AQUEOUS CHEMISRTY Chapter 4

General Properties of Aqueous Solutions Solution = a homogeneous mixture of two or more substances. Solvent = normally the component that is present in greater quantity Solute = dissolves in the solvent.

Electrolytes and Nonelectrolytes Electrolyte = a substance that dissolves in water to give an electrically conducting solution. In general ionic solids that dissolves in water are electrolytes. Some molecular substances are also electrolytes. eg. HCl (g) Nonelectrolyte = a substance that dissolves in water to give a nonconducting or very poorly conducting solution.

Strong and Weak Electrolytes Strong electrolyte  an electrolyte that exists in solution almost entirely as ions. Eg. Sodium Chloride NaCl (s) Na+(aq) + Cl-(aq) H2O Weak electrolyte  an electrolyte that dissolves in water to give a relatively small percentage of ions. Eg. ammonia NH3(aq) + H20(l) NH4+(aq) + OH-(aq)

However, in the solution we will also find the reverse reaction. NH3(aq) + H20(l) NH4+(aq) + OH-(aq) For this situation we generally write a single equation with a double arrow NH3(aq) + H20(l) NH4+(aq) + OH-(aq)

Types of Chemical Reactions Precipitation reactions when two ionic solutions are mixed and a solid ionic substance (precipitate) forms Acid-base reactions involve the transfer of a proton Oxidation-reduction reactions involve the transfer of electrons between reactants Other reactions

Precipitation Reactions A precipitation reaction occurs in aqueous solution because one product is insoluble. Solubility Rules Solubility of a substance  the amount of substance that can be dissolved in a given quantity of solvent. The solubility of substances varies widely. Precipitate  an insoluble compound formed during a chemical reaction.

Reaction of Potassium iodide solution and lead(II) nitrate solution

General Interest Shellfish form their shells from calcium carbonate. The organism secretes calcium ions from cells in contact with seawater, which contains dissolved carbon dioxide, some of which is present as carbonate ions. The ions combine to give a precipitate of calcium carbonate.

General Interest Kidney stones are generally also insoluble calcium compounds such as calcium carbonate , calcium phosphate or calcium oxalate.

Acid-Base Reactions Acids and bases are some of the most important electrolytes Acids taste sour Bases taste bitter and feel soapy

Another property of acids and bases is their ability to cause colour changes in certain dyes. An acid-base indicator is a dye used to distinguish between acidic and basic solutions by means of the color changes it undergoes in these solution. Probably the most important characteristic of acids and bases is how they react with each other.

Some Common Acids and Bases Name Formula Remark Acids: Acetic acid HC2H3O2 Found in vinegar Acetylsalicylic acid HC9H7O4 Aspirin Ascorbic acid H2C6H6O6 Vitamin C Citric acid H3C6H5O7 Found in lemon juice Hydrochloric acid HCl Found in gastric juice Sulfuric acid H2SO4 Battery acid Bases: Ammonia NH3 Often found in household cleaners Calcium hydroxide Ca(OH)2 Slaked Lime (used in building industry) Magnesium hydroxide Mg(OH)2 Milk of magnesia Sodium hydroxide NaOH Found in drain and oven cleaners

Acid and base definitions An acid is the species that donates a proton to another species in a proton-transfer reaction. A base is the species that accepts a proton in a proton-transfer reaction. NH3 (aq) + H2O (l) NH4+ + OH- (aq)

Reaction of nitric acid with water HNO3(aq) + H2O(l) NO3-(aq) + H3O+(aq) Note: the reaction simply involves the

Strong and Weak Acids and Bases A strong acid is an acid that ionizes completely in water, and is a strong electrolyte. HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq) A weak acid is an acid that only partially ionizes in water, and is a weak electrolyte. HCN (aq) + H2O (l) H3O+ (aq) + CN- (aq)

NaOH (s) Na+ (aq) + OH- (aq) A strong base is a base that is present in aqueous solution entirely as ions (one of which is OH-) and is a strong electrolyte. NaOH (s) Na+ (aq) + OH- (aq) H2O A weak base is a base that is only partially ionized in water. NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Neutralization Reactions One of the main properties of acid and bases is that they neutralize one another. A neutralization reaction is a reaction of an acid and a base that results in an ionic compound and possibly water. The ionic compound that is a product of a neutralization reaction is called a salt. Acid + Base Salt + Water 2HCl (aq) + Ca(OH)2 (aq) CaCl2 (aq) + 2H2O (l)

2HCl (aq) + Ca(OH)2 (aq) CaCl2 (aq) + 2H2O (l) To discuss the essential reactions occurring we have to write the reactions as ionic equations. Writing the strong electrolytes in the form of ions gives the following complete ionic equation: H+(aq) + 2Cl-(aq) + Ca2+(aq) + 2OH-(aq) Ca2+(aq) + 2Cl-(aq) + 2H2O(l) Canceling spectator ions and dividing by 2 gives the net ionic equation: H+(aq) + OH-(aq) H2O(l)

Example Writing an equation for a neutralization rxn Write the molecular equation and the net ionic equation for the neutralization of sulfurous acid by potassium hydroxide. sulfurous acid = potassium hydroxide = Molecular equation: Ionic equation: Net ionic equation:

Monoprotic acids  acids that have only one acidic hydrogen atom per acid molecule eg. HCl and HNO3 Polyprotic acid  an acid that yields two or more acidic hydrogens per molecule. Phosphoric acid is an example of a triprotic acid. By reacting it with different amounts of base, a series of salts can be obtained. H3PO4(aq) + NaOH(aq) NaH2PO4(aq) + H2O(l) H3PO4(aq) + 2NaOH(aq) Na2HPO4(aq) + 2H2O(l) H3PO4(aq) + 3NaOH(aq) Na3PO4(aq) + 3H2O(l)

Acid-Base reactions with Gas Formation Certain salts, notably carbonates, sulfites and sulfides, react with acids to form a gaseous product. Na2CO3(aq) +2HCl(aq) 2NaCl(aq) + H2O(l) + CO2(g) If we consider this reaction as an exchange, or metathesis reaction, then we obtain the following: Na2CO3(aq) +2HCl(aq) 2NaCl(aq) + H2CO3(aq) The last product in the above equation is carbonic acid, which is unstable and decomposes to water and carbon dioxide

Example Writing an equation for a reaction with gas formation. Write the molecular equation and the net ionic equation for the reaction of copper(II)carbonate with hydrochloric acid. Molecular equation: Ionic equation: Net ionic equation:

Working with Solutions Most chemistry you have encountered to date takes place in solution! In general, reactions involving solids proceed very slowly, if at all. When carrying out reactions in solutions it is very convenient to measure out the reactant by volume.

Molar Concentration Concentration = the quantity or amount of solute in a standard quantity of solution. Eg. Units: mol/L or g/ml We also use the relative terms : dilute and concentrated A solution is dilute, when the solute concentration is low A solution is concentrated when the solute concentration is high.

Molar concentration, or Molarity, M  the moles of solute dissolved in one liter of solution. Molarity = moles of solute liters of solution Units: mol.L-1 or M

Example Calculating molarity from mass and volume. A student prepared a solution by dissolving 1.345 g of potassium nitrate (KNO3) in enough water to prepare 25.00 mL of solution. What is the molarity (concentration) of the solution?

Example Or the other way around!! Calculate the mass of potassium permanganate needed to prepare 250 mL of 0.0380 M KMnO4 (aq) solution.

Dilution It is often necessary to reduce the concentration of a solution - that is to dilute it. This can be accomplished by taking a certain volume of the more concentrated solution and then diluting it with additional solvent. NB - during dilution the number of moles of solute remains constant.

The Dilution Equation For initial solution : C1 x V1 = n1 For final solution : C2 x V2 = n2 However n1 = n2 therefore : C1 x V1 = C2 x V2 or : C1V1 = C2V2

Example Diluting a solution. Calculate the volume of 0.0155 M HCl that we should use to prepare 100 mL of a 5.23x10-4 M HCl solution.

Quantitative Analysis The determination of the amount of a substance or species present in a material.

Volumetric Analysis A B A+B A titration is a procedure for determining the amount of substance A by adding a carefully measured volume of a solution with a known concentration of B until the reaction of A and B is just complete. A B A+B

Example Calculating the Quantity of a Substance in a Titrated Solution A dilute solution of hydrogen peroxide is sold in pharmacies as a mild antiseptic. A typical solution was analyzed for the percentage of hydrogen peroxide by titrating it with potassium permanganate. 5H2O2 + 2KMnO4 + 6H+ 8H2O + 5O2 + 2K+ + Mn2+ What is the mass percentage of H2O2 in a solution if 57.5 g of peroxide solution required 38.9 mL of 0.534 M KMnO4? Answer : 3.08%