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Preparation and Properties of Soap 1. INTRODUCTION Soap is made commercially from triacylglycerols or "triglycerides" that are found in fat or tallow. As triglycerides are esters, these can undergo a "hydrolysis" reaction which is the breaking apart of a molecule upon its addition reaction with water. Esters hydrolyze readily in the presence of strong acids or bases that act as catalysts. Hydrolysis of triacylglycerides with a strong alkali such as NaOH or KOH is called "saponification" as the products are glycerol and the sodium or potassium salts respectively of the long-chain fatty acids which are known as "soaps". A soap molecule has two parts: a lipid-compatible end, formed by the hydrocarbon chain, and a water- compatible end, formed by the carboxyl group. The lipid-compatible end readily dissolves in lipids such as grease, whereas the water-compatible end tends to remain dissolved in the water. In this way, soap emulsifies grease into small colloidal drops which can then be washed away. The cleansing power of soap is affected by "hard" water. Water that is "hard" contains calcium or magnesium bicarbonate or sulfate which form insoluble salts with soap. These salts precipitate to form soap scum and "bathtub ring" leaving less soap in the water to do the cleaning. In this experiment, a sample of soap will be prepared and some chemical properties of soap will be investigated. 2. EXPERIMENTAL SECTION 2.1 The Preparation of Soap Fat such as tallow or lard and 2 M NaOH will be made available. Caution: Care must be exercised when handling concentrated alkalis such as NaOH. If an alkali is accidentally spilled on your skin or clothing it must be washed immediately with copious amounts of water. The laboratory supervisor must also be notified in such cases. (1) Use a top-loading balance to weigh approximately 10 g of lard (fat) into a 150 mL beaker. Record this mass in the LRS. [See also Question (3) in Section 3.] (2) Place the beaker containing the fat and NaOH solution on a hot plate and set the heating control to about 50% of the maximum setting. (3) When the fat has melted place a magnetic stirring bar in the beaker and set the stirring speed to maintain a steady, gentle stirring of the fat in the beaker. (4) Maintain the steady stirring and slowly add 25 mL of 2 M NaOH to the fat. Take care to avoid "splattering" of the reaction mixture. Raise the temperature control to about 75% and watch for a solid to form. The formation of solid could occur quickly. (5) Add 20 mL of concentrated NaCl solution to the mixture and continue stirring over low heat until it is completely mixed, then remove the beaker from the hotplate stirrer and allow the mixture to stand. The mixture should then separate into two layers: (i) the upper layer consisting of soap "paste" and (ii) the lower, brine (aqueous) layer containing any excess NaOH and glycerol. (6) When the mixture has separated, collect the upper "soap cake" layer, dry it on filter paper, and use it to conduct the experiments detailed in Section 2.2. 2.2 The Properties of Soap (1) Take two clean 20 mL test tubes. Place a small piece of soap (about the size of a pea) in one of the tubes and a similar sized piece of the fat in the other. (2) Add 5 mL of deionized water to each tube. (3) Heat the tubes in a hot water bath and use disposable pipettes to agitate the mixtures so as to try to break up and "dissolve" the solid in each tube. Whilst heating, you should use the pipettes to suck up liquid and "squirt" it back into the tube in attempting to thoroughly mix the contents of each tube. To enable a comparison, try to agitate/mix each of the solutions to the same extent. After thorough heating and mixing allow the tubes to stand and assess the amount of lather (bubbles) that are formed in each case. Record these observations in Table 1 of the LRS. [See also Question (4) in Section 3.] (4) To each of the test tubes in Step (1) add four drops of "Universal" indicator. For each "solution", compare the resulting colour of the indicator with the colour chart available in the laboratory and hence determine the pH of each of the solutions. Make a note of these in the laboratory report sheet (LRS). [See also Question (5) in Section 3.] (5) Take three clean test tubes. Add small, equal quantities of soap (about the size of half a pea) to each tube. (6) Place 5 mL of deionized (i.e. Milli-Q) water in the first tube, 5 mL of tap water in the second, and 5 mL of a 1% w/v CaCl2 solution in the third. (7) Heat the tubes in a hot water bath. Agitate the mixtures as before and "squirt" the solutions with disposable pipettes until the soap dissolves. To enable a comparison, try to agitate/mix each of the solutions to the same extent. Allow the solutions to cool. When the solutions are cool, use the pipette to mix and agitate the mixtures as before by sucking up some liquid and "squirting" it back into the tube with a view to trying to create lather (bubbles) in each tube. To enable a comparison, try to agitate/mix each of the solutions to the same extent. Assess the relative amount of lather formed in each tube in terms of it being "large", "medium" or "small". Record these observations in Table 2 of the LRS. [See also Question (6) in Section 3.] Retain the solution of soap and distilled water for use in the next step. The other solutions can be discarded. (8) Place 5 mL of deionized water in a clean test tube and add 1 mL of peanut oil. To the test tube containing distilled water and soap retained in Step (4), add 1 mL of peanut oil. (9) Use disposable pipettes to agitate and mix the solutions in each of the tubes as was done previously for other test solutions. To enable a comparison, try to agitate/mix each of the solutions to the same extent. After agitation, immediately start a timer and observe the tubes. In Table 3 of the LRS record an estimate of tsep, the time (in seconds) taken for layers to form in each of the tubes. [See also Question (7) in Section 3.] 3. QUESTIONS AND CALCULATIONS Instructions Answer ALL questions using the space provided in the LRS. Attach a separate sheet, clearly labelled with the question number/s, if you have insufficient space in the LRS. Reports that are not submitted using the LRS will NOT be accepted. Answer Question (1) in Box (1), Question (2) in Box (2) and so on. Marks awarded for each question are shown at the bottom of each box along with the assessment criteria. Show all working and/or reasoning for every question. Answers that are given as a single word, number, letter, etc., will score zero. Final numerical answers should be quoted to the correct number of significant figures. Questions (1) Draw a schematic representation of a soap molecule, showing its "lipid compatible" (hydrophobic) end and its "water compatible" (hydrophilic) end. (2) Write an overall balanced chemical equation for the formation of soap assuming the starting material to be the triacylglycerol that is comprised of three molecules of the same fatty acid, 16:0 palmitic acid (CH3(CH2)14COOH). In the equation use NaOH as the alkali. (3) Using the balanced equation derived in Question (2), calculate the total mass of soap that is expected to be produced from the mass of the starting material you used in the experiment (assume an excess of alkali is used). (4) Refer to your observations made when performing Step (3) in Section 2.2. In the "Comments" column of Table 1 of the LRS offer a brief explanation for each of these observations (5) What does the test with "Universal" indicator (Step (4) of Section 2.2) suggest about the acidity or basicity of your sample of soap? Assuming your sample of soap is comprised of molecules of CH3(CH2)14COOFNa* write a chemical equation to explain the observed acidity or basicity. (6) Refer to your observations made when performing Step (7) of Section 2.2. In the "Comments" column of Table 2 of the LRS offer a brief explanation for each of these observations. (7) Assume your soap is the sodium salt of 16:0 palmitic acid (CH3(CH2)14COOH). Write an equation for its reaction with CaCl2. (8) Refer to your observations made when performing Step (9) of Section 2.2. In the "Comments" column of Table 3 of the LRS compare the approximate times for each of the layers to from in the tubes and offer. brief explanation for each of these observations. Detailed Marking Scheme for LRS Assessable Detail Allocation Aim Investigate soap (two matters @ 2 ea) 4 Total 4 Q1 Clear drawing 1 Required features shown (two @ 1ea) 2 Total 3 Q2 Clear molecular representation of reactants and products 2 Correct balanced chemical equation 2 Total 4 Q3 Correct stoichiometry 2 Correct FW of triglyceride 2 Correct moles of triglyceride 2 Correct FW of soap 2 Correct theoretical mass of soap produced 2 Total 10 Q4 Correct observation of lather (two mixtures @ 1/2 ea) 1 Reasonable comments (two mixtures @ 1 ea) 2 Total 3 Q5 Correct observation of acidity/basicity 2 Correct balanced chemical equation 2 Total 4 Q6 Correct observation of lather (three mixtures @ 1/3 ea) 1 Reasonable comments (three mixtures @ 2/3 ea) 2 Total 3 Q7 Correct equation for reaction of calcium ion with soap 2 Total 2 Q8 Reasonable observation of layer formation times 2 Reasonable conclusion about interaction of soap and oil 1 Total 3 Conclusion Mention of each of the aims (two @ 1 ea) 2 Reasonable spelling & sentence construction 2 Total 4 Grand Total 40 Aim The aim/s of this experiment is/are: Aim/s clearly identified and stated. 74 Method As per laboratory manual Changes: Results and Calculations (1) Schematic representation of a soap molecule. Clear sketch with end groups clearly identified. /3 (2) Balanced equation for saponification reaction. Correct equation. /4 (3) Calculation of expected mass of soap to be produced in saponification reaction. Mass of fat used: Correct calculation. /10 (4) Table 1. Observations made in Step (3) of Section 2.2. Test Mixture Lather Amount Comments Soap + deionized water Fat + deionized water Correct observations and explanations. /3 (5) The pH of soap/water "solution" and fat/water mixture. Observed pH (soap + water) = Observed pH (fat + water) = Equation: Correct observation, explanation and equation. /4 (6) Table 2. Observations made in Step (7) of Section 2.2. Soap + Lather Amount Comments Deionized water Tap water 1% w/v CaCl2 solution Correct observations and explanations. /3 (7) Reaction of soap with CaCl2: Correct equation. /2 (8) Table 3. Observations made in Step (9) of Section 2.2. Phase separation System time Comments tsep/s Deionized water + peanut oil Soap solution + peanut oil Correct observations and explanations. /3 Conclusions Succinct and accurate conclusions /4

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