Introductory Analytical Chemistry - Kinetics and Equilibrium

Information about Introductory Analytical Chemistry - Kinetics and Equilibrium

Published on August 5, 2014

Author: danlester

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Chem 14: Chem 14 Topics: Topics Introduction Chemical Kinetics Chemical Equilibrium Equilibria of Slightly Soluble Compounds Acids and Bases Gravimetric Analysis Topics: Topics Introduction Nature and scope of analytical chemistry Major areas of application The analytical method Analytical Techniques Rejection of outliers Terms frequently used Chemical Kinetics Chemical Equilibrium Equilibria of Slightly Soluble Compounds Acids and Bases Gravimetric Analysis Topics: Topics Introduction Chemical Kinetics Definition Rates of Reactions Importance of knowing reaction rates Expressing the rates of reactions Factors affecting rates of reactions Examples Chemical Equilibrium Equilibria of Slightly Soluble Compounds Acids and Bases Gravimetric Analysis Topics: Topics Introduction Chemical Kinetics Chemical Equilibrium The state of equilibrium Reaction Quotient Expressing equilibria with pressure Solving equilibrium problems Le Chatelier’s Principle IV. Equilibria of Slightly Soluble Solids Acids and Bases Gravimetric Analysis Topics: Topics Introduction Chemical Kinetics Chemical Equilibrium Equilibria of Slightly Soluble Compounds Concept Writing solubility product expression Determining Ksp from solubility Determining solubility from Ksp Predicting the formation of a precipitate Acids and Bases Gravimetric Analysis Introduction: Introduction Nature and scope of analytical chemistry It includes both qualitative and quantitative analysis. Analytical chemistry has bounds which are among the widest of any technological discipline. The craft of analytical chemistry is not in performing a routine analysis, but in improving established methods, extending existing methods to new types of samples, and developing new methods for measuring chemical phenomena. Introduction: Introduction B. Major areas of application Fundamental research Product development Product quality control Monitoring and control of pollutants Assay Medical and clinical studies Introduction: Introduction C. The analytical method Definition of the problem Choice of method Sampling Preliminary sample treatment Eliminating interferences Final measurement Method validation Assessment of results Introduction: Introduction D. Analytical Techniques Gravimetric – quantitative analysis by weight , usually involving the precipitation, filtration, drying, and weighing of a precipitate Photo: www.chemcollective.org Introduction: Introduction D. Analytical Techniques Volumetric – any method of quantitative chemical analysis in which the amount of the substance is determined by measuring the volume that it occupies, specifically the volume that reacts with the other substance. Also called titrimetric analysis . Photo: www.acdlabs.com Introduction: Introduction D. Analytical Techniques Spectrometric – measures the intensity of electromagnetic radiation emitted or absorbed by the analyte Photo: www.cnx.org Photo: www.directindustry.com Introduction: Introduction D. Analytical Techniques Electrochemical – studies the chemical response of a system to an electrical stimulation Photo: chemwiki.ucdavis.edu Introduction: Introduction D. Analytical Techniques Chromatographic – looks at physico-chemical properties after separation. Photo: en.wikipedia.org Photo: www.tutorvista.com Introduction: Introduction E. Rejection of outliers (Dixon’s Q-Test) In a set of replicate measurements of a physical or chemical quantity, one or more of the obtained values may differ considerably from the majority of the rest. The rejection of suspect observations must be based exclusively on an objective criterion and not on subjective or intuitive grounds. This can be achieved by using statistically sound tests for " the detection of outliers “. Introduction: Introduction E. Rejection of outliers (Dixon’s Q-Test) Method: The values are arranged in ascending order: Statistical Q-value ( Q exp ) is calculated. The obtained Q exp is compared to a critical Q-value ( Q crit ) found in tables. Data: 4.7,  6.2,  6.4,  6.5,  7.8 Null Hypothesis: There is no a significant difference between the suspect value and the rest of them, any differences must be exclusively attributed to random errors. 6.2 - 4.7 Q expt = -------------- = 0.484 7.8 - 4.7 Introduction: Introduction E. Rejection of outliers (Dixon’s Q-Test) Method: The values are arranged in ascending order: Statistical Q-value ( Q exp ) is calculated. The obtained Q exp is compared to a critical Q-value ( Q crit ) found in tables. Q expt < Q crit 0.484 < 0.710 Introduction: Introduction E. Rejection of outliers (Dixon’s Q-Test) Interpretation: Q expt < Q crit 0.484 < 0.710 * Null hypothesis is accepted at 95% CL. * The suspected value cannot be characterized as an outlier. * The suspected value must be retained in all subsequent calculations. Introduction: Introduction F. Terms frequently used Accuracy Analyte Assay Background Blank Calibration Clear solution Concentration Constituent Control test Detection limit Determination Equivalent Estimation Interference Matrix Method Positive test Precipitate Precision Primary standard Procedure Reaction Reagent Residue Sample Sensitivity Standard Standardization Technique Test Test solution Titration Validation of methods Chemical Kinetics: Chemical Kinetics What is Chemical Kinetics? Chemical Kinetics is the study of reaction rates, the changes in concentrations of reactants (or products) as a function of time. Shrdocs.com Chemical Kinetics: Chemical Kinetics Diverse rates of reactions Trickyzone.com Wikimedia.org Wallpaperup.com Nytimes.com Chemical Kinetics: Chemical Kinetics Importance of Knowing Reaction Rates How fast medicine acts How fast blood clots How long it takes for cement to harden How fast food spoils Etc… Chemical Kinetics: Chemical Kinetics Expressing Reaction Rates Reactants → Products A → B _ _ _ Chemical Kinetics: Chemical Kinetics Expressing Reaction Rates Chemical Kinetics : Chemical Kinetics Expressing Reaction Rates Chemical Kinetics: Chemical Kinetics Factors that affect reaction rate Nature of Reactants Concentration Physical State Temperature Presence of catalyst Chemical Kinetics: Chemical Kinetics Factors that affect reaction rate Nature of Reactants Concentration Physical State Temperature Presence of catalyst Under any given set of conditions, each reaction has its own characteristic rate, which is determined by the chemical nature of the reactants. Some substances are naturally more reactive than others, therefore, undergo rapid chemical changes. Chemical Kinetics: Chemical Kinetics Factors that affect reaction rate Nature of Reactants Concentration Physical State Temperature Presence of catalyst Molecules must collide to react. Rate α collision frequency α concentration Chemical Kinetics: Chemical Kinetics Factors that affect reaction rate Nature of Reactants Concentration Physical State Temperature Presence of catalyst Molecules must mix to collide. The frequency of collisions between molecules also depends on the physical states of the reactants. The more finely divided a solid or liquid reactant, the greater its surface area per unit volume, the more contact it makes with the other reactant, and the faster the reaction occurs. Chemical Kinetics: Chemical Kinetics Factors that affect reaction rate Nature of Reactants Concentration Physical State Temperature Presence of catalyst Molecules must collide with enough energy to react. Rate α collision energy α concentration Silberberg Chemical Kinetics: Chemical Kinetics Factors that affect reaction rate Nature of Reactants Concentration Physical State Temperature Presence of catalyst A catalyst increases the rate of a chemical reaction without undergoing a net chemical change. A catalyst lowers the activation energy for a reaction. Chemical Kinetics: Chemical Kinetics Examples Chang Chemical Kinetics: Chemical Kinetics Examples Chemical Kinetics: Chemical Kinetics Examples Chemical Kinetics: Chemical Kinetics Examples Chemical Kinetics: Chemical Kinetics Examples Chemical Kinetics: Chemical Kinetics Examples Chemical Kinetics: Chemical Kinetics Examples Because it has a nonpolluting combustion product (water vapor), hydrogen gas is used for fuel aboard the space shuttles and in earthbound cars with prototype engines: 2H 2 (g) + O 2 (g) → 2H 2 O (g) Express the rate in terms of changes in [H 2 ], [O 2 ], and [H 2 O] with time. When [O 2 ] is decreasing at 0.23 M/s, at what rate is [H 2 O] increasing? Chemical Equilibrium: Chemical Equilibrium The state of equilibrium Irreversible reactions - burning of paper (combustion) Reversible reactions - CuSO 4 ·5H 2 O ↔ CuSO 4 + 5H 2 O When the forward and reverse reaction rates are equal, the system has reached equilibrium. Chemical Equilibrium: Chemical Equilibrium The state of equilibrium http://www.chem.queensu.ca/ Chemical Equilibrium: Chemical Equilibrium Equilibrium constant expression Consider the following equilibrium system: w A + x B ⇄ y C + z D K c = The numerical value of K c is calculated using the concentrations of reactants and products that exist at equilibrium. Chemical Equilibrium: Chemical Equilibrium Equilibrium constant expression Examples: N 2 ( g ) + 3H 2 ( g ) ⇄ 2NH 3 ( g ) ; K c = PCl 5 ( g ) ⇄ PCl 3 ( g ) + Cl 2 ( g ) ; K c = CH 4 ( g ) + H 2 ( g ) ⇄ CO ( g ) + 3H 2 ( g ) ; K c = Chemical Equilibrium: Chemical Equilibrium Reaction quotient Reaction quotient, Q, is a specific ratio of product and reactant concentration terms. The value of Q changes continually until the system reaches equilibrium, Q = K. Chemical Equilibrium: Chemical Equilibrium Reaction quotient The direction of the reaction Chemical Equilibrium: Chemical Equilibrium Expressing equilibria with pressure Chemical Equilibrium: Chemical Equilibrium Solving equilibrium problems Summary: Read and understand the problem. Determine the reactants and products, setup the equation, then balance. Write the K or Q expression. Identify the given and required quantities. Make a table showing the initial, the change, and equilibrium concentrations, if applicable. Arithmetic and algebra is always involved in equilibrium calculations. Check your answer. Chemical Equilibrium: Chemical Equilibrium Le Chatelier’s Principle If a system at equilibrium is disturbed by a change in conditions (concentration, pressure, or temperature), it will temporarily not be at equilibrium, but will then undergo a net reaction to reach equilibrium again. Chemical Equilibrium: Chemical Equilibrium Le Chatelier’s Principle Effect of concentration Add more acetic acid When we add more of any component to a system in equilibrium, that addition constitutes a stress. The system relieves this stress by increasing the concentrations of the components on the other side of the equilibrium equation. We say that the equilibrium shifts in the opposite direction. Chemical Equilibrium: Chemical Equilibrium Le Chatelier’s Principle Effect of concentration Chemical Equilibrium: Chemical Equilibrium Le Chatelier’s Principle Effect of temperature An increase in temperature drives an exothermic reaction toward the reactants (to the left). A decrease in temperature drives an exothermic reaction toward the products (to the right). For an endothermic reaction, of course, the opposite is true. An increase in temperature drives an endothermic reaction toward the products (to the right). A decrease in temperature drives an endothermic reaction toward the reactants (to the left). Chemical Equilibrium: Chemical Equilibrium Le Chatelier’s Principle Effect of pressure An increase in pressure shifts the reaction toward the side with fewer moles of gas. A decrease in pressure shifts the reaction toward the side with more moles of gas. When the moles of gas are the same in a balanced reaction, an increase or decrease in pressure results in no shift of the reaction. Chemical Equilibrium: Chemical Equilibrium Le Chatelier’s Principle Effect of a catalyst For a reversible reaction, catalysts always increase the rates of both the forward and reverse reactions to the same extent. Therefore, the addition of a catalyst has no effect on the position of equilibrium. Equilibria of Slightly Soluble Compounds: Equilibria of Slightly Soluble Compounds Access denied… Acids and Bases: Acids and Bases Access denied… Gravimetric Analysis: Gravimetric Analysis Access denied… PowerPoint Presentation: References Bettelheim, Brown, Campbell and Farrell. Introduction to General, Organic and Biochemistry 8 th edition, 2007 Silberberg. Chemistry, The Molecular Nature of Matter and Change 5 th Edition, 2009 Chang. Chemistry 10 th Edition, 2010 Kealey & Haines. Instant Notes: Analytical Chemistry, 2002

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