Popular in Intro to chemistry
Popular in Chemistry
This 12 page Study Guide was uploaded by Gabrielle Notetaker on Monday April 25, 2016. The Study Guide belongs to CHM103 at University of Rhode Island taught by Dombi in Spring 2016. Since its upload, it has received 13 views. For similar materials see Intro to chemistry in Chemistry at University of Rhode Island.
Reviews for Chem Review
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 04/25/16
Chem Test 4 Topics: Reaction Rates, Equilibrium, Acids, Bases & Salts Chapter 8 Reactions: Spontaneous Processesprocesses that take place naturally with no apparent cause or stimulus. Exergonic Processes processes that give up energy as they take place. The energy appears on the right side of the balance equation as a negative number. Endergonic processes processes that gain or absorb energy as they take place. Energy appears on the right side of balanced equations as a positive number Entropy Entropy is a measurement of the disorder or randomness of a system. The more disorderly a system is, the higher its entropy o More order= lower entropy, higher potential energy, requires energy to exist o Less order= higher entropy, lower potential energy, releases energy to exist Entropy of a system can also be shown as the change in the number of molecules from the product to the reactants o More molecules in the product means increase in entropy Gibbs Free Energy G = H – TS o G= Gibbs free energy o H= enthalpy or the energy in the chemical bongs o T= temperature o S= entropy Both the reactants and products of a reaction contain some measure of free energy Delta G is the change in free energy products reactants A reaction is said to be spontaneous if the energy of the products is less than the energy of the reactants When Delta G is negative, the reaction is spontaneous Factors That Influence Spontaneity A process will always be spontaneous if it gives up energy and the entropy of the system increases. So if the energy in the bonds is less in the products and there is more disorder, then Delta H is a negative and o ΔG = ΔH – TΔS = ( number). A process that absorbs energy, positive ΔH, is spontaneous only if an increase in entropy is large and compensates for the increase in energy. (e.g. Ice spontaneous melts at 20 C.) o ΔG is negative because –TΔS > ΔH A process that causes an entropy decrease in the system will be spontaneous only if a decrease in energy also occurs that is large enough to compensate for the o entropy decrease. (e.g. Water spontaneously freezes at 0 C.) This makes ΔG is negative. o ΔH is negative and > –TΔS Entropy Changes with Phase Changes As the temperature increases the change in phase becomes spontaneous and the TΔ term dominates the equation. o Solids have the most order, lowest disorder,lowest entropy. o Liquids have a higher level of disorder, a higher entropy. o Gases have the most disorder, the highest entropy Stable Substances Stable substances are substances that do not undergo spontaneous changes at the prevailing conditions. Matter change states because ΔH and TΔS values change at different temperatures and pressures Reaction Rates Reaction rates can be determined experimentally by measuring the change in concentration of a reactant or product and dividing the change by the time required for the change to occur, using the following equation: o ∆C is the change in concentration of a reactant or a product that occurs in a measured amount of time, ∆t. o The value of ∆C is calculated by subtracting the initial concentration, C , 0 from the final concentration, C. t How Reactions Occur • An explanation of how reactions occur is called a reaction mechanism. A reaction mechanism is often expressed as a number of processes that must take place for reactions to occur. The following assumptions make up a reaction mechanism: o Assumption 1: Reactant particles must collide with one another in order for a reaction to occur. o Assumption 2: Reactant particles must collide with sufficient total energy for collision to result in a reaction. o Assumption 3: In some cases, especially molecules, reactant particles must collide with a specific orientation for the reaction to occur Why collisions are necessary • Reactant particles must collide if they are to react (assumption 1). With few exceptions, molecules cannot react with each other if they do not come in contact. During collisions, some bonds are broken, atoms are exchanged, and new bonds form. Why Minimum Collision Energy Is Necessary • The requirement that some bonds of reactant molecules must break if a reaction is to occur makes the requirement of minimum collision energies valid. • At all temperatures above absolute zero, the bonded atoms of molecules are vibrating and stretching the bonds. The energy associated with these vibrations is called internal energy. Why Specific Orientations are Necessary • When the reactant particles are ions, there is no requirement for a specific orientation because the charge is distributed evenly on the particle. So every orientation is equivalent for a reaction. • When the reactant particles are molecules, there is a requirement for a specific orientation because there is likely to be only one or two sites on the molecule that will allow for a reaction. In a molecule with a dipole, one the atom that are part of the dipole are likely to be the reaction center of the molecule. • In comparison, reactions that involve ions coming together are generally faster than reactions involving molecules because with ions every interaction is correctly oriented • Internal energy can be increased by collisions. This results in an increase in the vibrational amplitude which increases the chances for a bond to break. • Activation Energy • In some reaction mixtures, the average total energy of the molecules is too low at the prevailing temperature for a reaction to take place at a detectable rate. The reaction mixture is said to be stable. • For many stable mixtures, the addition of a small amount of energy starts the reaction which then continues without the addition of any more stimulus or energy from an outside source. • The small amount of outside energy needed to start a spontaneous processes is called the activation energy Molecular Orientation • Orientation effects are related to which side or end of a reacting particle actually contacts another particle during a collision. • The orientation of reacting particles is not important in some reactions such as 2+ 2 those between reacting ions in a solution. For example, Ca ions react with CO 3 in solution to form solid CaCO is 3nsoluble and settles out of the solution. Both ions can be considered to be spherical charged particles, so their orientation toward each other when they collide does not influence the reaction rate. • Orientation effects become important when the reacting particles are not spherical as in the following hypothetical reaction: AB + CD AC + BD • In this reaction it is obvious that the chances for a reaction to occur are better if the Aend of the first molecule hits the Cend of the second molecule. Energy Diagrams • Transition state • • The Nature of Reactions • Reactions between oppositelycharged ions in solution occur almost instantaneously. This is because the ions are strongly attracted to each other because of their opposite electrical charges. o Reactions between covalentlybonded molecules in which covalent bonds have to be broken often take place slowly. This is partially because the molecules collide only because of their random motion, and all collisions do not result in a reaction. o Other characteristics of reactants such as their physical state (gases, liquids or solids), their molecular sizes, and whether or not they are polar are also important influences of some reaction rates. The Temperature of Reactions • The effect of temperature on reaction rates can also be explained using the concept of molecular collisions. o An increase in the temperature of the reactants corresponds to an increase in the velocity and the kinetic energy of the molecules. o An increase in velocity will increase the number of molecular collisions that take place in a fixed amount of time and will thus increase the rate of the reaction. o An increase in the kinetic energy of the colliding molecules will increase the internal energy of the molecules and also increase the number of molecules with the required minimum activation energy. o Ion reactants work well with increases in temperature. o Molecular reactants have a limit to the increase in temperature as at some point the molecule decompose. The presence of catalysts • Catalysts are substances that speed up chemical reactions without being used up in the reaction. o Homogeneous catalysts are substances that are distributed uniformly throughout a reaction mixture. o Heterogeneous catalysts are substances normally used in the form of solids with large surface areas on which the reactions take place. o One explanation for catalytic behavior is that catalysts provide an alternate reaction pathway that requires less activation energy than the normal pathway. o Another explanation proposes that solid catalysts provide a surface on which reactant molecules adsorb with favorable orientations to each other. Adsorbed molecules with favorable orientations are located close enough to each other to react rapidly. Chemical Equilibrium • All chemical reactions can (in principle) go in both directions and products, located to the right of the arrow, can react to form reactants, located to the left of the arrow. This condition is indicated by the use of a double arrow pointing in both directions as shown below: H 2g) + I 2g) 2HI(g) o When the rate of the reaction toward the right is equal to the rate of the reaction toward the left, the reaction is said to be in equilibrium. o When a reaction is in equilibrium, the concentrations of reactants and products remain constant as time passes. o The unchanging concentrations of reactants and products in a reaction at equilibrium are called equilibrium concentrations. Equilibrium Expression • o In this equation, the brackets,[ ], stand for molar concentrations of the reactants, A and B, and the products, W and X. It is seen that each reactant concentration is raised to a power equal to the stoichiometric coefficient of that reactant in the equilibrium equation. Equilibrium constant o The K in equilibrium expressions is called the equilibrium constant. o As long as the temperature does not change, it has a constant value because none of the concentrations used to express it change with time once equilibrium is established. o A relatively large value for K indicates that the equilibrium position is toward the right or products side of the equilibrium. o A small K indicates an equilibrium position toward the left or reactant side of the equilibrium The position of equilibrium • The position of equilibrium is an indication of the relative amounts of reactants and products present in a reaction mixture at equilibrium. • The position is said to be to the right when the amount of product is significantly more than the amount of reactant. • The position is to the left when more reactant is present than product. • • The factors that effect equilibrium are o concentrations of reactants and products o reaction temperature o catalysts In general, Le Châtelier's principle predicts a shift away from the side to which something is added and toward the side from which something is removed. Catalysts cannot change the position of an equilibrium because it lowers the energy barrier for both the forward and reverse reactions; therefore, catalysts speed up both forward and reverse reactions and cannot change the position of equilibrium. Chapter 9 Arrhenius + An Arrhenius acid is any substance that provides hydrogen ions, H , when dissolved in water. An Arrhenius base is any substance that provides hydroxide ions, OH , when dissolved in water. BronstedLowry Acid • A Brønsted acid is any hydrogencontaining substance that is capable of donating a proton (H ) to another substance, same definition as Arrhenius acid. • A Brønsted base is any substance capable of accepting a proton from another substance. Conjugate Acids And Bases • The conjugate base (NO ) is 2ormed when a HNO acts as a2Brønsted acid by + donating a proton. Similarly, the conjugate acid (H O ) i3 formed when water (H 2) acts as a Brønsted base by accepting a proton. • A Brønsted acid (such as HNO ) and 2ts conjugate base (NO ) form wha2 is called a conjugate acidbase pair. • The same name is given to a Brønsted base (such as H O) and i2s conjugate acid (H 3 ). • A conjugate acidbase pair differ by 1 proton H + Binary Acids Solutions of binary acids such as HCl (aq) are formed by dissolving binary compounds such as HCl gas in water. The HCl gas before being dissolved in water is said to be anhydrous (meaning without water). The anhydrous gas is named hydrogen chloride. The water solution of the gas is called hydrochloric acid. Some hydrogencontaining compounds such as HCl, HI, HBr, and H S form acidic 2 solutions when they are dissolve in water. The names of the acid solutions, such as the hydrochloric acid given above, can be obtained by following four rules. Rules For Naming Binary Acids • Rule 1: Drop the word hydrogen from the anhydrous compound name. For example, HI, called hydrogen iodide becomes "iodide". • Rule 2: Add the prefix hydro to the result of step 1. "Iodide" becomes "hydroiodide". • Rule 3: Drop the suffix ide from the result of step 2 and replace it with the suffix ic. "Hydroiodide" becomes "hydroiodic". • Rule 4: Add the word acid to the end of the name as a separate word. The final name is "hydroiodic acid The SelfIonization Of Water • A sample of absolutely pure water does not contain only H O molecules.2 In addition, small but equal amounts of H O hyd3onium ion and OH hydroxyl ions are present. • The reason for this is that in one liter of pure water 1.0 x 10 moles of water molecules behave as Brønsted acids and donate protons to another 1.0 x 10 7 moles of water molecules, which act as Brønsted bases.. The reaction is: + H 2(l) + H O(2) 3 O (aq) + OH(aq) 7 + • As a result, absolutely pure water contains 1.0 x 10 mol/L of both H O 3 and OH. • The term neutral is used to describe any water solution in which the concentrations of H O a3d OH are equal. • Thus, pure water is neutral because each of the ions is present at a concentration of 1.0 x 10 M. 7 The Ion Product Of Water + • The reaction given earlier for the formation of H O and O3 in pure water is called the selfionization of water. The reversible nature of the reaction (indicated by the double arrow) means that an equilibrium is established and an equilibrium expression can be written for the reaction. The equilibrium expression is: Acidic Solution + • An acidic solution is a solution in which the concentration of H O hydron3um ion is greater than the concentration of OH hydroxyl ion. It is also a solution in which the pH, or power of the hydronium ion, is less than 7. Basic Or Alkaline Solution • A basic or alkaline solution is a solution in which the concentration of OH hydroxyl ion is greater than the concentration of H O . It 3s also a solution in which the pH is greater than 7 up to 14. The pH Concept + • It is often the practice to express the concentration of H O in 3n abbreviated form called the pH rather than to use scientific notation. + + • It is also a common practice to represent the H O ion3by the simpler H ion. + + • The pH notation is defined below, using H in place of H O : 3 pH = log[H ], or in alternate form [H ]= 1x10 pH • Thus, the pH is seen to be the negative of the exponent used to express the molar concentration of H using scientific notation. Properties Of Acids • All acids have certain properties in common such as tasting sour and producing + H 3 ions when dissolved in water. • In addition, all acids undergo characteristic doublereplacement reactions with solid oxides, hydroxides, carbonates, and bicarbonates • Acids react with and dissolve certain metals to yield hydrogen gas in a redox reaction. • The activity series is a tabular representation of the tendencies of metals to react with H .+
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'