CHM 1045 General Chemistry Chapters 4 5 6 Exam 2 Chapter 4 General Properties of Aqueous Solutions A solution is a homogenous mixture of two or more substances The solute is the substance present in a smaller amount and the solvent is the substance present in a larger amount o Solution may be Gaseous such as air Solid such as an alloy Liquid such as seawater o Aqueous solutions the solute initially is a liquid or a solid and the solvent is water Table 4 1 An electrolyte is a substance that when dissolved in water results in a solution that can conduct electricity A nonelectrolyte does not conduct electricity when dissolved in water o Figure 4 1 a A nonelectrolyte solution does not contain ions and the light bulb is not lit b A weak electrolyte solutions contains a small number of ions and the light bulb is dimly lit c A strong electrolyte solution contains a large number of ions and the light bulb is brightly lit The molar amounts of the dissolved solutes are equal in all three cases o Conductors Requires mobile charger Electrolyte Ex Gatorade Strong weak nonelectrolyte o Insulators o Ionic Compounds Dissociation Water is a very effective solvent for ionic compounds Although water is an electrically neutral molecule it has a positive region the H atoms and a negative region the O atom a positive and negative poles for this reason it is a polar solvent When sodium chloride dissolved in water The Na and Cl ions are separated from each other and undergo hydration the process in which an ion is surrounded by water molecules arranged in a specific manner We use the term ionization to describe the separation of acids and bases into ions The ionization of acetic acid is written with a double arrow to show that it is a reversible reaction that is the reaction can occur in both directions o Eventually a state is reached in which the acid molecules ionize as fast as the ions recombine Such a chemical state in which no net change can be observed although activity is continuous on the molecular level is called chemical equilibrium Covalent compounds o Do not dissociate Problems 6 8 10 12 Precipitation Reactions One common type of reaction that occurs in aqueous solution is the precipitation reaction which results in the formation of an insoluble product or precipitate o A precipitate is an insoluble solid that separates from the solution Precipitation reactions usually involve ionic compounds o Metathesis reaction also called a double displacement reaction a reaction that involves the exchange of parts between the two compounds Solubility o Predict whether a precipitate will form when a compound is added to a solution or when two solutions are mixed o Solubility of the solute which is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature A substance is said to be soluble if a fair amount of it visibly dissolves when added to water of not the substance is described as slightly soluble or insoluble All ionic compounds are strong electrolytes but they are not equally soluble Soluble one substance is observed to clearly dissolve in another Somewhat soluble some visual evidence of dissolution though dissolution is incomplete Insoluble no obvious evidence of dissolution Soluble salts Table 4 2 o Alkali metal salts group 1 metals Li Na K Rb Cs Fr NH 4 o Nitrates o Most halids Insoluble salts Exceptions MX where M Ag Hb Bp o Carbonates phosphates hydroxides Exceptions alkali metal salts NH4 barium Molecular Equations Ionic Equations and Net Ionic Equations hydroxide o Molecular equations The equation describing the precipitation of lead II iodide textbook page 121 is called a molecular equation because the formulas of the compounds are written as though all species existed as molecules or whole units A molecular equation is useful because it identifies the reagents that is lead II nitrate and potassium iodide o Phases o Double displacement reactions o Ionic equations Express dissociated species Ionic equation shows dissolved species as free ions Spectator ions are ions that are not involved in the overall reaction o Net ionic equations Eliminate spectator ions Shows only species that actually take place in the reaction Problems 20 22 24 Procedure for writing ionic and net ionic equations o Write a balanced molecular equation for the reaction using the correct formulas for the reactant and product ionic compounds Refer to Table 4 2 to decide which of the products is insoluble and therefore will appear as a precipitate o Write the ionic equation for the reaction The compound that does not appear as the precipitate should be shown as free ions o Identify and cancel the spectator ions on both sides of the equation Write the net ionic equation for the reaction o Check that the charges and number of atoms balance in the net ionic equation These steps are applied in Example 4 2 in textbook Acid Base Reactions Acids o Example HCl o Properties Acids have a sour taste for example vinegar owes its sourness to acetic acid and lemons and other citrus fruits contain citric acid Acids cause color changes in plant dyes for example they change the color of litmus from blue to red Acids react with certain metals such as zinc magnesium and iron to produce hydrogen gas A typical reaction is that between hydrochloric acid and magnesium 2HCl aq Mg s MgCl2 aq H2 g Acids react with carbonates and bicarbonates such as Na2CO3 CaCO3 and NaHCO3 to produce carbon dioxide gas Figure 4 6 For example 2HCl aq CaCO3 s CaCl2 aq H2O l CO2 g HCl aq NaHCO3 s NaCl aq H2O l CO2 g Aqueous acid solutions conduct electricity o Arrhenius s definitions of acids and bases are limited in that they apply only to aqueous solutions proton donors Bronsted acid is a proton donor Note Bronsted s definitions do not require acids and bases to be in aqueous solution Hydrochloric acid is a Bronsted acid because it donates a proton in water o HCl aq H aq Cl aq o Hydronium ion Bases o Example NaOH Ammonia NH3 o Properties Bases have a bitter taste Bases feel slippery for examples soaps which contain bases exhibit this property Bases cause color changes in plant dyes for example they change the color of litmus from red to blue Aqueous base solutions conduct electricity o Arrhenius Hydroxide ion donor Bronsted base is a proton acceptor Ex Identify each of the following species as a Bronsted acid base or both a PO4 3 Bronsted base b ClO2 Bronsted base c MH4 Bronsted acid d HClO4 both figure out
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