Why rate increases with concentration. If the concentration of an acid is increased, the reaction goes faster. Consequently if acid is added to a molecule the acid particles will try and collide with this molecule. If there is a low concentration of acid particles then there is less acid particles for reaction. This means there is not much chance of an acid particle hitting the molecule’s particles. Hence if the concentration is higher then there are more particles to collide. Therefore the higher the concentration, the more particles there are and the faster the rate of reaction. This idea also helps us to understand why the rate of reaction decreases with time.
At the start of a reaction, there is the optimum quantity of particles to react with. But they get used up during successful collisions, and as time goes on, there is a constant decrease in this number therefore the reaction slows down.
Why rate increases with temperature. When reacting substances are heated, the particles take in energy. They move faster, which means they collide more often and with more energy. So there are more successful collisions and the rate goes up. This is also true with reactions that depend on light energy such as photosynthesis.
Why rate increases with surface area. The reaction between a solid and a liquid is major. The acid particles can only collide with the outer most particles of the solid. Whereas, the acid particles can collide with any of the liquid particles as they are all free to move.
Method
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Place 50cm3 of Sodium Thiosulfate solution in a conical flask.
- Place the conical flask on a piece of paper that has a cross on it
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Place 10cm3 of dilute Hydrochloric acid in a measuring cylinder
- Add the dilute Hydrochloric acid to the Sodium Thiosulfate whilst at the same time starting a stopwatch
- Swirl the flask to mix the reaction
- Record the time (sec) for the cross to completely disappear.
Apparatus
Equation
Na2S2O (aq) + 2HCl (aq) ➔ 2NaCl (aq) + H2O (l) + S (s) + SO2 (g)
Safety
Hydrochloric acid – irritates
Wear safety glasses
Preliminary experiment
This is done to establish the range of value of temperature:
Preliminary results
Preliminary analysis
20°C was a lot quicker at reacting than I had originally thought. Subsiquently 80°C reacted too quickly which resulted in an inaccuracy in my timing. It is very hard to time accuratly to 2.7 seconds. To covercome this problem, I changed the concetration of the thiosulfate to 10ml3 of thiosulfate and 40ml3 of distilled water. This means that theoretically there are less thiosulfate molecules for the hydrochloric acid to collide with and therefore react slower.
Another commodity that I obtained, whilst performing the preliminary experiment, was the method that I had chosen to heat the chemicals, wasn’t very accurate. I have made it so that both the chemicals are heated together in a bigger water bath
Resultes table
