Rates of reactions

RESEARCH:

Chemical engineers exploit chemical reactions to produce materials on a commercial scale. One of their principal activities is the design and operation of chemical reactors. In order to accomplish design goals, the engineer often needs to know the kinetics of a reaction -- the factors that influence the rate of the reaction. In what follows, a brief discussion of the basic ideas in chemical kinetics is presented along with the mathematical models used to study the kinetics of chemical reactions. The close resemblance of the mathematical equations modeling chemical kinetics to those modeling population dynamics will become apparent. Appropriate links to topics in population dynamics will be indicated for you to explore if you so desire.
Reaction Rates
A chemical reaction involves one or more substances (reactants) that react to produce other substances (products). As the reaction proceeds, some chemical species are depleted while others are formed. Certain laws govern this process, and these laws can be expressed in terms of mathematical equations
Balance Laws
One of the assumptions made in chemical kinetics is that the number of atoms is preserved, i.e. atoms are neither created nor destroyed. For example, if there are atoms of Oxygen, , present before the reaction begins, then there will be the same number, , of atoms of Oxygen during all stages of the reaction. This is illustrated by the following stoichiometric equation describing the decomposition of nitrous oxide (a gas) into nitrogen and oxygen gases (do not confuse the nitrogen and oxygen gases, and , with the elements Oxygen, , and Nitrogen, ):

PLAN:

I must produce a piece of coursework investigating varrying 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 using the following caculation:

Rate of reaction = 1 divided by Time taken for rection to conclude.

According to the collision theory of reacting particles, there are five factors that can affect the rate of a reaction: temperature, concentration (of solution), pressure (in gases), surface area (of solid reactants), and catalysts. I have chosen to investigate the effect temperature has on a reaction. This is because temperature is practical and easy to investigate practical. Time is a serious element in my investigation, the preparation of a solid in powdered and unpowdered form would take longer to prepare, and it is difficult to get accurate readings due to the inevitabilities of human errors. A 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 cause a series of anomalous results.

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AIM: To observe the effects of a change in temperature on the rate of a reaction between Sodium Thiosulphate and Hydrochloric acid.

The reaction that will be used is:

Sodium Thiosulphate + Hydrochloric Acid --> Sodium chloride + water + Sulphur Dioxide.

(In symbol form)

This is a preview of the whole essay

AIM: To observe the effects of a change in temperature on the rate of a reaction between Sodium Thiosulphate and Hydrochloric acid.

The reaction that will be used is:

Sodium Thiosulphate + Hydrochloric Acid --> Sodium chloride + water + Sulphur Dioxide.

(In symbol form)

A series of experiments will be carried out changing the temperature while other factors remain constant. Both the sodium thiosulphate and the Hydrochloric acid are soluble in water. My first recording will be demonstrated at room temperature, this will be discovered by using a thermometre, I shall wait until the temperature remains constant before proceeding. When varrying the temperature I shall use a water bath to heat up the solution allowing it to reach the necessary temperature. A preliminary investigation was used to decide which apparatus was appropriate and the varriation of the temperatures. The results of my preliminary investigation are below:

My preliminary investigation allowed me to discover that any temperature below 20 C reacted too slowly, and 90¦C reacted too quickly to be worth including in my final investigation.

I have decided to operate the final investigation using the following apparatus:

1 thermometer = to allow me to test the temperature of the solution.
2 measuring cylinders = to measure the amounts of sodium thiosulphate and Hydrochloric acid used.
1 heatproof mat = for the experiment to place safely on.
1 stopwatch = to allow me to time the rate of reaction.
5 waterbaths = to be set at varried temperatures (ranging from 40-80 C) to heat the solution.
X board = to place the conical flask (containing Sodium thiosulphate/Hydrochloric acid solution) on allowing me to observe the rate of reaction.
1 Conical Flask = to contain the solution .
1 pair of tongs = to allow me to pick up the beaker safely after it has been heated.
1 pair of goggles = to protect my eyes from any rogue substances they may be subject to.

During this investigation I have decided to use Sodium Thiosulphate in the concentration of 0.03 moles per litre, 50cm shall be used in each experiment. I will be using Hydrochloric acid in batches of 5cm for each experiment.

METHOD:

collect nescessary apparatus and assemble in an appropriate fashion.
Using a measuring cylinder, collect 50cm of Sodium Thiosulphate and place into the conical flask.
Heat to appropriate temperate using the waterbath, check the accuracy of the water baths temperature by using a thermometre.
Place the X board under the flask.
Add 5cm of dilute hydrochloric acid swirling the flask to mix the two solutions. Using a stopwatch, start timing straight away.
Keep eyes on the cross and stop the clock when the solution has gone cloudy and the cross is no longer visible.
Repeat experiment varrying the temperature.
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 hydrochloric acid and thiosulphate will not be mixed up. The amount of hydrochloric acid will be 5cm each time, and the amount of thiosulphate will be fixed at 50 cm3. During the heating stage of the experiment,a constant temperature will be used throughout. 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. 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 temperature is increased. This can be justified by relating it 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. 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 AE. 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.

TABLE OF RESULTS:

ANALYSIS:

In this experiment I have found that as the temperature 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 reactions/collisions take place per second. In the temperature experiment the time taken for a reaction to take place decreased by roughly 10 seconds for every 10¦C increase in temperature. 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 against time taken for the reaction to take place) the graphs have negative correlation in both cases, meaning that as the temperature increased the time taken for the reaction to take place decreases. The time graph for the temperature experiment has a steep curve than, meaning that the decrease in time taken for the reaction was far more rapid.

Temperature graphs have a positive correlation as the temperature 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 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.

EVALUATION:

Overall I believe my investigation has been fairly sucessful the results I have obtained support my initial prediction, yet some of the errors I have made have left room for improvement. The anomalous reults shown on my graph could be due to a number of factors, for one I may not I have been entirely accurate with the timing system I have used. I only used one stopwatch (controlled by myself) and so time would have been lost (or made up) from the time I observe the first changes in the solution to the time I stop the clock. My own human reaction time may have been quicker or slower from experiment to experiment.

Another factor that could have been effecting the progress of my experiment was the fact that after taking the conical flask out of the waterbath I did not check the temperature again to make sure bath had maintained its acuracy. Any drop in temperature may have caused a varriation in my results.

If I carried out this investigation again I think I would pay more attention to degrees of accuracy, I would also like to investigate the effects of concentration on the rate of a reaction as this would supply a more varried aray of data and may help me to understand different aspects of the ‘collision theory’.

Although there are aspects that need improving in the experimentation rea of my coursework I am pleased with the overall product I have produced. I believe some aspects of my experiments were sucessful allowing a fairly high degree of accuracy, for instance I always took full care to make sure that the conical flask used was thoroughly cleaned in between each experiment and the area in which I worked was a safe environment, all these factors will help me improve any future investigations I carry out.

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