Factors that Affect the Rate of A Reaction

Factors that Affect the Rate of A Reaction
A chemical reaction occurs when two-reactant particles collide with sufficient energy, facing each other in the correct direction. There is always a transfer of energy and a new product formed. There is some times a change in the appearance of the new product. If chemical reactions did not take place there would only be about 100 different substances in the world. All other materials are created by chemical reactions. Mass is not loss when reactants become products. During a reaction bonds in the reactants are broken (energy required) and bonds in the product are formed (energy required). Reactions always involve a change/transfer in energy. The reactants always give out or take in energy. This is usually in the form of heat energy. Reactions involving a change/transfer of energy can be put into two categories - Endothermic reactions: ? require energy to proceed and will absorb heat form surroundings. Exothermic reaction: ? involves the release of energy and will tend to cause temperature to increase due to the formation of more stable products. In the reaction between Calcium Carbonate (limestone) and Hydrochloric acid, these are the reactants and the products are Calcium Chloride and Carbon Dioxide. This Balanced Word Equation shows the reaction CaCO3 + 2HCl CaCl2 + CO2 + H20 The rate of this reaction may be determined by measuring the amount of Calcium Carbonate, amount of Calcium Chloride or the amount of Carbon Dioxide produced. The simplest technique would involve measuring the amount of Carbon Dioxide given off.
The rates of these reactions must be carefully considered by chemists and chemical engineers in industrial situations for some reactions such as the production of a final material fast reactions will be more desirable than slower ones to increase the output, which will increase profits form sales. For some reactions such as the digression of a material, reducing the rate may be beneficial to increase the lifetime of the product. Altering Rates Adjusting several factors can increase the rate of a reaction. These include; - Concentration If we consider a reaction occurring in a solution the greater the concentration is the greater number of reactant particles are present or available. There for increasing the concentration will increase the rate of the reaction. The speed of the reaction is directly proportional to the concentration of the solution. This means, if the concentration is doubled them rate of the reaction will double. The low concentration of blue particles means that there is less chance of them colliding with the red particles. The high number of blue particles present means there is more chance of them coming into contact with the red ones. This means there is more chance of a reaction between the two. Temperature By altering the temperature of a reaction you can affect or control the rate at which it occurs. If we look at temperature, we know that the motion of the particles will increase because they have gained more energy. This will lead to an increase in the number of collision increasing the rate of the reaction. At a lower temperature the particles have less energy therefor they move more slowly and have less chance of colliding. At a high temperature the particles move more vigorously and have more chance of colliding and reacting. Pressure In gaseous reactions, changing the pressure will alter the rate at which it occurs. Increasing the pressure will increase the density of the number of particles per unit volume leading to more collisions at a faster rate as the particles have less room to move so more chance of coming into contact with each other and reacting. The particles are closer together and have more chance of coming into contact. There is more room in the sealed container so the particles have less chance of coming into contact. Surface area When solids are reacted, the surface area that is exposed to the reactant will greatly affect the rate of the reaction. If two equal weights of solid, one in the form of a fine powder and one in the form of one large lump are added to equal amounts of solution the powder will dissolve quicker because more surface area is exposed to the solution. This can be easily understood when looking at sugar. One large lump of sugar will dissolve much slower than caster sugar because more surface area is exposed to the reactant. In addition, the large lump is much harder to break down that the finer caster sugar. The reaction in this diagram is faster because the red particles come into contact with an overall larger surface area. The large lump can only react with the surface area exposed to the red particles therefor the reaction is slower. Catalysts Theses are chemicals, which are added in a reaction to increase the rate, which it occurs at. Yet they are not actually converted or consumed in the reaction. Specific catalysts are used for each type of reaction. They may often achieve the same product of yield as the normal reactants but with a lower temperature or pressure. Industry finds catalysts very useful and makes good use of them. Lower temperatures and pressure will always be favourable, economical, and safer. I think the most important factor that affects the rate of the reaction is the concentration because an increase or decrease in the concentration of the reactant will have a huge affect on the rate at which it occurs. I also think that the temperature is another important factor to control. Investigating the rate of the reaction between Sodium Thiosulphate and hydrochloric acid. Aim; The aim of this investigation is to see how the concentrations of reactants effect the rate at which a reaction occurs. The reaction between Sodium Thiosulphate and hydrochloric acid can be used to investigate how the concentration of reactants used can effect the rate at which a reaction occurs. When the two chemicals react, they form a yellow precipitate turning a transparent solution into an opaque one. By using a beaker with a marked piece of paper positioned underneath it we can react the two chemicals of different concentrations and time the duration of the reaction. The pattern (in this case a cross) will disappear after a specific quantity of Sulphur suspension has formed. Prediction: I think that increasing the concentration of the reactants will increase the rate at which the reaction occurs because I know that the greater the concentration the greater the number of reactant species available. Increasing the concentration will therefor increase the collision frequency. Increasing the collision frequency will therefor increase the chance the particles have of reacting. I think that doubling the concentration of the reactants will double the rate at which the reaction occurs this can be explained when looking at the concentrations more closely. If the concentration of hydrochloric acid is doubled and the Sodium Thiosulphate is kept at the same concentration then there are twice the number of Hydrochloric acid molecules in the solution when compared with the Sodium Thiosulpahte. This means the ratio of Hydrochloric acid to Sodium Thiosulphate is 2:1, which means there is a one In two chance of the coming into contact with the Sodium Thiosulphate particles and reacting. I think the Speed of the reaction will be proportional to the concentration. (Information obtained from gcse Text book by B.earl and L.D.R Willford, AirCom Education CDRom and Compton?s Interactive Encyclopaedia) Pre test: To ensure that the correct range of the concentration of the reactant is used in the experiment I will complete a pre test to check. I will do this by measuring out the Sodium Thiosulphate to concentrations of 20%, 40%, 60%, 80% and 100% and adding them separately to 100% concentrations of hydrochloric acid and time how long it takes for the solution to change from transparent to opaque. I will then change the experiment by changing the concentration of the Hydrochloric acid to the concentrations mentioned above and add the to a 100% concentration of Sodium Thiosulphate. Pre Test results. Concentration ( % ) Time taken (sec) for cross to disappear Hydrochloric Acid Sodium Thiosulphate 100% 100% 7.99 80% 100% 8.14 60% 100% 7.91 40% 100% 8.17 20% 100% 8.57 100% 80% 10.73 100% 60% 12.31 100% 40% 16.03 100% 20% 23.83 These results tell me that the different concentrations of Sodium thiosulphate I have decided to use should be fine for the experiment. The results are in line with my prediction although as the concentrations of Hydrochloric acid were changed it seemed it made little or no difference to the results. This could be due to a number of factors, which will need to be controlled during the experiment to ensure reliable results are gained. This anonymous result could have been caused by inaccurate timing, reading of the time or not rinsing out the beaker thoroughly enough (leaving some of the reactants from the previous test could give inaccurate results because it would affect the concentrations of the new one). These results may have been due to the concentration of Hydrochloric acid being too low in the first place. At 0.4m p/litre. To prevent this from happening in the actual experiment I will use 1M P/ litre of Hydrochloric acid. All of the above will need to be considered when doing the actual experiment. Factors to control To ensure that reliable results are gained form this experiment there are a number of factors which need to be controlled, these include: ? Only changing the concentration of the reactant to the required measurements there for I will need to ¼ ? Keep the temperature of the substances the same. Changing the temperature could effect the rate at which the reaction occurs. Increasing the temperature increases the kinetic energy the molecules have, therefor increasing the chance of collisions between the particles. Decreasing the temperature would have the opposite effect. ? The volume of the container needs to be kept the same. Increasing or decreasing would effect the rate at which the particles collide and react with each other. This is because the change in the volume of the container results in a change of the area the particles have to move. ? Use the same marked piece of paper, which is positioned under the beaker because the shading of the cross may effect the rate at which it disappears. Darker crosses may be a lot easier to see through the solution than lighter shaded ones. ? For each concentration, I will need to take the results three times and get an average of the time taken (in seconds) for the cross to disappear. This will give me more reliable results. Equipment 2x 25cm3 measuring 1x White tile (with cross drawn) cylinders 1x 50cm3 glass beaker The chemicals used for experiment 1x Stop clock Method: ? On white certamic tile draw a large cross in the centre and place a 50cm3 beaker on top of it. ? Measure out the different concentrations of Hydrochloric acid starting with 25cm3 to 5cm3(see table). With this, add the amount of water needed to create a total solution volume of 25cm3 e.g. if 15cm3 are measured out add 10cm3 of water etc. ? In turn, add the solutions to a beaker of 25cm3 of Sodium Thiosulphate that has been placed on the piece of paper with a cross on it. In-between tests ensure the beaker is thoroughly rinsed and dried to remove all reactants from the previous tests. ? Record the time (in seconds) it takes the solution to turn opaque and the cross to disappear. ? Repeat the experiment this time changing the concentration of the Sodium Thiosulphate making sure the total volume of the solution equals 25cm3 and add it to 100% concentration Hydrochloric acid. ? Repeat the whole experiment 3 times and record the average time (in seconds) it takes for the cross to disappear at each concentration of Hydrochloric acid and Sodium Thiosulphate. The Results Table Volume of solution (cm3) Time taken for cross to disappear (s) Sodium Thiosulpahte Hydrochloric acid Water Concentration (as %) Test 1 Test 2 Test 3 Average time taken 25 25 0 100 % HCl, 100% Na2S2O3 9.93 9.5 9.44 9.623 25 20 5 80% HCl 9.89 10.23 10.16 10.093 25 15 10 60% HCl 11.67 11.05 11.59 11.436 25 10 15 40% HCl 12.30 12.96 12.29 12.516 25 5 20 20 % HCl 13.65 13.12 13.85 13.423 20 25 5 80% Na2S2O3 9.33 10.38 9.92 9.876 15 25 10 60%Na2S2O3 13.49 13.80 13.86 13.71 10 25 15 40% Na2S2O3 16.93 19.83 17.83 18.07 5 25 20 20 % Na2S2O3 30.33 30.47 29.63 30.143

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