The aim of this investigation is to see how the concentration of a reactant in ratio to the reactor affects the rate of a reaction. When hydrochloric acid and Thiosulphate react together sulphur is liberated this means that as the reaction goes on the solution will become yellow and will change from being transparent to translucent to opaque. The sulphur is formed as a solid but not in the usual precipitation way.

Na2 + S2 + 2HCL 2NaCl+SO2 + S2 +H2O

To time the reaction I will draw a black cross on a piece of plain paper on which the beaker of reactants will be placed (HCL and Thiosulphate). When the chemicals come into contact with each other I will start timing with a stopwatch and will stop timing when the cross is longer visible through the beaker from above.

A chemical reaction between to chemicals can only happen if their molecules can collide into each other. Out of many collisions there will be a few successful collisions, which means that the two molecules will exchange electrons and that means that they have reacted. These molecules have to hit each other in the right direction and at the right speed; in short the rules for

Concentration of acid or thiosulphate

I will use separate measuring cylinders for both chemicals so I don't contaminate either of them until I'm ready to time the reaction. Because I' am (always) measuring 10ml of HCL I will use a small measuring cylinder, i.e. 100ml so I can accurately identify where the 10ml mark is. I will always use my eyes to tell me when to stop timing because the quality of eyes varies from person to person. For my big measurements of liquid I will use a large measuring cylinder.

HCL volume in cm cubed Volume of H20 in cm cubed Thiosulphate volume in cm cubed

If all input variables remained constant apart from concentration then fastest reaction rate possible would be to mix (assuming there isn't an excess) x and y together in their purest forms. This is because every time an x molecule collides into a molecule it has a 50% chance of it being a y molecule. But if you removed half of y and replaced it with a neutral chemical such as z the chances of a x molecule bumping into a foreign molecule decreases from 50% to about 25%. This in effect lowers reaction rate by lowering amount of "successful collisions"

The best way to measure the rate of reaction is by taking advantage of the sulphur being produced. Because the reaction liberates sulphur the solution goes form being transparent to opaque. This means that we could use an LDR or light dependent resistor and lamp to measure reaction rate. This could be done by placing and LDR to a voltmeter and having a lamp shine through the beaker on to the LDR. As the experiment goes on the LDR should have a lower resistance because there should now be little light shinning through onto the LDR form the lamp. You could time how long the resistance takes to go down to 6 Ohms, or what ever you decide to do from your preliminary results. Unfortunately we do not have access to these kinds of apparatus for this experiment so we will have to use different methods. We could place a piece of white paper under the beaker of reactants with a black cross underneath it. And time how long it takes so you can no longer see the cross from under the!

Keep head always at same height just before deciding when to stop timing, and always use the same pair of eyes, and keeping on any glasses.

I am not sure if light does affect the rate of reaction in this case but it may be absorbed as heat in which case it could quicken reaction rate. Light tends to affect experiments that involve salts such as silver nitrate. In some cases it helps to lower activation energy like a catalyst. But light intensity can be absorbed as heat so as light intensity increases so does temperature, and temperature does affect reaction rate so through -out the entire experiment I will treat light as an input variable to be controlled. See temperature as to how heat affects reaction rate.

Some common words found in the essay are:
Average Rate, Volume H20, Concentration Concentration, Apparatus Reference, Catalyst Activation, HCL Thiosulphate, Light Temperature, , reaction rate, S2 O2, rate reaction, measuring cylinder, successful collisions, percentage thiosulphate, stop timing, straight line, input variables, concentration thiosulphate, water molecules, affects reaction rate, increases reaction rate, Catalyst Concentration, volume cm 3, molecule decreases 50%, foreign molecule decreases,
Approximate Word count = 3268
Approximate Pages = 13 (250 words per page double spaced)