CH1010 Exam #3 Study Guide For reference see Chemistry: An Atoms-focused Approach by Gilbert, Kirss, and Foster

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1 Fall 2016_CH1010_Dr. Kreider-Mueller CH1010 Exam #3 Study Guide For reference see Chemistry: An Atoms-focused Approach by Gilbert, Kirss, and Foster (In addition to this study guide you need to review your powerpoints & ALL practice problems!) Chapter 7_Stoichiometry: Mass Relationships and Chemical Reaction *Your best method of studying this chapter is to practice problems! Look at the mock exam, old exam, and class problems/worksheets/hw* Definitions to know: o Stoichiometry: the mole ratios among the reactants and products in a chemical reaction. o Chemical Equation: a description of the identities & proportions of reactants and products. o Reactants: substances consumed during a chemical reaction. o Products: substances formed during a chemical reaction. o Molecular mass: the mass in amu of one molecule of a molecular compound. o Molar mass: the mass of 1 mole of a substance. A convenient measure for comparing quantities of substances because the number of atoms/molecules in a typical sample is huge! o Formula mass: the mass in amu of one formula unit of an ionic compound. o Law of conservation of mass: the principle that the sum of the masses of the reactants in a chemical reaction is equal to the sum of the masses of the products. o Percent Composition: the composition of a compound expressed in terms of the percentage by mass of each element in the compound. o Empirical formula: a formula showing the smallest whole-number ratio of elements in a compound. o Limiting Reactant: a reactant that is consumed completely in a chemical reaction. The amount of product formed depends on the amount of the limiting reactant available. o Theoretical Yield: the maximum amount of product possible in a chemical reaction for given quantities of reactants. o Percent Yield: the ratio, expressed as a percentage of the actual yield of a chemical reaction to the theoretical yield. Be able to do the following: o Balance chemical equations. Mass and number of atoms are conserved in a chemical reaction (Law of conservation of mass) Be able to interpret chemical reactions conceptually: Ex: The balanced chemical equation for the reaction below is: 2A2 + B2 2A2B o Calculate grams of a compound if you are given moles of a compound Moles can be used to convert between molecular level particles & macroscopic masses of compounds 1 of 12

2 o Calculate moles of a compound if you are given grams of a compound o Use the chemical formula of a compound to calculate the number of atoms in a sample of a given compound Be able to relate mass or moles of the compound to the mass or moles of the elements that comprise the compound o Perform stoichiometric calculations given a balanced chemical equation Mol/mol ratios allow us to convert between different compounds Be able to perform mass to mole conversions Be able to perform mole to mass conversions Such calculations are summarized in the table below: Be able to calculate grams of a compound if you are given moles of a compound Determine the limiting reagent and excess reagent (Above table from General Chemistry: Atom s First by McMurry & Fay) Calculate (or work backwards from) the percent yield of a reaction 2 of 12

3 o % Yield = (actual yield/ theoretical yield) 100% Calculate an empirical formula from percent composition o If you are given mass percentages, assume you have 100g of the sample. Using a sample size of 100g makes the conversion from mass percent of an atom to grams of the atom very easy. o For ex. If you have 100g of compound that has composition of 84.1% carbon and 15.9% hydrogen, then the sample contains 84.1 g of carbon and 15.9 g of hydrogen. Calculate a molecular formula using percent composition or the empirical formula along with molecular mass of the compound. Given a molecular formula, be able to determine percent composition. o If you are given molecular formula, assume you have 1 mole of the sample. o Use the molecular formula to determine the moles of each atom in the molecule. Ex. A 1 mole sample of C6H12O6 contains 6 moles of C, 12 moles of H, and 6 moles of O. o You can then convert from moles of atoms to grams of atoms using atomic weight as a conversion factor. o Divide the grams of each atom by the molecular weight, then multiply by 100% to obtain the mass percent of each atom in the molecule Given a percent composition, be able to determine mass of an element or a compound. Chapter 8_Aqueous Solutions Definitions to know: o Molarity (M): the number of moles of solute per liter of solution: M=n/V o Standard Solution: a solution of known concentration that is used in chemical analysis. o Dilution: the process of lowering the concentration of a solution by adding more solvent. o Electrolyte: a material that conducts electricity because it contains free ions; ionic solutions and molten salts are examples. o Strong Electrolyte: an ionic substance that dissociates completely when it dissolves in water. o Weak Electrolyte: a substance that only partly dissociates into ions when it dissolves in water. o Nonelectrolyte: a molecular substance that does not dissociate into ions when it dissolves in water. o Weak Acid: an acid that only partially dissociates in aqueous solutions. o Strong Acid: an acid that completely dissociates into ions in aqueous solution. o Strong Base: a base that completely dissociates into ions in aqueous solution. o Weak Base: a base that only partially dissociates in aqueous solutions. 3 of 12

4 o Neutralization Reaction: a reaction that takes place when an acid reacts with a base and produces a solution of a salt in water. o Molecular equation: a balanced equation that describes a reaction in solution in which the reactants are written as undissociated molecules. o Spectator Ion: an ion that is unchanged by a chemical reaction. o Net Ionic Equation: a balanced equation that describes the actual reaction taking place in solution; it is obtained by eliminating the spectator ions from the total ionic equation. o Precipitate: a solid product formed from a reaction in solution. o Titration: an analytical method for determining the concentration of a solute in a sample by reacting the solute with a solution of known concentration. Key Concepts: o All ionic compounds soluble in water are electrolytes. o If molecular compounds dissolve in water, they may ionize completely, partially, or not at all. If the molecular compounds are strong acids, then they ionize completely, if they are weak acids or weak bases then they ionize partially o Concentration of a solution is expressed in molarity. o A used in a dissociation equation indicates a rxn taking place simultaneously in both directions (equilibrium). It is used in dissociation of weak electrolytes. o Remember that the terms Dissolve and Dissociate are not the same thing!!!! Both nonelectrolytes and electrolytes can dissolve as long as they are soluble compounds, but only electrolytes can dissociate. o Electrolytes can conduct electricity, nonelectrolytes cannot conduct electricity o Precipitates form when the attraction between the cations and anions in solution are so strong that they overcome the attractive forces of the polar H2O solvent molecules o Ions with higher charges tend to have higher lattice energies, which means it is difficult to break the electrostatic interactions between the cations & anions. Therefore, these ion pairs tend to form precipitates. o H + reacts with H2O to form the hydronium ion (H3O + ) o If an acid contains several hydrogen atoms (ex. Acetic acid), not all of the hydrogen atoms are necessarily acidic o Most bases have the form M(OH)x (where M = metal, x = a value of 1or 2), but NH3 is a weak base Be able to do the following: o Identify three major types of reactions: 1) Precipitation Reactions Precipitation rxns result in the formation of an insoluble solid product (precipitate) Use solubility rules to predict products of precipitation reactions. 2) Neutralization Reactions Acid-base neutralization rxns result in the formation of H2O & a salt The salt generated in the rxn may be soluble or insoluble, so you need to check the solubility chart! 3) Oxidation-Reduction Reactions (Redox) Use changes in oxidation numbers to recognize when a species undergoes oxidation or reduction. 4 of 12

5 o Identify electrolytes (strong or weak) & nonelectrolytes o Identify Arrhenius acids and bases (both strong & weak) You need to memorize the following information in the tables below: (Above table from General Chemistry: Atom s First by McMurry & Fay) Identify soluble and insoluble salts o You will be given a solubility chart on the exam so you do not need to memorize it but you must know how to use it! Given a set of reactants, you must be able to predict the products & write a balanced molecular equation, ionic equation, and net ionic equation o Molecular equation: all substances involved in the rxn are written using their complete formulas as if they were molecules. The phases of reactants and products are determined from the solubility chart or your own understanding of standard states (example CO2 is (g)). o Ionic equation: all compounds that dissociate into ions in aqueous solution are written as discrete ions Any weak electrolytes or nonelectrolytes remains written as a molecule, not as a dissociated set of ions H2O (l) remains in its molecular form Any insoluble compounds are written as a formula unit, rather than a dissociated set of ions. Any solids, liquids or gases remain in their molecular form o Net ionic equation: a chemical equation written so that spectator ions are removed Be able to identify spectator ions 5 of 12

6 This equation shows the overall change that occurs (ex: the formation of H2O or the formation of a solid) Assign oxidation numbers (oxidation states) to elements in compounds or ions. Determine the Molarity given the moles (or mass) of the solute and the volume of solution. o Molarity = (moles of solute)/(liters of solution) Determine the volume of solution or mass/moles of solute given the molarity of the solution Be able to work with the dilution equation o MiVi = MfVf o Be able to determine procedures for producing solutions of a particular concentration given the pure substance, such as a solid compound, or given a more concentrated solution of the substance. Correctly perform various types of stoichiometric calculations for reactions that involve solutions 6 of 12

7 o Given the volume or molarity of one reactant be able to find the volume or molarity of the other reactant or product. o Given reactants volumes and concentrations be able to find the mass of a precipitate from the limiting reagent. o Given reactants volumes and concentrations be able to find concentration of aqueous products. o Ex: Titration problems Chapter 9_Thermochemistry: Energy Changes in Chemical Reactions For this chapter you need to spend time practicing problems (end of chapter problems, mock & old exam, in-class problems, HMWK & WKSH problems)!!!! Definitions to know: o Chemical Energy: potential energy stored in chemical bonds. o Kinetic Energy (EK): the energy of motion o Potential Energy (EP): energy that is stored, either in an object because of its position or in a molecule because of its chemical composition o Internal Energy (E): the sum of all the kinetic and potential energies of all of the components of a system. o Thermochemistry: the study of the changes in energy that accompany chemical reactions. o Thermodynamics: the study of energy and its transformations. o First Law of Thermodynamics: the principle that the energy gained or lost by a system must equal the energy lost or gained by the surroundings. o System: the part of the universe that is the focus of a thermochemical study. o Surroundings: everything in a thermochemical study that is not part of the system. o Thermal Energy: the portion of the total internal energy of a system that is proportional to its absolute temperature. o Exothermic Process: one in which energy (usually in the form of heat) flows from a system into its surroundings. o Endothermic Process: one in which energy (usually in the form of heat) flows from the surroundings into the system. o Pressure-Volume (P-V) Work: the work associated with the expansion or compression of a gas. W = PΔV o Enthalpy Change (H): the energy absorbed by the reactants (endothermic reaction) or the energy given off by the products (exothermic reaction) for a reaction carried out at constant pressure. ΔH = ΔE + ΔPV o Heat Capacity (CP): the energy required to raise the temperature of an object by 1 C at constant pressure. 7 of 12

8 o Molar Heat Capacity: the energy required to raise the temperature of 1 mole of a substance by 1 C at constant pressure. o Specific Heat Capacity (cp): the energy required to raise the temperature of 1 g of a substance by 1 C at constant pressure. o Thermodynamic Standard State: conditions under which thermodynamic measurements are reported (298 K & 1 atm for each gas, 1 M for solutions) o Enthalpy of Vaporization (Hvap): the energy required to convert 1 mole of a liquid substance at its boiling point into the vapor state. o Enthalpy of Fusion (Hfus): the energy required to convert 1 mole of a solid substance at its melting point into the liquid state. o Heat of Sublimation (Hsubl): the amount of heat required for sublimation of a solid to a gas o Enthalpy of Reaction (Hrxn): the enthalpy change that accompanies a chemical reaction; also called heat of reaction. o Hess s Law: The Hrxn for a reaction that is the sum of two or more reactions is equal to the sum of the Hrxn values of the constituent reactions. o Standard Enthalpy of Reaction (H rxn): the enthalpy change that accompanies a chemical reaction that takes place at standard conditions. o Standard Enthalpy of Formation (H f): the enthalpy change that accompanies the formation of 1 mole of a compound starting from elements in their standard states. o Bond Dissociation Energy (D): the amount of heat required to break a chemical bond in an isolated molecule in gaseous state. Key concepts: o Law of Conservation of Energy: Energy cannot be created or destroyed, it can only be converted from one form into another Be able to apply this law to explain energy transfer between the system & surroundings during chemical and physical changes. o First Law of Thermodynamics: the total internal energy E of an isolated system is constant o Be able to identify what is the system & what are the surroundings for a given problem o State functions do not depend on how a change occurs o H is a convenient measure of energy transfer at constant pressure, which is the condition under which most chemical reactions are run. o You will need to be able to calculate ΔHrxn using Hess s law, standard heats of formations (ΔH f), or bond dissociation energies (D). You will be provided with any necessary information unless you are solving for that value o In general, stable systems tend to lower energy states. o Bond breaking requires energy (endothermic) o Bond forming releases energy (exothermic) o More stoichiometric gas moles in products will increase the volume of system (expansion work) o Less stoichiometric gas moles in products will decrease the volume of system (contraction work) o We usually represent energy, work and enthalpy in terms of joules (J) or kj o If you reverse the arrow of a reaction, the magnitude of Hrxn remains the same, but the sign is switched 8 of 12

9 o If you multiply or divide a reaction by a factor, then the value of Hrxn will multiply or divide by the same factor. o If you are given a chemical equation and a value of H located to the right of the equation (see below), the values of H refer to the heat released/absorbed when reactants are converted to products in the molar amounts represented by coefficients in the balanced equation CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) H = 802 kj o Sublimation (s g), fusion (s l), vaporization (l g) are endothermic processes!!! The reverse processes are exothermic o Be able to explain why isvap> Hfus? Be able to do the following: Explain the changes in heat, work, & energy for a system in which a change occurs Identify state functions o Explain the characteristics of state functions You need to understand and be able to apply your knowledge of the information provided in the table below. Draw a heating or cooling curve for a given substance. o Identify phase changes from a heating or cooling curve o Why is phase change a constant temperature process? o What is thermal energy used for during heating of a liquid? o What is thermal energy used for during boiling of a liquid? Explain the difference between changes in internal energy (E) and changes in enthalpy (H) 9 of 12

10 Relate value & sign of H to the balanced equation & perform appropriate stoichiometric calculations Use standard molar enthalpy, heat of fusion and heat of vaporization in stoichiometric calculations. Use Hess s Law to determine ΔH for an overall reaction. o When reversing a reaction: the sign of H flips. o When multiplying/dividing the reaction with coefficient (#) the H is also multiplied/divided by that #. Explain why it is important to identify a standard state o Be able to write standard formation reactions o Use standard heats of formation to determine ΔH for a reaction and perform related calculations. o ΔHrxn = npδhf (products) nrδhf (reactants) Use Bond dissociation energies to estimate the value of ΔH for a given reaction Important Equations: o EK = (1/2)mv 2 (where m = mass, v = velocity) o E = Efinal-Einitial o V = Vfinal-Vinitial o H = Hfinal-Hinitial o w = PV (where P = pressure, V = volume) o E = q + w = q PV (where q = heat transfer) o qv = E (where qv = heat transfer at constant volume) o qp = E + PV = H (where qp = heat transfer at constant pressure) o Universe = system + surroundings o EUniverse = Esystem + Esurroundings o ΔHrxn = npδhf (products) nrδhf (reactants) o ΔHrxn = ΔHf (bonds broken) + ΔHf (bonds formed) or o ΔHrxn = D (Reactants bonds) D(Products bonds) D=bond dissociation energy 10 of 12