COLLISION THEORY
Two chemicals can only react if they collide with each other. . However, not all collisions result in a reaction. The collisions have to have enough energy. Particles in a liquid or gas have energy because they are always moving. This energy is called kinetic energy: the faster the particles are moving the more kinetic energy they will have. If low energy particles collide they will not react but simply bounce off each other. The minimum amount of energy for a reaction to take place is called its activation energy.
CONCENTRATION
The collision theory offers an explanation as to why the speed of a reaction increases as the concentration of the solutions increase. Particles can only react if they collide, in more concentrated solutions the particles are close together and therefore the distance between reactant particles is small and there will be more collisions per second. Although not collisions will result in the formation of product the rate of reaction will be greater that in a less concentrated solution.
SURFACE AREA
Everyone that has tried to light a fire knows that you need small pieces of wood to start with. Setting fire to a tree truck is very difficult. The collision theory explains why this is so. Burning wood is a chemical reaction between the gas oxygen and the solid wood. The particles of gas must collide with the particles of wood. These collisions can only take place on the surface of the wood. If the wood is cut or broken into smaller pieces then there is a greater surface area for the oxygen to collide with.
TEMPERATURE
Colliding particles must have enough energy if they are to react. Temperature is a measure of kinetic energy. If reactants are heated they possess more energy and will be moving faster and so the time between collisions will be reduced and the collisions will be more energetic. One way of explaining these twin factors is to think about dodgem cars. Imagine dodgem being driven by blindfolded people at a very slow speed. You would have to wait a long time for there was a crash and the crash would be very gentle. Now imagine these same dodgem cars being driven very fast. You wouldn’t have to wait very long for a crash and since they were going fast the crash would be violent.
CATALYST
A catalyst is a substance that speeds up the rate of a chemical reaction. The Lock & Key Theory offers an explanation as to how it works. Thee catalyst provides a surface on which the reactants ‘meet’ and then combine. The substrate provides a site for the reactants to meet. The reactant molecules have exactly the right shape to fit into the site like a lock and key. While held in this position the particles react.
RATE OF REACTION OF SODIUM THIOSULPHATE WITH HYDROCHLORIC ACID
When sodium thiosulphate reacts with hydrochloric acid a yellow precipitate is formed. The reactants are colourless but as the reaction takes place the liquid turns yellow and it is no longer possible to see through the liquid.
The equation for the reaction is
PREDICTION
I predict that the speed of the reaction will increase as the concentration of sodium thiosulphate increases. The reason for me making this prediction is that in a concentrated solution the reacting particles will collide more frequently and consequently the chances of reacting will be increased.
FAIR TEST
For this experiment I am going to change the concentration of the sodium thiosulphate I will do this by adding water to my solution. I will make the test fair by keeping the concentration and quantity of acid the same throughout the experiment. I will use the same size and shape flask. I will thoroughly clean the flask between experiments and I will draw the same size X.
PRELIMINARY WORK
APPARATUS
Stopwatch
Conical Flask
Sheet of white paper
Measuring cylinder
METHOD
A large X will be drawn on a sheet of white paper. 50 ml of sodium thiosulpate will be poured into the conical flask. 25 ml of dilute hydrochloric acid will be measured into a clean empty flask.
The acid will be poured into the conical flask containing the sodium thiosulpate AND simultaneously the stopwatch started. The flask will be placed on top of the X. The cross will be continuously observed by looking down through the top of the flask. As the reaction proceeds the cross will become less visible. The stopwatch will be stopped at the precise point when the cross is no longer visible.
The above procedure will be repeated using 40 ml of sodium thiosulphate and 10 ml of distilled water, then 30 ml of sodium thiosulphate and 20 ml of water. The results will be put into the following table
RESULTS
The graph shows that the more sodium thiosulphate there is in the solution the shorter the time for the cross to disappear.
I am now going to plot another graph to show how the rate of the reaction is affected by the concentration. The rate is found by dividing one by the time. The rate for 13.59 seconds is 1/13.59 which is 0.073583517. I will work out the other rates and put the results in another table.
The line of best fit for this graph is a straight line through the origin. This means that for this experiment the rate of reaction is directly proportional to the amount of thiosulphate.
Mandy the computer drew the above line. I believe you said you would draw the graphs by hand. Your line should look like the one above – but draw your line full length.
CONCLUSION
I predicted that the more concentrated the sodium thiosulphate the faster would be the reaction. This prediction was correct. For the concentrations of sodium thiosulphate used my results show that the rate of reaction is directly proportional to the concentration.
EVALUATION
I think my method worked well although it was not easy to decide the exact time at which the X disappeared. If I were to do the experiment again I would use a light meter. I would set up the meter to measure the amount of light passing through the solution. When the meter reached a particular point I would stop the stopwatch. If I were to do the experiment again I would use more varied concentrations and in particular would use 15 ml, as there is a big difference between the rate of reaction with 10 ml and the rate of reaction with 20 ml of sodium thiosulphate. I drew the graph when I had finished the experiment. Next time I do an experiment I will try and do the graph at the same time. This will help me decide if I need to more readings and, if so, where the readings should be.
