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Chemical Thinking Book Notes, Unit 1 Module 1

by: Jennifer

Chemical Thinking Book Notes, Unit 1 Module 1 Chem 151

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Notes from U1M1 of the Chemical Thinking textbook.
General Chemistry
John Pollard
Class Notes
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This 4 page Class Notes was uploaded by Jennifer on Sunday July 17, 2016. The Class Notes belongs to Chem 151 at University of Arizona taught by John Pollard in Fall 2015. Since its upload, it has received 104 views.


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Date Created: 07/17/16
Chemical Thinking, Volume I U1: How do we distinguish substances? Central goal: apply basic ideas to distinguish different substances Module 1: Searching for Differences Central goal: recognize distinctive properties of chemical substances that can be used to identify and separate them. Key: important terms, important concepts Differentiating Characteristics  Most systems are mixtures: can be homogeneous (combinations of substances with uniform composition/properties, looks like one substance, i.e. air) or heterogeneous (visibly different substances in same or different phases, i.e. a salad)  The assumption behind chemical analysis: Each substance, whether simple or complex, has one or more differentiating characteristic that makes it unique. Finding this differentiating characteristic lets us detect, identity, separate, and quantify substance.  Good differentiating characteristics: are intensive properties (don’t depend on how much of a substance we have, i.e. melting point, density, conductivity) and are not extensive properties (do depend on the amount of a substance, like mass or volume). Besides being intensive, they should be unique for each substance and should be able to be measured selectively.  Examples: boiling point (temperature at which substance changes from liquid to gas at a given pressure. At the pressure at sea level, this is the normal boiling point). To separate air, we could cool a sample at constant pressure until each gas condensed.  Transformation of one pure substance is a phase transition or phase change. The transition temperature is often used as a differentiating characteristic.  In phase changes at constant pressure, the temperature of the system is constant until one phase is fully transformed into another. The temperature where the transition occurs depends on pressure.  The diagram represents changes in temperature over a period of time. At 1, the substance is a gas and T (°C) 1 is cooling down, so the temperature 2 drops. At 2, the gas condenses into a 3 solid as energy (heat) is removed, but the temperature does not change 4 during this phase transition. At 3, the 5 liquid is cooling down. At 4, the liquid freezes into a solid, but does not change temperature during the phase time transition. At 5, the solid begins to cool.  Transitions that require energy to be added: melting (solid  liquid), boiling (liquid  gas), sublimation (solid  gas). Transitions that release energy/need energy removed: freezing (liquid  solid), condensation (gas  liquid), deposition (gas  solid).  This diagram shows energy changes during phase transitions. At 1, a T (°C) solid is heating up, and energy is slowly increasing as result. At 2, the 5 temperature is constant during a phase transition (melting), but there is 4 3 a “jump” in energy to move from the solid to liquid state. At 3, the 2 liquid is heating up and thus gaining 1 energy, until another phase transition (boiling) at 4, where again ΔE the temperature does not change but the energy “jumps” up. At 5 the gas is gaining energy as it heats up. By convention, the energy of the substance is negative.  Phase transitions lead to changes in physical properties of a substance, i.e. density, and is thus a physical change where the chemical properties are not altered. (I.e boiling2H O turns it from a liquid to a gas, so the substance is physically different, but the gas is sti2l H O NOT hydrogen and oxygen.) Phase Diagrams  Since temperatures of phase transitions change with pressure, range of stable temperatures for a phase will depend on the pressure.  This can be represented by a graph, where each area represents pressures and temperatures where a substance is stable in each phase, and where boundaries represent phase transitions; phase diagrams, different for each substance.  P 2 SOLID LIQUID 5 3 1 4 GAS T This is a phase diagram. Each combination of pressure and pressure can be treated like a point on the graph, with pressure on the y-axis and temperature on the x-axis. If a point falls in the area marked solid (high P, low T) then it will be a stable solid. If a point is in the liquid area (high P, high T), it’s a stable liquid. The gas area (low P, low to high T depending on substance) shows pressures and temperatures where the substance is a stable gas. If the point is on line 1, both solid and gas are present, and this is the temperature/pressure where the substance undergoes sublimation/deposition. If the point is on line 2, melting/freezing is occurring and both solid and liquid are present. On line 3, condensation/boiling occurs. 4 is the specific temperature and pressure of the triple point, where all three phases exist.  The phase change between liquid and gas doesn’t have a large density change, so beyond the critical point (marked 5 on the diagram), gas and liquid aren’t distinguishable and the phase is called a supercritical fluid.  If the liquid gas transition line (line 3 on the diagram) is viewed separate from the phase diagram, it’s called a vapor pressure curve. Vapor pressure is the pressure of evaporated gas from an enclosed liquid at a given temperature (higher at higher temperature, as more molecules have the energy to escape to the gas phase). When vapor pressure equals external pressure, the substance boils.  More volatile liquids (lower boiling points) have more vapor pressure at any given temperature. Comparing vapor curves is useful to separate substances because of this. Separations  Filtration is mechanical, and can separate solids from liquids with a physical barrier. Separation is not complete, as some solid passes through filter and some liquid is attached to the solid.  Crystallization induces the formation of a solid phase by changing temperature or pressure or by adding new substances to a mixture, affecting solubility and leading a solid to separate.  Distillation uses the differentiating characteristic of boiling points by changing a liquid to a gas, then recondensing the liquid substances. The boiling point of the most volatile component will be when the temperature remains constant and this component will change into a gas. The process is repeated until all liquids are separated. --- Fractional distillation is necessary when boiling points are close, and in this case evaporated fluid enters a vertical column where temperature decreases gradually, so substances will condense at different temperatures. Summary: By using differentiating characteristics, which must be intensive and unique to each substance, substances can be identified and separated. One common differentiating characteristic is the temperature/pressure at which phase changes occur, which can be analyzed with phase diagrams. Key skills to apply: Identifying differentiating characteristics, illustrating how temperature changes with time and how energy changes with temperature, creating/analyzing phase diagrams, developing strategies to separate mixtures (i.e. using distillation to separate a mixture of liquids based on boiling point).


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