CHEM 1120, Chapter 5 Notes
CHEM 1120, Chapter 5 Notes Chem 1120-001
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This 12 page Class Notes was uploaded by Callie Simpson on Tuesday February 23, 2016. The Class Notes belongs to Chem 1120-001 at East Carolina University taught by James E. Collins in Winter 2016. Since its upload, it has received 23 views. For similar materials see Introduction to Chemistry for the Allied Health Sciences in Science at East Carolina University.
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Date Created: 02/23/16
Chapter 5: “Classification and Balancing of Chemical Reactions" A Chemical Reaction: Consider the reaction involving carbon and oxygen: C (s) + O 2g) CO (g) 2 Read: “Elemental carbon combines with oxygen gas to yield carbon dioxide gas.” On the submicroscopic scale, we can imagine one atom of carbon combining with a molecule of oxygen to form a molecule of carbon dioxide. How many molecules of oxygen react with 2 atoms of carbon? 3 atoms of carbon? If we started with a dozen atoms of carbon, how many dozen Oxygen molecules will react? Another Chemical Reaction: Consider the reaction of elemental hydrogen with elemental oxygen to produce water: H 2g) + O (g2 H O 2 Unlike the previous reaction involving carbon, this reaction is not balanced with respect to oxygen! How can we balance it? Balancing Chemical Equations: When expressing a chemical reaction, the formulas for reactants and products must make “chemical sense”. Subscripts in formulas are not altered to provide mass balance. The coefficients in front of the formula is changed. Therefore: H 2g) + O (g2 2H O 2 But now “Hydrogen” is out of balance. We can add a coefficient of “2” in front of2H to bring it into balance: 2H 2g) + O (g)2 2H O 2 The Law of Conservation of Mass: The number of elements on both sides must balance and the mass on each side must balance 2H 2g) + O (g)2 2H O 2 2 equivalents = 4.0 2 equivalents = 36.0 g g 1 equivalent = 32.0 g Reactant’s Mass: Product’s Mass: 4.0 g 36.0 g + 32.0 g 36.0 g Tips for balancing chemical equations: Assume the most complicated species in the reaction has a coefficient of one. Consider grouping polyatomic ions as a single group. Balance elements that show up only once on both sides of the equation first. If free uncombined elements show up on either side of the equation, balance them last. *Your turn: Balance the following chemical equations. N H + H O N + H O 2 4 2 2 2 2 C 6 +6O CO2+ H O 2 2 Na 2O +A3Cl Al 3CO ) +2NaCl 3 3 Types of Reactions: 1. Precipitation Reactions 2. Acid-Base Neutralization Reactions 3. Oxidation-Reduction Reactions Solubility Guidelines for Ionic Compounds in Water: *Not all ionic compounds are considered soluble in water. *Your turn: After consulting the table of “Solubility for Ionic Compounds”, which of the following ionic compounds is(are) soluble in water? 1) AgNO 3 2) NaPO 4 3) BaCO 3 Dissolution of an Ionic Compound in Water: As a polar solvent, water can break the forces of an ionic bond. Note the orientation in which water aligns itself around each ion. Ion-Dipole Interactions: Pay careful attention to how water aligns itself around each ion. Dissolving MgCl in wate2: What happens when MgCl dissolves in water? 2 +2 - MgCl (s)2+ H O Mg 2 (aq) +2 Cl (aq) O H H H O H H O H H H H O H H O H O O Cl H O Mg+2 H H O Cl H H H H H H O O H H O H H O H H H H H O O Recognize that MgCl is 2n ionic compound. Note when chlorine is combined - with a metal as in this compound, it is a chloride anion, Cl . In the absence of the metal, chlorine would be neutral and there would exist a covalent bond between the two chlorine atoms. Dissolving Na SO in 2ater4 What happens when Na SO dissolv2s in4water? + -2 Na S2 (s) 4 H O 2Na (a2) + SO 4 (aq) In general, water can not break the forces of covalent bonds found in sulfate. H H O O H H H H H H H H O O H O H O Na+ H O SO42 H O H O Na+ H H H H O O H H H H O O O H H H H H O H H Sulfate remains associated because it is molecular. *Your turn: Predict the products of the following precipitation reactions and balance each: Pb(NO ) (aq) + NaI (aq) 3 2 FeCl 3aq) + NaOH (aq) Acids: Historically, acids were classified as substances that taste sour, react with active metals to produce hydrogen gas and turn certain organic compounds characteristic colors. Bases: Historically, bases were originally classified as substances that are slippery, taste bitter and turn certain organic substances a characteristic color. Arrhenius Perspective of Acids and Bases: An acid is a substance that increases the hydrogen ion, H + concentration when dissolved in water. For example, when HCl is dissolved in water, it ionizes as follows: - A base is a substance that increases the hydroxide ion, OH concentration when dissolved in water. For example, when NaOH dissolves in water, it ionizes to form hydroxide as follows: Common acids and bases Strength of Electrolytes: Strong acids Weak acids and bases are and bases are weak strong Strong acids completely dissociate when dissolved in water: H2O + HCl (g) H (aq) + Cl (aq) H O HNO3 (l) 2 H (aq) + 3O (aq) H O H2SO 4(l) 2 H (aq) + HSO (aq) 4 Strong bases completely dissociate when dissolved in water: H 2 + NaOH (s) Na (aq) + OH (aq) H2O + Ca(OH) 2(s) Ca (aq) + 2 OH (aq) An acid-base reaction (Neutralization Reaction): A closer look at an acid-base reaction: A gas forming reaction: Oxidation-Reduction Reactions: Originally oxidation referred to as the combination of oxygen with another substance. CH 4(g) + 2 2 (g) 2O (g) +22 H O 4 Fe (s) + 3 2 Fe O (s) When a 2 2 3 substance burns in the presence of air, the reaction is referred to as a Combustion Reaction. The Organic Perspective of Oxidation-Reduction Reactions: An Organic Molecule is oxidized when the molecule acquires an oxygen atom or loses a pair of hydrogen atoms. For Ionic compounds, an oxidation-reduction reaction results An Organic molecule is reduced when the molecule loses an oFor Molecular compounds, an oxidation-reduction reaction results when electron density is shifted towards an atom and away from another.example, reconsider: Oxidation occurs when an atom or group of atoms lose The carbon atom in methane loses hydrogen from its formula and. acquires oxygen in the product,2CO therefore methane is oxidized. In contrast, an oxygen atom of O acquires two 2 hydrogen atoms and is therefore reduced in the process. The Inorganic Perspective of Oxidation-Reduction Reactions: ***LEO the Lion says GER Loss of Electrons is Oxidation Gain of Electrons is Reduction Examples of an Oxidation-Reduction Reaction: Redox or Non-Redox? To determine whether a reaction is a redox or a non-redox reaction, we must compare the charges of all of those elements in the reactants versus those in the products. If the charges change, then a redox reaction has occurred. Oxidation Numbers “Hypothetical Charges”: We should be familiar with deducing the charge of the elements in an ionic compound, but what about molecular compounds? Elements in molecular compounds do not have a true charge? They share electrons. Therefore, we must consider a hypothetical charge for elements in a molecular compound. This hypothetical charge is referred to as an oxidation number. Predicting the charge of some common elements: General rules for assigning oxidation numbers: *Your turn: What is the oxidation number of Scandium in the compound Sc 2 3 What is the oxidation number of phosphorus in the species HPO ?2- 4 What is the oxidation number of Iodine in the compound IF ? 3 Oxidizing and Reducing Agents: An oxidizing agent is a substance that causes another substance to become oxidized. Therefore, it becomes reduced in the process; i.e.: it acquires electrons. Some common examples that are normally considered oxidizing agents include: H 2 ,2KMnO , Na 4r O 2 2 7 A reducing agent is a substance that causes another substance to become reduced. Therefore, it becomes oxidized in the process; i.e.: it gives up the electrons. Examples include: NaBH , 4n, Na, NaNO 2 Cautionary note: Oxidizing agents should not be stored next to reducing agents! Summary of terminology with respect to redox reaction: Oxidation of Copper Metal: Cu (s) + 4 HNO (aq) Cu(NO ) (aq) + 2 NO (g) + 2 H O 3 3 2 2 2 (l) Oxidation-Reduction Reactions: *Combining two elements: (Combination reaction) Potassium and chlorine react to form Potassium Chloride. *Decomposing a compound into its elements: Mercury (II) oxide is decomposed into its elements mercury and oxygen. *Displacing one element with another: (Single displacement) Silver (I) ions are displaced by Copper metal to form the compound, Copper (II) nitrate and Silver metal. *Lithium metal displaces Hydrogen gas from water.
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