notable exceptions are BaSO4, PbSO4, HgSO4 and CaSO4.Most hydroxide salts are only slightly soluble. Important soluble hydroxides are NaOH and KOH. Ca(OH)2, Sr(OH)2, and Ba(OH)2 are somewhat soluble*.Most sulfide, carbonate, chromate, and phosphate salts are only slightly soluble**.* Note Group 2 trends : As you go down the group sulfate solubility decreases and hydroxide solubility increases.** Slightly soluble compounds will form precipitates using "normal" concentrations.
35Describing Reactions in Solution For reactions involving ionic compounds, we can write the reaction as a molecular equation (or formula equation). This shows the normal (complete) formulas of all compounds:Example:K2CrO4(aq) + Pb(NO3)2(aq) PbCrO4(s) KNO3(aq)
36Describing Reactions in Solution We can rewrite the same reaction as a complete ionic equation - Shows a picture of all that actually occurs in solutionstrong electrolytes represented as ions in solutionweak and non- electrolytes still written in molecular (non-ionized) aqeuous state.Example:2K+(aq) + CrO42-(aq) + Pb2+(aq) NO3-(aq) PbCrO4(s) K+(aq) + 2 NO3-(aq)
37Describing Reactions in Solution A net ionic equation includes only the solution components involved in the reaction (spectator ions, which do not undergo change, are omitted)Pb2+(aq) + CrO42-(aq) PbCrO4(s)
38StoichiometryStoichiometry of Precipitation Reactions – based on Chapter 3 stoichiometry concepts, but using molarity (concentration) relationships.Practice with Chapter 3 & Molarity!Sample: How many grams of lead(II) hydroxide can be formed when 22.5 mL of M Pb(NO3)2 solution reacts with excess sodium hydroxide? (Hint: Use a BCA table).
39Acid-Base Reactions Definitions: Arrhenius: Brønsted-Lowry : Acid - forms H+ ions in solution (e.g HCl)Base - forms OH- ions in solution (e.g. NaOH)Brønsted-Lowry :Acid - proton (H+) donor (e.g. HCl)Base - proton acceptore.g. NH3: NH3 + H+ NH4+
41Acid-Base TitrationsAcid-base titrations (volumetric analysis) – determine an unknown quantity through titration.Titration involves adding a precisely measured volume of a solution of known concentration (the titrant) into a solution containing the substance being analyzed (the analyte).The titrant reacts with the analyte in a known manner, such as an acid-base reaction.
42Acid-Base TitrationsAn indicator marks the equivalence point (or stoichiometric point) where just the right amount of titrant has been added to completely react with the analyte.The endpoint is where the indicator actually changes color, which hopefully occurs near the equivalence point.
43Acid-Base Reactions Note the similarities to precipitation reactions. Acid-Base reactions are another variation of a double replacement reaction. The key is the production of water.Other common double replacement reactions produce gases.
44Acid-Base Stoichiometry There are numerous variations on the acid-base reaction. Be sure to read through the many examples in Section We will consider these examples now from a “modeling” perspective.
45Acid-Base ReactionsYou first want to examine the acid-base reaction (similar to predicting a precipitation reaction). Here are some general steps (they can and should vary depending on the problem):1. List the major species present in solution before the reaction occurs. Decide what reaction will occur (look for formation of water or gases)2. Write a balanced equation. (leave space for a BCA table)3. Calculate the moles of reactants. For solutions, use the volumes of the original solutions and their molarities (before mixing). Input into a BCA table.
46Acid-Base Reactions 4. Determine the limiting reactant if appropriate. 5. Analyze the problem and find the moles of reactant or product asked for.6. Convert to grams or volume of solution if asked for*All problems are different. Don’t “force” a problem into a particular solution method.
47Reaction Classes Combination Reactions 03m10an1element + element compoundmetal + nonmetal ionic compound2Na(s) + Cl2(g) 2NaCl(s)nonmetal + nonmetal covalent cmpd2H2(g) + O2(g) 2H2O(l)Draw a molecular diagram of this type of reaction01m11vd1
52Reaction Classes Decomposition Reactions Compound 2 elements or element + compound or 2 compoundsOxides, peroxides O2Nitrates NO2 or NO2-Carbonates CO2Hydrates H2OAmmonium salts NH3Draw a molecular diagram of this type of reaction04m03an1CHMVID06
56Reaction Classes Single-Displacement Reactions element + cmpd cmpd + element (The more metallic element in the compound is displaced.)carbon + metal oxides3C + Fe2O3 3CO + 2Femetals + waterCa(s) + 2H2O(aq) Ca(OH)2(aq) + H2(g)
62Reaction Classes Single-Displacement Reactions nonmetals + saltsCl2(aq) + 2KI(aq) 2KCl(aq) + I2(aq)What do all these types of reactions have in common???
63Oxidation-Reduction Reactions reactions in which electrons are transferredcauses a change in the charge of an ion or of oxidation state of an element in a moleculeOxidation states - numbers assigned to elementsused to keep track of electrons (not the same as charge, but related)
64Rules for Assigning Oxidation States (Table 4.2) The oxidation state of an uncombined element is zero (includes diatomic elements H2,N2, O2, F2, Cl2, Br2 and I2).The oxidation state of a monatomic ion is the same as its charge (e.g. the sulfide ion, S2-, has an oxidation state of -2).
65Rules for Assigning Oxidation States (Table 4.2) Oxygen has an oxidation state of -2 in covalent compounds (except in peroxides (O22-) where each oxygen is assigned an oxidation state of -1).In covalent compounds hydrogen is assigned an oxidation state of +1. (Hydrogen has a -1 charge in hydrides such as lithium hydride (LiH) or sodium hydride (NaH).
66Rules for Assigning Oxidation States (Table 4.2) In compounds, fluorine always has an oxidation state of -1.The sum of the oxidation states of the elements in a neutral compound must equal zero.The sum of the oxidation states of the elements in a polyatomic ion must equal the charge on the polyatomic ion.
67Rules for Assigning Oxidation States (Table 4.2) Oxidation states may be non-integers. For example in iron (III) oxide (Fe3O4), the iron has an oxidation state of 8/3 (eight-thirds).
68Rules for assigning oxidation states Practice: Identify the oxidation state of each atom in the following compounds:Magnesium nitrateLithium nitrideSodium nitrite
69Characteristics of Redox Reactions Oxidationa loss of electronsan increase in oxidation statethe substance oxidized is the reducing agent (gives electrons to another substance)Reductiona gaining of electronsa decrease in oxidation statethe substance reduced is the oxidizing agent (takes electrons away from another substance)
70Balancing Redox Reactions By the half-reaction method :In acidic solution1. Write separate oxidation and reduction reactions for the reaction.2. For each half reaction :balance all the elements except hydrogen and oxygenbalance oxygen atoms using H2Obalance hydrogen atoms using H+balance the charge using electrons
71Balancing Redox Reactions 3. If necessary, balance electrons lost and gained in each half reaction by multiplying one or both half reactions by an integer.4. Add the half-reactions and cancel out like species.5. Check to make sure charges and elements are balanced.
72Example 4.19Potassium dichromate is a bright orange compound that can be reduced to a blue-violet solution of chromium(III) ions. In acidic conditions, potassium dichromate reacts with ethyl alcohol as follows:Cr2O72-(aq) + C2H5OH(l) Cr3+(aq) + CO2(g) + H2O(l)Balance this equation using the half reaction method.
73Balancing Redox Reactions In Basic solution (see example 4.20):1. Balance as in an acidic solution (see above).2. Add a number of OH- ions equal to the H+ ions present to both sides of each half reaction to for H2O.3. Eliminate the number of H2O molecules that appear on both sides of the equation.4. Check to make sure charges and elements are balanced.
74Group (Partner) Quiz1. Give the oxidation state of each element in sodium chlorate (NaClO3)2. In the following reaction, identify the oxidizing agent, the reducing agent, the substance being oxidized, and the substance being reducedBr – (aq) + MnO4-(aq) Br2(l) + Mn2+(aq)3. Balance the above RedOx reaction that occurs in acidic solution.