Chapter 4 - Types of Chemical Reactions and Solution Stoichiometry

4.1 Water, the Common Solvent

• Water helps moderate the earth’s temperature: it cools automobile engines, nuclear power plants, and many industrial processes
• One of the most valuable properties of water is its ability to dissolve many different substances
• The “positive ends” of the water molecules are attracted to the negatively charged anions and that the “negative ends” are attracted to the positively charged cations
• The strong forced present among the positive and negative ions of the solid are replaced by strong water-ion interactions
• When ionic substances dissolve in water, they break up into the individual cations and anions
• The differences in the solubilities of ionic compounds in water typically depend on the relative attractions of the ions for each other and the attractions of the ions for water molecules
• Water dissolves many nonionic substances
  • Ethanol is very soluble in water
• “Like dissolves like” is a useful rule for predicting solubility

4.2 The Nature of Aqueous Solutions: Strong and Weak Electrolytes

• One useful property for characterizing a solution is its electrical conductivity, its ability to conduct an electric current
  • It can be checked conveniently by using an apparatus
• An electrolyte is a substance that when dissolved in water produces a solution that can conduct electricity
• The extent to which a solution can conduct an electric current depends directly on the number if ions present
• Sodium chloride produces ions in aqueous solutions that are strong electrolytes
  • NaCI is a strong electrolyte as well
  • Strong electrolytes dissociate completely in an aqueous solution
• Perchloric acid is a strong acid
• The Arrhenius definition of an acid is a substance that produces H+ ions in solution
• Weak electrolytes dissociate only to a small extent in aqueous solution
• Any acid, such as acetic acid, that dissociates only to a slight extent in aqueous solutions is called a weak acid
• Non-electrolytes are substances that dissolve in water but do not produce any ions

4.3 The Composition of Solutions

• To perform stoichiometric calculations, we must know the nature of the reaction and the amounts of chemicals present in the solutions
• Molarity: The amount of solute per volume of solution in liters
• Solution concentration is always given in terms of the form of the solute before it dissolves
• Standard solution: A solution whose concentration is accurately known
• Dilution with water does not alter the numbers of moles of solute present
  • A typical dilution calculator involves determining how much water must be added to an amount of stock solution to achieve a solution of the desired concentration
  • Dilution produce typically involves two types of glassware: a pipet and a volumetric flask
  • There are two common types of pipette: volumetric and measuring
• The central idea in performing the calculations associated with dilutions is to recognize that the moles of solute are not changed by the dilution
• The equation M1 V1 = M2 V2 always holds for a dilution

4.4 Types of Chemical Reactions

• ****Precipitation reactions
• Acid-base reactions
• Oxidation-reduction reactions

4.5 Precipitation Reactions

• **** Precipitation reaction: When two solutions are mixed, an insoluble substance sometimes forms: that is, a solid form and separates from the solution is
• A precipitation reaction also can be called a double displacement reaction
• When a solid containing ions dissolve in water, the ions separate and move around independently
• The phrase slightly soluble used in the solubility rules means that the tiny amount of solid that dissolves is not noticeable
• The solid appears to be insoluble to the naked eye
• The key to dealing with the chemistry of an aqueous solution is first to focus on the actual components of the solution begins nay reaction occurs and then to figure out how the components will react with each other

4.6 Describing Reactions in Solution

• In a complete ionic equation, all substances that are strong electrolytes are represented as ions
• The complete ionic equation represents as ions all reactants and products that are strong electrolytes
• A strong electrolyte is a substance that completely breaks apart into ions when dissolved in water
• The formula equation gives the overall reaction stoichiometry but not necessarily the actual forms of the reactants and products in the solution
• Net ionic equations include only those components that undergo changes in the reaction
• Chemists usually write the net ionic equation for a reaction in solution because it gives the actual forms of the reactants and products and includes only the species that undergo a change

4.7 Stoichiometry of Precipitation Reactions

• Solution reactions are sometimes difficult to tell immediately what reactions will occur when two solutions are mixed

  • The first step in this process always should be to write down the species that are actually present in the solution
  • The second point about solution reactions is that to obtain the moles of reactants we must use the volume of the solution and its molarity

• Solving Stoichiometry Problems for Reactions in Solution:

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  1. Identify the species present in the combined solution, and determine what reaction occurs

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  2. Write the balanced net ionic equation for the reaction

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  3. Calculate the moles of reactants

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  4. Determine which reactant is limiting

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  5. Determine the moles of product or products, as required

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  6. Convert to grams or other units, as required

4.8 Acid-Base Reactions

• ****An acid ****is a substance that produces H+ ions when dissolved in water, and a base is a substance that produces OH- ions

  • A base is a proton acceptor
  • With precipitation reactions, the focus is on the species present in the mixed solution

• The hydroxide ion is such a strong base that for purposes of stoichiometric calculations it can be assumed to react completely with any weak acid that will be encountered

• Steps to Performing Calculations for Acid-Base Reactions

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  1. List the species present in the combined solution before any reaction occurs, and decide what reaction will occur
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  1. Write the balanced net ionic equation for this reaction
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  1. Calculate the moles of reactants
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  1. Determine the limiting reactant where appropriate
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  1. Calculate the moles of the required reactant or product
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  1. Convert to gram or volume

• Volumetric analysis: A technique for determining the amount of a certain substance by doing a titration

• Titration: Involves delivery of a measured volume of a solution of known concentration into a solution containing the substance being analyzed

  • The exact reaction between titrant and analyte must be known
  • The stoichiometric point must be marked accurately
  • The volume of tyrant required to reach the stoichiometric point must be known accurately
  • An indicator very commonly used for acid-base titrations is phenolphthalein, which is colorless in an acidic solution and pink in a basic solution

• The endpoint and stoichiometric point should coincide

• Write down the components and focus on the chemistry of each one

• When a strong electrolyte is resent, write it as separated ions

4.9 Oxidation-Reduction Reactions

• Sodium chloride can be formed by the reaction of elemental sodium and chlorine
• Redox reactions: One or more electrons are transferred
• Photosynthesis is a very important oxidation-reduction reaction
• The concept of oxidation states provides a wary to keep track of electrons in oxidation-reduction reactions, particularly redox reactions involving covalent substances
• For a covalent bond between two identical atoms, the electrons are split equally between the two
• If two different atoms are involved, the shared electrons are assigned completely to the atom that has the stronger attraction for electrons
• Oxidation states of the atoms in a covalent compound as the imaginary changes the atoms would have if the shared electrons were divided equally between identical atoms bonded to each other
• Rules for the Assigning Oxidation States: The sum of the oxidation states must be zero for an electrically neutral compound
  • For an ion, the sum of the oxidation states must equal the charge of the ion
• The convention is to write actual charges ions as n+ or n-, the number being written before the plus or minus sign
• Oxidation states are written +n or -n, the number is written after the plus or minus sign
• Oxidation-reduction reactions are characterized by a transfer of electrons
• Reduction: A decrease in oxidation state
• Oxidation: An increase in oxidation state
• A helpful mnemonic device is OIL RIG (Oxidation Involves Loss: Reduction Involves Gain)
• Metallurgy, the process of producing metal from its ore, always involves oxidation-reduction reactions

4.10 Balancing Oxidation-Reduction Equations

• Half-reaction method: Balancing the equations of redox reactions that occur in aqueous solutions
  • The half-reaction method for balancing oxidation-reduction equations differs slightly depending on whether the reaction takes place in acidic or basic solution
• Rules for the Half-Reaction Method Occurring in Acidic Solutions: Write separate equations for the oxidation and reduction half-reactions
  • For each half-reaction, balance all the elements except hydrogen and oxygen, and balance using oxygen using H20
  • If necessary, multiply one or both balanced half-reactions by an integer to equalize the number of electrons transferred in the two half-reactions
  • Add the half-reactions, and cancel identical species
  • Check that the elements and charges are balanced
• Oxidation-reduction reactions can occur in basic solutions as well

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