Advaitha NairChemistry HLRate of bleaching using different concentrations of bleach with a constant concentration and volume of red food colouring.
Introduction: Bleach, also known as sodium hypochlorite (NaClO) is a widely used chemical for treating stains on clothes and disinfecting kitchen utensils. Fig 1: Sodium Hypochlorite Structure (PubChem)Bleach is used for the process of bleaching, which removes any colours (whitening) off of an object. Hypochlorite bleach solutions are “obtained by passing chlorine gas through cold, dilute, aqueous sodium hydroxide”.
The equation for this is:Cl2 + 2OH- ? OCl- + Cl- + H2OThe ingredients in hypochlorite bleaches vary with different pH levels. At pH < 2, Cl2 is the major molecule, whereas from pH 4-6, HOCl is the main component. At pH > 9, OCl- is ‘the only species present’.
In ‘household bleach’, hypochlorite ions in basic solution are the active ingredient (5-6% NaOCl). The OCl ion from this oxidises the ‘chromophomes’ (stains) in colored materials by removing one or more electrons from the chromophomes, changing its chemical structure and making it easier to remove from the material. In this process, the hypochlorite ion reduces itself to chloride and hydroxide ions. This process is also enhanced by using ‘optical brighteners, making fabrics appear brighter and whiter’ (Carter).
OCl- + H2O + 2e- ? Cl-+ 2OH-Kinetics plays an important role in this process as it determines the rate of bleaching between the hypochlorite ion and the chromophomes. It depends on the volume, concentration, pH, temperature and surface area of reactants, and also on the presence of a catalyst. Kinetic activity is bound to increase if the concentration of a reactant increases in a reaction as it increases the number of collisions between particles. This research topic is worth exploring since investigating the optimal concentration of bleach can be applied to real life situations such as bleaching stains in the most efficient manner, and other such applications like disinfecting utensils or getting rid of moulds or grouts in houses or other settings. In this experiment, the concentration of bleach is tested as an independent variable. By increasing or decreasing the concentration of bleach, there should be a potential increase or decrease in the rate of reaction. In order to establish an appropriate rate of reaction, changing the concentration of bleach will be tested by it color absorbance through spectrophotometry.
Hypothetically, if more bleach is added, color absorbance is weaker and the rate of reaction is faster. Aim:In this experiment, bleach acts as the reactant and its concentration is changed by adding different volumes of water. The total volume of the solution will be 10 mL, but the volume of water and bleach put in will be different. For example, one trial would be with 10 mL bleach and 0 mL water, and another trial would be with 9 mL bleach and 1 mL water. The aims of this experiment are to: Measure the absorbance of the reaction between red food colouring solution and bleach.
Measure the rate of reaction/bleaching between red food colouring solution and bleach Research Question: How does the concentration of bleach used affect the rate of bleaching of red food colouring solution?Independent Variable (IV): changing concentration of bleach (changing bleach+water ratio) – starting with 10 mL of bleach and 0 mL of water till 10 mL of water and 0 mL of bleach (total volume is 10mL) Dependent Variable (DV): rate of bleaching: measured by absorbance of bleach with red food colouring (calculated by initial rate of bleaching – final rate of bleaching/ time (100 seconds)Control Variable (CV): VariableSignificanceHow to controlValue Absorbance with water + food colouringSetting the absorbance of the food colouring situation to a constant wavelength for every trial run so that accurate values are obtainedCalibrate spectrophotometer in the beginning of experiment574.7 nmTemperature of roomTemperature of room needs to be constant so that it doesn’t affect kinetic energy of bleach and food colouring – to provide accurate readingsSet up air-conditioner in lab to room temperature25° Celsius Volume of red food colouring solutionVolume of red food colouring solution needs to be constant so that it doesn’t affect the rate of bleachingPut constant drops of food colouring for every trial10 dropsHypothesis: As the concentration of bleach increases, the rate of bleaching of red food colouring will increase, with a weaker colour absorbance. This is because as the concentration increases, there will be more frequent collisions between the molecules of bleach and red food colouring, making the rate of bleaching faster. However if the concentration of bleach decreases, there will be less frequent collision between said molecules, slowing down the rate of bleaching.Materials and Apparatus:Vernier Spectrophotometer Food colouring (red) Computer LoggerProCommercial bleach Cuvette Distilled water Plastic Beral pipet 100 mL beaker (± 0.1 ml) Stirring rod 10 mL graduated cylinder (± 0.1 ml)100 mL graduated cylinder (± 0.1 ml) Tissues (preferably lint-free) : to wipe cuvettes cleanPrecautions:Use safety goggles to protect eyes in case bleach or food colouring is spilledLocate nearest eye wash in case bleach or food colouring goes into eyesWear gloves to prevent any harmful effects of bleach in case it is spilled Make sure to dispose of bleach + food colouring in a sink and rinse cuvette thoroughly with waterCalibrate UV spectrophotometer properly Use lint-free tissues to clean cuvette so that it doesn’t have leftover food colouring solutionProcedure:1.
Connect a Vernier Spectrophotometer to a computer and start Logger Pro.2. To set up the spectrometer, open the Experiment menu and select Connect Interface ? Spectrometer ? Scan for Spectrometers. 3. Calibrate the UV spectrophotometer.Prepare the control variable by filling an empty cuvette ¾ full with distilled water and 10 drops of red food colouring. Open the Experiment menu and select Calibrate ? (Spectrometer). Place the cuvette in the spectrometer cuvette holder.
Align the cuvette so that the smooth sides are facing the light source of the spectrometer. Finish calibration. Do not touch the smooth surface of the cuvette as it can disrupt the data for absorbance.Fig 2: Calibration of Spectrophotometer (LoggerPro)4.
Determine the maximum wavelength (574.7 nm) for absorbance of red food colouring solution (CV) and set up the mode of data collection. 5. In a 10 mL graduated cylinder, put 10 mL of bleach. Put the bleach into a 100 mL beaker to mix this with 10 drops of food colouring by using the plastic Beral pipette. 6. Quickly transfer the bleach + red food colouring solution into a cuvette and put it inside the spectrophotometer (with the smooth sides facing the light source).
This is to make sure accurate readings are obtained as bleaching happens quickly.7. Collect absorbance-time data (for 100 seconds) for the reaction of food colouring solution and bleach. Once done, remove the cuvette from your spectrometer and pour out the solution. 8.
Discard the cuvette contents once done. Repeat the procedure to conduct 2 more sub-trials using 10 mL of bleach (0 mL of water). Use different cuvettes for each trial as used cuvettes might tamper with readings. (3 sub-trials for each trial) Trials and sub-trials: In this context, a trial is, for example: 10 mL of bleach with 0 mL of water, while a sub-trial would be doing this same trial 2 more times. This would give a total of 3 sub-trials for each trial.9. After this, repeat these procedures using different volumes of bleach and water; eg. 9 mL bleach and 1 mL water, 8 mL bleach and 2 mL water and so on.
Raw Data: Fig 3: Sample Graph of Trial 1 (Sub-trial 1) With 0 mL of Bleach UsedThe graph above is an example of how all trials and sub-trials were conducted. The rate of bleaching was taken by subtracting the final absorption from the initial absorption and dividing the total by the time each sub-trial took (100 seconds). For example: Trial 1 (Sub-trial 1): 0 mL bleachInitial absorption: 1.582Final absorption: 1.541Initial – final absorption = 0.
041 (1.582 – 1.541)Total / 100 = 0.041/100 = 0.00041These calculations were done for all trials and are shown in Table 1:Table 1: Volumes of water and bleach, Rate of Bleaching According to Trials and Sub-trials and Average Rates of Bleaching of Food Colouring Trial No.Volume of Water Used (± 0.
1mL)Volume of Bleach Used (± 0.1mL)Sub-trial 1 (nm/s)Sub-trial 2 (nm/s)Sub-trial 3(nm/s)Average rate of bleaching of food colouring w/ respect to time (100 secs)10100.000410.
00110.001395460.0 (max at 0.008)0.001210.002030.00162 (because 4B1 was 0)6550.000610.
007320.002120.005810.005083Analysis:For creating a graph, start with 0 mL of bleach until 10 mL of bleach (reversing the order of Table 1)Table 2: Volume of Bleach (x values) vs Average Rate of Bleaching (y values)Vol.
of Bleach (± 0.1mL)Avg. Rate of Bleaching00.
000936790.000743100.00037Fig 4: Graph with Best Fit Line for Volume of Bleach vs Rate of BleachingConclusion:As seen in the graph, as the concentration of bleach increases, the rate of bleaching decreases too. This means that as you add less bleach (more water) with the food colouring solution, the rate of bleaching of the food colouring increases. In the graph above, a linear line of best fit is used, which shows an R2 value of approximately -0.6 with an equation of -0.
0003x + 0.0031, which means that it is a moderate negative correlation, showing that the rate of bleaching decreases as the concentration of bleach increases. An error made here is the extrapolation of the first point on the scatter plot (0, 0.005083), which is the highest value for the rate of bleaching. This highest value for the rate of bleaching happens when there is 0 mL of bleach, which is an extremely significant error that disproves the hypothesis of the rate of bleaching increasing as the concentration of bleach increases. This means that there were faults during the experimental setup or during the data collection process.
Evaluation:The observations made above disprove the hypothesis, which stated that increased concentration of bleach would increase the rate of bleaching of red food colouring. To investigate this more and broaden the scope of analysis, more sub-trials can be used. Instead of 3 sub-trials for each trial, it could be increased to 5-10 sub-trials to check for consistency of values to calculate accurate average rates of bleaching and thus, create a more accurate graph with better readings for the line of best fit to analyse the trend. Sources of Errors and Improvements: Random ErrorSystematic ErrorSignificanceImprovementsTrial 4B1 – error that could be due to cuvette with solution put into the spectrophotometer too late (applies to other trials as well). Values for rate of bleach recorded would be inaccurate since bleaching takes place immediately and those values are lost, making any other calculations inaccurate as well. Analyse reaction faster(putting into the cuvette faster)Excel omitting/extrapolating(0, 0.005083) point on scatter plot for line of best fit. Results in an inaccurate line of best fit equation and R2 value.
More trials in order to interpolate such points Parallax: errors while measuring water and bleach with graduated cylinder and beaker. Wrong values recorded for the volumes of water and bleach. This is very significant since it affects the values for rates of bleaching. Check at least 3 times if readings are correct Real Life Applications:A real-life application of rates of bleaching is washing clothes. Usually adding more bleach can be used in order to reduce stains obtained on clothes, but too much can harm the clothing. Bleach is also used to clean out molds in bathrooms, sterilise kitchen utensils and disinfect trash cans. Learning about the rates of bleaching can be applied to the real world as it can be used to determine how much concentration of bleach one would need for doing any of the actions stated above, and as said before, too much can also be harmful.
The Kinetics of a Bleach Reaction. online Available at: http://www2.vernier.com/sample_labs/VSPEC-02-kinetics_bleach.pdf Accessed 18 Oct. 2017.
“Kinetics Activity.” The ChemCollective: Kinetics Studies of the Bleaching of Food Dyes, 7 Nov. 2016, chemcollective.org/chem/kinetics/.National Center for Biotechnology Information. PubChem Compound Database; CID=23665760, https://pubchem.ncbi.nlm.
nih.gov/compound/23665760 (accessed Jan. 29, 2018).Carter, Henry A. “Bleaches.” Chemistry Explained, 2016, www.chemistryexplained.com/Ar-Bo/Bleaches.html