7/16/2012. Chapter Four: Like Dissolve Like. The Water Molecule. Ionic Compounds in Water. General Properties of Aqueous Solutions

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General Properties of Aqueous Solutions Chapter Four: TYPES OF CHEMICAL REACTIONS AND SOLUTION STOICHIOMETRY A solution is a homogeneous mixture of two or more substances.

Solutions in which water is the solvent are called aqueous solutions. The Water Molecule Water is not a linear molecule.

1 General Properties of Aqueous Solutions Chapter Four: TYPES OF CHEMICAL REACTIONS AND SOLUTION STOICHIOMETRY A solution is a homogeneous mixture of two or more substances. A solution is made when one substance (the solute) is dissolved in another (the solvent). The solute is the substance that is present in the smallest amount. Solutions in which water is the solvent are called aqueous solutions. The Water Molecule Water is not a linear molecule. It is bent at an angle of about 105 Electrons are not evenly distributed around the atoms in water. Oxygen is more electronegative (EN) and pulls the e away from hydrogen. Oxygen is slightly negative; hydrogen is slightly positive. The molecule is polar because the charges are not distributed symmetrically. Notes 4.1 Like Dissolve Like In general, like dissolves like is a useful rule for predicting solubility. Polar Substances dissolve polar and ionic substances. Nonpolar substances dissolve nonpolar substances. However, not all ionic substances dissolve in water, particularly if the ion attraction is stronger than the attraction for the water molecules. Example 4.1 A&B Ionic Compounds in Water When an ionic compound dissolves in water, the ions are said to dissociate. This means that in solution the solid no longer exists as a well-ordered arrangement of ions in contact with one another. Instead, each ion is surrounded by several water molecules; it is called an aqueous ion or a hydrated ion. Figure 4.2 Polar Water Molecules Interact with the Positive and Negative Ions of a Salt Assisting in the Dissolving Process Oxygen atoms of water point towards positive ions; hydrogen atoms of water point towards negative ions. The transport of ions through the solution allows electric current to flow through the solution. 1

Electrolytic Properties All aqueous solutions can be classified in terms of whether or not they conduct electricity.

is a noneletrolyte and the solution does not conduct electricity. Examples are sucrose and water Molecular Compounds in Water When a molecular compound (e.g.

There are some important exceptions: NH 3 reacts with water to form NH 4+ and OH HCl reacts with water to form H + and Cl Strong vs Weak Electrolytes Compounds whose aqueous solutions conduct

Other strong electrolytes include strong acids and bases. Ex. NaCl(aq) Na + (aq) + Cl (aq) Single arrow means that ions have no tendency to recombine.

CH 3 CH 2 OH(aq) CH 3 CH 2 O (aq) + H + (aq) Double arrow means that the reaction is significant in both directions HCl in water NaOH in water HC 2 H 3 O 2 in water ionizes ionizes ionizes slightly

2 Electrolytic Properties All aqueous solutions can be classified in terms of whether or not they conduct electricity. If a substance forms ions in solution, then the substance is an electrolyte and the solution conducts electricity An example is NaCl If a substance does not form ions in solution, then the substance is a noneletrolyte and the solution does not conduct electricity. Examples are sucrose and water Molecular Compounds in Water When a molecular compound (e.g. ethanol) dissolves in water, only a very limited number of ions are formed. Therefore, there is nothing in the solution to transport electric charge and the solution does not conduct electricity. There are some important exceptions: NH 3 reacts with water to form NH 4+ and OH HCl reacts with water to form H + and Cl Strong vs Weak Electrolytes Compounds whose aqueous solutions conduct electricity well are called strong electrolytes. These substances exist in solution mostly as ions In general, soluble ionic compounds are strong electrolytes. Other strong electrolytes include strong acids and bases. Ex. NaCl(aq) Na + (aq) + Cl (aq) Single arrow means that ions have no tendency to recombine. Compounds whose aqueous solutions conduct electricity poorly are called strong electrolytes. These substances exist as a mixture of ions and un-ionized molecules in solution. Ex. CH 3 CH 2 OH(aq) CH 3 CH 2 O (aq) + H + (aq) Double arrow means that the reaction is significant in both directions HCl in water NaOH in water HC 2 H 3 O 2 in water ionizes ionizes ionizes slightly strong electrolyte strong electrolyte weak electrolyte Example 4.2 Solubility Guidelines The solubility of a substance at a particular temperature is the amount of that substance that can be dissolved in a given quantity of solvent at that temperature. A substance with a solubility of less than 0.1 mol/l is regarded as being insoluble. The differences in the solubility of ionic compounds in water typically depend on the relative attraction of the ions for each other and the attractions of the ions for water molecules. Pass out solubility table. Make Solubility Rules Table 2

3 Now Try This Use molecular level pictures to show the difference between adding ml of water to a beaker containing 5 g of NaCl and adding ml of water to a beaker containing 5 g of AgCl. Which of the following solutions contains the greatest number of ions? (Assume complete solubility for all salts.) One mole of potassium chloride dissolved in 1.0 L of water. One mole of sodium phosphate dissolved in 1.0 L of water. One mole of iron(ii) nitrate dissolved in 1.0 L of water. One mole of sodium hydroxide dissolved in 1.0 L of water. At least two of the above solutions have an equally great number of ions. Concentration The concentration is used to indicate the amount of solute dissolved in a given quantity of solvent or solution. When an ionic compound dissolves, the relative concentrations of the ions in the solution depend on the chemical formula of the compound. Molarity Solutions can be prepared with different concentrations by adding different amounts of solute to solvent. The amount (moles) of solute per liter of solution is the molarity or molar concentration (symbol M) of the solution. The definition of molarity contains three quantities. If you know any two, you can calculate the third. E.g. by knowing the molarity of a quantity of liters of solution, we can easily calculate the number of moles (and by using molar mass, the mass) of solute. Other Common Chemistry Conc. molality (m) = mass % (g/g) = volume % (v/v) = parts per million (ppm) = moles of solute kg mass of solvent mass of solute mass of solution volume of solute volume of solution mass of solute mass of solution x 106 Dilution A solution in concentrated form (stock solution) is mixed with solvent to obtaie a solution of lower solute concentration. This process is called dilution. An alternative way of making a solution is to take a solution of known molarity and dilute it with more solvent. Since the number of moles of solute remains the same in the concentrated and diluted forms of the solution, we can show: M conc V conc = M dil V dil Alternate forms of this equation are: M i V i = M f V f or C 1 V 1 = C 2 V 2 Examples 4.3 A-F 4.3D typo Ca(OCl) 2 3

Make a Solution Diluting an Stock Solution Types of Chemical Reactions Which of the following solutions contains the greatest number of ions? a) 400.0 ml of 0.10 M NaCl. b) 300.0 ml of 0.10 M CaCl 2.

You should also be familiar with five other categories: synthesis, decomposition, single-replacement, doublereplacement, and combustion.

Such a reaction is called a precipitation reaction and the solid that forms is called a precipitate. e.g.

4 Make a Solution Diluting an Stock Solution Types of Chemical Reactions Which of the following solutions contains the greatest number of ions? a) ml of 0.10 M NaCl. b) ml of 0.10 M CaCl 2. c) ml of 0.10 M FeCl 3. d) ml of 0.10 M sucrose. Your book divides chemical reactions into precipitation reactions, acid-base reactions, and oxidation-reduction reactions. You should also be familiar with five other categories: synthesis, decomposition, single-replacement, doublereplacement, and combustion. See Types of Chemical Reactions Handout Precipitation Reactions When tow solutions are mixed, an insoluble substance sometimes forms; that is, a solid forms and separates from the solution. Such a reaction is called a precipitation reaction and the solid that forms is called a precipitate. e.g. the reaction of K 2 CrO 4 (aq) and Ba(NO 3 ) 2 (aq) Exchange Reactions Exchange reactions, or metathesis reactions, involve swapping ions in solution: AX + BY AY + BX If one or more of the products in this doubledisplacement-type reaction produces a solid, gas, or molecular compound, then a reaction takes place. Many precipitation and acid-base reactions exhibit this pattern. 4

2KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) The complete ionic equation lists all strong soluble electrolytes in the reaction as ions.

5 Precipitation of Silver Chloride AgNO 3 (aq) + KCl(aq) AgCl + KNO 3 (aq) Ag + (aq) + NO 3 (aq) + K + (aq) + Cl (aq) AgCl + K + (aq) + NO 3 (aq) Ag + (aq) + Cl (aq) AgCl Describing Reactions in Solution Consider the reaction of potassium iodide and lead (II) nitrate. The molecular equation lists all species in their complete chemical forms. 2KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) The complete ionic equation lists all strong soluble electrolytes in the reaction as ions. 2K + (aq) + 2I (aq) + Pb 2+ (aq) + 2NO 3 (aq) PbI 2 (s) + 2K + (aq) + 2NO 3 (aq) The net ionic equation lists only those ions which are not common on both sides of the reaction. 2I (aq) + Pb 2+ (aq) PbI 2 (s) Note that spectator ions, ions that are present in the solution but play no direct role in the reaction, are omitted in the net ionic equation. Solving Stoichiometry Problems for Reactions in Solution Write the complete and balanced equation, the complete ionic equation, and the net ionic equation for the addition of aqueous potassium sulfate and aqueous barium nitrate. 1. Identify the species present in the combined solution, and determine what reaction occurs. 2. Write a balanced net ionic equation for the reaction. 3. Calculate the moles of reactants. 4. Determine which reactant is limiting. 5. Calculate the moles of product or products, as required. 6. Convert to grams or other units, as required. Example 4.6 You are given two solutions: Solution A consists of 2.00 L of 2.00 M AgNO 3 Solution B consists of 3.00 L of 1.00 M Na 2 CrO 4 Solution A: 2.00 L 2.00 M AgNO 3 (aq) Solution B: 3.00 L 1.00 M Na 2 CrO 4 (aq) a) What is the molar concentration of each solution b) Determine the total number of moles of each ion in each solution c) Now write a complete and balanced chemical equation for the mixing of these two solutions. d) Then write the net ionic equation e) Determine the mass amount of precipitate that will be formed f) What is the molar concentration of Na 1+ ions in the end solution? 5

Acids Acids are substances that are able to ionize in aqueous solution to form H + ions. Ionization occurs when a neutral substance forms ions in solution. An example is HC 2 H 3 O 2 (acidic acid).

6 Acids Acids are substances that are able to ionize in aqueous solution to form H + ions. Ionization occurs when a neutral substance forms ions in solution. An example is HC 2 H 3 O 2 (acidic acid). Since H + is a naked proton, acids are proton donors. Acids that ionize to form one H + are called monoprotic. Acids that ionize to form two H + are called diprotic and Acids that ionize to form three H + are called triprotic. Strong acids completely ionize in solution. The six strong acids: HCl, HBr, HI, HClO 3, H 2 SO 4, and HNO 3 HCl(aq) H + (aq) + Cl (aq) Weak acids partially ionize in solution; e.g. HF HF(aq) H + (aq) + F (aq) Bases Bases are substances that accept or react with H + ions formed by acids. We refer to bases as proton acceptors. Hydroxide ions, OH, react with H + ions to form water. H + (aq) + OH (aq) H 2 O(l) Compounds that do not contain OH can be bases. Proton transfer to NH 3 (a weak base) from water (a weak acid) is an example of an acid-base reaction. Since there is a mixture of NH 3, H 2 O, NH 4+, and OH in solution, we write: NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) Strong bases include group IA metal hydroxids, Ca(OH) 2, Ba(OH) 2, and Sr(OH) 2 Acid-Base Reactions A neutralization reaction occurs when an acid and a base react: HCl(aq) + NaOH(aq) H 2 O(l) + NaCl(aq) acid base water salt In general, an acid and base react to form a salt and water Acid-base reactions with gas formation Reaction of sulfides with acid gives rise to H 2 S gas. Na 2 S(aq) + HCl(aq) H 2 S(g) + 2NaCl(aq) Carbonates and hydrogen carbonates will form CO 2 NaHCO 3 (aq) + HCl(aq) H 2 O(l) + CO 2 (g) + NaCl(aq) Ammonium salts will form NH 3 when treated with hydroxide. NH 4 NO 3 (aq) + NaOH(aq) H 2 O(l) + NH 3 (g) + NaNO 3 (aq) NH 4+ (aq) + OH (aq) H 2 O(l) + NH 3 (g) net ionic Titrations A common way to determine the concentration of a solution is via titration. We determine the concentration of one substance by allowing it to undergo a specific chemical reaction, of known stoichiometry, with another substance whose concentration is known (standard solution). The point at which stoichiometrically equivalent quantities of acid and base are brought together is known as the equivalence point. In titration we often use an acid-base indicator to allow us to determine when the equivalence point of a titration has been reached. The indicator changes color at the end point of the titration. The indicator is chosen so that the end point corresponds to the equivalence point of the titration. Performing Calculations for Acid-Base Reactions 1. List the species present before any reaction occurs, and decide what reaction will occur. 2. Write a balanced net ionic equation. 3. Calculate moles of reactants. 4. Determine limiting reactant, if needed. 5. Calculate moles of required reactant or product. 6. Convert to grams or volume, as required. 6

What volume of 0.125 M HCl solution is need to neutralize 35.0 ml of 0.375 M Ca(OH) 2? Examples 4.

Ca Ca 2+ + 2e Calcium has been oxidized. When a substance gains electrons, in undergoes reduction. O 2 + 2e O Oxygen has been reduced.

Ca + O 2 CaO OIL (oxidation is loss of e ) RIG (reduction is gain of e ) Oxidation Numbers Electrons are not explicitly shown in chemical reactions.

7 What volume of M HCl solution is need to neutralize 35.0 ml of M Ca(OH) 2? Examples 4.8 Oxidation-Reduction Oxidation-reduction, or redox, reactions involve the transfer of electrons between reactants. When a substance loses electrons, it undergoes oxidation. Ca Ca e Calcium has been oxidized. When a substance gains electrons, in undergoes reduction. O 2 + 2e O Oxygen has been reduced. In all redox reactions, one species is reduced at the same time as another is oxidized. Ca + O 2 CaO OIL (oxidation is loss of e ) RIG (reduction is gain of e ) Oxidation Numbers Electrons are not explicitly shown in chemical reactions. Oxidation numbers (or oxidation states) help us keep track of electrons during chemical reactions. Reaction of Sodium and Chlorine Example 4.9A Find the oxidation states for each of the elements in each of the following compounds: K 2 Cr 2 O 7 CO 3 HClO 4 MnO 2 PCl 5 SF 4 Example 4.9B Which of the following are oxidationreduction reactions? Identify the oxidizing agent, and the reducing agent. Zn (s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g) Cr 2 O 7 (aq) + 2OH - (aq) 2CrO 4 (aq) + H 2 O(l) O 3 (g) + NO (g) O 2 (g) + NO 2 (g) 2CuCl (aq) CuCl 2 (aq) + Cu (s) Example 4.9C 7

Balancing Oxidation-Reduction Reactions The Half-Reaction Method (Acidic Solution) Recall the law of conservation of mass the amount of each element present at the beginning of the reaction must be

8 Balancing Oxidation-Reduction Reactions The Half-Reaction Method (Acidic Solution) Recall the law of conservation of mass the amount of each element present at the beginning of the reaction must be present at the end. Conservation of charge electrons are not lost in a chemical reaction. For many redox reactions we need to look carefully at the transfer of electrons. Half-reactions are a convenient way of separating oxidation and reduction reactions to see how many electrons are being transferred. Balancing Oxidation-Reduction Reactions Cr 2 O 7 (aq) + SO 3- (aq) Cr 3+ (aq) + SO 4 (aq) Let s balance this equation taking into consideration electrons? Break the reaction into two half reactions: Cr 2 O 7 (aq) Cr 3+ (aq) reduced SO 3- (aq) oxidized SO 4 (aq) Balance elements by adding coefficients Cr 2 O 7 2Cr 3+ Balance oxygens by adding H 2 O Cr 2 O 7 SO 3- SO 4 H 2 O + SO 3- SO 4 8

9 Balance hydrogens by adding H + Balance charges by adding e - 14H + + Cr 2 O 7 6e H + + Cr 2 O 7 H 2 O + SO 3- SO 4 + 2H + H 2 O + SO 3- SO 4 + 2H + + 1e - Balance each half reaction so that they have the same number of e - Balance each half reaction so that they have the same number of e - 6e H + + Cr 2 O 7 6e H + + Cr 2 O 7 (H 2 O + SO 3- SO 4 + 2H + + 1e - ) x 6 6H 2 O + 6SO 3-6SO H + + 6e - Add half reactions 6e H + + Cr 2 O 7 6H 2 O + 6SO 3-6SO H + + 6e - 14H + + Cr 2 O 7 + 6H 2 O + 6SO 3-2Cr H 2 O + 6SO H + Simplify 14H + + Cr 2 O 7 + 6H 2 O + 6SO 3-2Cr H 2 O + 6SO H + 2H + + Cr 2 O 7 + 6SO 3-2Cr 3+ + H 2 O + 6SO 4 Check that elements are balanced and charges equal each other 9

10 Lastly, add states of matter 2H + (aq) + Cr 2 O 7 (aq) + 6SO 3- (aq) 2Cr 3+ (aq) + H 2 O(l) + 6SO 4 (aq) Half-Reaction Method Balancing in Base 1. Balance as in acid. 2. Add OH to cancel H + ions (the same amount on both sides!) 3. Form water by combining H +, OH. 4. Simplify 5. Do a check - are elements and charges balanced. Example 4.10B Consider the titration of an acidic solution of Na 2 C 2 O 4 (colorless) with KMnO 4 (deep purple). MnO 4 is reduced to Mn 2+ (pale pink), while the C 2 O 4 is oxidized to CO 2. The end point is indicated by the presence of a pale pink color. If more KMnO 4 is added, the solution turns purple due to the excess of MnO 4. What is the balanced chemical equation for this reaction? 10

Chapter 4. Reactions in Aqueous Solution

Chapter 4. Reactions in Aqueous Solution 4.1 General Properties of Aqueous Solutions A solution is a homogeneous mixture of two or more substances. A solution is made when one substance (the solute) is