IRandMP2008 - C onfirm ation of theI de ntity of your...

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Confirmation of the Identity of your Unknown Acid Today you will measure the infrared (IR) spectrum and measure the melting point (mp) to verify the identity of your unknown acid You Will Write A Lab Report For This Lab- It Is Due Next Week !! IN CLASS QUIZ NEXT WEEK !! (Nov-13)
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Last Week: You based your identification on the measurement of two parameters, the molar mass and the pK a . But that may not be enough!!!! For example consider: Molar mass pKa Potassium bisulfate 136.2 1.9 Sodium bisulfate hydrate 137.4 1.9 Potassium dihydrogenphosphate 136.1 7.2 Sodium dihydrogenphosphate 138.0 7.2 We need to measure additional properties. The melting point and the infrared spectrum of a compound can be used to help identify an unknown. That’s your job for today
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First We’ll Talk About Melting Behavior & Melting Points Facts 1) A Pure Crystalline Material Has A Characteristic Melting Point and it Melts Over a Very Narrow Temperature Range. 2) A Different Pure Crystalline Material Will Have A Different Melting Temperature. E.g. Ice vs. Sugar 3) Impurities Lower the Melting Point and Make the Transition Broader SO We Can Use Melting Point Measurements to Identity Solids and/or Test If They Are Pure
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What Determines the Melting Point ? In a pure crystalline sample of a solid substance, all the molecules are held together in a regular three-dimensional array (a crystal) by various intermolecular forces. The stronger the forces, the more stable the crystal, and the higher its melting point. At the melting temperature, the thermal energy just equals the attractive forces that hold the crystal together. For example Ice. In this case the molecules, H 2 O, are held together by weak interactions and Ice has a low melting temperature.
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In a molecular solid, the units that make up the crystal are molecules , and the forces between them are weaker than covalent bonds. So the solid melts at a relative low temperature. Pure water (ice) has a
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IRandMP2008 - C onfirm ation of theI de ntity of your...

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