Types of Chemical Reactions and Solution Stoichiometry

Classification of Matter Solutions are homogeneous mixtures

Solute Solvent A solute is the dissolved substance in a solution. Salt in salt water Sugar in soda drinks Carbon dioxide in soda drinks Solvent A solvent is the dissolving medium in a solution. Water in salt water Water in soda

Saturation of Solutions A solution that contains the maximum amount of solute that may be dissolved under existing conditions is saturated. A solution that contains less solute than a saturated solution under existing conditions is unsaturated. A solution that contains more dissolved solute than a saturated solution under the same conditions is supersaturated.

Electrolytes vs. Nonelectrolytes The ammeter measures the flow of electrons (current) through the circuit. If the ammeter measures a current, and the bulb glows, then the solution conducts. If the ammeter fails to measure a current, and the bulb does not glow, the solution is non-conducting.

Definition of Electrolytes and Nonelectrolytes An electrolyte is: A substance whose aqueous solution conducts an electric current. A nonelectrolyte is: A substance whose aqueous solution does not conduct an electric current. Try to classify the following substances as electrolytes or nonelectrolytes…

Electrolytes? Pure water Tap water Sugar solution Sodium chloride solution Hydrochloric acid solution Lactic acid solution Ethyl alcohol solution Pure, solid sodium chloride

Answers… ELECTROLYTES: NONELECTROLYTES: Tap water (weak) NaCl solution   HCl solution   Lactate solution (weak)     Pure water   Sugar solution   Ethanol solution   Pure, solid NaCl   But why do some compounds conduct electricity in solution while others do not…?

Ionic CompoundsDissociate NaCl(s)  Na+(aq) + Cl-(aq) AgNO3(s)  Ag+(aq) + NO3-(aq) MgCl2(s)  Mg2+(aq) + 2 Cl-(aq) Na2SO4(s)  2 Na+(aq) + SO42-(aq) AlCl3(s)  Al3+(aq) + 3 Cl-(aq)

Some covalent compounds IONIZE in solution Covalent acids form ions in solution, with the help of the water molecules. For instance, hydrogen chloride molecules, which are polar, give up their hydrogens to water, forming chloride ions (Cl-) and hydronium ions (H3O+).

Strong acids such as HCl are completely ionized in solution. Other examples of strong acids include: Sulfuric acid, H2SO4 Nitric acid, HNO3 Hydriodic acid, HI Perchloric acid, HClO4 Hydrobromic acid, HBr

Weak acids such as lactic acid usually ionize less than 5% of the time. Many of these weaker acids are “organic” acids that contain a “carboxyl” group. The carboxyl group does not easily give up its hydrogen.

Because of the carboxyl group, organic acids are sometimes called “carboxylic acids”. Other organic acids and their sources include: Citric acid – citrus fruit Malic acid – apples Butyric acid – rancid butter Amino acids – protein Nucleic acids – DNA and RNA Ascorbic acid – Vitamin C This is an enormous group of compounds; these are only a few examples.

However, most covalent compounds do not ionize at all in solution. Sugar (sucrose – C12H22O11), and ethanol (ethyl alcohol – C2H5OH) do not ionize - That is why they are nonelectrolytes!

Molarity The concentration of a solution measured in moles of solute per liter of solution. mol = M L

Preparation of Molar Solutions Problem: How many grams of sodium chloride are needed to prepare 1.50 liters of 0.500 M NaCl solution? Step #1: Ask “How Much?” (What volume to prepare?) Step #2: Ask “How Strong?” (What molarity?) Step #3: Ask “What does it weigh?” (Molar mass is?) 1.500 L 0.500 mol 58.44 g = 43.8 g 1 L 1 mol

Serial Dilution It is not practical to keep solutions of many different concentrations on hand, so chemists prepare more dilute solutions from a more concentrated “stock” solution. Problem: What volume of stock (11.6 M) hydrochloric acid is needed to prepare 250. mL of 3.0 M HCl solution? MstockVstock = MdiluteVdilute (11.6 M)(x Liters) = (3.0 M)(0.250 Liters) x Liters = (3.0 M)(0.250 Liters) 11.6 M = 0.065 L

Single Replacement Reactions A + BX  AX + B BX + Y  BY + X Replacement of: Metals by another metal Hydrogen in water by a metal Hydrogen in an acid by a metal Halogens by more active halogens

The Activity Series of the Metals   Lithium   Potassium   Calcium   Sodium   Magnesium   Aluminum   Zinc   Chromium   Iron   Nickel   Lead   Hydrogen   Bismuth   Copper   Mercury   Silver   Platinum   Gold Metals can replace other metals provided that they are above the metal that they are trying to replace. Metals above hydrogen can replace hydrogen in acids. Metals from sodium upward can replace hydrogen in water

The Activity Series of the Halogens   Fluorine   Chlorine   Bromine   Iodine Halogens can replace other halogens in compounds, provided that they are above the halogen that they are trying to replace. 2NaCl(s) + F2(g)  2NaF(s) + Cl2(g) ??? MgCl2(s) + Br2(g)  No Reaction ???

Solubility Rules – Mostly Soluble Ion Solubility Exceptions NO3- Soluble None ClO4- Na+ K+ NH4+ Cl-, I- Pb2+, Ag+, Hg22+ SO42- Ca2+, Ba2+, Sr2+, Pb2+, Ag+, Hg2+

Solubility Rules – Mostly Insoluble Ion Solubility Exceptions CO32- Insoluble Group IA and NH4+ PO43- OH- Group IA and Ca2+, Ba2+, Sr2+ S2- Groups IA, IIA, and NH4+

Oxidation and Reduction (Redox)   Electrons are transferred   Spontaneous redox rxns can transfer energy   Electrons (electricity)   Heat   Non-spontaneous redox rxns can be made to happen with electricity

Oxidation and Reduction An old memory device for oxidation and reduction goes like this… LEO says GER Lose Electrons = Oxidation Gain Electrons = Reduction

Oxidation Reduction Reactions (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:

LEO says GER : Lose Electrons = Oxidation Sodium is oxidized Gain Electrons = Reduction Chlorine is reduced

Rules for Assigning Oxidation Numbers Rules 1 & 2 The oxidation number of any uncombined element is zero 2. The oxidation number of a monatomic ion equals its charge

Rules for Assigning Oxidation Numbers Rules 3 & 4 3. The oxidation number of oxygen in compounds is -2 4. The oxidation number of hydrogen in compounds is +1

Rules for Assigning Oxidation Number Rule 5 5. The sum of the oxidation numbers in the formula of a compound is 0 2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation Numbers Rule 6 6. The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its charge X + 4(-2) = -2 S O X + 3(-2) = -1 N O  X = +5  X = +6

Reducing Agents and Oxidizing Agents   The substance reduced is the oxidizing agent   The substance oxidized is the reducing agent Sodium is oxidized – it is the reducing agent Chlorine is reduced – it is the oxidizing agent

Trends in Oxidation and Reduction Active metals:   Lose electrons easily   Are easily oxidized   Are strong reducing agents Active nonmetals:   Gain electrons easily   Are easily reduced   Are strong oxidizing agents

Redox Reaction Prediction #1 Important Oxidizers Formed in reaction MnO4- (acid solution) MnO4- (basic solution) MnO2 (acid solution) Cr2O72- (acid) CrO42- HNO3, concentrated HNO3, dilute H2SO4, hot conc Metallic Ions Free Halogens HClO4 Na2O2 H2O2 Mn(II) MnO2 Cr(III) NO2 NO SO2 Metallous Ions Halide ions Cl- OH- O2

Redox Reaction Prediction #2 Important Reducers Formed in reaction Halide Ions Free Metals Metalous Ions Nitrite Ions Sulfite Ions Free Halogens (dil, basic sol) Free Halogens (conc, basic sol) C2O42- Halogens Metal Ions Metallic ions Nitrate Ions SO42- Hypohalite ions Halate ions CO2

Not All Reactions are Redox Reactions Reactions in which there has been no change in oxidation number are not redox rxns. Examples: