Presentation on theme: "WRITING AP EQUATIONS AP equation sets are found in the free-response section of the AP test. You are given three sets of reactants and you must write."— Presentation transcript:
1WRITING AP EQUATIONS AP equation sets are found in the free-response section of the AP test. You are given three sets of reactants and you must write balanced net ionic equations for the reaction that would occur. The equations are of mixed types. You will also answer a short question about each reaction. The section is worth 15 points and is 15 % of the free response grade. Free response is 50% of the total AP test grade.
2All AP equations "work". In each case, a reaction will occur All AP equations "work". In each case, a reaction will occur. These equations need to be written in net ionic form. All spectator ions must be left out and all ions must be written in ionic form. All molecular substances and insoluble compounds must be written together (not ionized!). Know your solubility rules!!!
3Ca(OH)2 and Sr(OH)2 are moderately soluble and can be written together or as ions. Ba(OH)2 is soluble and Mg(OH)2 is insoluble. CaSO4 and SrSO4 are moderately soluble and can be written together or as ions. BaSO4 is insoluble and MgSO4 is soluble.
4Weak electrolytes, such as acetic acid, are not ionized Weak electrolytes, such as acetic acid, are not ionized. Solids and pure liquids are written together, also. A saturated solution is written in ionic form while a suspension is written together.
5Each reaction will usually be worth a total of 5 points. General guidelines (may not always be true!): One point is given for the correct reactants and two points for all correct products. If a reaction has three products, one point is given for two correct products and two points for all correct products. One point will be given for correct balancing and the final point will be given for correctly and completely answering the question concerning the reaction.
6The best way to prepare for the equation section of the AP test is to practice lots of equations. The equation sets are similar and some equations show up year after year. Save the reactions that you write and practice them again before the AP test in May.
7When you are reading an equation, first try to classify it by type When you are reading an equation, first try to classify it by type. If it says anything about acidic or basic solution, it is redox. If you are totally stuck, look up the compounds in the index of your book or other reference books and try to find information that will help you with the equation. All reactions do not fit neatly into the five types of reactions that you learned in Chemistry I.
9Two compounds react to form two new compounds Two compounds react to form two new compounds. No changes in oxidation numbers occur. All double replacement reactions must have a "driving force" that removes a pair of ions from solution.
10Formation of a precipitate: A precipitate is an insoluble substance formed by the reaction of two aqueous substances. Two ions bond together so strongly that water can not pull them apart. You must know your solubility rules to write these net ionic equations!
19Reaction where one element displaces another in a compound Reaction where one element displaces another in a compound. One element is oxidized and another is reduced A + BC B + AC
20Active metals replace less active metals or hydrogen from their compounds in aqueous solution. Use an activity series or a reduction potential table to determine activity. The more easily oxidized metal replaces the less easily oxidized metal. The metal with the most negative reduction potential will be the most active.
21Ex. Magnesium turnings are added to a solution of iron(III) chloride. 3Mg(s) + 2FeCl3(aq) 2Fe(s)+3MgCl2(aq)3Mg + 2Fe3+ + 6Cl- 2Fe + 3Mg2+ + 6Cl-3Mg + 2Fe3+ 2Fe + 3Mg2+Always make sure that charge is balanced, as well as mass (atoms)!
22Ex. Sodium is added to water. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)2Na + 2H2O 2Na+ + 2OH- + H2Alkali metal demo
23Active nonmetals replace less active nonmetals from their compounds in aqueous solution. Each halogen will displace less electronegative (heavier) halogens from their binary salts.
24Ex. Chlorine gas is bubbled into a solution of potassium iodide. Cl2(g) + 2KI(aq) 2KCl(aq) + I2(s)Cl2 + 2K+ + 2I- 2K+ + 2Cl- + I2Cl2 + 2I- I2 + 2Cl-
26Anhydride means "without water" Anhydride means "without water". Water is a reactant in each of these equations.
27Nonmetallic oxides (acidic anhydrides) plus water yield acids.
28Ex. Carbon dioxide is bubbled into water. CO2 + H2O H2CO3
29Metallic oxides (basic anhydrides) plus water yield bases.
30Ex. Solid sodium oxide is added to water. Na2O + H2O 2Na+ + 2OH-
31Metallic hydrides (ionic hydrides) plus water yield metallic hydroxides and hydrogen gas.
32Ex. Solid sodium hydride is added to water. NaH + H2O Na+ + OH- + H2
33Phosphorus halides react with water to produce an acid of phosphorus (phosphorous acid or phosphoric acid) and a hydrohalic acid. The oxidation number of the phosphorus remains the same in both compounds. Phosphorus oxytrichloride reacts with water to make the same products.
34Ex. Phosphorus tribromide is added to water. PBr3 + 3H2O H3PO3 + 3H++ 3Br-
35Group I&II nitrides react with water to produce the metallic hydroxide and ammonia.
36Amines react with water to produce alkylammonium ions and hydroxide ions.
37Ex. Methylamine gas is bubbled into distilled water. CH3NH2 + H2O CH3NH3+ + OH-
39Redox reactions involve the transfer of electrons Redox reactions involve the transfer of electrons. The oxidation numbers of at least two elements must change. Single replacement, some combination and some decomposition reactions are redox reactions.
40To predict the products of a redox reaction, look at the reagents given to see if there is both an oxidizing agent and a reducing agent. When a problem mentions an acidic or basic solution, it is probably redox.
41Common oxidizing agents Products formed MnO4- in acidic solution Common oxidizing agents Products formed MnO4- in acidic solution Mn2+ MnO2 in acidic solution Mn2+ MnO4- in neutral or basic solution MnO2(s) Cr2O72- in acidic solution Cr3+ HNO3, concentrated NO2 HNO3, dilute NO H2SO4, hot, concentrated SO2
59Complex ion Name Formed from: [Al(OH)4]- tetrahydroxoaluminate ion (Al or Al(OH)3 or Al3++ OH-) [Ag(NH3)2]+ diamminesilver(I) ion (Ag+ + NH3) [Zn(OH)4] tetrahydroxozincate ion (Zn(OH)2 + OH-) [Zn(NH3)4] 2+ tetramminezinc ion (Zn2+ + NH3) [Cu(NH3)4]2+ tetramminecopper(II) ion (Cu2+ + NH3) [Cd(NH3)4] 2+ tetramminecadmium(II) ion (Cd2++ NH3) [FeSCN] thiocyanoiron(III) ion (Fe3+ + SCN-) [Ag(CN)2]- dicyanoargentate(I) ion (Ag+ and CN-) [Ag(Cl)2] dichloroargentate(I) ion (AgCl and Cl-)
60Adding an acid to a complex ion will break it up Adding an acid to a complex ion will break it up. If HCl is added to a silver complex, AgCl(s) is formed. If an acid is added to an ammonia complex, NH4+ is formed.
61Ex. Excess ammonia is added to a solution of zinc nitrate. 4NH3 + Zn2+ [Zn(NH3)4]2+(Other coordination numbers are acceptable, as long as correct charge is given.)
62[Ag(NH3)2]+ + Cl- + 2H+ AgCl + 2NH4+ Ex. A solution of diamminesilver(I) chloride is treated with dilute nitric acid.[Ag(NH3)2]+ + Cl- + 2H+ AgCl + 2NH4+
64Reaction where a compound breaks down into two or more elements or compounds. Heat, electrolysis, or a catalyst is usually necessary.
65A compound may break down to produce two elements. Ex A compound may break down to produce two elements. Ex. Molten sodium chloride is electrolyzed.2NaCl(l) 2Na + Cl2
66A compound may break down to produce an element and a compound. Ex A compound may break down to produce an element and a compound. Ex. A solution of hydrogen peroxide is decomposed catalytically. 2H2O2 2H2O + O2
67A compound may break down to produce two compounds. Ex A compound may break down to produce two compounds. Ex. Solid magnesium carbonate is heated MgCO3 MgO + CO2
68Metallic carbonates break down to yield metallic oxides and carbon dioxide.
69Metallic chlorates break down to yield metallic chlorides and oxygen.
70Hydrogen peroxide decomposes into water and oxygen.
71Ammonium carbonate decomposes into ammonia, water and carbon dioxide.
72Sulfurous acid decomposes into water and sulfur dioxide.
73Carbonic acid decomposes into water and carbon dioxide.
75Two or more elements or compounds combine to form a single product.
76A Group IA or IIA metal may combine with a nonmetal to make a salt.
77A piece of lithium metal is dropped into a container of nitrogen gas. 6Li + N2 2Li3N
78Two nonmetals may combine to form a molecular compound Two nonmetals may combine to form a molecular compound. The oxidation number of the less electronegative element is often variable depending upon conditions. Generally, a higher oxidation state of one nonmetal is obtained when reacting with an excess of the other nonmetal.
87Acids react with bases to produce salts and water Acids react with bases to produce salts and water. One mole of hydrogen ions react with one mole of hydroxide ions to produce one mole of water.
88Watch out for information about quantities of each reactant Watch out for information about quantities of each reactant! Remember which acids are strong (ionize completely) and which are weak (write as molecule).
89Sulfuric acid (strong acid) can be written as H+ and SO42- or as H+ and HSO4-. Concentrated sulfuric acid is left together because there is not enough water present for it to ionize.
90H+ + HSO42 - + Ba2+ + 2OH- BaSO4 + 2H2O Ex. A solution of sulfuric acid is added to a solution of barium hydroxide until the same number of moles of each compound has been added.H+ + HSO Ba2+ + 2OH- BaSO4 + 2H2O
91(S2- does not exist in water) Ex. Hydrogen sulfide gas is bubbled through excess potassium hydroxide solution.H2S + OH- H2O + HS-(S2- does not exist in water)
92Watch out for substances that react with water before reacting with an acid or a base. These are two step reactions.
93H2SO3 and 1 H2O cancel. SO2 + Ca2++ 2OH- CaSO3 + H2O Sulfur dioxide gas is bubbled into an excess of a saturated solution of calcium hydroxide.2-step reaction!SO2 + H2O H2SO3H2SO3 + Ca2+ +2OH-2H2O + CaSO3H2SO3 and 1 H2O cancel.SO2 + Ca2++ 2OH- CaSO3 + H2OAdd steps together!