Rates of reaction - Sodium Thiosulphate + Hydrochloric Acid.

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Sodium Thiosulphate + Hydrochloric Acid


I must produce a piece of coursework investigating the rates of reaction, and the effect different changes have on them. The rate of reaction is the rate of loss of a reactant during a chemical reaction There are five factors which affect the rate of a reaction:

  • Temperature,
  • Concentration (of solution),
  • Pressure (in gases),
  • Surface area (of solid reactants),
  • Catalysts.

I have chosen to investigate the effect of concentration on a reaction. This is because it  is the most practical to investigate. The others are very difficult to get accurate readings due to  human error. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.

The aim of my experiment is to see the effects of a change in concentration on the rate of a reaction. The reaction that will be used is:

Sodium Thiosulphate + Hydrochloric Acid
Na2S2O3 (aq) + 2HCl (aq)

Sodium Chloride + Water + Sulphur Dioxide + 2NaCl (aq) + H2O (l) + SO2 (g) +
S (s)

Both the sodium thiosulphate and the Hydrochloric acid are soluble in water, so the concentration of either can be changed. However I have chosen to vary the Hydrochloric acid as it is available to me.  I will also try to keep a constant temperature in the room as temperature effects chemical reactions as well.

I decided which concentrations to use during my preliminary experiment –

50ml of NASO/10ml of HCL/0ml of water

50ml of NASO/1ml of HCL/9ml of water

Using my preliminary experiments I decided on using the following apparatus:

3 measuring cylinder

1 conical flask

1 stopwatch
1 pair of goggles.1 apron
1 piece of paper with an cross in black pen on it.

Method: -
Experiment 1 - Changing the concentration
sodium thiosulphate, water and HCL ( at varying concentrations) are poured out into 3 measuring cylinders and then poured into a conical flask, which is placed on top of a  piece of paper with letter X. The stopwatch will now be started. When the mixture has turned sufficiently cloudy so that the letter X can no longer be seen the stopwatch will be stopped and the time will be recorded. The experiment is repeated with all the concentrations 3 times each so that I can get an average.

Repeat results and averages will be taken to improve the credibility of the findings, and present solid grounding for the final conclusion. The repeat results will help to iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a third experiment will be performed to replace the anomalous set of results.

Safety – A pair of goggles will be worn during f the experiment in order to protect my eyes. An apron will also be worn to protect my skin and clothing. I will also make sure that no-one is near me, so that I won’t be distracted from the experiment.
 We must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals.

Fair Test - In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time when the X has disappeared. I will make sure that the measuring cylinders for the Hydrochloritic acid and thiosulphate will not be mixed up. I will used water to keep each experiment at a constant volume. The amount of thiosulphate will be the same each time. I must also make sure that I put all of the chemicals in at the time, as that could effect my results.  Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment. All of these precautions will make my final results more reliable and keep anomalies at a minimum so making the entire investigation more successful. I must also try and keep the temperature the same as A small increase in temperature causes significant changes to the distribution energies

Prediction –
I  predict that as the concentration of the  Hydrochloric acid increases the rate of reaction will increase. This means that my graph drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the concentration is increased. This can be justified by If solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. All this can be understood better with full understanding of the collision theory itself:
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.
Concentration – If the concentration of a solution is increased there are more reactant particles per unit volume. This increases the probability of reactant particles colliding with each other.
Surface Area – If a solid is powdered then there is a greater surface area available for a reaction, compared to the same mass of empowered solid.

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Only particles on the surface of the solid will be able to undergo collisions with the particles in a solution or gas.

Effect of a catalyst - A catalyst works by providing an alternative reaction pathway that has lower activation energy. A catalyst provides a reaction route of lower activation energy, however, a greater proportion of particles will have energy greater than the activation energy.



In this experiment I have found that as the concentration is increased the ...

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