Chemistry: Cooling of Water - Lab Report Example

EXPERIMENT NUMBER: COOLING OF WATER Aim The objective of this experiment is to examine the cooling of water when certain ionic substances are added to it. Safety Precautions In the course of the experiment, there are certain measures that ought to be put into consideration to ensure the safety of all the people in the laboratory. This particular experiment involves the dissolution of ionic substances in water. Sodium Hydroxide is one of the reagents that will be used. It is corrosive in nature, and caution must be taken to prevent skin contact with NaOH. The other reagents; ammonium sulphate, sodium nitrate, and potassium nitrate are acidic in nature. Contact with skin is harmful since they cause irritation. Aside from being careful in conducting the experiment, protective wear is recommended. Since this experiment involves a lot of measuring of reagents, the hands and eyes stand the most risk of being splashed on with the chemicals. In this light, use of gloves and wearing eye protection is advised. It is also advisable to wear closed to the laboratory since spilled chemicals can pour on the feet. These measures involve clothing, but it is necessary to exhibit proper laboratory conduct. Measuring of reagents should be done from the working surface and not in the air; this reduces the chances of chemicals falling. Introduction What temperature changes are noted, when NaOH, NaNO3, KNO3, and (NH4)2SO4 are dissolved in water? This question formed the basis of the experiment involving ionic substances as solutes, and water as the solvent. Water, in its pure state, shows specific cooling behaviour. It is characterized by a heat capacity of 4.2Jg-1K-1. Owing to this high heat capacity, it means that for the temperature of an aqueous solution to be raised by one unit (0C or K, which gives the same change), a lot of energy will be required (Reger, Goode and Ball). Similarly, for the temperature of an aqueous solution to fall by one unit, a lot of energy has been released to the surrounding. Water is a di-ionic molecule made up of a hydroxyl ion (OH-), and a hydrogen ion (H+) also referred to as a proton. The oxygen of water has a pair of dissociated electrons which gives it a partial negative charge. Oxygen, being more electronegative than hydrogen, gives the hydrogen a partial positive charge. Due to these properties, water exists as a dipole. Different water molecules form hydrogen bonds amongst themselves, between the partial negative of one molecule, and partial positive charge of another molecule. Presence of partial charges in a water molecule makes it possible for ionic substances to dissolve in water. When anionic compound is added to water, it goes into solution. This process is accompanied by temperature change where either the temperature rises, or falls. When an ionic substance is dissolved in water, the process is referred to as hydration. In a case whereby hydration is accompanied by temperature increase of the solution, the process is exothermic. When the reverse is true, the hydration process is characterized as endothermic (Reger, Goode and Ball). Variables: Since the experiment measure temperatures, this is the dependent variable; it keeps changing throughout the experiment. The water is the constant since its volume and composition remains unchanged in the entire period. The ionic substance is the dependent variable because whatever temperature changes are noted, are dependent on the ionic compound. Hypothesis: The type, and magnitude of temperature change occurring when an ionic substance dissolves in water, will depend on the intensity of bonds within the structure of the ionic compound. Materials and reagents Sodium hydroxide Ammonium Sulphate Sodium Nitrate Potassium Nitrate Distilled water 100mL measuring cylinder Thermometer 250mL glass beakers Gloves Procedure 1. 0.1 moles of the ionic solute were measured. This required calculations to be made, to determine what volume of solute liquid would contain 0.1 moles of the salt. 2. 100mL of distilled water was measured using a measuring cylinder and put in a 500mL glass beaker. 3. The temperature of the water was measured and recorded in the data table. 4. The ionic substance was added to the water while using a thermometer to stir the mixture. 5. After stirring, the final temperature of the mixture was noted too, and recorded in the data table. Table 1. Data of temperature change recorded when 0.1 moles of ionic substance was added to 100mL of water at room temperature and pressure. Initial temperature was that recorded before adding of solute. The final temperature is the reading obtained after mixing the solution. Trial 1 Trial 2 Trial 3 Average Average ΔTemp. (0C) Initial (0C) Final (0C) Initial (0C) Final (0C) Initial (0C) Final (0C) Initial (0C) Final (0C) NaOH 23 31.5 23 33 24 37.5 23.3 34 10.7 (NH4)2SO4 23 22 23.2 23 24 23 23.4 22.7 -0.7 NaNO3 22.2 20.8 23.4 22 25 22 23.5 21.6 -1.9 KNO3 24 20 24 20.5 24 19.8 24 20.1 -3.9 Results From the data obtained from the dissolving of the above ionic salts in water, one thing is evident. When the compound is dissolved in water, the temperature of the water can either increase or decrease. Though different trends are noted in the temperature change of water, all the solutes in this experiment are ionic in nature. This means that another factor other than nature of bonds existing are responsible for the direction in which the temperature changes (Hill, Holman and Holman). From the collected data, it is evident that of the four compounds dissolved in water. Only NaOH resulted in the overall water temperature being raised. This is an example of an exothermic reaction. Such kind of a reaction is characterized by the release of energy to the surrounding. (NH4)2SO4, NaNO3, and KNO3, on the other hand, are endothermic in nature. The ions of these compounds interact with polar water molecules, and energy is absorbed. Discussion and evaluation When the structure of water molecule is considered it is noted to be bipolar. Since there are both positive and negative charges in the water molecules, albeit partial, they allow for the formation of hydrogen bonds (Hill, Holman and Holman). Dissolving an ionic substance in water, results in the formation of an aqueous solution. An ionic compound has both negative and positive charges in its structure. The structure in question is referred to as a lattice if the compound is in solid form. In this particular experiment, four different compounds were used. For NaOH, the constituent ions are Na+ and OH-. In (NH4)2SO4, the cation is NH4 +, whereas the anion is the sulphate, SO42-. For the compound NaNO3, the cation in the compound is Na+, and the nitrate ion NO3- is responsible for the negative charge. In KNO3, positive charge is due to K+ cation, while the NO3- gives the compound the negative charge. When an ionic substance is dissolved in water, there are two processes that take place. The first is the separation of ions in the lattice, if the compound is in solid state. NaOH for instance can either be in solid or liquid state. When in solid, the lattice between sodium, and hydroxyl ions would require to be broken down first. This process of bond breaking requires energy thus is accompanied by a fall in temperature of water. The second process is the interaction of water molecules with ions of the solute. Depending on the type of interaction taking place between the different ions in the mixture, the temperature of the water will either rise or fall (Hill, Holman and Holman). When Na+ ions go into solution, they interact with OH- ions. Since the ionic compound is itself contributing similar anions, OH-,then the other reaction is favoured more. This is the combination of H+ ions with OH- to form water. The process releases energy into the surrounding, causing an increase in temperature to be noted. When (NH4)2SO4 goes into solution, NH4+ is attracted to OH-. It is vital to note that both ions have delocalized electron pair in their structure thus the degree on interaction is reduced. H+ interacts with SO42- which has two electrons it can use in forming the new bonds. As for the nitrates NaNO3 and KNO3, both the ion and cation are monovalent. NaNO3, however, causes a lesser temperature fall than KNO3. Potassium has nineteen protons in its nucleus, whereas sodium has eleven. Because of this, the bond strength to be broken in the case of KNO3 is stronger thus the greater temperature change recorded. Errors and limitations The set-up used is prone to interference from the external environment. This would limit the accuracy of the readings obtained in the experiment. The errors that were encountered in this experiment were systematic, where measurements and temperature readings could be done inaccurately (Hill, Holman and Holman). Improvements Using a calorimeter or a set-up that would ensure no heat energy gain or loss to the environment would be an improvement on the experimental set-up. A Styrofoam cup that has a lid would serve as an efficient option for the set-up. This would allow for stirring to be done safely, and the insulation would reduce interferences. When pellets of NaOH are used, they should be weighed in a sealed container. NaOH is hygroscopic in nature and when exposed, it absorbs water vapour from the atmosphere. Such an occurrence would interfere with the results of the experiment. Works Cited Hill, Graham, John Holman and John Stranger Holman. Chemistry in context: laboratory manual. Cheltenham, GL: Nelson Thornes, 2001. Print. Reger, Daniel L., Scott R. Goode and David W. Ball. Chemistry: Principles and Practice. Stamford: Cengage Learning, 2009. Print. Read More