An investigation into the factors affecting the rate of a reaction: The write up of an experiment using Sodium Thiosulphate and hydrochloric acid

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Skill Area P: Planning

What I going to investigate is how different factors affect the rate of the chemical reaction of Sodium Thiosulphate and Hydrochloric acid. In which a precipitate is formed and causes the solution to become opaque. They react as in the equation below:

Sodium Thiosulphate + hydrochloric acid -->; sodium chloride + sulphur + sulphur



Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

I plan to change the concentration of the Sodium Thiosulphate by diluting it with water but I plan to keep the same volume of liquid in the beaker to ensure that it is a fair test because the volume of liquid in the beaker may affect the reaction time. Also in order to make it a fair test I am going to keep the following variables the same: the temperature; the volume of the hydrochloric acid; the same distance between the light sensor and the solution; the same distance from the light source and the solution; the voltage going into the light bulb; same concentration of acid; same beaker and apparatus; and the same total volume.

I have identified the following hazards and I will deal with them as shown below:

Hazard 1: Dealing with Acids -- I will wear safety specs to protect my eyes and care will be taken while handling the acid.

Hazard 2: Electrical Equipment - I will not touch the electrical equipment while handling liquids and care will be taken to make sure that all electrical equipment will not come into contact with liquids.



A beaker



Light Source (I used a 6V bulb)(Surround the light with a ring of black cardboard to focus the light

Light sensor (again surround the sensor with black cardboard to irradiate light from other sources)



Clamp stand

Sodium Thiosulphate

Hydrochloric Acid

Firstly I set the computer up and loaded the software, which I used to record the data. I then attached the light sensor onto the clamp stand and also attached it to the computer. After that I attached the beaker to the clamp stand the same distance away from the light sensor each experiment and made sure that the light source fitted underneath the beaker and that I could pour the liquids into it without hassle. I then placed the light source directly underneath the light sensor and beaker, and attached it to a Lab-Pak as a supply of electricity. I then started preparing for the experiment. I used a syringe to measure 10ml of Hydrochloric acid for each experiment. And I also used 2 burettes one for water the other for Sodium Thiosulphate; I poured these separately and then mix them together in the beaker attached to the clamp. Once everything was ready, I poured the hydrochloric acid into the SodiumThiosulphate and my partner placed a marker on 'Log-it Lab' to show when I had had put the hydrochloric acid in, so that we could work out the length of time it took for the light level to drop 40%. The experiment was repeated three times.

When planning my investigation I made use of the following scientific knowledge:

Collision Theory: In order for a reaction to take place particles must collide but certain factors affect the amount of effective collisions. This is called collision theory. The factors that affect the rate of a reaction are:

1. Temperature

2. Concentration (the number of given particles in a given volume in grams/litre

3. Size of particles

The higher the temperature the more kinetic energy the particles have the more times the particle will collide and will have more activation energy when they collide and therefore a reaction will take place faster and the rate will be higher. Therefore I will have to be wary of keeping the temperature the same or similar. As otherwise I may find faults in my results. However, changes of temperature often accompany reactions with temperature increasing in exothermic reactions such as this one, so that although I may keep the external...
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