The variable I am going to change will be the temperature - Sample Essay
I am investigating the rate of reaction between hydrochloric acid and sodium thiosulphate. The variable I am going to change will be the temperature; I will raise the temperature by approximately 10oc each time. Other possible variables that could be changed are; changing the concentration of the acid, increasing the pressure by reducing the volume of the container which means the particles will be closer together which would result in faster collisions. Using a catalyst would speed up a reaction and changing the surface area could speed up or slow down the reaction.
When the two chemicals are added together, they start to react. It will give off sulphur as one of the products. This is called a precipitation reaction. The equation between sodium thiosulphate and hydrochloric acid is: Na2 S2 O3(aq) + 2HCl(aq) 2NaCl(aq) + S(s) + SO2(g) A chemical reaction can only occur between particles when they collide. The faster the particles are moving, the more energy they have. Fast moving particles are more likely to react when they collide. Particles will move more quickly when the temperature is raised. The same amount of sulphur will always be produced, but at different rates.
There is a certain amount of minimum energy needed in order for colliding particles to react with each other. If the colliding particles have less than this minimum energy, (because they aren’t moving fast enough), then they just bounce off each other and no reaction occurs, this is called elastic energy. The minimum energy is called the activation energy. The temperature will speed up the rate of reaction because the hotter the particles are, the more energy they have which means they will move faster and are therefore more likely to collide into each other.
I predict that when the temperature of a reactant is raised, the particles will gain energy to move faster and collide with each other, which will increase the rate if reaction. As I only recorded one set of results, I feel that the outcome of my experiment may not be completely accurate. If I had a second set of results, my evidence may be more reliable Apparatus 2 measuring cylinders 1 small beaker of sodium thiosulphate 1 bottle of hydrochloric acid 2 teat pipettes 1 conical flask Goggles Bunsen Tripod Thermometer Stop clock Paper with cross on it
To ensure that it is a fair test, the only variable that is going to be changed will be the temperature, I will try to raise it by approximately 10oC each time and I will repeat the experiment to check that I don’t have any incorrect results. I will use the same bottles of acids and keep them at the same concentrations. I won’t change the surface area of the chemicals. I will check there is no contamination on the equipment that could affect my results and I will use separate pipettes so that the chemicals don’t start reacting before the stop clock is started.
Method Collect apparatus and using the teat pipettes, pour out into measuring cylinders, 25cm3 of Sodium Thiosulphate and 25cm3 of Hydrochloric acid. Make sure the two chemicals do not come into contact with each other at this point. Using a pencil, draw a cross on a small piece of paper and put it under the conical flask. Pour one of the chemicals into the conical flask and then pour the other chemical in (it doesn’t matter which order), start the stop clock immediately. Give the flask a quick shake to ensure the chemicals mix.
Keep watching the cross until it is no longer visible, then stop the clock and record the time. For the next experiments, before the chemicals are mixed together, one of them has to be heated up over the Bunsen. Using the thermometer, raise the temperature by approximately 10oC each time. 25cm3 thiosulphate 25cm3 Hydrochloric acid Mix==> ==> Ongoing==> Watch stopped Temp (oC) Time (s) Rate (1/t) .
My temperature, time graph shows that if the temperature is low then it takes longer for the mixture to cloud over, if the temperature is higher then it takes a shorter time for the sulphur to form. The rate of reaction graph shows that the higher the temperature is the faster the rate of reaction is. If the temperature is low then the rate is slow. My predictions matched my results; this is because I had previously learnt about and researched rates of reaction. I expected to get some anomalous results because when I did the experiment in class I had anomalous results. Evaluation
My results weren’t 100% accurate because although I did repeat them to check that they weren’t way out of proportion, I didn’t record the second set of results. I had a bit of trouble reaching the correct temperature when heating up the hydrochloric acid so the temperature wasn’t raised by exactly 10oC each time. Too improve my practical I would repeat the results one more time and raise the temperature more accurately. I would also try out different temperatures, for example I could lower the temperature below room temp, or I could find out if there is an optimum temperature for the rate of reaction. Clare Apps Year 11 assessed practical.