Unformatted text preview: 1/19/2009 Order changed slightly
We We will talk about types of reactions before talking about solutions Chapter 4
Types of reactions, solution chemistry and REDOX Simple chemical reactions
Combination A + B AB AB Decomposition AB A+B Combustion CxHy + “Z” O2 XCO XCO2 + Y/2 H2O Single Replacement redox A + BC AC + B BC AC Double replacement AB + CD AD AD + CB precipitation neutralization unstable products Balancing reactions
The The number of atoms of each type need to “add up” on both sides of the reaction. At At this point, we do it by examination. C6H12(l) + O2(g) CO CO2(g) + H2O(g) Start Start with the largest molecule, define as one, use “if” “then” on the other side, then finish. Combination:
Elements Elements can directly combine to form compounds C + O2 CO CO2 Compounds Compounds can combine to form other compounds: Plaster Plaster of Paris CaSO4*.5H2O + H2O CaSO CaSO4*2H2O CaO CaO + CO2 CaCO CaCO3 These These may be in solution but often are direct combinations. Decomposition:
It It gets smaller……. Compounds Compounds break down to other compounds or elements: Second Second term: Organics can break down to CO2 and H2O by natural agents Water Water breaks down to hydrogen and oxygen when electricity is applied. Liquid Liquid NaCl breaks down to Na and Cl2 CaCO CaCO3 + heat CaO CaO + CO2 1 1/19/2009 Decomposition
2 H2O 2NaCl 2NaCl 2 H2 + O2 2Na 2Na(s) + Cl2 (g) Combustion
Organic Organic + oxygen gives CO2 + H2O Key Key trick to balance: CxHy CxHy + “Z” O2 X CO2 + y/2 H2O CO Z = (2X + y/2)/2 (2X The The organic can contain oxygen, nitrogen or sulfur (or other elements), which end up as an oxide. Metal Reactivity Single Replacement Redox
An An element reacts, in solution, with a compound to replace the cation in the compound. A + BC BC AC + B AC How How do we know it happens? Look at the reactivity table MOST MOST Reactive Lithium Lithium (Li) Potassium Potassium (K) Barium Barium (Ba) Calcium Calcium (Ca) Sodium Sodium (Na) Magnesium Magnesium (Mg) Aluminum Aluminum (Al) Manganese Manganese (Mn Mn2+) Mn2+) Zinc Zinc (Zn) Chromium Chromium (Cr Cr 3+ Cr 3+ ) Iron Iron (Fe Fe Fe2+) Cadmium Cadmium (Cd) Cobalt Cobalt (Co Co 2+ Co 2+) Tin Tin (Sn Nickel Nickel (Ni Sn 2+ Sn 2+) Ni 2+ Ni 2+) Lead Lead (Pb Pb 2+ Pb 2+) Hydrogen Hydrogen (H2) Copper Copper (Cu Silver Silver (Ag) Mercury Mercury (Hg Platinum (Pt Cu 2+ Cu 2+) Hg Hg +) Pt 2+ Pt 2+) Gold Gold (Au Au 3+ Au 3+) LEAST LEAST Reactive Note:
Note: Note: All reactivities are for the most common ion formed. In the cases of elements that can form more than one ion, the ion indicated is the most common ion. Reactivity Reactivity of a nonmetal: Fluorine is most reactive, oxygen, chlorine….less Double Replacement ppt, neutralization, gas forming
You You must specify the driving force (what makes the reaction go forward. It It can be one of three reasons: Formation of a precipitate Neutralization Neutralization Formation Formation of an unstable (gas) product 2 1/19/2009 How do we know what’s soluble?
Anions Soluble (aq) Precipitate (s)
nitrate `Most cations No common cations Acetate Most cations Ag+ Fluoride chloride bromide iodide Most cations Ag+, Pb2+,4+, Hg22+, Tl+ sulfate Most cations Ba2+, Sr2+,Pb2+,4+, Ag+,Ca2+ chromate Most cations cations Ba2+, Sr2+, Pb2+,4+, Ag+ Ba Sr Pb Ag sulfide hydroxide oxide NH4+, cations of columns 1 and 2 Most other cations carbonate phosphate NH4+, cations of column 1, except Li+ Most other cations Net ionic equations
Many Many compounds break apart in water Water Water is polar (more later) and can pull ions into solution Strong Strong Electrolyte= fully ionized in solution Weak Weak Electrolyte = barely ionized in solution Nonelectrolyte Nonelectrolyte = compound does not ionize We We use a light bulb: if there are ions in solution, the light goes on. IONIC COMPOUNDS
Compounds in Aqueous Solution
Many reactions involve ionic compounds, especially reactions in water — aqueous solutions.
KMnO4 in water K+(aq) + MnO4-(aq) An Ionic Compound, CuCl2, in Water CCR, page 149 Aqueous Solutions
How do we know ions are present in aqueous solutions? The solutions conduct conduct electricity! They are called They are called ELECTROLYTES ELECTROLYTES HCl, MgCl2, and NaCl are strong strong electrolytes. They dissociate completely (or nearly so) into ions. HCl, MgCl2, and NaCl are strong strong electrolytes. They dissociate completely (or nearly so) into ions. Aqueous Solutions 3 1/19/2009 Aqueous Solutions
Acetic acid ionizes only to a small extent, so it is a weak electrolyte. weak CH3CO2H(aq) ---> CH3CO2-(aq) + H+(aq) ---> Aqueous Solutions
Acetic acid ionizes only to a small extent, so it is a weak weak electrolyte. CH3CO2H(aq) ---> ---> CH3CO2-(aq) + H+(aq) Aqueous Solutions
Some compounds dissolve in water but do not conduct electricity. They are called called nonelectrolytes. nonelectrolytes.
Examples include: sugar ethanol ethylene glycol ol Writing Net Ionic Equations
Molecular Equation: A chemical equation that includes the complete, molecular formulas for all compounds in a chemical reaction. Ionic Equation: A chemical equation in Equation: chemical equation in which which aqueous, ionic compounds are written as free ions in solution. Net Ionic Equation: A chemical equation in which only the species (ions, atoms, molecules) involved in the reaction are written. Spectator ions are cancelled. Consider Consider calcium chloride dissolved in water: CaCl2(s) CaCl2(aq) CaCl2(aq) Ca aq) Ca2+(aq) + 2 Cl-(aq) aq) Consider the Reaction of CaCl2 with AgNO3: Molecular: Molecular:
CaCl2(aq) + 2 AgNO3(aq) Ca(NO3)2(aq) + 2 AgCl(s) Wh di When dissolved in water, aqueous ionic compounds are generally dissociated and solvated by the water molecules. Note: Note: one mole CaCl2 produces two moles Cl-. In solution, the chloride Clconcentration is 2x the Ca2+ concentration Ionic: Ionic:
Ca+2(aq) + 2 Cl-(aq) + 2 Ag+(aq) + 2 NO3-(aq) Ca+2(aq) + 2 NO3-(aq) + 2 AgCl(s) Net Net Ionic:
2 Ag+(aq) + 2 Cl-(aq) 2 AgCl(s) Ag+(aq) + Cl-(aq) AgCl(s) 4 1/19/2009 What happens in the TV crime show at the scene of the crime?
Get Get rid of the spectators so the “good guys” can concentrate on the action. We We do the same thing. Any ion showing up on both sides of the reaction is a spectator…..we get rid of it. The The net ionic equations shows only the species involved in a chemical change. Ca Ca2+ and NO3- are spectator ions. They do not participate in the reaction. Additional Additional Example: Write the molecular, ionic, and net ionic equations for the reaction of magnesium metal with hydrochloric acid. Mg(s)+ HCl(aq) MgCl MgCl2(aq) + H2(g) Note: Note: solids and gases don’t break apart Balance Balance the equation first. Mg Mg(s)+ 2HCl(aq) MgCl2(aq) + H2(g) MgCl Full Full ionic: Mg Mg(s)+ 2H+ 2Cl-(aq) Mg2+ + 2Cl-+ H2(g) Mg Drop Drop the spectators for net ionic: Mg Mg(s)+ 2H+ Mg Mg2+ + H2(g) Solutions & Concentration Concentration Solutions
A solution is a homogeneous mixture solution composed of a solvent and one or more solutes. solutes. A solute is a substance dissolved in solute the solvent. The The solvent is the substance that dissolves the solute or solutes. Note: Note:
Sometimes it is not clear what is the solvent or the solute. The solvent is generally considered to be the most abundant substance. Degrees of Saturation:
Unsaturated Unsaturated – more solute may be dissolved in a solution. Saturated Saturated – the maximum amount of solute solute is dissolved in a solution. Supersaturated Supersaturated – more solute is dissolved in a solution than is stable at that temperature. Aqueous Aqueous means "dissolved in water". 5 1/19/2009 Concentration Concentration is the measure of the amount of solute in a solution (part / whole). Molar concentration, Molar concentration, or molarity is a measure of the moles of a solute in one liter of solution.
Molarity = M = n moles of solute mol solute = = V volume of solution L solution Molarity Calculating Concentration
Example Example #1: What is the concentration of a solution found to contain 0.0834 mol of BaCl2 in a 20.0 mL sample of solution? Example #2: What is the concentration of #2 Wh th sodium sodium hypochlorite solution prepared by dissolving 5.66 g of NaOCl in enough water to make 250.0 mL of solution? Brackets Brackets around a formula indicate the concentration of the substance is being discussed:
[NaCl] means “the molarity of NaCl” Preparing a Solution
To To prepare a solution of known concentration from a solid substance soluble in water:
1. Determine the mass required to make the Determine the mass required to make the desired desired volume of the solution. 2. Dissolve that quantity of solid in the appropriate volumetric flask. Dilutions
When When you have a stock solution of known concentration, you may prepare less concentrated solutions by diluting the stock solution with water. When When a quantity of solution is diluted, the number of moles of solute does not change number of moles of solute does not change, only the total volume, therefore:
mol solute in the concentrated solution = mol solute in the dilute solution Example: Example: An experiment calls for 500.0 mL of 0.333 M solution of NaOH. Describe how to prepare this solution. Because: concentration Because: concentration x volume = mol We We can use the relationship: M1V1 = M2V2 Dilution Examples
1. What volume of a 0.333M NaOH solution is required to make 250.0 mL of a 0.100M NaOH solution? Describe its preparation. 2. 2. What is the concentration of a solution prepared by diluting 2.00 mL of a 0.250M solution of sucrose to 25.0 mL? Compounds Compounds and Ions in solution
Ionic Ionic compounds often dissociate into their ions when dissolved in water. Compounds Compounds that undergo complete (100%) dissociation (like NaCl) are called strong electrolytes electrolytes because their solutions are good electrical conductors. Some Some ionic compounds only partially dissociate in water (like H3PO4) and are called weak electrolytes, because their solutions are poor electrical conductors. 6 1/19/2009 Covalent Covalent compounds (like glucose, C6H12O6) do NOT dissociate in water and are called nonelectrolytes, nonelectrolytes, because their solutions do not conduct electricity.
Note: Note: Pure water (which we rarely actually have) is a very poor electrical conductor. Mass Percent mass/mass, common in industry
5% 5% NaCl solution 5 g NaCl/100g solution
5 g NaCl/5 grams NaCl+95 g H2O) NaCl/5 Concentrated nitric acid 68 (1.41g/mL) Concentrated nitric acid 68 % (1.41g/mL)
68 68 g HNO3/100 g solution or 68g HNO3/ 68g HNO3+32 grams H2O Example: Example: Consider the strong electrolyte, MgCl2. What are the concentrations of Mg2+ ions and Cl- ions in a 0.100 M solution of MgCl2 ? Concentrated Concentrated sulfuric acid 98% (1.84g/mL) Bleach: Bleach: 6% NaClO (1.084g/mL) Mass/volume Common in medicine
Liquid Liquid medicine is delivered by volume, with the grams, or milligrams/unit volume stated Volume/volume
Used Used with liquids in liquids: IPA IPA alcohol is sold as 70% or 70 ml in 100 ml solution Parts per million (ppm) (ppm)
Measured Measured as parts/1x106 Polluntants: Polluntants: lead and mercury Lead: 10>µg/dL Lead: 10>µg/dL 10 micro grams/100 mL From From FDA: Mercury: FDA limit for human consumption (in fish) of ppm human consumption (in fish) of 1 ppm (large tuna/swordfish). The The average concentrations of total mercury in non-exposed people is about non8 parts per billion (ppb) in blood and 2 ppm in hair. http://www.fda.gov/fdac/reprints/mercury. http://www.fda.gov/fdac/reprints/mercury. html Predicting States of Matter
Elements Mercury (Hg) and Bromine (Br2) are the only elemental LIQUIDS at STP (standard temperature and pressure). All Metals (except Hg) and metalloids are Metals (except Hg) and metalloids are SOLIDS in their elemental states at STP. C, P, S, Se, and I2 are SOLID non-metals at STP. The other non-metals are GASES in their elemental forms at STP. 7 1/19/2009 Ionic Compounds
Ionic Ionic compounds are SOLIDS at STP if there is no water around. Ionic Ionic compounds that are soluble in water will will be AQUEOUS if water is available. In In electrolysis decomposition reactions, ionic compounds must be melted (LIQUIDS). Other Other Compounds
Acids Acids will be aqueous. NonNon-metal oxides (eg. CO2, SO3) are generally gases at STP. ll STP Water Water is a liquid (unless at high temperature, as in a combustion). Water Solubility of Ionic Compounds If one ion from the “Soluble Compd.” list is present in a compound, the compound is water soluble. Water Solubility of Ionic Compounds Common minerals are often formed with anions that lead to insolubility: sulfide fluoride carbonate oxide Iron pyrite, a sulfide Azurite, a copper carbonate Orpiment, arsenic sulfide ACIDS
An acid -------> -------> H+ in water ACIDS
An acid -------> H+ in water -------> Some strong acids are strong HCl H2SO4 HClO4 HNO3 hydrochloric sulfuric perchloric HNO3 nitric HCl(aq) ---> H+(aq) + Cl---> (aq) 8 1/19/2009 Weak Acids
WEAK ACIDS = weak electrolytes
CH3CO2H acetic acid acid H2CO3 carbonic acid H3PO4 phosphoric acid HF hydrofluoric acid ACIDS
Nonmetal oxides can be acids CO2(aq) + H2O(liq) ---> ---> H2CO3(aq) SO3(aq) + H2O(liq) O(liq) ---> ---> H2SO4(aq) and can come from burning coal and oil. Acetic acid BASES
Base ---> OH- in water ---> Ammonia, NH3 NaOH(aq) NaOH(aq) OH OH-(aq)
NaOH is a strong base ---> Na+(aq) + --- Na (aq) An Important Base BASES
Metal oxides are bases CaO(s) + H2O(liq) --> --> Ca(OH)2(aq)
CaO in water. Indicator shows solution is basic. Know the strong acids & bases! 9 1/19/2009 Chemical Reactions in Water
Sections 5.2 & 5.4-5.6—CD-ROM Ch. 5 5.4-5.6—CDAX + BY AY + BX Precipitation Reactions
The “driving force” is the formation of an insoluble compound — a precipitate. Pb(NO3)2(aq) + 2 KI(aq) -----> (aq) KI(aq) ----2 KNO3(aq) + PbI2(s) Net ionic equation Pb2+(aq) + 2 I-(aq) ---> PbI2(s) ---> Pb(NO3) 2(aq) + 2 KI(aq) ----> ----> PbI2(s) + 2 KNO3 (aq) We will look at
The anions exchange places between cations. EXCHANGE EXCHANGE REACTIONS or Double replacement reactions AcidAcid-Base Reactions
The The “driving force” is the formation of water. NaOH(aq) + HCl(aq) ---> ---> NaCl(aq) + H2O(liq) Net Net ionic equation OH-(aq) + H+(aq) ---> ---> H2O(liq) This This applies to ALL reactions of STRONG acids and bases. AcidAcid-Base Reactions
A-B reactions are sometimes called NEUTRALIZATIONS NEUTRALIZATIONS because the solution is neither acidic nor basic at the end. The The other product of the A-B reaction is a ASALT SALT, MX. HX + MOH ---> MX + H2O ---> Mn+ comes from base & Xn- comes from acid This is one way to make compounds! GasGas-Forming Reactions
This is primarily the chemistry of metal carbonates. carbonates. CO2 and water ---> H2CO3 ---> H2CO3(aq) + Ca2+ ---> ---> 2 H+(aq) + CaCO3(s) (limestone) Adding acid reverses this reaction. ---> MCO3 + acid ---> CO2 + salt GasGas-Forming Reactions
CaCO3(s) + H2SO4(aq) ---> ---> 2 CaSO4(s) + H2CO3(aq) Carbonic acid is unstable and forms CO2 & H2O H2CO3(aq) ---> CO2 + water ---> (Antacid tablet has citric acid + NaHCO3) 10 1/19/2009 Quantitative Aspects of Reactions in Solution Terminology
In solution we need to define the SOLVENT SOLVENT the component whose the component whose physical physical state is preserved when solution forms SOLUTE SOLUTE the other solution component PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O in enough water to make 250 mL of solution. Calculate molarity.
Step 1: Calculate moles of NiCl2•6H2O
5.00 g • 1 mol mol = 0.0210 mol 237.7 g Step 2: Calculate molarity
0.0210 mol = 0.0841 M 0.250 L [NiCl2•6 H2O ] = 0.0841 M NiCl The Nature of a CuCl2 Solution Ion Concentrations
CuCl2(aq) --> --> Cu2+(aq) + 2 Cl-(aq) USING MOLARITY
What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500 M solution? Because Conc Conc (M) = moles/volume = mol/V this means that If [CuCl2] = 0.30 M, then [Cu2+] = 0.30 M [Cl-] = 2 x 0.30 M moles = M•V 11 1/19/2009 USING MOLARITY
What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500 M solution? Preparing Solutions
Weigh Weigh out a solid solute and dissolve in a given quantity of solvent. solvent. Dilute Dilute a concentrated solution to give one that is less concentrated. moles = M•V Step 1: Calculate moles of acid required. (0.0500 mol/L)(0.250 L) = 0.0125 mol Step 2: Calculate mass of acid required. (0.0125 mol )(90.00 g/mol) = 1.13 g PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do?
H2 O Add water to the 3.0 M solution to lower its concentration to 0.50 M But how much water do we add? Dilute Dilute the solution! 3.0 M NaOH Concentrated 0.50 M NaOH Dilute PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? How much water is added? The important point is that ---> --->
moles of NaOH in ORIGINAL solution = moles of NaOH in FINAL solution PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Amount of NaOH in original solution = M•V = (3.0 mol/L)(0.050 L) = 0.15 mol NaOH Amount of NaOH in final solution must also = 0.15 mol NaOH Volume of final solution = (0.15 mol NaOH)(1 L/0.50 mol) = 0.30 L or 300 mL 12 1/19/2009 PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Conclusion:
H2 O Preparing Solutions by Dilution
A shortcut 3.0 M NaOH Concentrated 0.50 M NaOH Dilute add 250 mL of water to 50.0 mL of 3.0 M NaOH to make 300 mL of 0.50 M NaOH. Minitial • Vinitial = Mfinal • Vfinal SOLUTION SOLUTION STOICHIOMETRY
Section 5.10 Zinc reacts with acids to produce H2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? Zinc Zinc reacts with acids to produce H2 gas. Have Have 10.0 g of Zn What What volume of 2.50 M HCl is needed to convert the Zn completely? Step 1: Write the balanced equation Zn(s) + 2 HCl(aq) --> ZnCl2(aq) + --> H2(g) Step 2: Calculate amount of Zn
10.0 g Zn • 1.00 mol Zn = 0.153 mol Zn 65.39 g Zn Step 3: Use the stoichiometric factor Zinc reacts with acids to produce H2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? ACIDACID-BASE REACTIONS Titrations
H2C2O4(aq) + 2 NaOH(aq) ---> ---> acid base Na2C2O4(aq) + 2 H2O(liq) Carry out this reaction using a TITRATION. TITRATION. Step 3: Use the stoichiometric factor
0.153 mol Zn • 2 mol HCl = 0.306 mol HCl 1 mol Zn Step 4: Calculate volume of HCl req’d
1.00 L 0.306 mol HCl • = 0.122 L HCl 2.50 mol Oxalic acid, H2C2O4 13 1/19/2009 Setup for titrating an acid with a base Titration 1. Add solution from the buret. 2. Reagent (base) reacts with compound (acid) in solution in the flask. 3. Indicator shows when exact stoichiometric reaction has occurred. ti 4. Net ionic equation H+ + OH- --> H2O --> 5. At equivalence point moles H+ = moles OH- CCR, page 186 LAB PROBLEM #1: Standardize a solution of NaOH — i.e., accurately determine its concentration. 1.065 g of H2C2O4 (oxalic acid) requires (oxalic acid) requires 35.62 35.62 mL of NaOH for titration to an equivalence point. What is the concentraconcentration of the NaOH? 1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? Step 1: Calculate amount of H2C2O4
1.065 g • 1 mol = 0.0118 mol 90.04 g Step 2: Calculate amount of NaOH req’d
0.0118 mol acid • 2 mol NaOH = 0.0236 mol NaOH 1 mol acid 1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? Step 1: Calculate amount of H2C2O4 = 0.0118 mol acid Step 2: Calculate amount of NaOH req Step 2: Calculate amount of NaOH req’d = 0.0236 mol NaOH Step 3: Calculate concentration of NaOH
0.0236 mol NaOH = 0.663 M 0.03562 L [NaOH] = 0.663 M LAB PROBLEM #2: Use standardized NaOH to determine the amount of an acid in an unknown. Apples contain malic acid, C4H6O5. C4H6O5(aq) + 2 NaOH(aq) ---> (aq) NaOH(aq) --Na2C4H4O5(aq) + 2 H2O(liq) 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid? 14 1/19/2009 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid?
Step 1: C•V = = Step 2: Calculate amount of NaOH used. (0.663 M)(0.03456 L) 0.0229 mol NaOH Calculate amount of acid titrated.
1 mol acid 2 mol NaOH 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid?
Step 1: = Step 2: = Calculate amount of NaOH used. 0.0229 mol NaOH Calculate amount of acid titrated 0.0115 mol acid 0.0229 mol NaOH • Step 3: Calculate mass of acid titrated.
0.0115 mol acid • 134 g = 1.54 g mol = 0.0115 mol acid 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid? Step 1: = Step 2: Step 2: = Step 3: = Calculate amount of NaOH used. 0.0229 mol NaOH Calculate amount of acid titrated Calculate amount of acid titrated 0.0115 mol acid Calculate mass of acid titrated. 1.54 g acid
1.54 g • 100% = 2.01% 76.80 g Oxidation – Reduction Chemistry
REDOX Step 4: Calculate % malic acid. First Nightmare of Chem 1A You You already can do several redox equations Combination Combination Decomposition Decomposition Single Single Replacement Combustion Combustion There There is another class, which can’t be done “by inspection” We We call this “redox” There are different ways to solve redox problems, we will use the technique of Half Cells
You You are welcome to use whatever way works for you works for you. I will be using the half cell technique, as it will is found to work better. 15 1/19/2009 Balance Balance the following equation by inspection:
(Note: this is a net ionic equation - charge and number of atoms must be balanced.) number Here is the balanced Net Ionic Equation: 16 H+ + 2 Cr2O72- + 1 C2H5OH 4 Cr3+ + __ H+ + __ Cr2O72- + __C2H5OH __ __ Cr3+ + __ H2O + __ CO2 11 H2O + 2 CO2 Party Popper Reaction
A mixture of potassium chlorate, antimony mixture sulfide, and phosphorus. The overall reaction is as follows: __ KClO3 (s) + __Sb2S3 (s) + __ P4 (s) __ __ KCl (s) + __ P4O10 (s) + __ Sb2O3 (s) + __ SO2 (g) How can we solve that????? We break it down into smaller pieces, solve each one separately, than put them back together
Net: Net: 28 28 KClO3 (s) + 6 Sb2S3 (s) + 3 P4 (s) -> 28 KCl (s) + 3 P4O10 (s) + 6 Sb2O3 (s) + 18 SO2 (g) No, No, I will not ask you to solve this one. OxidationOxidation-Reduction Reaction
A reaction in which electrons are reaction transferred from one species to another. Many Many common chemical reactions are REDOX REDOX reactions:
“Rusting” “Rusting” or oxidation of metals Combustion Combustion reactions Single Single replacement reactions Metabolism Metabolism of sugars by our bodies 2 Na(s) + Cl2(g) 2 NaCl(s) NaCl In In this synthesis reaction, electrons are transferred from sodium to chlorine: Two Two Na atoms lose an electron (oxidation):
2 Na Na 2 Na+ + 2 eNa Each Each Cl gains an electron (reduction): Cl2 + 2 e2 ClCl The The above reactions are called halfhalfreactions, as each represents ½ of the actual reaction. 16 1/19/2009 Oxidation Oxidation Numbers are a convenient way to keep track of electrons (changing place) in REDOX reactions. Oxidation Oxidation numbers are SIMILAR to charge, but they are NOT the same thing. We can but they are NOT the same thing. We can call call them “formal charges”-a way to keep charges”track of what changes. Oxidation Oxidation numbers may be assigned to elements in compounds in which electrons are shared – not just to charged species. Oxidation States (Numbers) Assigning Oxidation Numbers
1. The oxidation number of atoms in their elemental form is zero. The oxidation state of monatomic ions is the same as the charge. The oxidation state of fluorine is always 1 in its compounds. The oxidation state of other halogens (Cl, Br, I) is -1 unless combined with O, F, or a more reactive halogen. In these cases assign using rule #7. 2. 3. 4. Assigning Oxidation Numbers (cont.)
5. Assigning Oxidation Numbers (cont.)
7. Oxygen is almost always assigned an oxidation number of -2 in its compounds. (Exceptions: OF2, peroxides and superoxides) peroxides and superoxides) The oxidation state of Hydrogen in compounds is usually +1 unless it is a metal hydride = -1 All other oxidation numbers may be assigned by the following principles:
The The sum of the oxidation states in a neutral compound must equal zero neutral compound must equal zero. The The sum of the oxidation states in a polyatomic ion must equal the charge on the ion. 6. CH4 + 2 O2 CO CO2 + 2 H2O In In the above reaction, assign oxidation numbers to all elements. Carbon is OXIDIZED. Its oxidation number is being increased. Carbon is losing electrons: increased Carbon electrons: Oxygen Oxygen is REDUCED. Its oxidation number is being reduced Oxygen being reduced. Oxygen is gaining electrons: Wait Wait a minute! Why is gaining electrons “reduced”? Metals Metals oxidize to metal oxides. Metal ores (oxides, sulfides) are reduced to their metals in the manufacture of metals. We keep the same words But……. Watch out
LEO LEO = Losing Electrons Oxidation= Reducing agent GER GER = Gaining Electrons Reduction = oxidizing oxidizing agent Oxygen Oxygen gains electrons (reduction) and causes metals to be oxidized=oxidizing agent 17 1/19/2009 SiCl SiCl4 + 2 Mg 2 MgCl2 + Si MgCl Al + MnO4- MnO MnO2 + Al(OH)3 In In the above reaction, identify the atoms that are oxidized and reduced. Si Si4+ Si Si Mg Mg ClMg2+ Cl- doesn’t change The The species (atom, ion, or molecule) that is oxidized is called the reducing agent. The The species that is reduced is called the oxidizing agent. Identify Identify the oxidizing and reducing agents in the above reaction. Consider the above, UNBALANCED redox reaction. What What is being oxidized? What is being reduced? What is being reduced? What What is the oxidizing agent? What What is the reducing agent? Balancing Redox Equations (cont.) Balancing Redox Equations:
The Method of ½ - Reactions Balance Balance the reaction: MnO4- + Fe+2 Fe Fe Fe+3 + Mn+2 Mn Step Step 2 Separate into ½-reactions. ½Oxidation: Reduction: Step 3 Balance the ½-reactions for mass (# of th reac (# atoms). atoms).
1) in acidic (H+) aqueous solution (H2O). Step Step 1 Identify as an oxidation-reduction reaction. oxidationAssign oxidation numbers. 2) 3) Balance all atoms except H or O (unless they are the oxidized or reduced species). Balance O with waters (H2O). Balance H with acid (H+ ions). Balancing Redox Equations (cont.) Step Step 4 Balance ½-reactions for charge ½using electrons. (e- is a product in oxidation reactions and a reactant in reduction reactions.) Step 5 Multiply ½-reactions by appropriate reactions by appropriate factors factors to equalize electrons. Step 6 Add ½-reactions together to get the ½overall reaction. Step 7 CHECK to see that reaction is balanced for charge and # of atoms. Balance Balance the following Reactions:
1. Cu(s) + NO3-(aq) Cu Cu2+(aq) + NO(g) in acidic aqueous solution. 2. Cr2O72-(aq) + C2H5OH(aq) CO2(g) Cr Cr3+(aq) + in acidic aqueous solution. 18 1/19/2009 Disproportionation
An oxidationAn oxidation-reduction reaction in which the reactant is both oxidized and reduced. Balance the following redox reaction in acidic Balance the following redox reaction in acidic aqueous aqueous solution:
1) 2) Balancing redox reactions in BASIC (OH-) aqueous (H2O) solution: Balance as if in ACIDIC solution. Add OH- ions equal to the number of H+ ions to BOTH SIDES of the equation. 3) Combine OH- and H+ ions on the same side into waters Cancel waters waters. Cancel waters. 4) Check balancing for charge and # of atoms. Balance the following reactions in BASIC solution.
a) Cl2(g) Cl Cl-(aq) + ClO2-(aq) NO2-(aq) + Al(s) NH NH3(g) + AlO2-(aq) AgCN AgCN(s) + H2O(l) b) Ag(s) + CN-(aq) + O2(g) 19 ...
View Full Document