Investigating the reaction between Sodium Thiosulphate and Acid.

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Experiment Aim:

To investigate how the concentration of acid affects the rate of reaction with Sodium Thiosulphate.

Background Knowledge:

The reaction that will be set-up will occur as follows:

During a reaction, bonds between atoms must be broken before new molecules can be made. This means that for every chemical reaction a minimum amount of energy is needed to go into breaking these bonds. This is called the ‘activation energy’. The molecules also have to collide in order for a reaction to take place, and the collisions between particles have different amounts of energy. If colliding molecules have enough energy, then it is successful and a reaction occurs. The reactions are fastest if the energy of the molecules is greater than the activation energy of the reactants. (RESOURCE 2) Different factors can affect the rate of reaction, as explained below:

  • Concentration

In solutions of higher concentrations, particles are closer together, and have a greater chance of colliding. There are consequently more collisions, and the rate of the reaction is higher (as concentration of a solvent is increased).

  • Temperature

At higher temperatures, particles are moving faster. Therefore, at a higher temperature, there are more collisions, and the collisions are also more energetic. More collisions therefore have energy greater than the activation energy, meaning that the rate of reaction is faster when temperature is increased.

  • Presence of a catalyst

Catalysts generally lower the activation energy needed for a reaction to take place. This means that more collisions are likely to be successful (because more then have energy greater than the activation energy), and the rate of reaction will increase.

  • Surface area

This factor is of a solute in a reaction, and it can affect the rate of reaction, but this cannot be changed in this reaction, because sodium thiosulphate and hydrochloric acid are both liquids. With a greater surface area of the solid in a reaction, collisions are more frequent and the reaction rates greater.

When conducting this experiment, one should be careful, as a concentrated solution of hydrochloric acid will burn skin or damage clothing; skin contact with acid should be avoided, even though the hydrochloric acid to be used is diluted. This is a potential safety hazard / concern for the reaction. (RESOURCE 1)

Planning:

The rate of reaction, as with any reaction must be measured continuously, by taking frequent measurements of something throughout the experiment. With this reaction in particular, the following methods can be used to judge how fast the reaction is:

  • By measuring the volume of gas produced (i.e.: the sulphur dioxide produced by the effervescence of the reaction).
  • By measuring the mass of the reaction lost (i.e.: when sulphur dioxide gas is given off from the reaction).
  • By measuring the time taken for the sulphur produced to obscure a mark, or cross underneath the clear flask / beaker in which the reaction will take place.

After contemplating which of the following methods to actually use, I have chose to measure the rate of reaction by measuring the time taken for the sulphur produced to obscure a cross / mark underneath the flask in which the reaction should take place. The reason for this decision is that for each of the other choices, where were a few downfalls of the method:

  • If I were to measure the rate of reaction by measuring the volume of gas produced (i.e.: the sulphur dioxide produced), there would have been a problem such that the equipment that I would have had to use would not be very effective. Also, accurate equipment for this procedure may not have been available.
  • If I were to measure the rate of reaction by measuring the mass of the reaction lost (continuously when the sulphur dioxide is given off), problems may have arisen, and also, this procedure is fairly complicated, as the sulphur dioxide given off is only a small quantity. Therefore, the results may not have been very spread out, or have a range from which my conclusion would be valid.
  • Therefore, I have chosen to measure the rate of reaction by measuring the time taken for the sulphur to obscure a cross underneath the conical flask. The reason for this choice is that the sulphur produced in the reaction goes particularly cloudy, and therefore obscures the cross well, once enough sulphur has been produced by the reaction. This procedure is therefore very effective, and relatively simple, aswell as precise (in terms of the results).
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For this investigation, I shall be using the most precise and accurate equipment that is required for the results to be accurate and reliable. There may, as one may expect, be a few problems concerning the accuracy of the equipment that might be used. For example, if one was going to use a 100cm3 measuring cylinder to measure –out 10cm3 hydrochloric acid, there may be an accuracy problem. This is because a 100cm3 measuring cylinder does not show a reading of 99.7cm3 (for example), it only shows 99cm3 and 100cm3. Therefore, measuring instruments that are accurate to the volume / mass being measured ...

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