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Sodium Thiosulphate Coursework Essay

We must produce a piece of coursework investigating the rates of reaction, and the effect different changes have on them. The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. It is measured by dividing 1 by the time taken for the reaction to take place. There is five factors which affect the rate of a reaction, according to the collision theory of reacting particles: temperature, concentration (of solution), pressure (in gases), surface are (of solid reactants), and catalysts. I have chosen to investigate the effect temperature and concentration have on a reaction. This is because they are the most practical to investigate – it would take longer to prepare a solid in powdered and unpowdered form, and it is difficult to get accurate readings due to the inevitabilities of human errors, and as gas is mostly colourless it is difficult to gauge a reaction changing the pressure, and if a substance is added to give the gas colour, it may influence the outcome of the experiment. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.

Experiment 1 – Changing the concentration

5 cm3 of HCl (at concentration 1 mol./dm3) and 15 cm3 of sodium thiosulphate (at varying concentrations – 10 to 35 g/dm3) are poured out into two measuring cylinders and then poured into a conical flask, which is placed on top of a board marked with letter X. The stopwatch will now be started. When the mixture has turned sufficiently cloudy so that the letter X can no longer be seen the stopwatch will be stopped and the time will be recorded. The experiment is repeated with all the concentrations. The whole procedure is then repeated.

Experiment 2 – Changing the temperature

5 cm of HCl (at concentration 1 mol./dm3) and 15 cm of sodium thiosulphate (at varying concentrations – 10 to 35 g/dm3) are poured out into two measuring cylinders. A beaker is half filled with hot water from a tap. The water is placed on top of a Bunsen on a blue flame and the two measuring placed inside the water bath. The water is heated to the necessary temperature (30?C to 70?C) then the two measuring cylinders are taken out and the contents of both are poured into a conical cylinder. The time it takes for the X to disappear is timed and recorded. The experiment is repeated using all the temperatures. The entire procedure is the repeated.

Repeat results and averages will be taken to improve the credibility of the findings, and present solid grounding for the final conclusion. The repeat results will help to iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a third experiment will be performed to replace the anomalous set of results.

Safety – A pair of goggles will be worn during the heating part of the experiment in order to protect the eyes. An apron will also be worn to protect the skin and clothing. When handling hot beakers and measuring cylinders a pair of tongs will be used. A gauze and heatproof mat will be used while heating to avoid any damage to the equipment.

Fair Test – In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the HCl and thiosulphate will not be mixed up. The amount of HCl will be 5 cm3 each time, and the amount of thiosulphate will be fixed at 15 cm3. During the heating stage of the experiment, a blue flame will be used throughout. Also the same Bunsen burner and gas tap will be used to maintain continuity. All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful.

Prediction –

I predict that as the temperature is increased the rate of reaction will increase. I also predict that as the concentration of the sodium thiosulphate increases the rate of reaction will increase. This means that both graphs drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the concentrationtemperature is increased. This can be justified by relating to the collision theory. When the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully. If solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. All this can be understood better with full understanding of the collision theory itself:

For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.

Concentration – If the concentration of a solution is increased there are more reactant particles per unit volume. This increases the probability of reactant particles colliding with each other.

Pressure – If the pressure is increased the particles in the gas are pushed closer. This increases the concentration and thus the rate of reaction.

Surface Area – If a solid is powdered then there is a greater surface area available for a reaction, compared to the same mass of unpowdered solid. Only particles on the surface of the solid will be able to undergo collisions with the particles in a solution or gas.

The particles in a gas undergo random collisions in which energy is transferred between the colliding particles. As a result there will be particles with differing energies. Maxwell-Boltzmann energy distribution curves show the distribution of the energies of the particles in a gas.

The main points to note about the curves are:

1. There are no particles with zero energy.

2. The curve does not touch the x-axis at the higher end, because there will always be some particles with very high energies.

3. The area under the curve is equal to the total number of particles in the system.

4. The peak of the curve indicates the most probable energy.

The activation energy for a given reaction can be marked on the distribution curve. Only particles with energy equal or greater than the activation energy can react when a collision occurs.

Although Maxwell-Boltzmann distribution curves are for the particles in a gas, the same distributions can be used for the particles in a liquid or solid.

Effects of a temperature change – The graph below shows Maxwell-Boltzmann distribution graphs for a fixed mass of gas at two temperatures – T1 and T2, where T2 is roughly 10?C higher than T1. The total area under the curve remains the same, since there is no change in the number of particles present.

A small increase in temperature causes significant changes to the distribution energies. At the higher temperature:

1. The peak is at a higher energy.

2. The peak is lower.

3. The peak is broader.

4. There is a large increase in the number of particles with higher energies.

It is the final change that results increase in rate, even with a relatively small increase in temperature. A small increase in temperature greatly increases the number of particles with energy greater than the activation energy. The shaded areas on the energy distribution curves show this.

Effect of a catalyst – A catalyst works by providing an alternative reaction pathway that has lower activation energy. A catalyst does not alter the Maxwell-Boltzmann distribution. Because a catalyst provides a reaction route of lower activation energy, however, a greater proportion of particles will have energy greater than the activation energy.

Analysis

In this experiment I have found that as the temperature and concentration is increased the time taken for the reaction to take place decreases. This means the rate of reaction increasers as it takes less time for a reaction to take place, so more take place per second. In the temperature experiment the time taken for a reaction to take place decreased by roughly 10 to 15 seconds for every 10?C increase in temperature, with the one anomaly being the 30?C reading. There is also a trend in the increase in rate of reaction as the temperature increases. The difference is always more or less 0.02 s-1, with the same exception.

Using the graphs, with lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that with the “time” graphs (that plot temperature and concentration against time taken for the reaction to take place) the graphs have negative correlation in both cases, meaning that as the temperatureconcentration increased the time taken for the reaction to take place decreases. The time graph for the temperature experiment has a much steeper curve than the one for the concentration experiment, meaning that the decrease in time taken for the reaction was far more rapid.

Naturally, the above means that the both the graphs plotting rate against temperature and concentration have positive correlation – as the temperature and concentration are increased so does the rate of reaction. This is because when the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully, and when solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur.

The graph for concentration shows that when the concentrations were relatively low (10, 15, 20 g/dm3), the increase of rate x1000 was also fairly small (increasing from 4.47 to 6.71 to 9.47). There was then a gradual increase in the difference, and between 30 and 35 g/dm3 the rate more than doubled from 17.90 to 37.56s-1. This shows that there are far more collisions at a concentration of 35 g/dm3 than at 30 g/dm3.

The graph plotting time against the rate of reaction x1000 shows that the difference of rate between increasing temperatures (excluding the anomaly of 30?C) was pretty much regular, increasing in steps of 6-10 (9.17 to 15.37 to 24.28 to 31.67). However, once again there is a giant gap in the last temperature increase – at 60?C the RoR x1000 is 31.67 s-1, and at 70?C it is 57.03 s-1.

For this to fully make sense it is necessary to recap the collision theory briefly:

For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.


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