Aims:
I aim to find out the order of reaction with respect to [H2O2] and [2I-]. I aim to find out the activation enthalpy of the reaction by finding the rate of reaction at different temperatures using the Arrhenius Equation. The experiment will go as follows:
Into a conical flask put 15cm3 of distilled water and add 2cm3 of [X]moldm-3 potassium iodide (KI) solution and 1cm3 of 2moldm-3 sulphuric acid. Then add to this 2.5cm3 of 5vol (0.42moldm-3) hydrogen peroxide (H2O2). For the second part of my investigation, the KI solution will remain a constant 0.3moldm-3 and the H2O2 solution will vary. H2O2 + 2I- + 2H+ -> 2H2O + I2
Methods to find the rate:
1 – Use a colorimeter to monitor the change in absorbance as the reaction progresses, showing how the concentration of Iodine changes with time and how varying concentrations of KI and H2O2 solutions affects the rate of reaction. 2 – Use an iodine clock: add 2cm3 of 0.1moldm-3 sodium thiosulphate (Na2S2O3) and 1cm3 of 1% starch solution at the start of the reaction. The added sodium thiosulphate reacts with the iodine as it’s formed: I2 + 2Na2S2O3 -> 2NaI + Na2S4O6
When the sodium thiosulphate is all used up the iodine remains in solution and reacts with the starch indicator to make the blue colour. I will vary the temperature and time how long it takes for a colour change of blue to appear, and from this calculate the activation enthalpy of the Harcourt-Essen reaction. I will keep the temperature’s constant with the use of water baths and fresh ice. The KI wil be kept at a constant 0.3moldm-3 and the H2O2 will be kept at a constant 5 vol. Concentrations to be used:
0.3, 0.25, 0.2, 0.15, 0.1 and 0.05moldm-3 potassium iodide – from solid KI (constant 0.3moldm-3 for varying H2O2) 6, 5, 4, 3, 2 and 1vol hydrogen peroxide – from 20vol (1.68moldm-3) H2O2 via dilution (constant 5vol for
varying KI)
Chemical Ideas:
Harcourt-Essen reaction:
The iodine clock reaction (also known as the Harcourt-Essen reaction) is a classical chemical clock demonstration experiment to display chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two colorless solutions are mixed (usually with a catalyst such as sulfuric acid) and at first there is no visible reaction. After a short time delay, the liquid suddenly turns to a shade of dark blue. reference 1 The iodine clock reaction times how long it takes for a fixed amount of thiosulphate ions to be used up i.e. the time taken for the iodide ions to reach a fixed number of moles produced in the reaction between potassium iodide and an oxidising agent (usually hydrogen peroxide). reference 2 The system is as follows:
The oxidising agent reacts with the iodide ions (usually introduced in the form of KI). H2O2 + 2I- + 2H+ -> 2H2O + I2
The iodine produced is then absorbed by reaction with a fixed amount of thiosulphate ions: 2S2O32- + I2 -> S4O62- + 2I-
As soon as the thiosulphate ions are used up the free iodine then reacts with some starch indicator that is added right at the beginning. The reaction produces an almost instantaneous blue/black colour. Thus it is possible to time fairly accurately the time taken for a fixed amount of iodine to be produced (from the thiosulphate moles present initially). reference 2 Colorimeter:
The colorimeter is a simple type of spectrophotometer and is used to measure the intensity of absorption of coloured compounds over a narrow range of frequencies. You can use the colorimeter to measure the concentration of a coloured compound at different times as a reaction proceeds in a rate experiment to find the order of the reaction reference 3 – as with my Harcourt-Essen reaction. Light from an LED light source passes through a cuvette containing a sample of solution. Some of the incoming light is absorbed by the solution. As a result, light of a lower intensity strikes a photodiode. The Colorimeter measures the amount of light transmitted through a sample at a user-selectable wavelength. The amount of light that passes through a solution is known as transmittance. Transmittance can be expressed as the ratio of the intensity of the transmitted light (It) and the initial intensity of the light beam (Io), as expressed by the formula: reference 4 T = It /Io
Experimental Methods:
Making up the solutions:
The potassium iodide solution will be made up by first calculating the amount (in moles) of KI needed from the equation: concentration = amount/volume as I know the desired concentration(s) and how large a volume I want to make up. Then I can calculate how exactly how much solid KI (in grams) is needed for that concentration by using the equation: amount = mass/Mr – given the Mr of KI is 166.0028. Then to actually make up the solution, I will first brush the balance, place a plastic weighing container on it, and then tare the balance. I will then proceed to accurately weigh out the required mass of KI with a clean spatula. The hydrogen peroxide solution will be diluted from 20vol to [X]vol by using 20/[X] the volume of 20vol H2O2 in distilled water. Methods:
Potassium iodide solution:
1. Using the equations; amount = mass/Mr and concentration = amount/volume, I calculated the correct mass of KI needed to make up the required concentrations of KI solution (0.05 through to 0.3moldm-3). I used the complete values throughout all the equations, and then rounded my final volume to an appropriate decimal place relative to the precision of the scientific balance I will use to make up the solution. 2. Solid KI (mass m) was weighed on a scientific balance.
The balance was brushed clean, a light plastic container was placed on it, and then it was tared before weighing. The solid KI was placed on the balance and the side doors and lid were closed. I continued to add and remove solid KI until the desired mass was achieved. I recorded its mass to the maximum possible decimal places once the number had settled. 3. Solid KI (mass m) was placed into a 250cm3 beaker.
I tipped the KI from the plastic container into a 250cm3 glass beaker, making sure not to spill any and to ensure that as little as possible stuck to the sides of the beaker. 4. I used a 25cm3 glass filling pipette to pipette 100cm3 of distilled water into the 250cm3 beaker. I rinsed out the filling pipette with distilled water before use, carefully attached a pipette filler, and then measured up to the 25cm3 mark, reading where the meniscus was. I then repeated this 4 times for a total of 100cm3. When I pipetted the water into the beaker I made sure to rinse down any KI that had stuck to the sides of the beaker. I then proceeded to dissolve the KI by swirling the solution gently.
Hydrogen peroxide solution:
1. H2O2 (volume V, 20vol (1.68moldm-3)) was pipetted into a 250cm3 beaker using a 25cm3 glass filling pipette. I rinsed out the filling pipette with H2O2 before use, carefully attached a pipette filler, and then measured up to the required mark, reading where the meniscus was for the required volume. 2. Distilled water (20/Vcm3) was added to the 250cm3 beaker using a 25cm3 glass filling pipette. I rinsed out the filling pipette with distilled water before use, carefully attached a pipette filler, and then measured up to the required mark, reading where the meniscus was for the required volume.
Risk Assessment:
Sulfuric acid – H2SO4 (2moldm-3): reference 5
Corrosive above 1.5moldm-3 but irritant below 1.5moldm-3. Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out and a glass or two of water should be swallowed. Vomiting should not be induced. Splashes on the skin should be dealt with by flooding the area with large quantities of water. If blistering occurs, seek medical attention. As it will be a corrosive concentration, I will wear laboratory goggles and gloves to protect my eyes and skin from potential splashes. Wipe up small amounts with a damp cloth and rinse it well. For larger amounts, and especially for (moderately) concentrated acid, cover with mineral absorbent (e.g. cat litter) and scoop into a bucket. Neutralise with sodium carbonate and rinse with plenty of water. Potassium iodide solution – KI (0.05 through to 0.3moldm-3): reference 6 Harmful above 1moldm-3 but a low hazard below 1moldm-3.
Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out and a glass or two of water should be swallowed. Vomiting should not be induced. Splashes on the skin should be dealt with by flooding the area with large quantities of water. If blistering occurs, seek medical attention. As it will be low hazard concentrations, I will wear lab specs and gloves to protect my eyes and skin from potential splashes. Wipe up small amounts with a damp cloth and rinse it well. For larger amounts add sodium thiosulfate solution (20%, 1 M) to the remaining spill and leave for 1 hour. Mop up and rinse with plenty of water.
Hydrogen peroxide – H2O2 (0.084 through to 0.504moldm-3 diluted from 1.68moldm-3): reference 7 Harmful above 2.3moldm-3, irritant between 1.5moldm-3 and 2.3moldm-3, but a low hazard below 1.5moldm-3. Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out no more. Vomiting should not be induced. Splashes on the skin should be dealt with by flooding the area with large quantities of water. If blistering occurs, seek medical attention. As it will be diluted from a harmful concentration, I will wear laboratory goggles and gloves to protect my eyes and skin from potential splashes. Wipe up small amounts with a damp cloth and rinse it well. For larger amounts, and especially for (moderately) concentrated solutions, cover with mineral absorbent (e.g. cat litter) and scoop into a bucket. Rinse with plenty of water. Using a glass filling pipette:
Injury due to broken glass.
Pipette could break due to excessive when attaching to pipette filler. If a minor cut occurs, flood the area with running water to remove traces of chemicals and apply firm pressure with an absorbent lint-free dressing until bleeding stops. Seek first aid advice. Sit down in case of feeling faint. Hold the end of the pipette nearest the pipette filler and ease gently into the pipette filler, then tighten the pipette filler’s grip. Carefully sweep up larger pieces of glass and place in the glass only bin in the lab. Electricity to power colorimeter
Electric shock.
Water/solution(s) splashed near electrical appliances.
Turn off electrical supply. Seek first aid advice immediately. Handle with dry hands. Keep electrical equipment away from sinks and taps. Avoid trailing leads through water.
Sodium thiosulphate – Na2S2O3 (0.1moldm-3): reference 8
Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out no more. Vomiting should not be induced. Splashes on the skin should be dealt with by rinsing the area with water. As it is a low hazard, I will wear lab specs and gloves to protect my eyes and skin from potential splashes. Wipe up spills with a damp cloth and rinse it well.
Starch (solution): reference 9
Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out no more. Vomiting should not be induced. Splashes on the skin should be dealt with by rinsing the area with water. As it is a low hazard, I will wear lab specs and gloves to protect my eyes and skin from potential splashes. Wipe up spills with a damp cloth and rinse it well.
Sodium Iodide – NaI (PRODUCT): reference 10
Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out no more. Vomiting should not be induced. Splashes on the skin should be dealt with by rinsing the area with water. As it is a low hazard, I will wear lab specs and gloves to protect my eyes and skin from potential splashes. Wipe up spills with a damp cloth and rinse it well.
Sodium tetrathionate – Na2S4O6 (PRODUCT): reference 8
Could be accidently swallowed or splashed in eye or on skin. Splashes in the eyes should be flooded with water for 10 minutes and you should seek medical attention. If swallowed, the mouth should be washed out no more. Vomiting should not be induced. Splashes on the skin should be dealt with by rinsing the area with water. As it is a low hazard, I will wear lab specs and gloves to protect my eyes and skin from potential splashes. Wipe up spills with a damp cloth and rinse it well.
References:
Reference 1: Harcourt-Essen reaction (last accessed 10/11/2013) http://en.wikipedia.org/wiki/Iodine_clock_reaction
Reference 2: Iodine clock reaction (last accessed 11/11/2013) http://openstudy.com/study#/updates/4fa0c4a0e4b029e9dc31cdb1 Reference 3: “Salter’s Advanced Chemistry: Chemical Ideas”; 3rd Ed; Heinemann Educational Publishers: 2008; pg. 157 “Using ultraviolet and visible spectra” Reference 4: Colorimeter (last accessed 15/11/2013)
