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Melting Point and Boiling Point Determination

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    The objective of this experiment was to determine the elting point and boiling point of malic acid and ethyl acetate through the rough and accurate method, and compare them to the theoretically obtained ones. The objective was achieved seeing as the melting and boiling point of the samples were obtained, however, there were deviations from the theoretically obtained ones which could be attributed to factors like human error (inaccuracies), Impurity of the sample and laboratory condition.

    INTRODUCTION

    Melting point, by definition Is the range oftemperature at which a substance changes from solid to liquid state, while boiling point refers to the temperature at orces that hold them in the liquid state. These properties are greatly affected by intermolecular forces of attraction which could be held responsible for how compounds will react to external forces like pressure or temperature. The size and structure orientation of the molecules are also a huge factor in the melting point and boiling point because the more complex the structure, and the smaller the structure, the harder it is to break the bonds between molecules.

    The purity of the sample is also another factor because theoretically speaking, when a sample is contaminated, its melting and boiling point would be ifferent from the pure ones, thus causing inaccuracies in the values obtained. Malic Acid is a solid, white crystalline granular powder which has a strong acidic taste, is combustible and is slightly hygroscopic. Its molecular weight is 134. 1 g/mol, and it is soluble in water and alcohol, slightly soluble in ether and insoluble in benzene. The molecule forms a Hydrogen bond with other molecules. It is bound by a strong force that keeps the molecules in the solid state at room temperature.

    The temperature range needed to reach to be able to break down this force is the melting point. The theoretical melting point of this compound is 130C.  It is an industrial acid and is slightly immiscible with water, but miscible in ethanol, acetone, diethyl ether and benzene. Its molecular weight is 88. 1 g/mol, and it is flammable. It forms a dipole-dipole attraction between molecules.

    This force is less strong than the H- bonding like malic acid but strong enough for it to be in the liquid state at room emperature. The temperature needed to overcome this force is the boiling point. The experiment is achievable in a three-hour time frame seeing as the materials are readily available in a laboratory, however the limited Mel-temp (apparatus) available during the time the experiment was conducted proved to be a barrier to the time efficiency seeing as several groups had to take turns borrowing it.

    MATERIALS AND METHODS

    We were asked to determine the melting point and boiling point of two reagents, eakers (250 mL) for the water bath and oil bath, a thermometer, rubber bands, three capillary tubes, a bulblet (5ml test tube), a paper clip or safety pin, wire gauze, forceps, a simple distillation apparatus which includes a distilling flask, iron stands, iron clamps, rubber tubing, a Liebig condenser, a receiving flask and cork stopper to properly seal the set-up. Cooking oil and water were used as the liquids for the hot baths.

    The Melting Point Determination of Malic Acid The melting point of malic acid was first determined using the rough method which is a prerequisite in identifying the exact melting point of the sample. A one end sealed capillary tube was used to contain the sample with dimensions of 1 mm in diameter and 5 cm in length. Using the paper clip or safety pin, the malic acid powder was inserted 1 cm inside the tube. The prepared capillary tube was then attached to a thermometer using a rubber band or rubber ring. The prepared set-up was immersed in the oil bath. With the use of a stove, the temperature of the oil bath increased.

    The temperature range recorded was from when the substance began to liquefy until it has completely melted. Melting Points and Boling Points In the next set-up in which the melting point of the substance is determined using the accurate method, the Mel-temp apparatus was used. Another capillary tube with malic acid and one end sealed is prepared and inserted inside the Mel-temp. The temperature was increased slowly at a rate of 1-2 degrees per minute. When the capillary tube is already clear, the temperature was recorded.

    The Boiling Point Method Determination of Ethyl Acetate In the determination of the boiling point of ethyl acetate using the rough method, a et-up for simple distillation was prepared. 50 mL of ethyl acetate was placed inside the distilling flask after rinsing it with the sample. The thermometer was attached with the cork stopper. The Liebig condenser was connected with the distilling flask using a cork stopper to properly seal it. A receiving flask was prepared at the end of the condenser. Water was placed inside the condenser and the sample was heated. At the first few drops (that will reserve 2mL) of the distillate, it was disposed.

    The range of temperature recorded was when the first drop of the distillate arrives at the eceiving flask until the temperature of the sample becomes constant after continuously heating.  The distillate was used for the last part of the experiment which involves the determination of the boiling point of ethyl acetate using the accurate method. A capillary tube with one sealed end is placed inside a bulblet, with the sealed side on top. The distillate or ethyl acetate was placed inside the bulblet until it reached the top of the capillary tube.

    It was then attached to a thermometer using a rubber band. The set-up was then submerged in a water bath. The temperature was increased slowly using a stove until the capillary tube emitted a continuous and rapid stream of bubbles. Then, the heat was removed. The range of temperature was recorded from when the capillary tube stopped giving out bubbles until the ethyl acetate went inside the capillary tube.

    Determination of the Melting Point of Malic Acid In the determination of the melting point of malic acid, the data in table 1 has been obtained. Using the rough method, the initial temperature has been obtained when the malic acid started to melt or liquefy, which was 130C. The final temperature was recorded when the malic acid completely melted which was 136C. By definition, the melting point of a substance is a range. Thus, using the rough method, malic acid has melting point of 130-136C. In the accurate method, the Mel- [email protected] was used.

    By the time the capillary tube became clear, a temperature of 142 C was recorded. Comparing the experimental results to the theoretical results, the data showed close values with a 6. 34% error in the accurate method and verlapping results in the rough method. The inaccuracy of the actual result as slightly hygroscopic and is reactive with oxygen. Once a container of laboratory synthesized malic acid had been opened, it starts oxidizing which then forms Oxaloacetic Acid.

    In the rough method, the temperature 69C, was recorded when the distillate first dropped in the receiving lask. The final temperature, 76C, was obtained when the temperature remained constant while continuously heating the ethyl acetate. It committed a percentage error of 1. 43% from the theoretical boiling point. In the accurate method, an initial temperature of 68 C was recorded when the continuous stream of bubbles stopped coming out of the capillary tube with a percentage error of 7. 1%.

    The data gathered from the experiment was relatively close to the theoretical data although with minor discrepancies. These errors may be attributed mostly to human error. Such are naccurate reading and measurement, exposure of the substance to contamination or impurities, or a delayed reaction time in recording the data. It also shows how the rough method and accurate method differs from each other. Ideally, the accurate method would show a closer value to the theoretical values because from definition, accuracy is the correctness or exactness of a measurement to a fixed and absolute value. But, experimentally, it shows that the rough method produced a result closer to the theoretical values.

    CONCLUSION AND RECOMMENDATIONS

    The determination of the melting point of Malic Acid using the Rough Method was ble to show that the melting range of the compound is This obtained result is accurate since the theoretical melting point of Malic Acid is 130C. However, by using the Accurate Method, results showed that the melting point of the experimented compound is 142 C. This 6. 34% discrepancy can be attributed to the time elapsed in which the compound was exposed to oxygen and lack of resources.

    A probable and effective solution in mitigating this change in melting point is to perform the accurate method immediately after the rough method was performed and the compound has solidifies. Also, the opening of the capillary tube in which the acked compound is contained should be covered at all times to prevent the number of apparatus (Mel-temp). On the other hand, in the determination of the boiling point of Ethyl Acetate, the boiling range of the compound is 69C to 76C as obtained using the rough method and the boiling point obtained using the accurate method is 68 C.

    The percent error committed was 1. 43% (rough method) and 7. 91% (accurate method). These discrepancies might have occurred due to human errors and unpractised laboratory techniques. The experiment was able to achieve its objectives. By deducing, it is safe to say hat it showed good results regardless of the minor discrepancies because the results were relatively close to the theoretical data.

    ASSOCIATED CONTENT

    The temperature at which a solid melts and becomes a liquid is the melting point. Since this requires that the intermolecular forces that hold the solid together have to be overcome, the temperature at which melting occurs will depend on the structure of the molecule involved – an example of the relationship between structure and properties. Hence, different compounds tend to have different melting points. A pure, nonionic, crystalline organic compound usually has a sharp and characteristic elting point (usually 0. 5-1 . OC range). A mixture of very small amounts of miscible impurities will produce a depression of the melting point and an increase in the melting point range.

    Consequently, the melting point of a compound is a criterion for purity as well as for identification. The melting point of an organic solid can be determined by introducing a tiny amount into a small capillary tube, attaching this to the stem of a thermometer centred in a heating bath, heating the bath slowly, and observing the temperatures at which melting begins and is complete. Pure samples sually have sharp melting points, for example 149. 5-1 50C or 189-190C; impure samples of the same compounds melt at lower temperatures and over a wider range, for example 145-148 C or 186-189C.

    It is standard practice (in order to make the most effective use of time) to carry out a rapid melting point determination initially (by heating rapidly) to establish an approximate melting point and then carry out at least two further careful determinations (by heating more gently, i. e. temperature changing only about 2 C/min) until you obtain two consistent values. The general ethod is to the heat the sample indirectly by placing the prepared sample (either packed in a glass capillary or on a glass cover slip) in or on a heated medium, these days this is most commonly a heated metal block such as a Mel-Temp apparatus.

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