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What is the melting point of a compound?
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It is the temperature at which the crystalline solid and corresponding liquid phase are at equilibrium at external pressure of 1 atm
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Where is the melting point range measured from?
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The temp when the first drop of liquid appears amongst crystals of the compound to the temp at which complete liquification takes place
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Look at the diagrams in the lab manual to see this
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Ok.
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What is a characteristic of pure organic compounds?
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They have sharp melting points (m.p. range approximately 1-2 degrees celsius)
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How do impurities (including residual solvent) affect melting point of compound?
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It lowers the m.p of a compound but also cause a widening of the range
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What is a criterion of purity for organic compounds that exist as solids at room temperature?
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A sharp m.p that is unchanged by recrystallization
-This could be used to idenfity compounds
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If we have 2 solid samples w/ same M.P. What conditions would they be identical and what conditions will they be different?
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Identical if mixed melting point is undepressed
(Not identical if mixed m.p is below that of the individual components and occurs over a wide range)
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If approx m.p of sample is known what can we do to save time?
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We can set the temperature to max until it is 15-20 degrees below expected m.p.
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At what rate should the dial be adjusted? To what degree/ minute do we want?
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We want the temperature to be raised at approx 2 degree per minute until melting process is completed
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What happens if our temperature goes up way too quickly as you approach melting point?
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Temperature observed when you see the first drop of liquid appear amongst crystals will probably be lower than true m.p
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Why is there such a difference?
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It is due to a lag (at this rapid rate of heating)
Lag exists between the thermometer reading and actual temperature of heating block
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If approx m.p is unknown what do we do?
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Prepare 2 samples of compound, first one raise temp as quickly as you can until melting is observed
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If approx m.p is unknown what do we do?
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For our 2nd sample, cool temperature until it is at least 20* below observed melting point
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We must correct the thermometers using the equation
Y= MX + B , what do the variables mean?
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Y= true temperature
X = Experimental temperature reading
M,B are provided constants
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What is the most common error ppl make in m.p analysis?
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They see the crystal become transparent and think this is when they record the rating. No. Record first temp until u see a drop of liquid
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