The temperature of the reaction - When the temperature is low, the particles in the reaction do not have much energy so collision is unlikely to occur. However on heating, particles take in energy causing them to move faster and collide more often. Because the collisions have more energy they are more likely to be successful in breaking and reforming bonds. Therefore the rate of the reaction will increase. When the temperature increases by 10C, the rate of the reaction roughly doubles.
Concentration of the Hydrochloric Acid and Sodiumthiosulphate - The more concentrated the solvent, the more particles are present and the greater the chance of a successful collision. In dilute acids, with fewer particles the chance of successful collisions occurring is reduced. Therefore if concentration is increased, the reaction rate also increases. This also applies in reactions between two gasses when increasing the pressure has the same effect as increasing the concentration.
Amount of the Reactants - It is important to keep the amount of reactants constant in the experiment because, as the reactant is used up, so the reaction slows down. This means a small amount of solid will have a different reaction time to a large amount. A small amount of solid will have fewer atoms so will be less likely to successfully collide with acid particles. The limiting factor would be the amount of the smaller reactant.
Presence Of A Catalyst - A catalyst increases the rate of a chemical reaction without itself being used up. Catalysts are usually transition metals. In the presence of a catalyst, less energy is needed by a collision in order to be successful. Therefore there are relatively more successful collisions and so the reaction rate increases.
In order to ensure a fair test with reliable results, I will need to keep all variables constant apart from the one I am intending testing.
Apparatus: The apparatus used will be:- a conical flask, hydrochloric acid, sodiumthiosulphate, stopwatch, beaker, measuring cylinders, plain paper, and safety glasses.
Safety: To ensure that the experiment is safe always wear safety goggles to protect eyes from the reactants. Keep all ties and long hair away from the solution.
Preliminary Experiment: I constructed a preliminary experiment to determine the concentration of Hydrochloric acid I was going to use. I kept the amount of sodiumthiosulphate constant at 25cm3 and added hydrochloric Acid from 1cm3 to 7cm3. I measured the volumes of the solutions using a measuring cylinder, so it was consistently accurate. When the reactants were put to together I timed it until the solution when cloudy and I couldn’t see the black cross drawn underneath the conical flask on a piece of paper.
From these results I have come to a conclusion of what concentration I will use in the experiment, and it is 5cm3. This is because it is near average of the results and therefore will give average results in the main experiment. Also this is an easier quantity to measure and would be more accurate than 6 or 7 ml.
Method:
Step 1
Gather all the apparatus.
Step 2
Make sure all safety precautions are covered.
Step 3
Draw a black cross on a piece of paper and place it under the cylinder/
Step 4
Measure out 5cm3 of HCL and put it in the cylinder.
Step 5
Put in 25cm3 of sodiumthiosulphate.
Step 6
Immediately start the stop clock and stop it when the cross is no longer visible. This measures how fast the sulphur is formed as a product of the reaction (Na2 S2 03+ 2HCL -→ 2Na Cl + H20 + S + S02). Work out the rate of reaction.
Step 7
Repeat again but replace 2cm3 of sodiumthiosulphate with 2cm3 of water. Between each repetition wash the equipment thoroughly so the results will be accurate and will not be affected by extra chemicals.
Step 8
Keep repeating with the same pattern until there is no sodiumthiosulphate in the reaction. Distilled water is used to dilute the sodium thiosulphate as tap water will cause inaccuracies as there are many chemical impurities in the water.
Step 9
Clean all apparatus and put it all away.
Results:
Analysis: From the results I have obtained evidence that can be taken to prove scientific theory. From the results I have plotted a graph of average results for every reaction. The results clearly show that the higher concentration of sodiumthiosulphate there is, the faster the reaction will occur and the higher the rate of reaction will be. This is because there are more sodiumthiosulphate molecules in the solution and therefore there are more chances of bonds being broken by these. The results show a trend. This is that the rate of reaction rises in proportion with the concentration of the sodiumthiosulphate. The results plotted on the graph show a very strong negative correlation, again proving that the higher the concentration of sodiumthiosulphate, the faster the reaction will happen. This relates back to my hypothesis (which has been proven) – the more concentration of sodiumthiosulphate the more bonds will be broken in the faster time and therefore the rate of reaction will be higher. From the average results I have plotted on the graph I have inserted a line of best fit which intercepts a majority of the points. Along the line of best fit I have put one line representing the ‘mystery solution’. I tested the solution and the results are in the tables below and from this result inserted a line of best fit and predicted the concentration of sodium thio sulphate.
Evaluation: In my experiment I managed to fulfill the initial aim sufficiently which was to see if the concentration of sodiumthiosulphate affected the rate of reaction with hydrochloric acid once it’s been diluted with water. It is obvious from my results that the concentration of sodiumthiosulphate does affect the rate of reaction and when water is added the reaction consequently decreases in speed. A majority of my results were fairly accurate as I carried the experiment out well, in my opinion. There are generally no odd results, and I think that is because the experiment was very fair and all precautions were carried out with care. The precautions that were paid attention to were thorough cleaning of the beaker and conical flask, as if this was not done sufficiently it could have influenced the next set of results, the exact measurements of hydrochloric acid, water and sodiumthiosulphate. To improve the experiment I could use all different equipment to make sure that there were definitely no influences on the result or use different equipment simultaneously so the results are entirely independent. If I was to create another experiment I would investigate whether the application of thermal energy affected the rate of reaction, and if it did in what way. The equipment used would be a conical flask, measuring cylinder, pipette, sodiumthiosulphate, hydrochloric acid, stopwatch, plain paper, and safety glasses. Also a water bath would be needed. I would predict that when higher amounts of heat were applied to the solution the faster the rate of the reaction would be. This is because more energy is applied and therefore the molecules would be ‘propelled’ and would break the bonds faster, with more force, and consequently all the bonds would be broken faster. Precautions would be to make sure that the substance didn’t boil and leave deposits in the conical flask. The method would be as follows:
Step 1: Gather all the apparatus.
Step 2: Make sure all safety points are covered.
Step 3: Measure the rate of reaction at room temperature.
Step 4: Then use a water bath to raise the temperature. This will maintain the temperature of the solution.
Step 5: Put the conical flask in icy water so the temperature decreases to 5 degrees and then measure the rate of reaction.
Step 5: Put the conical flask in icy water so the temperature decreases to 5 degrees and then measure the rate of reaction.
In this experiment I would not judge the time by when the cross disappears as this is inaccurate so I will measure the volume of gas produced using a graduated syringe.
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