The aim of this experiment is to answer the following question: What is the effect of temperature on the equilibrium constant for the hydrolysis of an ester?

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Scott Mabbutt                  

                           Individual Investigation                    

Aim:      The aim of this experiment is to answer the following question: What is the effect of temperature on the equilibrium constant for the hydrolysis of an ester?

    The reaction I aim to investigate is a reversible reaction where an ester (an organic compound with RCOOR group) is produced and. Esters are generally insoluble in water but are soluble in other solvents. Esters are formed in an esterification reaction where and carboxylic acid (RCOOH group) and an alcohol (ROH), react to form an ester. Below is the esterification reaction/ester hydrolysis reaction.

  Alcohol   + Carboxylic acid  Ester   + Water

The Equipment and apparatus I will be using in my investigation are as follows:

  • Safety Goggles
  • Test Tube x 5
  • Test tube stoppers x 5
  • Water Bath with thermostatic control
  • Test tube holder (suitable for use in water baths)
  • Burette
  • Clamp stand with clamp fixings
  • Safety Mat
  • Funnel
  • 250ml Beaker
  • 250ml Conical Flask
  • 10ml Measuring cylinder
  • Thermometer
  • White tile

Safety goggles will be worn so that the risk of chemicals coming into contact with the eyes is lessened (see risk assessment). Test tubes are being used as the environment for the reaction to take place, they are glass so can be monitored and can be easily stored in a test tube holder in the water bath. Test tube stoppers will be used so that none of the water or ethanol can evaporate out of the tube, thus giving inaccurate results. Water baths will be used so that the temperature, can be maintained at different temperatures. Test tube holders will be used so that all the test tubes can be held safely without risk of tipping over. A burette will be used in the titration because it will less small amounts of liquid at a time, and also has an accurate scale up the side, so volumes will be easier to obtain.

     A Clamp stand will be used so that the burette is held in place above the solution of unknown concentration securely. A safety mat will be used so that any spillages will be kept off of the tables. A funnel will be used so that any pouring of liquids will be safer from the risk of spillage. Beakers of 250ml are used to contain liquid yet will have lips on them to aid pouring. Conical flasks will be used in the titration to stop any of the liquid hitting the side. A 10ml measuring cylinder will be used so that the reactants can be measured accurately. A thermometer will be used so that the temperature of the water bath can be verified.

Risk assessment

The chemicals I will be using are:

  • Hydrochloric acid (known concentration of 0.1moldm-3-150ml approx.
  • Sodium Hydroxide Solution (unknown concentration)- 150ml approx.
  • Sulphuric acid (unknown concentration)- 150ml approx.
  • Ethanol (95% concentration with 5% water)-500ml approx.
  • Ethanoic Acid (Glacial Acetic Acid 100%)-500ml approx.
  • Methyl Orange
  • Phenolphthalein

The Hydrochloric acid I am using is corrosive, so if it comes into contact with skin it will burn. Therefore when I am transferring the chemical from the sealed jar to the burette, I will ensure that safety goggles are on; so that any spillage will not come into contact with the eyes. I will also ensure that I am using a funnel on the burette. This is so that spillages will be less likely. Also when cleaning I will make sure I am using excess water to clean anything where the acid has contacted because it may react violently with water in small quantities.

      Similarly the sulphuric acid I am using is corrosive, therefore I must be careful when adding it to the test tube in the reaction, to prevent overfilling and spillages.

   The Sodium Hydroxide I am using is also corrosive, so when pouring into beakers and the burette, I will use a funnel and wear safety glasses.

   The ethanol I am using is highly flammable so I will keep it away from any naked flames. It is also very toxic if ingested, therefore I must make sure that it will be kept well away from the face at all times. As the ethanol is used in the reaction it I will make sure it is measured out accurately and that no spillages take place.

   Ethanoic acid is its concentrated form is also corrosive and will cause burns if it comes into contact with the skin or eyes. There I will be wearing safety goggles when using this chemical to add to the test tube or when it is used in the titration.

    Phenolphthalein is very harmful even in small quantities, it causes water imbalance and is a strong laxative. Therefore this substance must be kept off of tables, hands or any equipment that is likely to be handled.

    When doing a acid-base titration, the reaction is exothermic so therefore I must make sure nobody touches the beaker when the two chemicals are mixing.

   Also the water baths will be set so that there is an element heating the water. This must not be touched as it can cause burns. Also the water may too hot to touch so, the water must be touched as little as possible. Another reason for not touching the water too much is that if water gets onto the floor there is a risk of people falling over; therefore this risk should be minimized

Variables

Although these do not affect the value of the reaction constant I will still aim to control some variables. The variables I am aiming to control are that of concentration and  that of total pressure. As all reactants and products are in the liquid phase, keeping the total pressure constant will be just the atmospheric pressure. To control the variable of concentration I will measure accurately different reactants each time so that the same concentration and amounts will be obtained each experiment.

   I will also be controlling the chain length of the acid and alcohol I use in the reaction. If I change the chain length of the reactants different products are going to be produced and may affect the equilibrium constant of the reaction. For this reason I will use ethanol and ethanoic acid, as they are easily obtainable and they produce an ester that is also a colourless liquid.

   I will be varying temperature as feel this is feasible to do, by using a water bath. Also because concentration does not actually affect the value for Kc, it will just shift the equilibrium position, away from the part of the reaction with the highest concentrations.

    The temperature ranges I will be using for this experiment will be in divisions of five degrees Kelvin from 293K, 298K, 303K , 308K , 313K , 318K , 323K. In terms of degrees Celsius this equal to 20oC, 25oC, 30oC, 35oC, 40oC, 45oC, 50oC. I am using this range because I feel I will be able to notice a marked difference in the equilibrium constant. Also I do not want to use any value of temperature above 60oC as above this temperature ethanol will boil, and will be in the gaseous state, separated from this rest of the reaction (condensed on the sides of the test tube) and therefore unable to take part in it. Also if I go above 100oC then the water would boil and the water bath would then not be as effective at keeping temperature constant, as well as damaging the heating element.

   

    Other ways I will ensure that I will make my experiment fair, accurate and able to produce repeatable data will now be explained.

  •  I will make sure that the water bath I am using is set to that temperature for about 15minutes before the experiment, so that the water is an even temperature throughout.
  • I will also store the chemicals that I will be using in the water bath so that the reaction starts at the desired temperature, and does not have to be raised to that temperature.
  • I will use sterilised equipment throughout so that there is no chance of contamination, as this could affect the reaction.
  • I will also use the same equipment all the way through to ensure that no other contaminants can enter the mixtures. After each experiment I will clean out all equipment with both distilled water and the substance that is to be placed in the equipment after cleaning.
  • I will always use the same initial volumes and concentration of the alcohol and carboxylic acid. This is so that the
  • I will also use the same amount of concentrated sulphuric acid catalyst for each experiment. This amount will be four drops from a small pipette. This is so that the overall concentration of acid will be easier to work out if the drops are constant.
  • When titrating my sample I will make sure I use the same concentration of sodium Hydroxide solution. This amount will be 0.4molsdm-3. This will be done so that the same concentration will be used in calculations throughout, my analysis, and will make the spotting of patterns easier, if the sodium Hydroxide solution is of the same concentration all the way through.
  • I will use the same amount of phenolphthalein in the sample to measure the colour. This amount will be 6 drops from a small pipette. This will be done because if different amounts were added there may be a colour change at a different point to when I expect one, so making the amount of indicator constant should eliminate this.
  • When moving the sample I take from the reaction vessel to be titrated I will move it quickly as possible so that the temperature will not go down, if it does then that would change the amount of acid and make the experiment inaccurate. I will help to maintain accuracy by placing the titration apparatus close to the water bath. I will also have the titration equipment already set up, so the titration could be done immediately.
  • I will also graph my results as I get them. This way I can make sure that any patterns can be spotted early and any errors in the investigation will be made clear at an early stage.
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The thing I will be measuring is the amount of Sodium Hydroxide solution it takes to neutralise. The point at which I know that the solution is neutralised is the point when the phenolphthalein changes colour from colourless to pink. In order to find out this I will use a conical flask on top of a white tile. The colour will be deemed changed when I can no longer shake away the pink colour. This will indicate there are excess hydroxyl ions present in the solution. These ions are present in the Sodium Hydroxide solution and when they come ...

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