Melting Point Why are melting points carried out? to help characterize the sample if new compound, allows future characterizations by others indicates purity of the compound What is the definition of the melting point of a pure compound? the temperature at which the vapor pressures of the solid and liquid are equal What is the criteria for impurity to depress melting point? must be soluble in the compound can be solid or liquid (water or organic solvent) What are some examples of impurities that don?t depress melting point? insoluble impurities such as sand or charcoal What forces affect melting point? (Also, consider the effect on melting point of larger molecules, symmetrical molecules and R,S isomers, and hydrogen bonding molecules) ionic attractions, London forces, hydrogen bonding, dipole-dipole attractions larger molecules melt at higher temperatures than smaller molecules symmetrical isomers have higher melting points R and S enantiomers have the same melting point but racemic mixture and diastereomers will have different melting points Hydrogen bonding molecules have higher melting points than counterparts of similar molecular weight What are the eutectic point and the eutectic temperature? in melting point composition diagram, EP is the lowest point where mixture has very narrow melting range with 2 solids in liquid solution, EP is the point at which 2 solids of liquid solutions are at equilibrium ET is the lowest temperature at which a mixture of a certain concentration will start melting What are some features of a eutectic point? How do you read and interpret the melting point composition graph? What is the purpose for the mixed melting points experiment? to see if two compounds that have the same melting points are identical to confirm the identity of an unknown How should the sample be prepared? - unknown compound is mixed with a known compound with similar melting point Why should the sample be prepared in a specific manner? if the compounds are identical, the melting point with stay the same if the compounds are different, the melting point will be lower What are the criteria for the sample before packing? dry because moisture is an impurity finely powdered so it can be packed tightly to avoid air pockets How should it be packed? tightly so there is efficient transfer of heat and about 2-3 mm high in the capillary How should sample be heated and why? gradually, at about 1(C per minute because the heat should be transferred equally to the sample and thermometer What may happen if you heat the sample too quickly? the sample and thermometer will be at different temperatures melting range will be inaccurate What is the definition of melting range? the point at which the first drop of liquid appears to the point at which entire sample has melted Why does the sample shrink before melting? because the crystalline structure changes What happens if you have too much sample? - the sample will not heat instantaneously and will throw off the melting range What happens if sample is coarse (not powdered)? the heat is not transferred through the sample efficiently and equally which results in an inaccurate melting range Why is eutectic point difficult to detect? because it is hard to see when the first infinitesimal drop formed from the sample What are some sources of error when taking the melting point? moisture in the air sample contamination Mel-Temp thermometer not calibrated Prelab/Postlab Questions What does the Mel-Temp apparatus consist of? electrically heated aluminum block that accommodates 3 capillaries lower port illuminates sample upper port observes sample with 6-power lens Why do ionic compounds typically have high melting points? because the strong electrostatic attractions between the negative and positive ions must be overcome List 2 kinds of organic molecules that decompose before melting. ionic organic molecules organic molecules with strong hydrogen bonds (sucrose) Naphthalene has a higher molecular weight than succinic acid and yet, succinic acid has a higher melting point that naphthalene. What major factor accounts for the higher melting point of succinic acid? succinic acid has a higher molecular symmetry You have an unknown sample that has a melting range of 161( - 165(C. The three possibilities for this unknown are salicylic acid (mp: 158.5( - 159(C), sulfanilamide (mp: 165( - 166(C), and 4-acetaminophenol (mp: 169( - 170(C). Which of the 3 standards can you automatically eliminate? Why? salicyclic acid because the melting range is lower than the observed range and even if it was an impure sample of salicylic acid, the range would have to be under 158.5( - 159(. What are the 2 effects of recrystallizing an impure compound on the compound?s melting point? decreases melting point range shifts range upwards Apparatus Mel-Temp
Want to see the other 4 page(s) in Melting Point?JOIN TODAY FOR FREE!