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CHEM 1211 - Principles of Chemistry I
Chapter 4 - Types of Chemical Reactions
and Solution Stoichiometry
4.1 Water, the Common Solvent
! water is one of the most important substances on earth ! humans are approximately 70% water ! water helps moderate the earth’s temperature; it cools automobile engines, nuclear power plants, and many industrial processes ! many substances dissolve in water, e.g., salt and sugar ! what happens when a solid dissolves " in order to understand the process must understand the nature of water " see Figure 4.1, p
- water has 2 covalent bonds
- water is a polar molecule " see Figure 4.2; this process is called hydration (have hydration sphere)
- when salts (ionic compounds) dissolve in water they break up into individual cations and anions
! aq designated hydrated ions ! solubility of ionic substances varies greatly, e.g., sodium chloride is soluble but silver chloride only slightly soluble ! solubility depends on the relative attractions of the ions for each other and the attraction of the ions for water molecules ! water also dissolves nonionic substances, e.g., ethanol (CH CH OH) 3 2 " see Figure 4. ! many substances sill not dissolve in water, e.g., fats, because they are nonpolar molecules and do not effectively interact with polar molecules " “like dissolves like”
4.2 The Nature of Aqueous Solution: Strong and Weak Electrolytes
! recall the terms solute and solvent ! electrical conductivity can be used to characterize a solution " can have strong electrolytes, weak electrolytes, and non-electrolytes " see Figure 4.
Strong Electrolytes
! are completely ionized when dissolved ! will consider " (1) soluble salts, " (2) strong acids, and " (3) strong bases ! NaCl is a strong electrolyte; see Figure 4.5, p 135
NaCl(s) ˆ Na (aq) + Cl (aq)+^!
! strong acids-HCl, HNO , H SO , HClO ; see text for dissociation reactions 3 2 4 4 " note Arrhenius definition of an acid - a substance that produces protons when it is dissolved in water ! strong bases-NaOH, KOH; see text for dissociation reactions " note Arrhenius definition of an base - a substance that produces hydroxide ions when it is dissolved in water
Weak Electrolytes
! weak electrolytes dissociate (ionize) only to a small extent in aqueous solution " most common weak electrolytes are weak acids and bases
- acetic acid, CH COOH, is a weak acid, weak electrolyte 3
- ammonia, NH , is a weak base, weak electrolyte 3
Nonelectrolytes
! ethanol, CH CH OH, and sugar (sucrose), C H O , do not break up into ions but dissolve 3 2 12 22 11 as molecules in water
4.3 The Composition of Solutions
! chemical reactions often take place when two solution are mixed ! to perform stoichiometric calculations in such cases, must know two things " (1) the nature of the reaction " (2) the amounts of chemical present, usually expressed as concentrations ! the most commonly used expression of concentration is molarity (M) " molarity is defined as the number of moles of solute per liters of solution
4.7 Stoichiometry of Precipitation Reactions
- do examples from the end of the chapter
4.8 Acid-Base Reactions
! recall the Arrhenius concept of acids and bases; acid are substances that produce hydrogen ions (H ) in water and bases are substances that produce hydroxide ions (OH ) in water+^! ! although these ideas are fundamentally correct, it is convenient to have a more general definition of a base, which includes substances that do not contain hydroxide ions; such a definition was provided by Johannes N. Brønsted (1879-1947) and Thomas M. Lowry (1874-1936), who defined acids and base as follows: " an acid is a proton donor " a base is a proton acceptor ! consider mixing an aqueous solution of hydrogen chloride (HCl) with an aqueous solution of sodium hydroxide (NaOH)
" net reaction
" what if the acid is a weak electrolyte?
- mix an aqueous solution of acetic acid (HC H O ) and an aqueous solution of 2 3 2 potassium hydroxide (KOH); very little of the acid has dissociated (<1%) " the reaction is
" the net ion equation is
" this reaction illustrates a very important general principle; the hydroxide ion is such a strong base that for the purposes of stoichiometric calculations it can be assumed to react completely with any weak acid that we will encounter ! do examples from the end of the chapter
Titrations
! volumetric analysis is a technique for determining the amount of a certain substance by doing a titration " a titration involves delivery (from a buret) of a measured volume of a solution of known concentration (the titrant) into a solution containing the substance being analyzed (the analyte) " the point in the titration where enough titrant has been added to react exactly with the analyte is called the equivalence or the stoichiometric point; use indicators to determine this point
- do a problem from the end of the chapter
4.9 Oxidation-Reduction Reactions
! note the following reaction (see Figure 4.18)
! reactions, like the one above, in which one or more electrons are transferred, are called oxidation-reduction reactions or redox reactions " photosynthesis, which stores energy from the sun in plants by converting carbon dioxide and water to sugar, is a very important redox reaction " in humans, the oxidation of sugars, fats, and proteins provides the energy necessary for life ! combustion reactions also involve oxidation and reduction; an example
" note the neither reactants nor products are ions; reaction still assumed to involve a transfer of electrons from carbon to oxygen; to explain this use the concept of oxidation states
Oxidation States
! concept of oxidation states (or oxidation numbers) provides a way to keep track of electrons in oxidation-reduction reactions, particularly redox reactions involving covalent substances ! define the oxidation states of the atoms in a covalent compound as the imaginary charges the atoms would have if the shared electrons were divided equally between identical atoms bonded to each other or, for different atoms, were all assigned to the atom in each bond that has the greater attraction for electrons ! see Table 4. ! do problems from the end of the chapter
