Transcript M1_1
Unit 1 How do we distinguish substances? The central goal of this unit is to help you understand and apply basic ideas that can be used to distinguish the different substances present in a system. Chemistry XXI In your groups, discuss your answers to the following two questions: Why do we care? In which relevant contexts would be important for humans to distinguish between different substances? What would the consequences be of failing to do so? Unit 1 Chemistry XXI How do we distinguish substances? Pollution detection and control Health monitoring Drug development Food control Resource exploration and management Crime investigation Unit 1 How do we distinguish substances? Chemistry XXI FOUR MAIN MODULES M1. Searching for Differences Identifying differences that allow us to separate components. M2. Modeling Matter Using the particulate model of matter to explain differences. M3. Comparing Masses M4. Determining Composition Characterizing differences in particle’s mass and number. Characterizing differences in particle’s composition. Integration To illustrate the power of chemical ideas and models in distinguishing and identifying chemical substances we will focus our attention in our own planet’s atmosphere. What is it made of? Chemistry XXI How do we separate its components? How do we identify them? How do we explain their properties? How do we model their behavior? Unit 1 How do we distinguish substances? Chemistry XXI Module 1: Searching for Differences Central goal: To identify distinctive properties of substances present in a system that can be used to identify and separate them. The Challenge Analysis What is this? Chemistry XXI Imagine that you are interested in investigating the properties and composition of certain system, from the Earth’s atmosphere to an unknown material found in a crime scene or polluting the ocean. How would you analyze the system? What properties would you measure? How would you separate its different components? Central Problems Most systems, natural or synthetic, are complex. Chemistry XXI They tend to be a combination of hundred or thousands of different substances (mixture vs. pure substance) Smell > 800 different substances Their properties change as a function of position (heterogeneous vs. homogeneous). Their properties change as a function of time (dynamic vs static); How do we characterize the state of a system? Chemistry XXI We need to identify and measure properties that allow us to determine how a system changes in time and space. Consider, for example, our atmosphere. Let′s think! What properties could we use to study its state and behavior? Let′s think! We use properties such as: Chemistry XXI Temperature Pressure Density Concentration Measurable Properties For some systems, its is useful to measure properties that are size dependent (extensive properties): Chemistry XXI Mass Volume Energy In general, it is more useful to measure properties that do not depend on the size of the system (intensive properties): Temperature Density Pressure Concentration Chemical Composition From a chemical point of view, it is of crucial interest to find the “composition” of the system (the nature of the substances present in it). How do we do it? Chemistry XXI A first common step is to try to separate the different components. How would you propose to separate the different substances present in our atmosphere? Let′s think! Differentiating Characteristics Chemistry XXI All chemical analysis is based on the assumption that each substance, no matter how simple or complex, has a “differentiating characteristic” that can be used to separate/detect/identify it. For example, in the case of the atmosphere, we can take advantage of the different boiling/condensation points of the substances present in it. Let′s explore! Substance oC K Water 100 373.15 Oxygen -182.95 90.20 Argon -189.35 87.36 Nitrogen -195.79 77.36 T(K) = T(oC) + 273.15 Liquid Nitrogen Differentiating Characteristics What other properties of a substance can be used as “differentiating characteristics”? Physical Properties Chemistry XXI Density Boiling Point Melting Point Surface Tension Heat Capacity Conductivity Viscosity Solubility We look for properties that: Do not depend on the size of the sample (Intensive Properties); Can be measured without changing the chemical nature of the substances. Phase Behavior Chemistry XXI The separation of the main components of our atmosphere can be accomplished using boiling/condensation points as “differentiating characteristic.” However, to implement a successful separation we need to understand the phase behavior of substances. Let′s think! Phase Transitions A sample of solid water at -20 oC is heated at a constant rate to 120 oC. During the experiment you measure the temperature of the system as a function of time, as well as the amount of energy absorbed by the system during the heating process. What would you expect to see? E (kJ) Chemistry XXI T(oC) Absorbed (+) 100 0 t 0 0 100 T(oC) Physical Change The observed behavior is characteristic of all substances undergoing phase changes. T (oC) Chemistry XXI 100 0 During a phase change, the physical properties of the substance change Constant T t Energy Transfer Chemistry XXI During a phase transition, energy is transferred between the system and the surroundings. Convention Energy added (+) Energy released (-) Energy added (kilojoules- kJ) Phase Stability For any given substance, different phases are stable at different values of temperature and pressure. Chemistry XXI P S S S S S S S S S S L L L S S S S S S S S L L L L S S S S S S S S S L L L L S S S S S S S S L L L L L L L S S S S S S L L L L L L G S S S S S L L L L L G G G S S S S L L L G G G G G G S S S G G G G G G G G G G S G G G G G G G G G G G G G G G G G G G G G G G G G T Chemistry XXI Phase Changes and Phase Diagrams Phase Diagrams Important Points for Water Chemistry XXI Vapor Pressure Curve Normal boiling point Normal freezing point Triple point T(°C) 100 0 0.0098 P(mmHg) 760 =1 atm 760 4.58 Two Different Substances Chemistry XXI Notice differences in scales Carbon dioxide Let′s think! Water What are the major differences? Separating Air A variety of people are interested in separating air components for different purposes: Purification, environmental analysis, mining. How do they do it? What differentiating characteristics may they use? STEP 1 Chemistry XXI Filtration – Based on differences in particle size. Used to separate solids from liquids too. Separating Air STEP 2 Distillation- Based on the difference in boiling points. Chemistry XXI Air is normally cooled in stages until it reaches –200°C. Water vapor condenses, and is removed using absorbent filters; Carbon dioxide freezes at –79 °C, and is removed; Oxygen liquefies at –183 °C; Nitrogen liquefies at –196 °C. Air Composition Substance % by V Nitrogen 78.1 Oxygen 20.9 Argon 0.934 Argon Fractional Distillation Chemistry XXI Let′s apply! Assess what you know Let′s apply! Chemistry XXI Boiling of a liquid occurs when its vapor pressure (the pressure exerted by the vaporize substance) equals the atmospheric pressure. Which liquid, A or B, is more volatile (higher vapor pressure at same T)? Liquid Which liquid boils at a lower temperature? What is its normal boiling temperature? Notice log scale A B Gas Let′s apply! Fuel Separation A rich fuel mixture containing the following hydrocarbons has been extracted from underground: Propane Butane Neo-Pentane 2-Heptene Chemistry XXI The mixture is at an initial temperature of 5 oC. Based on the data provided, design a procedure to separate each component. What differentiating characteristic would you use? Propane Butane Neo-Pentane 2-Heptene Fuel Separation At 5 oC and 1 atm: Chemistry XXI Propane (G) Butane (G) Neo-Pentane (L) 2-Heptene (L) We can distill the liquids (heat up) and the gases (cool down). Chemistry XXI In your groups, come up with one question about something you don’t understand at this point? Searching for Differences Summary Chemistry XXI The identification of a differentiating characteristics is crucial to: identify, detect, separate, or quantify the amount of substances is a system. Phase properties are particularly useful because they can be measured/induced by changing temperature and pressure. Chemistry XXI For next class, Investigate what model of matter would allow us to explain why two substances, such as water and carbon dioxide, have different phase behaviors. What assumptions about the submicroscopic structure of matter do we need to make to explain it?